Moonbounce at W2PU
Adaptive Polarization at 432 MHz
Joe Taylor, K1JT
We have built a highly capable 432 MHz EME station for the Princeton University AmateurRadio Club. Its dual-polarization (“X-pol”) antenna is lightweight, modest in size, rugged,
and easy to point in azimuth and elevation. An important station feature is its adaptive
polarization capability, rendering moot the problems of Faraday lockout and one-way
propagation often experienced by single-polarization EME stations on this band. Withtransmitter power of several hundred watts at the antenna, a station like ours is capable ofworking any comparably equipped station over the EME path whenever the moon is visible
to both. We hope that some of our design ideas will be useful to others considering entry
into the EME fraternity on the 70 cm band.

Why 432 MHz ?
The 222 and 902 MHz bands are not available world-wide, and EME antennas for
144 MHz are too large for our purpose. We have a 2.3 m dish equipped for anundergraduate advanced lab experiment observing galactic neutral hydrogen at 1420
MHz. In principle this dish could be outfitted for 1296 MHz EME; however, for a number ofreasons we decided to develop EME capability at 432 MHz instead. As illustrated in
Figure 1 and discussed in detail in reference [1], achievable system noise temperatures
are nearly as low there as at 23 cm. Equipment for 70 cm is relatively easy to build andwidely available off the shelf. It seems that in many ways 70 cm is the most under-utilizedof all EME bands, and we hope to encourage a reversal of this trend.

Fig 1 – Typical contributions tosystem noise temperature, Ts, fromthe cosmic microwave background(CMB), the Earth’s atmosphere, thewarm surface of the moon, galactic
noise entering through the main
antenna beam, and sky and groundnoise from an antenna’s side andrear lobes. Antenna temperature Ta
is a combination of all thesecontributions, appropriately weighted
by antenna pattern; Ts is the sum ofTa and receiver noise temperature Tr.

What sort of antenna? How much power?
Linear polarization is the standard on 70 cm, so in principle dual-polarization (“X-pol”)
antennas can provide the same well-known advantages that they do for 2 m EME. As
described in reference [2], an adaptive-polarization receiver yields an average sensitivity
improvement of 3 dB, and much more in extreme cases of polarization angle mismatch.
However, X-pol Yagis with good performance are harder to build at 70 cm than at 2m, andthey have not heretofore been widely used. The main problems are achieving accurate
symmetry between the X and Y planes of Yagi elements and fitting the necessary
feedlines, baluns, etc., into available space around the feedpoint.
A lunchtime conversation with Justin Johnson, G0KSC, at the 2012 EME Conference inCambridge persuaded us that with sufficient care, excellent X-pol Yagis could be built anddeployed for 432 MHz. With the help of Justin’s company, InnovAntennas, we decided toproceed in that direction. After a few iterations we settled on a basic antenna design that
we call 15LFA-JT: a 15-element, dual polarization Yagi about 3.5 m (11.5 ft) long, rearmounted and built on a one-inch square hollow fiberglass boom. Simulation with theantenna modeling software NEC4 shows that an array of four such Yagis should provide
22.4 dBi gain in each of two orthogonal linear polarizations, with extremely low side- andrear-lobe responses. Clean patterns are very important for achieving the low system noise
temperatures required for EME on the UHF and higher bands.
Among all of the propagation modes used by amateurs, the Earth-Moon-Earth path is oneof the very few that allow accurate and reliable predictions of signal strength. The linkbudget for EME communication was described in detail in reference [1]. It can besummarized in the following equation for, the received signal-to-noise ratio in dB:(1)
Here and are the received and transmitted powers expressed in dBW (dB above onewatt); is the Earth-Moon-Earth path loss in dB, assuming isotropic antennas; andare gains of the transmitting and receiving antennas in dBi; and is the received noise
power in dBW. For average moon distance the path loss can be written as(2)
Received noise power is equal to, whereconstant, is the system noise temperature in Kelvins, andHz. Thus, in units of dBW,

Joules/K is Boltzmann’s
the received bandwidth in(3)
Let’s assume equal gains
for the transmitting and receiving antennas, as
would be the case for our station communicating with its “twin”. We can then calculate thetransmitter power required for any specified system noise temperature, bandwidth, and, using the equation

.

