Chapter

30

Space Communications
Radio amateurs point theirantennas skyward to talk via
communications satellites built
by groups of interested andtalented hams. In the first part ofthis chapter, Steve Ford, WB8IMY,
provides some background andshows what you need to get
involved with amateur satellites.
Later we’ll discuss Earth-MoonEarth (EME) communications,also known as moonbounce.
The EME section, written by
Joe Taylor, K1JT, discusses therequirements for successful
contacts via the EME path. K1JT
gratefully acknowledges input andadvice for the EME section from
Allen Katz, K2UYH, Marc Franco,
N2UO, Peter Blair, G3LTF,
Graham Daubney, F5VHX, Lionel
Edwards, VE7BQH, Ian White,
GM3SEK, Leif Asbrink, SM5BSZ
and Rex Moncur, VK7MO.
A Glossary of space communications terms may be found at theend of this chapter.

30.1 Amateur Satellite History
Many people are astonished to discover that Amateur Radio satellites are not new. In fact,
the story of Amateur Radio satellites is as old as the Space Age itself.
The Space Age is said to have begun on October 4, 1957. That was the day when the Soviet
Union shocked the world by launching Sputnik 1, the first artificial satellite. Hams throughout
the world monitored Sputnik’s telemetry beacons at 20.005 and 40.002 MHz as it orbited theEarth. During Sputnik’s 22-day voyage, Amateur Radio was in the media spotlight since hams
were among the few civilian sources of news about the revolutionary spacecraft.
Almost four months later, the United States responded with the launch of the Explorer 1
satellite on January 31, 1958. At about that same time, a group of Amateur Radio operatorson the West Coast began considering the possibility of a ham satellite. This group later organized itself as Project OSCAR (OSCAR is an acronym meaning Orbiting Satellite Carrying
Amateur Radio) with the expressed aim of building and launching amateur satellites. (Seethe sidebar “When Does a Satellite Become an OSCAR?”)
After a series of high-level exchanges with the American Radio Relay League and theUnited States Air Force, Project OSCAR secured a launch opportunity. The first AmateurRadio satellite, known as OSCAR 1, would “piggyback” with the Discoverer 36 spacecraft
being launched from Vandenberg Air Force Base in California. Both “birds” (as satellites arecalled among their builders and users) successfully reached low Earth orbit on the morningof December 12, 1961.
OSCAR 1 weighed only 10 pounds. It was built, quite literally, in the basements and garages of the Project OSCAR team. It carried a small beacon transmitter that allowed groundstations to measure radio propagation through the ionosphere. The beacon also transmitted
telemetry indicating the internal temperature of the satellite.
OSCAR 1 was an overwhelming success. More than 570 amateurs in 28 countries forwarded observations to the Project OSCAR data collection center. OSCAR 1 lasted only 22days in orbit before burning up as it reentered the atmosphere, but Amateur Radio’s “lowtech” entry into the high tech world of space travel had been firmly secured. When scientificgroups asked the Air Force for advice on secondary payloads, the Air Force suggested they
study the OSCAR design. What’s more, OSCAR 1’s bargain-basement procurement approachand management philosophy would become the hallmark of all the OSCAR satellite projects
that followed, even to this day.
Since then, amateurs have successfully built and launched dozens of satellites, each one
progressively more sophisticated than the last.

30.1.1 AMSATMuch of the amateur satellite progress has been spearheaded by the Radio Amateur Satellite Corporation, better known as AMSAT. The original AMSAT was formed in Washington,
DC in 1969 as an organization dedicated to fostering an Amateur Radio presence in space.
The AMSAT model quickly became international with many countries creating their own
AMSAT organizations, such as AMSAT-UK in England, AMSAT-DL in Germany, BRAMSATin Brazil and AMSAT-LU in Argentina. All of these organizations operate independently but
may cooperate on large satellite projects and other items of interest to the worldwide AmateurRadio satellite community. Because of the many AMSAT organizations now in existence,
Space Communications   30.1

30.7 Satellite References
Books
S. Ford, WB8IMY, The ARRL Satellite
Handbook (Newington: ARRL, 2008)
Getting Started with Amateur Satellites,
available from AMSAT-NA at www.
amsat-na.com/store/.
Digital Satellite and Telemetry Guide,available from AMSAT-NA at www.
amsat-na.com/store/.

ArticlesP. Alvarez-Cienfuegos, EB4DKA and
J. Montana, EA4CYQ, “A Better Way toWork Low Earth Orbit FM Satellites,”
QST, Feb 2009, pp 41-43
L.B. Cebik, W4RNL, “Circularly Polarized
Aimed Satellite Antenna,” Antenna
Options, QEX, Sep/Oct 2007, pp 51-56.
P. Antoniazzi, IW2ACD, and M. Arecco,
IK2WAQ, “Low-Profile Helix Feed forPhase 3E Satellites: System Simulationand Measurements,” QEX, May/Jun
2006, pp 34-39.
M. Spencer, WA8SME, “A Satellite
Tracker Interface,” QST, Sep 2005,
pp 64-67.
L. Smith, W5KQJ, “An Affordable Az-El
Positioner for Small Antennas,” QST,
Jun 2003, pp 28-34.

30.8 Earth-Moon-Earth (EME) Communication
EME communication, also known as
moonbounce, has become a popular form ofamateur space communication. The EME
concept is simple: the moon is used as apassive reflector for two-way communication between two locations on Earth (seeFig 30.61). With a total path length of about
half a million miles, EME may be considered
the ultimate DX. Very large path losses suggest big antennas, high power and the best
low noise receivers; however, the adoption ofmodern coding and modulation techniques
can significantly reduce these requirementsfrom their levels of just 10 years ago. Evenso, communication over the EME path presents unusual station design challenges andoffers special satisfaction to those who canmeet them.

HBK0108

Antenna
Radiation Patterns

Earth 300 K*

30.8.1 Background
EME is a natural and passive propagation
phenomenon, and EME QSOs count toward
WAC, WAS, DXCC and VUCC awards. EME
opens up the bands at VHF and above to a
new frontier of worldwide DX.
Professional demonstrations of EME capability were accomplished shortly after WW II.
Amateurs were not far behind, with successful
reception of EME echoes in 1953 and pioneering two-way contacts made on the 1296, 144
and 432 MHz bands in the 1960s. Increased
EME activity and advances to other bands
came in the 1970s, aided by the availabilityof reliable low-noise semiconductor devices
and significant improvements in the design ofYagi arrays and feed antennas for parabolic
dishes. These trends accelerated further in the
1980s with the advent of low-noise GaAsFETand HEMT preamplifiers and computer-aided
antenna designs, and again after 2000 with
the introduction of digital techniques. See thesidebar, “Amateur EME Milestones.”
EME QSOs have been made on all amateur
bands from 28 MHz to 47 GHz. Many operators have made WAC, WAS and even DXCC
on one or more of the VHF and UHF bands.
EME is now within the grasp of most serious
VHF and UHF operators.
30.30   Chapter 30

Moon
210 K*XMTR
Transmitter
Feed Line

RCVR
ReceiverFeed Line* Noise temperature, Kelvins
Fig 30.61 — Schematic representation of major system components for a (one-way)
EME path. This illustration also shows some of the factors contributing to system
noise temperature, which is discussed later in the chapter.

Amateur EME Milestones
1953
1960
1964
1964
1970
1970
1972
1987
1987
1988
1988
2001
2005
2005
2009

W3GKP and W4AO detect lunar echoes on 144 MHz
First amateur 2-way EME contact: W6HB works W1FZJ, 1296 MHz
W6DNG works OH1NL, 144 MHz
KH6UK works W1BU, 432 MHz
WB6NMT works W7CNK, 222 MHzW4HHK works W3GKP, 2.3GHz
W5WAX and K5WVX work WA5HNK and W5SXD, 50 MHz
W7CNK and KA5JPD work WA5TNY and KD5RO, 3.4 GHz
W7CNK and KA5JPD work WA5TNY and KD5RO, 5.7 GHz
K5JL works WA5ETV, 902 MHz
WA5VJB and KF5N work WA7CJO and KY7B, 10 GHz
W5LUA works VE4MA, 24 GHz
AD6FP, W5LUA and VE4MA work RW3BP, 47 GHz
RU1AA works SM2CEW, 28 MHzGDØTEP works ZS6WAB, 70 MHz

30.9 EME Propagation
30.9.1 Path Loss
Path loss in free space is caused by nothing
more than the spherical expansion of a radio
wave as it propagates away from an antenna.
An EME signal is attenuated as 1/d2 (inverse
distance squared) over the quarter-million
mile path to the moon, and again as 1/d2 on
the return trip, for a net 1/d4 path loss. Radio
waves incident on the surface of the moon
are often said to be “reflected,” although infact they are partly absorbed and partly scattered by the irregular lunar surface. A full
expression giving the EME path loss as aratio of received power to transmitted power,
assuming isotropic antennas at each end of
the path, is=

h r 2l264 π2 d 4

(1)

where
r is the radius of the moonl is the wavelengthd is the distance to the moonh is the lunar reflection coefficient.

