EME with Adaptive Polarization at 432 MHz
Joe Taylor, K1JT, and Justin Johnson, G0KSC
Linear polarization is standard for both terrestrial and EME communication on the 70 cm
band. For EME this convention ensures that ionospheric Faraday rotation and spatial
polarization offsets often combine to cause “Faraday lockout” and apparent one-way
propagation. On the 2-meter band, dual-polarization (“X-pol”) Yagis and software-based
adaptive-polarization receivers are widely used to solve this problem. As described inreference [1], such a receiver yields an average sensitivity improvement of 3 dB, and much
more in extreme cases of polarization mismatch. However, up to now these techniques
have not been much used at 70 cm because X-pol Yagis with good performance areconsiderably harder to build at UHF. The main problems are achieving accurate
symmetry between the X and Y element planes and fitting the necessary feedlines, baluns,
etc., into available space around the feedpoint.
In this paper we describe development and implementation of an excellent dualpolarization EME antenna for 70 cm using four mid-sized LFA Yagis. The array built at
W2PU (the Princeton University Amateur Radio Club station) is shown in Figure 1. By
EME standards it is small, lightweight, rugged, and easy to point in azimuth and elevation.
The antenna is suitable and practical for nearly any QTH where EME is feasible. As
outlined below, two stations using these antennas should be able to work each other by
EME at nearly any time the moon is visible to both. We hope that some of our designideas will be useful to others considering entry into the EME community on the 70 cm
band.

Fig. 1. – Four-Yagi dual-polarization 432 MHz array at W2PU. The Yagis are rear-mounted; boom
length is 3.5 m, and stacking distance is 1.2 m in each direction.

Antenna Gain Requirements
Of all propagation modes used by amateurs, EME is one of the few that allow accurate
and reliable predictions of signal strength. The link budget for the Earth-Moon-Earth pathis well understood, and described in detail in reference [2]. It can be summarized in thefollowing equation for, the received signal-to-noise ratio in dB:(1)
Here and are the received and transmitted powers expressed in dBW (dB above
1 W); 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 a station communicating with its “twin”. We can then rearrange
equation (1) to solve for the antenna gain required for communication with any specified
signal-to-noise ratio, path loss, system noise temperature, bandwidth, and transmitter
power:.
Communication in the JT65 digital mode is nearly 100% reliable [3, 4] if
bandwidth
Hz. FordB,
dB,
K, andequation (4) reduces to
.

(4)
dB in
Hz

(5)

For convenience this relation is plotted in Figure 2. It’s clear that for transmitter powers
around 100 W, antenna gains of 22 dBi should be sufficient. With a few hundred watts at
the antenna, two stations equipped in this way will have several extra dB in hand — that is,they 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.

Fig. 2. – Minimum antenna gain for EME communication between twin EME stations using JT65 at
432 MHz, as a function of transmitter power. The dashed line corresponds to the four-Yagi arrayat W2PU.

Model 15LFA-JT Yagi
Forward gain is not everything. Clean antenna patterns with low side and rear lobes inelevation as well as azimuth are very important for achieving the low system noise
temperatures required for EME, especially on the UHF and higher bands. One of thebiggest challenges when building Yagis for 70 cm is a mechanical one. Matching devices,
folded dipoles, etc., are much larger in terms of wavelength than at lower frequencies. Fora single-plane antenna the consequences may be minimal — minor pattern distortions, forexample. However, when two orthogonal Yagis are placed on the same boom moreundesirable interactions will occur, as shown in Figure 3. To avoid such effects thecenters of all elements must fall in a single straight line. Ideally, no part of either antennashould extend into the plane of the orthogonal polarization.
With these considerations in mind, after a few iterations we settled on a basic antenna
design that we call 15LFA-JT: a 15-element, dual polarization, LFA-fed Yagi 3.5 m long,
rear-mounted and built on a one-inch square hollow fiberglass boom. Simulation withantenna modeling software EZNEC v5 Pro/4 and 4NEC2 (both using the latest NEC4.2
calculation engine) shows that an array of four such Yagis should provide 22.4 dBi in eachof two orthogonal linear polarizations, with extremely low side- and rear-lobe responses(Figure 3, left).