(4)

Communication in the JT65 digital mode is nearly 100% reliable [3, 4] if
bandwidth
Hz. FordBi and typical system noise temperature
K, the required transmitter power fordB is then
dBW,

dB in

(5)

or 66 watts.
With transmitters producing at least several hundred watts at the antenna, two stations
equipped in this way will have several extra dB in hand, and should nearly always be able
to work each other by EME. Audible self-echoes and CW communication with a twin
station will require something likedB in 50 Hz bandwidth, and thus about 10 dB
more transmitter power — say 700 W at the antenna.

Model 15LFA-JT Yagi
To allow us to verify the antenna design and make initial test measurements, G0KSC sent
us materials for a pair of 15LFA-JT Yagis following his design. Some important
construction details can be made out by studying Figures 2 through 4. Reflectors anddirectors are made from 1/4-inch aluminum rod, each one passing through the boom
center to maintain full polarization symmetry. Element positions for the two orthogonal
polarizations are offset by 30 mm along the boom. Long sides of the LFA driven elements
are made from 10 mm brass tubing with 1 mm wall thickness; the U-shaped end pieces
are made from 8-mm brass tuning, cut to 45-degree angles and hard-soldered at the 90degree corners. These end pieces fit snugly into the long straight sides, and are solderedin place after final tuning. The forward side of each LFA loop is split with a 6 mm gap filledwith a nylon insulator. We drilled all the way through the brass and nylon and tapped theholes for 4-40 machine screws. Ends of the RG142 feedlines are stripped, fitted with eyelugs, treated with liquid rubber sealant, and secured to the feedpoints with 4-40 screws.
The screws are accessible through 6-mm holes drilled through one side of the boom,
immediately above the feedpoints. Full details of element lengths and spacings will be
published soon.

Fig 2. – A pair of 15LFA-JT rear-mounted Yagis on a simple mount with azimuth and elevation
control. Light-duty Dacron ropes serve as forward guys, to prevent boom sag.

Fig 3. – Separate feedlines carry signals for horizontal and vertical linear polarization from each
Yagi to the power splitters.

Fig. 4. – Rear view of a single Yagi showing construction details around the driven elements. The
brass loops are fed at the split centers of their forward sides, inside the hollow boom. RG142
coaxial feedlines pass out to the rear. All elements are held in place with tight-fitting holes and a
few drops of fiberglass resin.

Antenna tests
One of the most sensitive indicators of how closely a real antenna resembles its computermodel is a plot of return-loss versus frequency. Therefore, after assembling each Yagi wemade a number of swept-frequency return-loss measurements, following advice from
G0KSC to take advantage of two “tuning controls” for each polarization: the telescoping
ends of the driven loops, and the length of the first director. We adjusted both in steps as
small as 0.5 mm; in practice, this required us to cut several extra D1 elements with slightly
different lengths, to find the best match. We checked carefully for evidence of feedline
radiation possibly caused by connecting coax directly to the driven loop. We found none.
With optimum tuning we obtained return losses better than 25 dB over the range 427 to
437 MHz, in excellent agreement with the simulations. Compared with some designs,
these Yagis are relatively wideband antennas.
Measurements of angular power patterns are difficult without a professional antenna
range. Nevertheless, we attempted such a measurement using purely amateur-radio
techniques. Our test signal was the W3CCX/B beacon located in Philadelphia, 42 miles
away. We drove the antenna at constant rate over a full 360 degrees in azimuth, and thenrepeated in the opposite direction. With the receiver AGC turned off and the uppersideband bandwidth set to 5 kHz, audio output from the station’s TS-2000X transceiver
was sampled by a computer soundcard at 48 kHz, squared, and averaged over 1-secondintervals. The averages were recorded along with readout azimuths. Plots in Figure 5
show the simulated and measured azimuth patterns for our phased pair of 15LFA-JT
Yagis, for horizontal polarization.