Average Path Loss(dB)
–242.9–252.1–255.8–261.6–268.0–271.2–276.2–279.7–284.1–289.2–293.5where pt is the transmitted power. The standard
expression for an antenna’s power gain iswhere A is the effective aperture or collecting
area. Gain in dBi (dB over an isotropic
antenna) may therefore be written as(2)

Inserting values r = 1.738 × 106 m, d =3.844 × 108 m and h = 0.065 gives the average path losses quoted in Table 30.7 for the
principal amateur EME bands. The need toovercome these very large attenuations is ofcourse the main reason why EME is so challenging. The moon’s orbit is an ellipse, and its
distance d varies by ±6.8% over each month.
Because of the inverse-fourth-power law inEquations (1) and (2), this change results in
path-loss variations of ±1.1 dB at the extremesof lunar distance, independent of frequency.
The reflection of radio waves is of course notaffected by the optical phases of the moon.
The dependence of path loss on l2 suggeststhat EME should be nearly 20 dB more difficult at 1296 MHz than at 144 MHz. Thisconclusion is misleading, however, becauseof the assumption of isotropic antennas. If
one uses transmitting and receiving antennas of gain gt and gr, expressed as ratios, theexpected power pr received as a lunar echo
may be written as the productp r = pt gt g r Frequency
(MHz)
50
144
222
432
902
1296
2304
3456
5760
10368
24048

g = 4π A / l 2

In this section we use the convention of lowercase letters to denote dimensionless ratios,
and the corresponding upper-case letters togive equivalent values in dB. Thus, the EME
path loss in dB is given for isotropic antennas
by the expression
 h r 2l2 
L = 10 log  = 10 log  64 π2 d 4 Table 30.7
Two-Way EME Path Loss with
­Isotropic Antennas(3)

G = 10 log (4π A / l 2 )With Pr and Pt expressed in dB relative to somereference power, for example 1 W, we have
Pr = Pt + G t + L + G r(4)Thus, assuming a fixed size of antenna,such as a parabolic dish or Yagi array of effective frontal area A, the frequency dependence
is reversed: for a given transmitted power,
lunar echoes would be 20 dB stronger for
every decade increase in frequency, rather
than 20 dB weaker. Most practical situations
fall somewhere between these two extremesof frequency dependence.
For reasons explained in detail below, amateur EME communication is feasible withroughly comparable degrees of difficultyover nearly two decades of frequency, from
144 MHz to 10 GHz. Not surprisingly, somevery different techniques must be mastered inorder to do successful EME at the lower andupper extremes of this wide frequency range— so the final choice of band(s) for EME isoften determined by the interests, skills andresources of an individual operator.

30.9.2 Echo Delay and Time
SpreadRadio waves propagate at speed c, thespeed of light, very nearly equal to 3 × 108
m/s. Propagation time to the moon and backis therefore 2d/c or about 2.4 s at perigee,
2.7 s at apogee and 2.56 s on average. The
moon is nearly spherical, and its radius cor-

responds to r/c = 5.8 ms of wave travel time.
The trailing parts of an echo, reflected from
irregular surface features near the edge ofthe lunar disk, are delayed from the leading edge by as much as twice this value. In
practice, most of the moon’s surface appears
relatively smooth at the radio wavelengths
used for amateur EME. Lunar reflections
are therefore quasi-specular, like those froma shiny ball bearing, and the power usefulfor communication is mostly reflected froma small region near the center of the disk.
At VHF and UHF frequencies, the effective
time spread of an echo amounts to no morethan 0.1 ms.
Reflection from a smooth surface preserves
linear polarization and reverses the sense of
circular polarization. At shorter wavelengths
the lunar surface appears increasingly rough,so reflections at 10 GHz and above containa significant diffuse component as well asa quasi-specular component. The diffuse
component is depolarized, and significant
portions of it arise from regions farther out
toward the lunar rim. The median time spread
can then be as much as several milliseconds.
In all practical cases, however, time spreading
is small enough that it does not cause significant smearing of CW keying or intersymbolinterference in the slowly keyed modulations
commonly used for digital EME.
Time spreading does have one very significant effect. Signal components reflectedfrom different parts of the lunar surface travel
different distances and arrive at Earth withrandom phase relationships. As the relative
geometry of the transmitting station, receiving station and reflecting lunar surfacechanges, signal components may sometimes
add and sometimes cancel, creating large
amplitude fluctuations. Often referred to as
libration fading, these amplitude variations
will be well correlated over a coherence
bandwidth of a few kHz, the inverse of thetime spread.

30.9.3 Doppler Shift andFrequency Spread
EME signals are also affected by Doppler
shifts caused by the relative motions of Earthand moon. Received frequencies may be higher or lower than those transmitted; the shift is
proportional to frequency and to the rate ofchange of total path length from transmitter toreceiver. The velocities in question are usually
dominated by the Earth’s rotation, which at
the equator amounts to about 460 m/s. For the
self-echo or “radar” path, frequency shift will
be maximum and positive at moonrise, fallingthrough zero as the moon crosses the local
meridian (north-south line) and a maximum
negative value at moonset. The magnitude ofSpace Communications   30.31

shifts depends on station latitude, the declination of the moon and other geometrical factors. For two stations at different geographic
locations the mutual Doppler shift is the sumof the individual (one-way) shifts. Maximum
values are around 440 Hz at 144 MHz, 4 kHz
at 1296 MHz and 30 kHz at 10 GHz.
Just as different reflection points on the
lunar surface produce different time delays,
they also produce different Doppler shifts.
The moon’s rotation and orbital motion are
synchronized so that approximately the same
face is always toward Earth. The orbit is eccentric, so the orbital speed varies; since therotation rate does not vary, an observer on
Earth sees an apparent slow “rocking” of themoon, back and forth.
Further aspect changes are caused by the
5.1° inclination between the orbital planesof Earth and moon. The resulting total lineof-sight velocity differences are around
0.2 m/s, causing a frequency spread of order
0.2 Hz at 144 MHz. Like all Doppler effects,
these shifts scale with frequency. However,measured values of frequency spread increaseslightly more rapidly than frequency to thefirst power because a larger portion of thelunar surface contributes significantly to echo
power at higher frequencies. Linear scaling
would suggest frequency spread around
15 Hz at 10 GHz, but measurements show it
to be several times larger.
From a communication engineering pointof view, libration fading is just another example of the so-called Rayleigh fading observedon any radio channel that involves multiple
signal paths — such as ionospheric skywave,
tropospheric scatter and terrain multipath
channels with reflections from buildings,
trees or mountains. Interference effects thatcause signal fading depend on frequency
spread as well as time spread. Signal amplitudes remain nearly constant over a coherencetime given by the inverse of frequency spread.
In general, fading rates are highest (shortest
coherence times) when the moon is close to
the local meridian and lowest near moonrise
and moonset. They also depend on the moon’s
location in its elliptical orbit.
Typical coherence times are several seconds at 144 MHz, a few tenths of a second at
1296 MHz and 20 ms at 10 GHz. At 144 MHz,intensity peaks lasting a few seconds can aid
copy of several successive CW characters,
but at 432 MHz the timescale of peaks anddropouts is closer to that of single characters. At 1296 MHz the fading rates are often
such that CW characters are severely chopped
up, with dashes seemingly converted to several dits; while the extremely rapid fading at
10 GHz can give signals an almost “auroral”
tone. Skilled operators must learn to deal withsuch effects as best they can. As describedfurther below, modern digital techniques canuse message synchronization as well as errorcorrecting codes and other diversity tech30.32   Chapter 30

niques to substantially improve the reliability of copy on marginal, rapidly fading EME
signals.

30.9.4 Atmospheric andIonospheric Effects
Propagation losses in the Earth’s troposphere
are negligible at VHF and UHF, although rain
attenuation can be an important factor above
5 GHz. Tropospheric ducting of the sort that
produces enhanced terrestrial propagation canbend signals so that the optimum beam heading for EME is directed away from the moon’s
center. Even under normal conditions, enoughrefraction occurs to allow radio echoes whenthe moon is slightly below the visible horizon.
In practice, these problems are usually overshadowed by other complications of doing
EME at very low elevations, such as blockage
from nearby trees or buildings, increased noisefrom the warm Earth in the antenna’s main
beam and man-made interference.
The Earth’s ionosphere causes several propagation effects that can be important to EME.
These phenomena depend on slant distancethrough the ionospheric layer, which increases
at low elevations. At elevation 10°, attenuation
through the daytime ionosphere is generally
less than 0.5 dB at 144 MHz, and nighttime
values are at least 10 times lower. These numbers scale inversely as frequency squared, so
ionospheric absorption is mostly negligible
for EME purposes. Exceptions can occur at
50 MHz, and under disturbed ionosphericconditions at higher frequencies. Ionosphericrefraction can also be important at 50 MHz,at very low elevations. Ionospheric scintillations (analogous to the “twinkling” of stars in
the Earth’s atmosphere) can exhibit significant
effects at VHF and UHF, primarily on EME
paths penetrating the nighttime geomagnetic
equatorial zone or the auroral regions. Again,
disturbed ionospheric conditions magnify theeffects. The multipath time spread is verysmall, less than a microsecond, while frequency
spread and fading rate can be in the fractionalhertz to several hertz range. These scintillations
can increase the fading rates produced by Earthrotation and lunar librations.
Much more important is the effect of Faraday rotation in the ionosphere. A linearly polarized wave will see its plane of polarization
rotate in proportion to the local free-electron
density, the line-of-sight component of the
Earth’s magnetic field and the square of wavelength. The effect is therefore greatest during
the daytime, for stations well away from theequator, and at low frequencies. A mismatch
Dq between an incoming wave’s polarization
angle and that of the receiving antenna willattenuate received signal power by an amountcos2Dq. As shown in Fig 30.62, polarization
losses increase rapidly when the misalignment exceeds 45°.
Because of the l2 dependence, Faraday

Fig 30.62 — Attenuation caused by
misalignment of a linearly polarized
signal with the polarization angle ofa receiving antenna. The attenuation
increases rapidly for alignment errors
greater than 45°.

rotation is generally important for EME operation only at 432 MHz and below. The effectis cumulative for an outgoing signal and its
returning echo, so a station transmitting andreceiving with the same linearly polarized
antenna will see its own echoes disappear
whenever the total Faraday rotation is close toan odd integral multiple of 90°. Faraday rotation in the daytime ionosphere can amountto as much as a full turn at 432 MHz andmany turns at 144 MHz. At 432 MHz the rotation may be essentially constant over several
hours; on lower bands significant changes canoccur in 30 minutes or less. Variations areespecially noticeable near sunrise or sunset
at one end of the path, where ionization levels
are changing rapidly.
The Earth’s spherical shape determines
the orientation in space of a wave emitted or
received by an antenna with horizontal (orother locally referenced) polarization angle.
As discussed in detail below, when combinedwith Faraday rotation this effect can cause
users of fixed-linear-polarization antennas toexperience apparent one-way propagation.
A polarized radio signal reflected from the
moon’s rough surface is partially scatteredinto other polarization states, and a disturbed
ionosphere can sometimes generate a mixtureof polarization angles. As a consequence, fading caused by 90° polarization misalignments
will not always produce deep nulls. Measurements show that at UHF and below, the crosspolarized scattered signal is usually 15 dB
or more below the principal polarization. On
the other hand, at 10 GHz and higher, where
the lunar surface is much rougher in terms ofwavelength, cross-polarized diffuse echoes
may be only a few dB below the principal
reflected polarization. These comments apply
to both linear and circular polarization.