Fig. 3 – Simulated single antenna patterns for the 15LFA-JT in both E and H planes. Left: allelements are perfectly aligned. Right: elements are offset as though mounted above and to one
side of a 1’’ (25mm) square boom.

The LFA Yagi (Loop Fed Array) is a traditional style Yagi with an extended folded-dipole
type feed laid flat on the boom. Thus, both sides of the driven loop are in line with theparasitic elements, rather than extending above or below the boom. Unlike a traditional
folded dipole, even the feedpoint is in line. This leads to a perfectly symmetrical pattern inboth E and H planes. However, this mechanical arrangement adds certain construction
complications. With all elements in line the feedpoints lie inside the boom, centered on its
axis. Workable construction options include using a metallic boom, although for optimum
long-term performance the elements must be welded to the boom. Insulated elements
passing through a metallic boom are another option, but this practice can lead to eddy
currents in the boom, detuning the antenna and causing deterioration of both pattern andsystem temperature. With these problems in mind, we decided to use a hollow fiberglassboom for these antennas.
As shown in Figure 4, simulations of the 15LFA-JT show that its gain and pattern are wellmaintained over a bandwidth of at least 5 MHz. As a result, we expect the antenna’s
performance to remain good even in the presence of un-modeled external influences suchas rain or ice loading.

Fig. 4 – Simulated gain (blue), front to back ratio (red), and front-to-rear (green) plotted asfunctions of frequency.

Important construction details of the individual Yagis can be gleaned by studying Figure 5.
Reflectors and directors are made from 1/4-inch aluminum rod, each one passing throughthe boom center to maintain full polarization symmetry. 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 90-degree corners. These end pieces fit snugly into the long straight sides, and are
soldered in place after final tuning. Element positions for the two orthogonal polarizations
are offset by 30 mm along the boom.
The NEC electromagnetic calculation engine assumes zero-length, “tail-less” connections
to the feedline. The forward side of each LFA loop is split with a 6 mm gap filled with anylon insulator, and very short pigtails on the coax are compensated by adjusting the“trombone” end pieces of the driven loops. For the initial antennas we opted to drill all theway through the brass and nylon and tap the holes for 4-40 machine screws. Ends of the
RG142 feedlines were stripped, fitted with eye lugs on very short tails, treated with liquidrubber sealant, and secured to the feedpoints with 4-40 screws. The screws areaccessible through 6-mm holes drilled through one side of the boom, immediately above
the feedpoints. Soldering the feedline to the driven element would be an even better
option, ensuring minimal resistive losses and a long service life. Soldering would require
somewhat larger holes in the boom.
Rear-mounting the Yagis ensures that coax feedlines between each feed point and the
3/2-wavelength power dividers can be as short as possible. Rear-mounting also meanshaving no H-frame or other un-modeled metal parts (including the elevation positioning
mechanism) inside the active regions of the Yagis. Such conductive parts could detune
the antennas and degrade both gain and system noise temperature.

Fig. 5. – Rear view of a single LFA15-JT 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.

For the benefit of home constructors, element lengths and spacings for the 15LFA-JT Yagi
are presented in Table 1. For best results you should use a fully insulated boom or ametallic boom with welded elements. The specified element lengths assume a nonconducting boom. Corrections must be applied for a metallic boom: for example, a ¾-inchsquare boom with electrically connected elements requires 11.5 mm added to eachelement length. Offset between elements in the two polarizations is 30 mm along theboom. High accuracy is required to replicate the simulated antenna at 70 cm. Use aquality caliper to measure the elements.

Table 1. – Element lengths and spacings for the 15LFA-JT Yagi.
Element
R
DE1
DE2
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
D12
D13

Spacing (mm)
0.0
48
120
179.5
310.5
454.5
648.5
891
1157.51442
1738.52037
2330
2617.52918
3183

Length (mm) uncorrected
338.5
299*
299**
314
300.5
292
290.5
287
283.5
279
276
275
273
268
257
238.5

*Total length of loop. The long straight sections should be reduced in size to allow for thetelescoping end sections.
** The forward section of the loop has the feed point at its center. Loop size is based on 10 mm
brass tube with 8 mm end sections. Adjustments will be required for changes in these
dimensions.