Fig 5. – Simulated (left) and measured (right) azimuth patterns of a pair of 15LFA-JT Yagis stacked
horizontally, in horizontal polarization. The stacking distance was 1.2 m.

The slightly asymmetric sidelobes are almost certainly the result of reflections from nearby
structures — in particular, a permanent steel ladder the top of which is only about 1 m
away from the forward ends of the (temporarily mounted) Yagis when pointed toward thehorizon, close to the direction of W3CCX. This near-field distortion should disappear when
the antennas are elevated, and especially after the array is raised to its final position at the
top of its mast (see Figure 2). Other than this (presumably temporary) distortion, themeasured array pattern closely approximates the very low side- and rear-lobe responsesof the computer model, especially when allowance is made for likely multi-path
propagation at very low signal levels. We concluded that the completed array should haveexcellent performance.
Our next test of the temporary two-Yagi array was a measurement of Sun noise. Software
was written to control the azimuth rotor in a repetitive Off-On-Off pattern, centered on the
Sun’s current position. The offset positions were set atdegrees.
Once again, we recorded 1-second averages of the receiver output power. Results after
normalization to the mean off-source power level are plotted in Figure 6. The On- to-Offsource power ratio was found to bedB, very close to the expected value
7.4 dB calculated for that day’s solar flux at 432 MHz and our measured system noise
temperature (44 SFU and 118 K, respectively). Essentially identical results were obtainedfor each of the two polarizations.

Fig. 6. – Measurements of Sun noise with a phased pair of 15LFA-JT Yagis. The dotted line
represents the expectedon the day of measurement.

Final Station Configuration at W2PU
Satisfied with the performance of our temporary two-Yagi array, we proceeded to build two
more Yagis and configure them in the 2 × 2 array shown in Figure 7. The completed arrayis very compact, with stacking distance only 1.2 m (47 inches) in each plane.

Fig. 7. – The four-Yagi 432 MHz array at W2PU.

Optimum configuration of a dual-polarization EME system requires independent
preamplifiers for each polarization mounted at the antenna and feeding separate Rx
feedlines. A separate low-loss feedline is used for the Tx side. As illustrated in Figure 8,
the front-end configuration at W2PU uses three coaxial relays: K1 and K3 accomplish thenecessary T/R switching, while K2 selects horizontal or vertical polarization fortransmitting. All three relays must handle the full transmitter power. In order to protect thepreamplifiers, K1 and K3 must have better than 60 dB isolation on their “off” port.

Fig. 8 – Front-end
switching arrangement
used at W2PU.

Major components of the two-channel receiver are summarized in the block diagram of
Figure 9. The indicated combination of hardware and software provides adaptivepolarization reception of CW, SSB, and JT65 signals and a highly sensitive band-scope
covering a 90 kHz portion of the band. The IQ+ receiver hardware (manufactured by
LinkRF, see ref. [5]) converts incoming signals to an in-phase and quadrature (I/Q)
baseband pair for each polarization. These two pairs are sampled at 96 kHz by a
4-channel sound card, in our case an M-Audio Delta44. All subsequent signal processing
is done by computer, using the programs Linrad [6] and MAP65 [7]. Many additional
details concerning the receiver can be found in the documentation for these programs andin reference [2].
For transmitting we use a Kenwood TS-2000X followed by a Beko HLV-1100 solid-state
power amplifier.

Fig. 9 – Block diagram of the 432 MHz receiving setup at W2PU.