30.10 Fundamental Limits
30.10.1 Background NoiseEME signals are always weak, so considerations of signal-to-noise ratio are paramount.
A received signal necessarily competes withnoise generated in the receiver as well as thatpicked up by the antenna, including contributions from the warm Earth, the atmosphere,
the lunar surface, the diffuse galactic and cosmic background and possibly the sun andother sources. (Refer to Fig 30.61, and thinkof adding a warm atmosphere just above theEarth, the sun somewhere beyond the moon,and galactic and extragalactic noise sources ateven greater distances, filling the whole sky.)If Pn is the total noise power collected fromall such noise sources expressed in dBW, we
can write the expected signal-to-noise ratioof the EME link asaxy, the Milky Way. An all-sky map of noise
temperature at 144 MHz is presented in the
top panel of Fig 30.63. This noise is strongest
along the plane of the Galaxy and toward thegalactic center. Galactic noise scales as frequency to the –2.6 power, so at 50 MHz thetemperatures in Fig 30.63 should be multipliedby about 15, and at 432 divided by 17. At 1296
MHz and above galactic noise is negligible
in most directions. During each month themoon follows a right-to-left path lying close to
the ecliptic, the smooth solid curve plotted in

SNR = Pr − Pn = Pt + G t + L + G r − Pn (5)Since isotropic path loss L is essentially fixedby choice of a frequency band (Table 30.7),
optimizing the signal-to-noise ratio generally
involves trade-offs designed to maximize Pr
and minimize Pn — subject, of course, tosuch practical considerations as cost, size,
maintainability and licensing constraints.
It is convenient to express Pn (in dBW) interms of an equivalent system noise temperature Ts in kelvins (K), the receiver bandwidth
B in Hz and Boltzmann’s constant k = 1.38
× 10–23 J K–1:
Pn = 10 log (kTs B)(6)

The system noise temperature may in turnbe written as
Ts = Tr + Ta

(7)

Here Tr is receiver noise temperature, related
to the commonly quoted noise figure (NF)
in dB by()

Tr = 290 10 0.1NF − 1

Fig 30.63 — Top: All-sky contour map of sky background temperature at 144 MHz. The
dashed curve indicates the plane of our Galaxy, the Milky Way; the solid sinusoidal
curve is the plane of the ecliptic. The Sun follows a path along the ecliptic in one year;
the moon moves approximately along the ecliptic (± 5°) each month. Map contours areat noise temperatures 200, 500, 1000, 2000 and 5000 K. Bottom: One-dimensional plotof sky background temperature at 144 MHz along the ecliptic, smoothed to an effective
beamwidth 15°.

(8)

Antenna temperature Ta includes contributions from all noise sources in the field ofview, weighted by the antenna pattern. The
lunar surface has a temperature around 210 K;
since most antennas used for amateur EME
have beamwidths greater than the moon’s angular size, as well as sidelobes, the moon’s
effect will be diluted and noise from othersources will also be received. Sidelobes areimportant, even if many dB down from themain beam, because their total solid angle islarge and therefore they are capable of collecting significant unwanted noise power.
At VHF the most important noise source isdiffuse background radiation from our Gal-

Table 30.8
Typical Contributions to System Noise Temperature
Freq
(MHz)
50
144
222
432
902
1296
2304
3456
5760
10368
24048

CMB
(K)
3
3
3
3
3
3
3
3
3
3
3

Atm
(K)
0
0
0
0
0
0
0
1
3
10
70

Moon
(K)
0
0
0
0
1
2
4
5
13
42
170

Gal
(K)
2400
160
50
9
1
0
0
0
0
0
0

Side
(K)
1100
100
50
33
30
30
30
30
30
30
36

Ta
(K)
3500
260
100
45
35
35
37
40
50
85
260

Tr
(K)
50
50
50
40
35
35
40
50
60
75
100

Ts
(K)
3500
310
150
85
70
70
77
90
110
160
360

Space Communications   30.33

Fig 30.63. Sky background temperature behind the moon therefore varies approximately
as shown in the lower panel of Fig 30.63, regardless of geographical location on Earth. For
about five days each month, when the moon isnear right ascension 18 hours and declination–28°, VHF sky background temperatures near
the moon are as much as 10 times their averagevalue, and conditions for EME on the VHF
bands are poor.
By definition the sun also appears to an
observer on Earth to move along the ecliptic,
and during the day solar noise can add significantly to Pn if the moon is close to the sun or
the antenna has pronounced sidelobes. At frequencies greater than about 5 GHz the Earth’s
atmosphere also contributes significantly. An
ultimate noise floor of 3 K, independent of
frequency, is set by cosmic background radiation that fills all space. A practical summaryof significant contributions to system noise
temperature for the amateur bands 50 MHz
through 24 GHz is presented in Table 30.8
and Fig 30.64 and Fig 30.65, discussed in
the next section.

30.10.2 Antenna and Power
Requirements
The basic circumstances described so
far ensure that frequencies from 100 MHz to
10 GHz are the optimum choices for EME andspace communication. Over this region anda bit beyond, a wide variety of propagation
effects and equipment requirements provide a
fascinating array of challenges and opportunities for the EME enthusiast. The enormous
path-loss variability encountered in terrestrial HF and VHF propagation does not occur
in EME work, and some of the remaining,smaller variations — for example, those arising from changing lunar distance and different
sky background temperatures — are predictable. We can therefore estimate with someconfidence the minimum antenna sizes andtransmitter powers required for EME communication on each amateur band.
The necessary information for this task issummarized in Table 30.8 and Figs 30.64
and 30.65. Columns 2 through 6 of thetable give typical contributions to system
noise temperature from the cosmic microwave background (CMB), the Earth’s atmosphere, the warm surface of the moon, galactic
noise entering through the main antenna beamand sky and ground noise from an antenna’sside and rear lobes. Antenna temperature Ta
is a combination of all these contributions,appropriately weighted by antenna pattern;
the system noise temperature Ts is then thesum of Ta and receiver noise temperature Tr,
referred to the antenna terminals. Numbers
in Table 30.8 are based on the fundamentals
described in the previous section and on hypothetical antennas and receivers that conformto good amateur practice in the year 2009;30.34   Chapter 30

Fig 30.64 — Typical contributions to system noise temperature Ts as function of
frequency. See text for definitions and descriptions of the various sources of noise.

they have been rounded to two significant
figures. It is possible to do slightly better thanthese numbers — for example, by building
antennas with lower sidelobe response or preamplifiers with still lower noise figure — but
it’s not easy!
The topmost curve in Fig 30.64 illustrates clearly why the frequency range
from 100 MHz to 10 GHz is optimum for
EME communication. Fig 30.65 shows thatfurther reductions of Ts must come fromlower Tr or better suppression of antenna sidelobes. There is nothing you can do about noisefrom the CMB, atmosphere, moon or Galaxy
entering your main beam! For comparison,
the very best professional receiving equipment achieves system noise temperatures
around 20 K in the 1-2 GHz region — only
a few dB better than current amateur practice. These systems generally use cryogenic
receivers and very large dish antennas that can
provide better suppression of sidelobes.
Having established reasonable target figures for system noise temperature, we can
now proceed to estimate minimum antenna
and power requirements for an EME-capable
station on each amateur band. Rearrangementof Equations (5) and (6) yields the following
relation for transmitter power Pt in dBW:

Pt = SNR − G t − G r − L + 10 log(kTs B)(9)

Values for L and Ts can be taken for eachamateur band from Tables 30.7 and 30.8.
For illustrative purposes let’s assume SNR =3 dB and B = 50 Hz, values appropriate fora good human operator copying a marginal
CW signal. (The 50 Hz effective bandwidth
may be established by an actual filter, or morecommonly by the operator’s “ear-and-brain”
filter used together with a broader filter.) For
antennas we shall assume bays of four long
Yagis for the 50 through 432 MHz bands,
parabolic dishes of diameter 3 m on 1296
and 2304 MHz and 2 m dishes on the higher
bands. Representative gains and half-power
beamwidths for such antennas are listed incolumns 3 and 4 of Table 30.9. Column
5 then gives the necessary transmitter powerin watts, rounded to two significant figures.
A station with these baseline capabilities
should be self sufficient in terms of its ability
to overcome EME path losses — and thus able
to hear its own EME echoes and make CW
contacts with other similarly equipped EME
stations. Note that the quoted minimum valuesof transmitter power do not allow for feed line
losses; moreover, a CW signal with SNR =Fig 30.65 — Percentage contributions to system noise temperature as a function of
frequency.

3 dB in 50 Hz bandwidth hardly represents“armchair copy.” At the highest frequencies, ­issues of oscillator stability and Doppler spreading might make the assumed
50 Hz bandwidth unrealistically narrow,thus requiring somewhat more power ora larger antenna. On the other hand, lower
power and smaller antennas can be sufficientfor working stations with greater capabilities
than those in the table.
Other factors can reduce the minimum
power or antenna gains required for successful
EME, at least some of the time. One possibility, especially effective at 50 and 144 MHz at
low moon elevations, is to take advantage ofreflections from reasonably smooth ground(or better still, water) in front of your antenna.
­Often referred to as ground gain, these reflections can add as much as 6 dB to an antenna’s effective gain at elevations where thereflections are in phase with the direct signal.
Another possibility is to use more efficientcoding and modulation schemes than provided
by Morse coded CW.