Initial 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. After assembling each Yagi we madeswept-frequency return loss measurements as a tuning aid, adjusting the telescoping endsof the driven loops and the length of the first director for best match. We used length steps
as small as 0.5 mm; in practice, this required us to cut several extra first-director elementswith slightly different lengths, to find the best (and flattest) return-loss curve. We checked
carefully for evidence of feedline radiation potentially caused by connecting coax directly tothe driven loop. We found none, but nevertheless we installed two UHF-rated ferrite cores(Laird Technologies 28A0593-0A2) on each feedline where they exit the boom. 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.
We built a temporary two-Yagi array and mounted it with full azimuth and elevation control
in the planned location atop the Princeton University Physics building. With this array wemade measurements of return loss, angular pattern, and sun noise; we also did a bit ofsuccessful SWL-only EME operation. Satisfied with the performance of the temporary twoYagi array, we proceeded to build two more Yagis and configure them in the 2 × 2 arrayshown in Figure 1.

Optimum use of a dual-polarization EME system requires independent preamplifiers foreach 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 6, the front-end
configuration at W2PU uses three coaxial relays: K1 and K3 accomplish the necessary T/R
switching, while K2 selects horizontal or vertical polarization for transmitting. All three
relays must handle the full transmitter power. In order to protect the preamplifiers, K1 andK3 must have better than 60 dB isolation at the receive port, when transmitting.

Fig. 6 – Front-end switching arrangement used at W2PU.

Major components of our two-channel receiver are summarized in the block diagram of
Figure 7. The indicated combination of hardware and software provides adaptivepolarization reception of all modes and a highly sensitive band-scope covering a 90 kHz
portion of the 70 cm band. The IQ+ receiver hardware (manufactured by LinkRF, see ref.
[5]) converts incoming signals to in-phase and quadrature (I/Q) baseband pairs for eachpolarization. The two pairs are sampled at 96 kHz by a 4-channel sound card, in our casean 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 canbe found in the documentation for these programs and in reference [1]. For transmitting
we use a Kenwood TS-2000X followed by a Beko HLV-1100 solid-state power amplifier.

Fig. 7 – Block diagram of the 432 MHz receiving setup at W2PU.

Worksheet for System Noise Temperature
Optimizing the receive performance of an EME station at UHF and above requires carefulattention to every contribution to system noise temperature. Unlike the situation for a welldesigned 23-cm station using a parabolic dish and circular polarization, a 70-cm setup witha Yagi array necessarily has a number of lossy, ambient-temperature items in front of thefirst preamplifier. We highly recommend the use of VK3UM’s EME Calculator software [8],or alternatively a simple spreadsheet like the one shown below as an aid to minimizing
in a step-by step manner. Reference [9] provides a link to the spreadsheet file, which iseasily 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 current 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
30 cm, inside the hollow booms. On the other hand, the estimated 48 K contribution fromantenna noise is likely to be overly pessimistic, especially at higher elevations. Overall, we
believe the worksheet numbers are conservatively realistic for typical EME conditions.
Two stations equipped similarly to W2PU, each with four X-Pol Yagis similar to the
15LFA-JT and Tx power of 100 W or more at the antenna, should be able to work eachother by EME more-or-less any time the moon is available.

Tsys Worksheet

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

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+
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
Figure(dB)

Noise Contribution

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 Capability
The Tsys worksheet can provide good estimates of station capability, but many of thenumbers shown in the example 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 8 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 nearly 10 dB, close to the predicted value.

Fig. 8 – Measurement of Sun noise with the W2PU four-Yagi array using Linrad’s calibrated,recording S-meter. Blue and red curves correspond to horizontal and vertical polarizations,
respectively.