Worksheet for System Noise Temperature
Optimizing the receive performance of an EME station requires careful attention to every
contribution to system noise temperature. Unlike the situation for a well-designed 23-cm
station using a parabolic dish and circular polarization, a 70-cm setup using a Yagi array
necessarily has a number of lossy, ambient-temperature items in front of the firstpreamplifier. I highly recommend the use of VK3UM’s EME Calculator software [8], or
alternatively a simple spreadsheet like the one shown on the next page as an aid tominimizing in a step-by step manner. Reference [9] provides a link to the spreadsheet
file, which is easily adaptable for your own use. Spreadsheet items highlighted in yellow
are input by the user, while all remaining numbers are calculated from the input data. As
displayed here, the spreadsheet reflects the 432 MHz EME setup at W2PU.
We have not yet done a particularly good job of minimizing before-the-preamp
contributions to system noise. For example, the 22 K contribution from RG142 feedline
segments could certainly be reduced; this small-diameter cable is needed only for about
one foot, inside the hollow booms. On the other hand, the estimated 48 K contribution
from antenna noise is likely to be overly pessimistic, especially at higher elevations.
Overall, we believe the numbers are conservatively realistic for typical EME conditions.
Two stations equipped similarly to W2PU, each with four X-Pol Yagis similar to the 15LFAJT and Tx power of 100 W or more at the antenna, should be able to work each other by
EME more-or-less any time the moon is available.

Tsys Worksheet
4 ft RG-142
Power splitter
3 ft LDF 4-50A
T/R relay
LNA1 (DB6NT)
10 ft LMR400
100 ft LMR240
10 ft RG58
LNA2 (ARR)
LinkRF IQ+

Gain
(dB)
-0.32
-0.05
-0.04
-0.05
23.00
-0.27
-5.20
-1.00
20.00

Tr at antenna feedpoint
Antenna and feed losses
Sky noise (main beam, on ecliptic)
Side and rear lobes
Total antenna noise, Ta
System noise temperature, Ts

0.06

Frequency (MHz)
Lossless antenna gain (dBi)
Solar Flux at 432 MHz (SFU)
Tx power at antenna (W)

432
22.40
44.0
100

EME path loss (dB)
G/Ta (dB/K)
G/Ts (dB/K)
Y Sun (dB)
EME S/N in B=2500 Hz (dB)
EME S/N in B=50 Hz (dB)

261.6
5.5
1.6
9.9
-23.0
-6.0

Noise
FigureNoise Contribution

(dB)

0.50
9.00

(K)
22.2
3.6
2.9
3.7
30.8
0.1
3.9
1.5
0.9
0.5

% Total
18.7%
3.1%
2.5%
3.1%
26.0%
0.1%
3.3%
1.2%
0.7%
0.4%

0.94

70.0

59.2%

4.0
20.0
25.0
48.4
118.4

3.4%
16.9%
21.1%
40.8%
100.0%

0.40

Quantitative Tests of Station Capabilities
The Tsys worksheet can provide good estimates of station capabilities, but many of thenumbers shown in the example above are little more than estimates based on
manufacturers’ data sheets. In order to confirm the important bottom-line performance
numbers, we conducted a number of astronomical measurements and EME echo tests.
As one example, the spreadsheet calculates a value for YSun, the ratio (expressed in dB) of
sun noise plus system noise to system noise alone. On one particular day ofmeasurement the solar flux at 432 MHz was 44 SFU [see ref. 10] and the resulting
calculated YSun was 9.9 dB. Figure 10 shows a screen snapshot of the Linrad recording
S-meter, calibrated in dB, during a series of Off-On-Off scans of the Sun. It’s easy to seethat the measured YSun is close to the predicted value, nearly 10 dB.

Fig. 10 – Measurement of Sun noise with theW2PU 4-Yagi array. Blue and red curves
correspond to horizontal and verticalpolarizations, respectively.