30.10.3 Coding andModulation
International Morse code with on-off keying(OOK) is an excellent general purpose communication mode. It is easy to implement andperforms well in weak-signal conditions. EME
operating procedures for CW usually include
multiple repetitions so that essential parts ofa bare-minimum QSO can be assembled fromfragments copied on signal peaks. However,modern communication theory points the way
toward modulation schemes significantly more
efficient than OOK, codes better than Morse,and error-control methods more effective thansimple repetition. Amateur experiments withthese ideas have led to the current popularityof digital EME on the VHF and lower UHFbands. In general, an efficient digital mode
designed for basic communication with weaksignals will compress user messages into a
compact form and then add redundancy inthe form of a mathematically defined errorcorrecting code (ECC). Such codes can ensurethat full messages are recoverable with highconfidence, even when significant portions ofa transmission are lost or corrupted.
A number of distinct sources may contribute to the improved performance of such amode over CW. Multitone FSK (MFSK) is amore efficient modulation than OOK, in part
because each received symbol is roughly theequivalent of a full character, rather than asingle dot or dash. For equivalent messages,
MFSK can therefore be keyed much more
slowly than CW and detected in a muchsmaller bandwidth. Morse code is self-syn-

Fig 30.66 — TF/DL3OCH used a ruralroad sign to help him give contacts with
Iceland to a number of EME operatorson 1296 MHz. Bodo has activated many
DXCC entities on 1296 MHz EME by using
JT65, a single long Yagi and a 100 W solidstate amplifier.

chronizing at the character level (if a signal
is strong enough for letters to be recognized),
but a Morse transmission contains no useful information for synchronizing a wholemessage. This fact makes it difficult to piece
together copied fragments of a CW message
being sent repeatedly.
In contrast, a synchronized digital transmission with ECC can encode the complete
message into a new data format designed
to enhance the probability that successful
decoding will produce the message’s full

Table 30.9
Typical Antenna and Power
Requirements for CW EME
Freq
(MHz)
50
144
432
1296
2304
3456
5760
10368

Ant
Type1
4×12 m
4×6 m
4×6 m
3m
3m
2m
2m
2m

G
(dBi)
19.7
21.0
25.0
29.5
34.5
34.8
39.2
44.3

HPBW
(deg)
18.8
15.4
10.5
5.5
3.1
3.0
1.8
1.0

TxPwr
(W)
1200
500
250
160
60
120
60
25

1Example antennas for 50, 144 and 432 MHz

are Yagi arrays with stated lengths; those for1296 MHz and higher are parabolic dishes of
specified diameter.

Space Communications   30.35

information content, with everything in its
proper place. For the limited purpose of exchanging call signs, signal reports and modest
amounts of additional information, digital
EME contacts can be made at signal levels
some 10 dB below those required for CW,
while at the same time improving reliability
and maintaining comparable or better rates of

information throughput. Depending on yourskill as a CW operator, the digital advantage
may be even larger.
Thus, digital EME contacts are possible
between similar stations with about 10 dB
less power than specified in Table 30.9, orwith 5 dB smaller antenna gains at both transmitter and receiver. An excellent example isthe highly portable EME setup of DL3OCH,
shown in Fig 30.66. With a single long Yagi
(59 elements, 5 m boom, 21.8 dBi gain), a
100 W solid state amplifier and the JT65C
digital mode,1 this equipment has helped
to provide dozens of new DXCC credits on
1296 MHz from countries with little or no
regular EME activity.

30.11 Building an EME Station
30.11.1 Antennas
The antenna is arguably the most importantelement in determining an EME station’s capability. It is not accidental that the baseline
station requirements outlined in Table 30.9
use Yagi arrays on the VHF bands and parabolic dishes at 1296 MHz and above: one ofthese two antenna types is almost always thebest choice for EME. The gain of a modern,well designed Yagi of length  can be approximated by the equation
G = 8.1 log ( / l) + 11.4 dBi,

(10)

and stacks of Yagis can yield close to 3 dB
(minus phasing line losses) for each doublingof the number of Yagis in the stack.
For comparison, the gain of a parabolic dishof diameter d with a typical feed arrangement
yielding 55% efficiency is
G = 20 log (d / l) + 7.3 dBi(11)

The gains of some nominal antennas of eachtype are illustrated graphically in Fig 30.67,
which helps to show why Yagis are nearly
always the best choice for EME on the VHF
bands. They are light, easy to build and haverelatively low wind resistance. Stacks offour Yagis are small enough that they can bemounted on towers for sky coverage free ofnearby obstructions. Larger arrays of 8, 16

Fig 30.67 — Representative gains of practical Yagi antennas, arrays of Yagis andparabolic dishes as a function of frequency. Yagi arrays make the most cost-effective
and convenient antennas for EME on the VHF bands, while parabolic dishes are
generally the best choice above 1 GHz.

30.36   Chapter 30

or even more Yagis are possible, although
the complexity and losses in phasing lines
and power dividers then become importantconsiderations, especially at higher frequencies. Long Yagis are narrowband antennas,
usable on just a single band.
We usually think of the linear polarizationof a transmitted signal as being “horizontal”
or “vertical.” Of course, on the spherical Earththese concepts have meaning only locally.
As seen from the moon, widely separated
horizontal antennas may have very different
orientations (see Fig 30.68). Therefore, in theabsence of Faraday rotation an EME signaltransmitted with horizontal polarization bystation A will have its linear polarization misaligned at stations B and C by angles known
as the spatial polarization offset. In Fig 30.68
the signal from A arrives with vertical polarization at B and at 45° to the horizon at C.
Suppose C is trying to work A and qs = 45° is
the spatial polarization offset from A to C. The
return signal from C to A will be offset in the
opposite direction, that is, by an amount −qs

Fig 30.68 — The spherical Earth createsspatial polarization offsets for wellseparated stations with horizon-oriented
linear polarization. Here, a signaltransmitted horizontally at A arrived withvertical polarization at B and midway
between horizontal and vertical at C.
When combined with Faraday rotation,offsets close to 45° can lead to apparent
one-way propagation. See text for details.

Computer and Internet Resources for EME
Software for finding and tracking the moon:
MoonSked, by GM4JJJ, www.gm4jjj.co.uk/MoonSked/moonsked.htm
EME System, by F1EHN, www.f1ehn.org/
EME2008, by VK3UM, www.ve1alq.com/vk3um/index.html
SkyMoon, by W5UN, www.w5un.net/
GJTracker, by W7GJ, www.bigskyspaces.com/w7gj/
WSJT, by K1JT, physics.princeton.edu/pulsar/K1JT/
Web calculator: www.satellite-calculations.com/Satellite/suncalc.htm
Software for EME performance calculations:
EMECalc, by VK3UM, www.ve1alq.com/vk3um/index.html
Digital EME:
WSJT, MAP65: physics.princeton.edu/pulsar/K1JT/
Topical email reflectors:
Moon-Net: www.nlsa.com/nets/moon-net-help.html
Moon: www.moonbounce.info/mailman/listinfo/moon
Beginner information:
W5UN: www.w5un.net/
EA6VQ: www.vhfdx.net/jt65bintro.html
W7GJ (EME on 50 MHz): www.bigskyspaces.com/w7gj/
Technical references:
SM5BSZ: www.sm5bsz.com/linuxdsp/linrad.htm
W1GHZ: www.w1ghz.org/antbook/contents.htm
GM3SEK: www.ifwtech.co.uk/g3sek/eme/pol1.htm
and www.ifwtech.co.uk/g3sek/stacking/stacking2.htm
F5VHX: www.rfham.com/g8mbi/g8mbi/pol1.htm
Dubus: www.marsport.org.uk/dubus/eme.htm
Chatrooms and Loggers:
NØUK: www.chris.org/cgi-bin/jt65emeA
ON4KST: www.on4kst.com/chat/
HB9Q: hb9q.ch/joomla/
Monthly Newsletters:
144 MHz, by DF2ZC: www.df2zc.de/newsletter/index.html
432 and Above, by K2UYH: www.nitehawk.com/rasmit/em70cm.html
Solar Flux data:
Archival: www.ngdc.noaa.gov/stp/SOLAR/ftpsolarradio.html#noonflux
Current: www.ips.gov.au/Solar/3/4/2

Fig 30.69 — Arrayof four 10-element,
dual-polarization
144 MHz Yagis
at KL7UW. Alaskan
frost makes thehorizontal andvertical elements
stand out clearly.
A pair of loop
Yagis for 1296 MHz
can be seen inside
the 2-m array.

Fig 30.70 — This 3 m TVRO dish withaluminum frame and mesh surface wasoutfitted for 1296 MHz EME as a joint
effort by VA7MM and VE7CNF. The dualcircular polarization feed is a VE4MA/
W2IMU design.