We also made measurements of the astronomical radio source known as Sagittarius A,
located at the center of our Milky Way galaxy. The antenna was pointed to azimuth 180°
(due south, on the local meridian) and elevation 21°. For the latitude of Princeton, 40°, thiselevation corresponds to celestial declination −29°. With the receiver AGC turned off andthe upper-sideband bandwidth set to 5 kHz, audio output from the station’s TS-2000X
transceiver was sampled in a soundcard at 48 kHz, squared, and averaged over 1-second
intervals. We recorded these relative power measurements 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 9 shows the observed power levels (expressed as
equivalent noise temperatures) after further averaging into 5-minute intervals. We
expected a maximum of about 160 K additional noise temperature from Sagittarius A, very
close to the amount observed.

Fig. 9 – Measurement of excess antenna noise from Sagittarius A, at the center of our Galaxy.

Arguably the most important tests of a station’s EME capability are quantitative
measurements of its own lunar echoes. The freely available open-source program WSJT
[ref. 11] includes a feature for making automated echo measurements. Measured echo
returns from the W2PU station, around –24 dB for Tx power 100 W, are consistent with thestrengths predicted by the Tsys worksheet.

EME Operation with Adaptive Polarization
As outlined above, MAP65 works together with a two-channel SDR-style receiver thatmixes RF signals to baseband. Conversion from analog signals to digital data takes placeat the hardware-to-software boundary indicated by the horizontal dashed line in Figure 7.
We use Linrad as a “front end” for MAP65, mainly because of its superb noise blanker andflexible capabilities for CW operation. We normally set the Rx center frequency to 432.045
MHz, and the displayed passband (for both Linrad and MAP65) to the range 432.000 –432.090. Linrad and MAP65 can run in the same computer; however, both programs
display a lot of information, so a separate video screen for each is essentially arequirement. We actually use two separate computers, with Linrad running under Linuxand MAP65 under Windows 7.
MAP65 uses the so-called “timf2” network data forwarded from Linrad to locate anddecode all JT65 signals in its displayed passband. In practice this allows us to locate CW
signals in the lower portion of the frequency range and JT65 signals in the upper portion.
For every JT65 signal, MAP65 makes an optimized linear combination of the two receiverchannels so as to match the signal’s actual polarization angle, whatever that might be. If
the decoded JT65 message includes the transmitting station’s grid locator, the program
calculates and recommends the transmitted polarization (H or V) that will produce theclosest approximation to the polarization that was used for transmitting at the DX location.
In practice, we usually respond to a CQ by using the recommended Tx polarization. If we
get no answer, we call again using the opposite polarization. When calling CQ we
generally alternate between H and V in subsequent transmissions.

Conclusions
By now we have made many EME QSOs with the system described. We find it to be every
bit as capable as we had hoped. Our decision to rear-mount the 15LFA-JT Yagis has thedistinct advantage of keeping all extraneous metal out of the active region of the array,
thereby preserving the excellent angular patterns of the model simulations. We note thatrear-mounting requires suitable counterweights extending behind the antennas, thereby
complicating the structure mechanically. A possible alternative would be to build an Hframe of fiberglass or other non-conducting material, attaching the individual Yagis near
their balance points. Such an arrangement could be much lighter and particularly
attractive for portable or DXpedition use — especially if the simple “armstrong” method isused for tracking the moon. It should be obvious that 8- or 16-bay arrays of these Yagis

would make even more capable EME antennas. Further information on the 15LFA-JT canbe found at reference [12].

Acknowledgments
We thank Princeton University Physics Department colleagues Dan Marlow (K2QM), Norm
Jarosik, and Geoff Gettelfinger for their essential help throughout the course of this project.

References
1. 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.
2. 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 .
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. Current data and other details canbe found at http://www.swpc.noaa.gov/ftpmenu/lists/radio.html, and archival data at
http://www.ngdc.noaa.gov/stp/space-weather/solar-data/solar-features/solarradio/noontime-flux/.
11. WSJT: http://physics.princeton.edu/pulsar/K1JT/wsjt.html.
12. The 15LFA-JT antenna: http://g0ksc.co.uk.