2m EME AZEL Array = Initial Test

2m EME Azimuth Elevation Array             VK4AMG   27May13

After a good start to the world of JT65 EME contacts in 2011, reliable performance was difficult to find.

My setup was a modest 12 over 12 YAGI (6m boom) with fixed elevation. That is intended for tropo operation. The antenna was supported by a masthead LNA (about 0.6dB nF), low loss coax (25m LMR600) to an IC746Pro. A SSPA provided healthy legal output while allowing continuous 1:1 minute operation of JT65.

Contact to Europe (clear horizon to moonset) had been easy. Most contact complete in four periods around peak ground gain at 5 and 10 degrees moon elevation.

Contacts to Americas were a different thing. Rising ground crowded with residences not only broke up the ground gain but provided local QRM. With the help of  Lance KB8RQ, I was able to identify early moonrise (around 2 deg) as ideal to work stateside 2m EME stations.

On some occasions, local noise was evident by a 10+ dB increase in noise floor. On other occasions, the waterfall level and content looked "normal" but contacts were rare and typically with "BIG GUNs" only.

A degree of frustration also arose from the process of identifying DX station's frequency, doppler, etc and finding their "trace" to allow WSJT application to do its thing. Often these problems were minimised by calling CQ and seeing what Lady Luck brought. This was frustrating with many sessions of 1 to 2 hours spent without adding a new call to the log.

Time for Change

There is room over the roof of our entertainment area for a modest 3 x 3m AZ EL antenna array. Hopefully it could be mounted so it met Executive approval (not visible outside the property boundary when the array was "parked".

Which YAGI Design???

Much research and observation of the selection of others lead me to select DK7ZB (Lightweight 7 element 50 Ohm 3m boom Yagi).

DK7ZB 7element 50 Ohm 144MHz SSB 3m Boom                as Built by VK4AMG 25May13
	Ref	Dvr	D1	D2	D3	D4	D5
Vertical Position	2990	2670	2530	2055	1360	605	10
Horizontal Position	3000	2680	2540	2065	1370	615	20
Element Length	1027	985	955	935	921	926	897
	3250mm Long		25mm Square Boom			144.3 MHz
			6mm Elements			10.7 dB

Distance in vertical plane is 2,85m and in horizontal plane is 3,00m between the Yagis.

The four Yagis with a harness of  50-Ohm cables of the same length. At the points X the feedpoint impedance is 25 Ohm. A quarter-wave transformer of 50 Ohm-coax (I2) transforms each side to 100Ohm. When T'd the feed point Z is the desired 50Ohm.

An array of 4 crossed YAGI with each polarisation separately fed to a masthead LNA with Tx changeover and separate receive feed two Funcube Pro SDRs and MAP63 allow full band monitoring and internet reporting.

Construction

20mm square aluminium boom (1.2 mm wall) with 6mm (0.9mm wall) tube was selected to provide light yet sturdy construction. The 50 Ohm design was to be fed with a 1/4 wave coax balun run within the boom. This proved to be simple and provided a feed point at the mounting point of the rear mounting point of the X-Yagi.

Through boom insulated elements were chosen. Electrical cable clamps secured with silastic provided the insulation. Correction factor as per DK7ZB instructions was applied. The SWR of each Yagi was 1:1 from 144 to 144.5 MHz. The 1.3:1 SWR points were around 142 and 145 MHz.

The four Yagi were phased again following DK7ZB approach of equal lengths of 50 Ohm coax (RG213) between each pair. This produces a 25 Ohm point which is combined with 5/4 wave (corrected for velocity factor) lengths of 50 Ohm (RG213) to provide a 50 Ohm feed point.  The SWR of the array matched that of each Yagi.

The array is connected to the shack with 15m of LMR600 low loss coax. A masthead LNA feeding the shack SDR receivers with LMR400 is proposed.

Preliminary Results

A test of the array without rotators was conducted at 2am 27May13. This was the point where the moon was at 0 deg azimuth and 80 degrees elevation.  The H frame was laid on the ground and supported for the required pointing.

While it was expected and a little disappointing that no 2m EME stations were QRV, an echo test using WSJT application proved conclusive. Echos were detected with just 50W transmit power. Echos were detected after just two transmission (with the decodes and average cleared). While Q of 10 had been shown on other occasions, this is the first time correlation provided a definitive echo lock.

Note the degradation factor during this test was high at -6dB. At low noise times of 2 to 3 dB contacts should be possible with 25W transmit power.

The only advantage of the 2am Local time of the test and the full moon were the resulting photo of the moon over the array Yagi.

The glint under the Yagi is the tropo array for 6m thru 23cm (see VK4AMG on QRZ.com).

Conclusion

While I had some reservations about an AZ EL 2m array of 3m boom Yagi on a H frame of only 3 x 3m, the  performance of 17.2 dBd together with the minimisation of local QRM provides performance not seen previously on the 12 over 12 tropo array even at peak ground gain.

A good LNA on each polarisation combined with FuncubePro SDRs should provide excellent receive performance while providing an ability to monitor the whole 2m EME segment.

This array also provides support for favourable EMR analysis to support a high power special permit for EME. It is hoped that the W1SL (2x4CX250B) amplifier that has been sitting under the bench partly completed will provide the transmit power to take advantage of the special permit.

To be done (May13)

AZ & EL rotators need to be mounted and wired. The units, collected over time are KR400 and KR500. A homebrew microprocessor controller interfaced to the shack PC is proposed to keep the array correctly pointed.

A lot of learning to implement the Funcube SDR and MAP63 receive system.

Manufacture of two masthead LNAs (0.6dB or better) to support the dual SDR configuration yet stand the rigors of sharing space with a 1kW transmitter.

Some time to sit back and enjoy working some new stations and countries with the new system.

Watch this space.