= –45°. The Faraday rotation angle qF, on theother hand, has the same sign for transmission
in both directions. Thus the net polarization
shift from A to C is qF + qs , while that fromC to A is qF – qs. If qF is close to any of thevalues ±45°, ±135°, ±225°, …, then one of
the net polarization shifts is nearly 90° while
the other is close to 0°. The result for stations
with fixed linear polarization will be apparent
one-way propagation: for example, A cancopy C, but C cannot copy A.
Obviously no two-way contact can be madeunder these conditions, so the operators must
wait for more favorable circumstances or else
implement some form of polarization control
or polarization diversity. One cost-effective
solution is to mount two full sets of Yagi
elements at right angles on the same boom.
Arrays of such cross-polarized or “Xpol” Yagis make especially attractive EME antennas
on the VHF and lower UHF bands because
they offer a flexible solution to the linearpolarization misalignment problem. As an
example, Fig 30.69 shows the 4 × 10 element,
dual-polarization EME array at KL7UW. Thisantenna and a 160 W solid-state amplifier
have accounted for hundreds of EME contacts
with the state of Alaska on 2 meters.
At 1296 MHz and above, gains of 30 dBi
and more can be achieved with parabolic dishes of modest size. As a result, these antennas
are almost always the best choice on these
Space Communications   30.37

Fig 30.71 — Mounting arrangement, counterweights and az/el control
system for the VA7MM 3 m dish.

bands. Their structure does not depend on any
radio frequency resonances, so in many ways
dishes are less critical to build than ­Yagis.
Element lengths in high-gain Yagis must be
accurate to better than 0.005 l, while the reflecting surface of a dish need be accurate only
to about 0.1 l. Mesh surfaces are attractiveat frequencies up to at least 5 GHz, becauseof their light weight and lower wind resis-

tance. Openings in the mesh can be as large as
0.05 l without allowing much ground noise
to feed through the surface. A parabolic antenna has a single feed point, so there are no
losses in phasing lines or power splitters. You
can use a dish on several bands by swapping
feeds, and with suitable feed designs you can
produce either linear or circular polarization,
including dual polarizations. A very attractive(A)and convenient option is to transmit in one
sense of circular polarization and receive inthe opposite sense. Transmitting in right-handcircular (RHC) and receiving in LHC has
become the standard for EME at 1296 and2304 MHz, and will probably become thestandard on higher bands as well.
As made clear in Fig 30.67, the 432 MHzband lies in a transition region where both
Yagis and parabolic dishes have attractive
features. Either four long Yagis or a 6 m dishcan produce enough gain (about 25 dBi) to
let you work many other EME stations on
this band. Many linear-polarization systems
are already in use — for good reason, sincemost amateur use of this band is for terrestrial
communi­cation — so converting everyone tocircular polarization is impractical. Therefore, schemes have been devised to physicallyrotate dish feeds and even whole Yagi arrays tocope with the resulting polarization alignment
problems. Another scheme is to use a dualpolarization dish feed or dual-polarization­Yagis, as described above and increasingly
used on 144 MHz. This approach has not
yet gained wide popularity on 432 MHz,however.
ANTENNA PATTERN
A clean pattern with good suppression ofside and rear lobes is important for all EME
antennas — especially at 432 MHz and above,
where excessive noise pickup through sidelobes can significantly increase Ts. For Yagi
arrays you should use modern, computeroptimized designs that maximize G/Ts, theratio of forward gain to system noise temperature. Be sure to pay attention to maintaining
a clean pattern when stacking multiple antennas. First sidelobes within 10-15° of the main
beam may not be a major problem, because(B)

Fig 30.72 — N4MW outfitted this 2.6 m offset parabolic dish for
10 GHz EME. Equipment mounted at the focus (close-up at B)
includes low noise preamplifier, transverter, traveling wave tube
(TWT) power amplifier and feed horn for 10 GHz.

30.38   Chapter 30

their solid angle is small and they will lookmostly at cold sky when EME conditions arefavorable. Side and rear lobes farther fromthe main beam should be suppressed as muchas possible, however. Remember that evenclose-in sidelobes will degrade your receiving
performance at low elevations.
For parabolic dishes, G/Ts is optimized byusing a feed with somewhat larger taper in illumination at the edge of the dish than would
yield the highest forward gain. Best forward
gain is generally obtained with edge taper
around −10 dB, while best G/Ts occurs around−15 dB. Edge taper of −12 dB is usually agood compromise. Some good reproducible
designs for dish feeds are described in references at the end of this section.2
ANTENNA MOUNTS
EME antennas have high gain and narrow
main beams that must be properly aimed at
the moon in two coordinates. Although polar
mounts (one axis parallel to the Earth’s axis)
have sometimes been used, by far the mostpopular mounting scheme today is the elevation-over-azimuth or az/el mount. Readily
available computer software (see the sidebar,“Computer and Internet Resources for EME”)
can provide azimuth and elevation coordinates for the moon, and a small computer canalso control antenna positioning motors toautomate the whole pointing system.
For mechanical reasons it is desirable toplace the antenna’s center of gravity close to
the intersection of the vertical (azimuth) andhorizontal (elevation) axes. On the other hand,
the mounting structure must not interfere withcritical active regions of the antenna. Stacked
Yagis are generally mounted so that metallic
supporting members are perpendicular to theradiating elements or located at midpoints
where the effective apertures of separate Yagis meet. Feed lines and conducting support
members must not lie in the active planes
containing Yagi elements, unless they run
wholly along the boom. For dual-polarization
Yagis, feed lines should be routed toward therear of each Yagi and any mid-boom support members must be non-conducting. For
EME there is nothing magical about using
horizontal and vertical for the two orthogonal
polarizations, and there are some advantages
to mounting cross-Yagis with elements in the“×” rather than “+” orientation.3
Parabolic dishes are usually mounted frombehind, with counterweights extending rearward to relieve torque imbalance on the elevation axis. Screw-jack actuators designed for
positioning 1980s-style TVRO dishes can bereadily adapted for elevation control.4 Standard heavy-duty antenna rotators can be used
for azimuth positioning of dishes up to about
3 m in size. Larger dishes may require heavier,
one-of-a-kind designs for pointing control.
Figs 30.70 through 30.72 show examples ofparabolic dishes in the 2-3 m range, neatly andsuccessfully outfitted for EME at VA7MM
and N4MW.

30.11.2 Feed lines,
Preamplifiers and TR
Switching
Any feed line between the antenna andreceiver introduces attenuation and noise, so

at UHF and above it is vital that the lownoise preamplifier (LNA) be mounted very
close to the antenna terminals. At ambient
temperature, every 0.1 dB of loss in frontof the LNA adds at least 7 K to the effective
Tr, and therefore to Ts. On bands where Ta ismuch lower than ambient, even 0.1 dB of attenuation can result in 0.5 dB loss of receiversensitivity.

Fig 30.73 — Recommended “front ends” for EME systems include low-noise
preamplifiers (LNAs) mounted at the antenna and use separate feed lines fortransmitting and receiving. Relay K2 shown in (A) may be omitted if K1 provides
adequate isolation of the LNA while transmitting. An arrangement like that in (B)is recommended for a dual-polarization antenna such as an array of cross-Yagis.
Transmitter power is sent to one polarization or the other, but received signals in bothpolarizations are amplified and sent on to a dual-channel receiver. Part (C) showsa suitable arrangement for circular polarization implemented with a two-port, dualcircular-polarization feed horn. In this case, no high-power TR relay is required.

Space Communications   30.39

LNA gain should be sufficient to overcome feed line losses and dominate the noise
contributed by subsequent stages by at least
15-20 dB. Current practices usually employ
one or, especially at 432 MHz and above,
two low-noise GaAsFET or HEMT transistors in the preamplifier, with only a simple
noise impedance matching circuit between
the first active device and the antenna. Only
if severe out-of-band interference is present
should a narrow filter be placed ahead of thefirst LNA. Bandpass filtering is often desirable between LNA stages, and may be usedwithout significant impact on system noise
temperature. The feedhorn of a dish antenna
can have a valuable high-pass effect that attenuates signals at lower frequencies.
The same antenna is generally used forboth transmitting and receiving, so the LNA
must be out of the line and protected whentransmitting. Fig 30.73A illustrates a preferred switching arrangement that uses two
separate feed lines: a low-loss line to carrytransmitter power and a relatively inexpensive
feed line from LNA to receiver. A high-power
relay K1 at the antenna handles TR switching;a second relay, K2, may be used to protect
the LNA in case the isolation at K1’s receive
port is inadequate.
Additional relays are required in order tomake best use of a dual-linear-polarization
system. Fig 30.73B shows an arrangementthat lets you select either horizontal or verticalpolarization for transmitting (via relay K5)
and use both polarizations simultaneously
for receiving. A dual-channel receiver canform a linear combination of signals in thetwo channels to match the polarization ofa desired signal exactly, whatever its angle.
Such optimization is readily accomplished ina receiver whose last stages are defined andimplemented in software, as in the highly effective Linrad system designed by SM5BSZ.5
For digital EME using dual-polarization antennas, Linrad and a software program called
MAP65 make an especially powerful combination for digital EME.6
With circular polarization you may notneed a high-power TR relay at all. Fig 30.73C
shows a typical arrangement with transmitterconnected to one port of a feed horn providing both senses of circular polarization, andthe LNA to the other port. Since the isolation between ports may be only 20 or 30 dB,
a low-power relay protects the LNA during30.40   Chapter 30

transmit periods. In all of these schemes, suitable sequencing should be used to assure that
the LNA is disconnected before transmission
can begin. Many amateurs find it best to use
the energized position of TR relays on receive.
When the station is not in use, preamplifiers
will then be disconnected from the antenna.

30.11.3 Transmitters andPower Amplifiers
FREQUENCY STABILITY
Weak signals are best detected in a bandwidth no wider than the signal itself. As aconsequence, EME systems must use stableoscillators. For best results with CW yourfrequency drift over a minute or so should
be no more than 10 Hz at the operating frequency; with digital modes the ideal target
may be several times smaller. Most moderntransceivers are stable enough at VHF, butsome only marginally so at UHF.
The crystal oscillators used in transverters may need temperature compensation foradequate stability; better still, they can bephase-locked to an external high-stability reference oscillator. A number of digital EME
QSOs have been made on the 23 cm and 13 cm
bands using only 5-10 W transmitter power
and parabolic dishes in the 2-3 m range —much smaller systems than the baseline examples listed in Table 30.9 — but such contacts generally depend on having stabilized
local oscillators. Especially for digital EME,
where detection bandwidths less than 10 Hz
are used, unstable oscillators lead directly toloss of sensitivity.
POWER OUTPUT
After frequency stability, the most important specification for an EME transmitter is power output. The maximum power
practically achievable by amateurs ranges
from 1500 W on the VHF and lower UHFbands down to the 100 W range at 10 GHz.
Fortunately, the required power levels forEME (Table 30.9) are compatible with thesenumbers. At 432 MHz and below, triode or
tetrode vacuum tubes with external-anodeconstruction can provide ample gain andpower output. Some popular tubes include
the 4CX250, 8930, 8874, 3CX800, 8877,
GU-74B, GS-23B and GS-35B. Amplifiers
using one or a pair of these (or other simi-