We also made measurements of the astronomical source known as Sagittarius A, locatedat the center of our Milky Way galaxy. The antenna was pointed to azimuth 180° (duesouth, on the local meridian) and elevation 21°. For our latitude of 40° this elevation
corresponds to celestial declination −29°. Using the simple computer program describedearlier, we sampled the receiver output and calculated relative power at 1-second intervals
over a period of nearly 20 hours. Owing to Earth rotation, our antenna beam passed
across the galactic center at about 17:45 local sidereal time. Figure 11 shows theobserved power levels (expressed as equivalent noise temperatures) after further
averaging into roughly 5-minute intervals. We expected a maximum of about 160 K
additional noise temperature from Sagittarius A, very close to the amount observed.

Fig. 11 – Measurementof excess antennanoise from the center of
our Galaxy.

Arguably the most important tests of a station’s EME capability are quantitative
measurements of its own lunar echoes. My freely available open-source program WSJT
[ref. 11] includes a feature for making automated echo measurements. The results of one
such test are summarized in the screen shot below, showing a measured signal-to-noise
ratio –19 dB for Tx power 450 W at the antenna.

Fig. 12 – Measurement of lunar echoes from the W2PU station. The blue curve is corrected forthe expected Doppler shift at the start of the run; the red curve also includes corrections forchanges in Doppler shift over the full measurement interval, about 1 minute.

Conclusions
By now we have now made many EME QSOs with the system described herein. We find it
to be every bit as capable as we had hoped.
Our decision to rear-mount the 15LFA-JT Yagis has the distinct advantage of keeping all
extraneous metal out of the active region of the antenna, thereby preserving the excellent
angular patterns of the model simulations. However, rear-mounting requires suitablecounterweights extending to the rear, complicating the structure mechanically. A possible
alternative would be to build the necessary H-frame of fiberglass or other non-conducting
material, attaching the individual Yagis to the H-frame near their balance points. Such anarrangement would be much lighter and could be particularly attractive for portable or
DXpedition use — especially if the “armstrong” method is used for tracking the moon.

Acknowledgments
I thank Justin Johnson (G0KSC) and my Physics Department colleagues Geoff
Gettelfinger, Dan Marlow (K2QM), and Norm Jarosik for their essential help throughout thecourse of this project.

References
1. J H Taylor, K1JT, “Earth-Moon-Earth (EME) Communication.” ARRL Handbook forRadio Communications, Chapter 30 (2010); also available online at
http://physics.princeton.edu/pulsar/K1JT/Hbk_2010_Ch30_EME.pdf
2. J H Taylor, K1JT, “MAP65 Version 2: A Panoramic, Polarization-Matching Receiverfor JT65.” Proceedings of the 15th International EME Conference, Cambridge, pp.
101-108 (2012); http://physics.princeton.edu/pulsar/K1JT/K1JT_EME2012.pdf.
3. J H Taylor, K1JT, “The JT65 Communications Protocol.” QEX (September-October2005); http://physics.princeton.edu/pulsar/K1JT/JT65.pdf.
4. J H Taylor, K1JT, “Open Source WSJT: Status, Capabilities, and Future Evolution.”
Proceedings of the 12th International EME Conference, Wurzburg (2006);
http://physics.princeton.edu/pulsar/K1JT/K1JT_eme2006.pdf.
5. LinkRF web site: http://www.linkrf.ch/IQ+.html.
6. Linrad, by SM5BSZ: http://www.sm5bsz.com/linuxdsp/linrad.htm.
7. MAP65: http://physics.princeton.edu/pulsar/K1JT/map65.html.
8. VK3UM EME Calculator, http://www.vk3um.com/eme%20calculator.html
9. The Excel spreadsheet is available at
http://physics.princeton.edu/pulsar/K1JT/W2PU_432.xlsx.
10. Solar flux measurements are made by the US Air Force every day at a number of
frequencies and at four sites around the world. Details can be found at
http://www.swpc.noaa.gov/ftpmenu/lists/radio.html.
11. WSJT: http://physics.princeton.edu/pulsar/K1JT/wsjt.html.

© 2014 by Joe Taylor, K1JT