lar tubes) can provide output powers ranging up to 1000 or 1500 W on the 50 to 432
MHz bands. VHF and UHF power amplifiers
based on solid state power devices have also
become viable alternatives. Many amateurs
have built amplifiers using these techniques,
and commercial designs are available.
At frequencies above 1 GHz, transit-time
limitations and physical structures prevent
most high-power vacuum tubes from performing well. For many years planar triodes in
the 2C39/7289/3CX100 family have been themainstays of amateur 1296 MHz power amplifiers. Some higher power tubes for this band
include the GI-7B, GS-15, TH347, TH308
and YL1050. The 2C39/7289/3CX100 tubes,
as well as the GI-7B and GS-15, generally
require water cooling at the power levels desirable for EME.
Solid-state power amplifiers are also available for the microwave bands. As prices comedown and power levels increase, these units
are becoming more popular with EME operators. Surplus solid-state amplifiers usable on
the 902, 2304 and 3456 MHz bands have been
attractive buys over the last few years, providing output power up to several hundred watts
at reasonable cost. Two, four or even moreunits are sometimes used together to achieve
higher output. At the highest EME frequencies surplus traveling wave tube (TWT) amplifiers can provide power levels up to several
hundred watts.
RF SAFETY
One final point must be made in a discussion of high power amplifiers. Anyone who
has thought about how a microwave ovenworks should know why RF safety is an
important issue. Dangerous levels of radio
frequency radiation exist inside and at shortdistances from power amplifiers and antennas. In normal operation, power density ishighest in the immediate vicinity of small,
low-gain antennas such as feeds for parabolic
dishes. EME operators should be aware that
ERP (effective radiated power) can be highly
misleading as a guide to RF hazards. Somewhat counter-intuitively, a large, high-gain
antenna helps to reduce local RF hazards by
distributing RF power over a large physicalarea, thus reducing power density. Be sure
to read the RF Safety section in the Safety
chapter and pay attention to the RF protection
guidelines there.

30.12 Getting Started
Perhaps you already have a weak-signal
VHF or UHF station with somewhat lesser capabilities than those recommended in Tables
30.8 and 30.9, and would like to make a few
EME contacts before possibly undertaking
the task of assembling a “real” EME station.
On 144 and 432 MHz — probably the easiest
EME bands on which to get started — you
can visually point your single long Yagi at
the rising or setting moon and work some of
the larger EME stations. Your daily newspaper probably lists the approximate timesof moon rise and moon set in your vicinity;
many simple web-based calculators can give
you that information as well as the moon’s
azimuth and elevation at any particular time(see the sidebar, “Computer and Internet Resources for EME”).
These aids may be all you need to makeyour first EME contacts, especially if you takeadvantage of the weak-signal capabilities ofan efficient digital mode. To optimize yourchances of success, adapt your operating procedures to the prevailing standards that other
EME operators will be using, as discussed
below. For your first attempts you may wantto make prearranged schedules with someestablished stations.

30.12.1 Tracking the Moon
For serious EME work you’ll want a moregeneral way of keeping your antenna pointed
at the moon. The Earth and moon complete
their mutual orbit every 27.3 days, one sider­eal month. The lunar orbit is inclined to theplane of the ecliptic, the Earth-Sun orbital
plane, by 5.1°. Since the Earth’s equator isitself inclined at 23.5° to the ecliptic, themoon’s path through the sky swings northand south of the equator by as much as 28°
in a monthly cycle (see Fig 30.63).
Predicting the moon’s exact position is acomplicated problem in celestial mechanics,
but is readily handled to sufficient accuracy
by simple computer software. In general the
problem can be reduced to 1) calculating the
moon’s position on the sky, as seen by a hypothetical observer at the center of the Earth;2) applying a parallax correction to yield thelunar position at a specified location on the
Earth’s surface; and 3) converting the astronomical coordinates of right ascension anddeclination to azimuth and elevation at the
specified terrestrial location.
Of course, the Earth’s rotation and moon’s
orbital motion imply that the moon’s direction is constantly changing. Suitable computer software can follow these changes andgenerate the necessary commands to keep
your antenna pointed at the moon. Several
free or inexpensive software packages withmoon-tracking features are listed in the side-

Fig 30.74 — Monthly median values of solar flux density at six frequencies, over the
11 years of solar cycle 23. Original data are from the Sagamore Hill Observatory in
Massachusetts. At the lower frequencies, and especially near solar maximum, day-today and even hour-to-hour upward variations can be an order of magnitude larger thanthose in the monthly medians.

bar, “Computer and Internet Resources for
EME,” and such facilities are built into theprograms WSJT and MAP65 widely used for
digital EME. For good results you should aim
for pointing accuracies of about 1⁄4 of yourhalf-power beamwidth or better.

30.12.2 System Evaluation
Careful measurements of your EME system’s performance can help to determine
where station improvements can be made.
Transmitter performance is essentially determined by power output, feed line losses
and antenna gain. The first two can be measured in standard ways, but antenna gain ismuch more difficult. The most useful figureof merit for receiving performance is G/Ts,
and while absolute measurements of either G
or Ts separately are difficult, you can measure
their ratio with useful accuracy and compare itwith expectations. One technique particularly
useful at 432 MHz and above is to use thesun as a broadband noise source. Point yourantenna at cold sky and then at the sun, andmeasure y, the ratio of received noise powerin the two directions. Operate your receiverat maximum bandwidth with the AGC off,
and if possible make the observations withthe sun at elevation 30° or higher.
To calculate G/Ts from y, you will needa contemporaneous estimate of solar flux
density at your operating frequency. Daily
measurements of solar flux are obtained ata number of standard frequencies and madeavailable online (see the sidebar, “Computerand Internet Resources for EME”); you caninterpolate an approximate flux value for theamateur band in question. Solar flux densities
vary with sunspot activity, and day-to-day oreven hour-to-hour variations are especiallylarge at lower frequencies and near solar
maximum. Representative monthly medianvalues for six frequencies are presented in
Fig 30.74 over sunspot cycle 23. As a starting point, you can estimate a value of solar
flux for your band and a similar point in thesunspot cycle directly from Fig 30.74. Then,
Space Communications   30.41

if S* is the solar flux density in units of W
m-2 Hz-1 and y is expressed as a dimensionless ratio, the corresponding value of G/Ts indB is given by 8π k (y − 1) G
= 10 log  S* λ 2 
Ts



(12)

Solar fluxes are usually quoted in Solar Flux
Units (1 SFU = 10–22 W m-2 Hz-1), and with
S in those units Equation (12) can be reduced
to the simpler relation
 3.47 (y − 1) G
= 10 log 


Ts
S λ2



(13)

Representative values of G/Ts and Ysun = 10
log y for the example antennas of Table 30.9
are given in columns 4 and 5 of Table 30.10.
The tabulated values of Ysun are based on
quiet-sun flux densities typical of years nearsunspot minimum.
Of course, receiving your own echoes fromthe moon provides the best guarantee thatyour equipment is capable of EME communication with comparable stations. Transmitting
a few dashes, then standing by to hear yourlunar echo some 2.5 s later, brings a thrill thatmost hams never forget. With suitable signalprocessing software, echoes can be detected
and measured even with relatively low-power
equipment. Version 4.9.8 of the WSJT program (see the sidebar, “Computer and Internet
Resources for EME”) includes an automatedecho-testing facility that is useful for quantitative measurements even if your echoes aremany dB below the audible threshold.

30.12.3 Operating Procedures
The more that’s known about likely structure, content and timing of a transmitted message, the easier it is to copy. EME signals areoften near the threshold of readability, so it
is highly desirable to standardize operatingprocedures and message structure, and to provide transmissions with enough redundancy

Table 30.10
Representative G/Ts and Ysun for
Example Antennas
Freq
(MHz)
144
432
1296
2304
3456
5760
10368

Ant
Type1
4×6 m
4×6 m
3m
3m
2m
2m
2m

G
(dBi)
21.0
25.0
29.5
34.5
34.8
39.2
44.3

G/Ts
(dB)
–3.8
5.8
11.1
15.6
15.3
18.8
22.4

Ysun
(dB)
4
12
11
11
9
9
11

1Example antennas for 144 and 432 MHz are

Yagi arrays with stated lengths; those for1296 MHz and higher are parabolic dishes of
specified diameter.

30.42   Chapter 30

to capitalize on signal peaks and bridge likely
gaps in reception.
You may wish to make your first EME
QSOs with the aid of explicit schedules: that
is, arrangements to attempt a contact witha particular station at a specified time andfrequency. EME schedules usually state theduration of timed transmissions as well as
starting time, transmitter frequency, and an
indication of which station will transmit first.
For a minimal QSO, message information isoften reduced to the bare essentials of callsigns, signal reports and acknowledgments.
The signal report is sometimes reduced to a“yes or no” indication of whether both callsigns have been successfully copied.
Remember to allow for Doppler shifts,
especially at higher frequencies where theoffset may exceed your received bandwidth.
Most moon-tracking software used for EME
can display the expected frequency shift foryour own echoes as well as that for a distantstation. In a scheduled QSO attempt, keep
your transceiver set to the schedule frequency
and use its RIT control to search for the otherstation around his expected Doppler shift.
When looking for contacts at random, especially at 432 MHz and above, set your RIT
to the expected Doppler shift of your own
echoes. If a station you copy does likewise,
you will find each other’s signals on the same
frequency as your own echoes. Many operators also benefit from using a software-driven
panoramic spectral display of the “waterfall”
variety, which can make signal acquisition
much easier.
CW EME FORMAT
By convention, a minimal CW EME contact usually follows a format something likethe sequence of messages in Table 30.11.
If timed TR sequences are being used, theessential information is repeated for the full
duration of a sequence.
The standard QSO procedures involvea number of different messages sent in sequence, and operators do not proceed to thenext message until they have copied the essential information (call signs, signal report,
acknowledgements) in previous messages.
After call signs have been copied, a signal
report is sent. Because CW dahs are easier todiscern than dits, a default EME signal report(essentially meaning “I have copied both callsigns”) is the letter O. A station receiving callsigns and O responds with RO, and a final acknowledgment of a valid contact is signified
by sending RRR …. On 432 MHz and above,the letter M is sometimes used as an alternative signal report meaning “both call signs
copied with difficulty.” Of course, when signals are adequate for reasonably good copy
normal RST signal reports can be used andother restrictions on message structure andtiming relaxed.
In non-scheduled operation, it frequently

Table 30.11
Typical Messages in a Minimal EME
CW Contact
Period1
2
3
4
5
6

MessageCQ CQ CQ DE W6XYZ W6XYZ …
W6XYZ DE K1ABC K1ABC …
K1ABC DE W6XYZ OOO OOO …
W6XYZ DE K1ABC RO RO RO …
K1ABC DE W6XYZ RRR RRR …
W6XYZ DE K1ABC TNX 73 …

happens (for example, in response to yourCQ) that you can recognize and copy yourown call more easily than the other station’scall. The sequence YYY… (for “Your call”)
can be sent to ask a calling station to send hiscall only, repeating it many times. A contact
is considered complete and valid when RRR
has been received (after message number 5
in Table 30.11 has been received). However,at this point the other station does not knowthat his acknowledgment was copied, so it
is normal to finish with something like message 6.
The conventional duration of transmit andreceive periods is different on different bands
and has evolved somewhat over time. On 50
and 144 MHz, stations usually transmit for
one full minute and then receive for a fullminute. On 432 MHz and above, schedules
with 2.5-minute transmissions have beenstandard. The longer period gives stations
with mechanically variable polarization adequate time to peak a received signal.
CW sending speed is generally around 12
to 15 WPM. Some operators find it helpful
to use greater-than-normal spacing between
complete letters. Keep in mind that characterssent too slowly may be chopped up by typical
EME fading, while code sent too fast will bejumbled. When transmitting call sequences,send the other station’s call once, followed by“DE” and your own call once or twice. Thenpause and repeat the sequence. This cadence
sets a rhythm so that the receiving operator
can anticipate when the missing parts of amessage can be expected to arrive. Send withproper spacing; the use of a programmablekeyer is especially helpful and encouraged.
A signal buried in the noise and accompanied
by fading is hard enough to copy, without theadded complication of irregular sending.
DIGITAL EME WITH JT65
Most digital EME is presently done with
the JT65 protocol, as implemented in the computer programs WSJT and MAP65 (see thesidebar, “Computer and Internet Resources
for EME”). Like other popular digital modes,
this one requires a personal computer with asound card for audio input and output. JT65
uses structured message formats, a Reed Solomon error-correcting code with slightly morethan five-fold redundancy and 1-minute TR

Table 30.12
Typical Messages in a JT65 EME Contact
Period12
3
456

Fig 30.75 — Screen shot of computer
program WSJT.

sequences. Signal peaks and dropouts due tomultipath fading do not affect individual characters or words, but rather the probability of decoding the whole message. The modulation is65-tone frequency shift keying (65-FSK) withcomputer-generated audio tones modulating asingle-sideband transmitter in USB mode.
As expected from the theory describedbriefly above, results show that JT65 contacts can be made at signal levels about 10
dB less than those needed for CW. The detection bandwidth implemented in the receiving
software for JT65 sub-modes A, B and C is
2.7, 5.4 and 10.8 Hz, respectively. These may
be compared with minimum bandwidths of
25-50 Hz for 15 WPM CW. Requirementson oscillator stability are therefore somewhat
more stringent for JT65 than for CW.
Basic procedures for minimal JT65 contacts are very similar to those for CW, and atypical message sequence is shown in Table
30.12. Standard messages include space reserved for Maidenhead grid locators, so thisinformation is normally exchanged alongwith call signs. Following CW practice, you
should proceed to the next message in the
QSO sequence only when information fromthe previous step has been copied. A default
signal report may be sent as OOO, but many
operators prefer to send and receive numerical signal reports (giving the signal strength

Using Shorthand Messages
CQ W6XYZ CN87
W6XYZ K1ABC FN42
K1ABC W6XYZ CN87 OOO
RO
RRR
73

in dB, relative to noise power in a standard
2500 Hz bandwidth) as measured by the receiving software. Messages using such reports are shown in the final column of Table
30.12. Many additional details on the usage of
JT65 can be found in documentation distributed with the WSJT program. Fig 30.75 shows
an example screen shot of K1JT finishing acontact with RU1AA via 144 MHz EME.
Keep in mind that digital EME techniques
have been used for less than a decade and arelikely to be still evolving. New protocols may
be designed in the future using different typesof ECC, different message structures, longer
or shorter TR sequences, larger or smaller
bandwidths, and so on. The latest information
will likely be found online — for example,
at some of the internet addresses listed in the
sidebar, “Computer and Internet Resources
for EME.”

30.12.4 Finding QSO Partners
At the time of this writing (early 2009)
levels of EME activity are highest on
144 MHz. Almost any time the moon isabove your horizon (excluding a few days
each month when it is near the plane of
the Milky Way), you can find JT65 EME
signals in the frequency range 144.100 to
144.160 MHz — often dozens of them, especially on ­weekends. Hundreds of operators,
worldwide, are “on the moon” regularly using
JT65 on 144 MHz, and some of the larger
stations have made EME contacts with morethan 2000 other stations. Operating frequencies of particular stations are often posted onreal-time loggers (see the sidebar, “Computer

Using Long-Form Messages
CQ W6XYZ CN87
W6XYZ K1ABC FN42
K1ABC W6XYZ –21
W6XYZ K1ABC R–19
K1ABC W6XYZ RRR
TNX RAY 73 GL

and Internet Resources for EME”). Explicit
schedules can also be made on the loggers,
and this is a good way to initiate your first
EME contacts. Don’t compromise a true EME
contact by resorting to the logger after a QSO
attempt has been started, however.
CW activity on the 144 MHz band, as well
as most EME activity on higher bands in any
mode, tends to be concentrated in activity
weekends scheduled once each month whenthe moon is in a favorable location. After
144 MHz, the bands with most EME activity are 1296 and 432 MHz. On those bands
random CW (and occasionally SSB) activity is mostly found between 5 and 30 kHz
above the nominal sub-band boundary (forexample, between 1296.005 and 1296.030),
and digital activity is between .060 and .090,
concentrating around .065 or .070. A hundredor more stations are typically active on thesebands during the annual ARRL International
EME Competition and in other major operating events.
The higher microwave bands, especially
2.3 and 3.4 GHz, have at least several dozen
regularly active stations. On these and higher
bands activity tends to concentrate around.100. Keep in mind that different band segments may be assigned in different parts of
the world.7 For example, North American
stations use 2304.100, but some Europeans
work around 2320.100. EME activity on
the 50, 222 and 902 MHz bands is mostly
done with ­prearranged schedules. The sidebar, “Computer and Internet Resources for
EME” lists some resources that can help you
to find schedule partners and the dates of EME
contests and activity weekends.

30.13 EME Notes and References
1J. Taylor, K1JT, “The JT65 Communications

Protocol,” QEX, Sep/Oct 2005, pp 3-12.
2M. Franco, N2UO, “Computer Optimized
Dual Mode Circularly Polarized Feedhorn,”in Microwave Update 2008, ARRL, 2008,
and ok1dfc.com/EME/technic/septum/
N2UO%20opt.pdf; P. Wade, W1GHZ,“VE4MA and Chaparral feeds with Septum
Polarizers,” www.w1ghz.org/antbook/conf/
VE4MA_Chaparral_septum_feeds.pdf.;
G. Kollner, DL4MEA, “Designing a SuperVE4MA feed horn for 23 cm and 13 cm,”
Dubus, 1/2007, p 33.

3L. Åsbrink, SM5BSZ, “The ‘+’ Configuration vs.

the ‘X’ Configuration,” www.sm5bsz.com/
polarity/simplesw.htm.
4D. Halliday, KD5RO (now K2DH), “Microwave
EME Using a Ten-Foot TVRO Antenna,” The
ARRL UHF/Microwave Projects Manual, Vol
1, 1996; M. Mattila, VE7CMK, and T. Haynes,
VE7CNF, “Project Moonbounce: EarthMoon-Earth Station VA7MM,” www3.telus.
net/public/va7mm/eme/.
5L. Åsbrink, SM5BSZ, “Linrad: New Possibilities
for the Communications Experimenter,” QEX,Part 1, Nov/Dec 2002 pp 37-41; Part 2, Jan/Feb 2003 pp 41-48; Part 3, May/Jun 2003
pp 36-43; Part 4, Sep/Oct 2003 pp 29-37; “Linrad with High Performance Hardware,” QEX,Jan/Feb 2004, pp 20-31; “Linrad Home Page”,
www.sm5bsz.com/linuxdsp/linrad.htm.
6J. Taylor, K1JT, “MAP65: A Panoramic, Polarization-Matching Receiver for JT65,”
in Microwave Update 2007, ARRL, 2007,
and physics.princeton.edu/pulsar/K1JT/
MAP65.pdf.
7Y. Mataka, JA4BLC, “Full Spectrum Receiverfor 13 cm EME,” Dubus 3/2005, p 21.

Space Communications   30.43

30.14 Glossary of Space Communications Terminology
AMSAT — A registered trademark of theRadio Amateur Satellite Corporation,a nonprofit scientific/educationalorganization located in Washington, DC.
It builds and operates Amateur Radio
satellites and has sponsored the OSCAR
program since the launch of OSCAR 5.
(AMSAT, PO Box 27, Washington, DC
20044.)
Anomalistic period — The elapsed time
between two successive perigees of asatellite.
Antenna temperature — Used in EME
system evaluation, a measure of thenoise picked up by the antenna fromall noise sources in the field of view,
weighted by the antenna pattern.
AO-# — The designator used for AMSATOSCAR spacecraft in flight, by
sequence number.
AOS — Acquisition of signal. The time atwhich radio signals are first heard from asatellite, usually just after it rises above
the horizon.
Apogee — The point in a satellite’s orbit
where it is farthest from Earth.
Argument of perigee — The polar angle
that locates the perigee point of asatellite in the orbital plane; drawn
between the ascending node, geocenter,and perigee; and measured from theascending node in the direction ofsatellite motion.
Ascending node — The point on the
ground track of the satellite orbit where
the sub-satellite point (SSP) crosses theequator from the Southern Hemisphere
into the Northern Hemisphere.
Az-el mount — An antenna mount thatallows antenna positioning in both theazimuth and elevation planes.
Azimuth — Direction (side-to-side in thehorizontal plane) from a given point
on Earth, usually expressed in degrees.
North = 0° or 360°; East = 90°; South =180°; West = 270°.
Circular polarization (CP) —Describes
an electromagnetic wave in which
the electric and magnetic fields arerotating. If the electric field vector isrotating in a clockwise sense, then it iscalled right-hand circular polarizationand if the electric field is rotating in acounterclockwise sense, it is called lefthand circular polarization. Polarization
sense is determined looking in the
direction in which the wave is traveling.
Cross-polarized — Antennas that arealigned with their polarization at rightangles or accept opposite senses ofcircular polarization.
Descending node — The point on the
ground track of the satellite orbit where
30.44   Chapter 30

the sub-satellite point (SSP) crosses theequator from the Northern Hemisphere
into the Southern Hemisphere.
Desense — A problem characteristicof many radio receivers in which astrong RF signal overloads the receiver,reducing sensitivity.
Doppler effect — An apparent shift
in frequency caused by a change indistance between transmitter andreceiver.
Downlink — The frequency on whichradio signals originate from a satellite
for reception by stations on Earth.
Earth station — A radio station, on ornear the surface of the Earth, designed totransmit or receive to/from a spacecraft.
Eccentricity — The orbital parameterused to describe the geometric shape ofan elliptical orbit; eccentricity values
vary from e = 0 to e = 1, where e = 0
describes a circle and e approaches 1 fora very long, thin ellipse.
EIRP — Effective isotropic radiatedpower. Same as ERP except the antennareference is an isotropic radiator.
Elevation — Angle above the local
horizontal plane, usually specified
in degrees. (0° = plane tangent to the
Earth’s surface at your location; 90° =straight up, perpendicular to the plane of
the Earth).
EME — Earth-Moon-Earth (see also
Moonbounce)
Epoch — The reference time at which aparticular set of parameters describing
satellite motion (Keplerian elements) aredefined.
EQX — The reference equator crossing of
the ascending node of a satellite orbit,
usually specified in UTC and degrees oflongitude of the crossing.
ERP — Effective radiated power. System
power output after transmission-line
losses and antenna gain (referenced to a
dipole) are considered.
ESA — European Space Agency. A
consortium of European governmentalgroups pooling resources for space
exploration and development.
Faraday rotation — A rotation of thepolarization of radio waves when the
waves travel through the ionosphere,
in the presence of the Earth’s magnetic
field.
FO-# — The designator used for Japaneseamateur satellites, by sequence number.
Fuji-OSCAR 12 and Fuji-OSCAR 20
were the first two such spacecraft.
Geocenter — The center of the Earth.
Geostationary orbit — A satellite orbit atsuch an altitude (approximately 22,300
miles) over the equator that the satellite

appears to be fixed above a given point.
Ground gain — Reflections from theground in front of an antenna can add
as much as 6 dB to the effective gain at
certain elevation angles.
Groundtrack — The imaginary line
traced on the surface of the Earth by the
subsatellite point (SSP).
Inclination — The angle between theorbital plane of a satellite and theequatorial plane of the Earth.
Increment — The change in longitude ofascending node between two successive
passes of a specified satellite, measured
in degrees West per orbit.
Iskra — Soviet low-orbit satellites
launched manually by cosmonauts
aboard Salyut missions. Iskra means“spark” in Russian.
JT65 — A digital mode used for EME
communications.
Keplerian Elements — The classicalset of six orbital element numbers
used to define and compute satellite
orbital motions. The set is comprisedof inclination, Right Ascension of
Ascending Node (RAAN), eccentricity,
argument of perigee, mean anomaly andmean motion, all specified at a particularepoch or reference year, day and time.
Additionally, a decay rate or dragfactor is usually included to refine thecomputation.
LEO — Low Earth Orbit satellite such as
the Phase 1 and Phase 2 OSCARs.
LHCP — Left-hand circular polarization.
Libration fading — Rapid fading of EME
signals caused by changing relative
geometry of reflecting features on the
moon’s surface and an observer on
Earth.
LNA — Low noise amplifier — a
preamplifier mounted as close to theantenna terminals as possible for satellite
and EME communication.
LOS — Loss of signal — The time when asatellite passes out of range and signals
from it can no longer be heard. This
usually occurs just after the satellite goes
below the horizon.
Mean anomaly (MA) — An angle that
increases uniformly with time, startingat perigee, used to indicate where asatellite is located along its orbit. MA isusually specified at the reference epoch
time where the Keplerian elements aredefined. For AO-10 the orbital time
is divided into 256 parts, rather thandegrees of a circle, and MA (sometimes
called phase) is specified from 0 to
255. Perigee is therefore at MA = 0 with
apogee at MA = 128.
Mean motion — The Keplerian element toindicate the complete number of orbits asatellite makes in a day.
Microsat — Collective name given to aseries of small amateur satellites having
store-and-forward capability (OSCARs
14-19, for example).
Molniya — Type of elliptical orbit, firstused in the Russian Molniya series, thatfeatures a ground track that more or lessrepeats on a daily basis.
Moonbounce — A common name for
EME communication in which signals
are bounced off the moon before being
received.
NASA — National Aeronautics and Space
Administration, the US space agency.
Nodal period — The amount of time
between two successive ascending nodesof satellite orbit.
Orbital elements — See Keplerian
Elements.
Orbital plane — An imaginary plane,
extending throughout space, that
contains the satellite orbit.
OSCAR — Orbiting Satellite Carrying
Amateur Radio.
PACSAT — Packet radio satellite (seeMicrosat and UoSAT-OSCAR).
Parabolic (dish) antenna — An antennareflector that is a portion of a paraboloidof revolution. Used mainly at UHF andhigher frequencies to obtain high gain
and narrow beamwidth when excitedby one of a variety of driven elements
placed at the dish focus to illuminate thereflector.
Pass — An orbit of a satellite.
Passband — The range of frequencies
handled by a satellite translator ortransponder.
Path loss — The total signal loss between
transmitting and receiving stations
relative to the total radiated signal
energy.
Perigee — The point in a satellite’s orbit
where it is closest to Earth.
Period — The time required for a satellite
to make one complete revolution about
the Earth. See Anomalistic period andNodal period.
Phase I — The term given to the earliest,
short-lived, low Earth orbit (LEO)
OSCAR satellites that were not equippedwith solar cells. When their batteries

were depleted, they ceased operating.
Phase 2 — LEO OSCAR satellites.
Equipped with solar panels that powered
the spacecraft systems and recharged
their batteries, these satellites have beenshown to be capable of lasting up to five
years (OSCARs 6, 7 and 8, for example).
Phase 3 — Extended-range, highelliptical-orbit OSCAR satellites withvery long-lived solar power systems(OSCARs 10 and 40, for example).
Phase 4 — Proposed OSCAR satellites ingeostationary orbits.
Polar Orbit — A low, circular orbit
inclined so that it passes over the Earth’s
poles.
Precession — An effect that ischaracteristic of AO-10 and
AO-40 orbits. The satellite apogee
SSP will gradually change over time.
Project OSCAR — The California-based
group, among the first to recognize thepotential of space for Amateur Radio;
responsible for OSCARs I through IV.
QRP days — Special orbits set aside forvery low power uplink operating throughthe satellites.
RAAN — Right Ascension of Ascending
Node. The Keplerian element specifying
the angular distance, measured eastward
along the celestial equator, between thevernal equinox and the hour circle of
the ascending node of a spacecraft. Thiscan be simplified to mean roughly thelongitude of the ascending node.
Radio Sputnik — Russian Amateur Radio
satellites (see RS #).
Reference orbit — The orbit ofPhase II satellites beginning with thefirst ascending node during that UTC
day.
RHCP — Right-hand circular polarization.
RS # — The designator used for mostRussian Amateur Radio satellites (RS-1through RS-15, for example).
Satellite pass — Segment of orbit during
which the satellite “passes” nearby andin range of a particular ground station.
Sidereal day — The amount of timerequired for the Earth to rotate exactly
360° about its axis with respect to the“fixed” stars. The sidereal day contains
1436.07 minutes (see Solar day).

Sidereal month — The Earth and mooncomplete their mutual orbit (with respect
to fixed stars) every 27.3 days, one
sidereal month.
Solar day — The solar day, by definition, contains exactly 24 hours(1440 minutes). During the solar day
the Earth rotates slightly more than 360°
about its axis with respect to “fixed”
stars (see Sidereal day).
Spin modulation — Periodic amplitude
fade-and-peak resulting from therotation of a satellite’s antennas about
its spin axis, rotating the antenna peaks
and nulls.
SSP — Subsatellite point. Point on thesurface of the Earth directly between thesatellite and the geocenter.
System noise temperature — Used in
EME system evaluation, a measure ofnoise picked up by the antenna plus
noise generated in the receiver.
Telemetry — Radio signals, originating
at a satellite, that convey informationon the performance or status ofonboard subsystems. Also refers to theinformation itself.
Transponder — A device onboard asatellite that receives radio signals in
one segment of the spectrum, amplifies
them, translates (shifts) their frequency
to another segment of the spectrum andretransmits them. Also called linear
translator.
UoSAT-OSCAR (UO #) — Amateur Radio
satellites built under the coordinationof radio amateurs and educators at the
University of Surrey, England.
Uplink — The frequency at which signals
are transmitted from ground stations toa satellite.
Visibility Circle — The range of area on
the Earth that are “seen” by a satellite.
This is also called the “footprint” forthat satellite.
Window — Overlap region between
acquisition circles of two groundstations referenced to a specific satellite.
Communication between two stations is
possible when the subsatellite point iswithin the window.
WSJT — A suite of computer software that
includes tools for EME communication.

Space Communications   30.45