2m EME Antenna Array

Some photos of my 144MHz EME Moonbounce Anntenna build


Design Intent

In the quest for more gain (and lower noise) for my Earth-Moon-Earth (EME) radio experiments, I decided to investigate a design by Justin, G0KSC. The so-called "Loop-Fed Array". This looks to be an optimised yagi with a very wide "folded dipole" driven element, which is claimed to act more like a loop, reducing sidelobes.
Simulation comparisons seem to indicate that, for a given boom length, the Gain to Noise Temperature (G/T) ratio of this design equals or exceeds that of all other yagi designs.
A tall claim, but given my noisy suburban environment, and the extremely weak echos from the moon, it was most tempting to attempt an "Earth-Moon-Earth" radio contact.
Looking at his website, I decided to try a 9-element design. This provided me with a reasonable compromise between gain and antenna size. I had to re-model the antenna in NEC4, as I only had 8mm diameter aluminium tube on hand. The modelling results looked so good that I felt confident enough to commit to the build.

NEC Listing

For anyone interested in modelling the antenna that I built, here's the NEC listing. Bear in mind that I used the NEC4 engine for the simulations.



lfa.nec

Feedpoint Considerations

Much has been written about feeding yagis with coax, and a great many ideas still abound about the importance of feed symmetry and decoupling. My approach is a simple one - use a single decoupling choke on the coax, and leave the folded dipole free to find it's own balance point - don't force the opposite end to ground. Instead, use a quarter-wave section of coax, connected to the feed at one end, shorted to the boom at the other to maintain a DC short and noise immunity.

Results

I am amazed at how "quiet" these antennas are. I have had great success in receiving signals bounced off the moon, and while my power limitation (100W) limits my signals received by others, I have managed quite a few EME contacts into Europe and North America.


NEC4 Elevation Pattern

Free-space modelling of the Elevation Pattern of the array.

NEC4 Azimuth Pattern

Free-space modelling of the Azimuth Pattern of the array.

Return Loss measurements

The 4 antennas tuned individually. I'm still not sure why #2 is a bit different....

4 Yagis

Four yagis, ready to go.


Feed Detail

Feed detail, showing a quarter-wave dc short to ground the feed.

In Operation

Pointing at the moon.


If Wishes were Fishes

If I had the time and resource, here's what I'd be contemplating:


8 by 12 element array

25dBi of low side-lobe directivity.

Useful Links


News

1296MHz Activities
26/06/16 Antenna Improvements.
Moon Tracker Ver 2
23/06/16 A new version.
1296MHz Activities
29/09/15 Build a small dish.
LNA
26/07/15 Low-noise Preamplifier Design.
1296MHz Activities
19/04/15 Power Amplifier Design.
1296MHz Activities
13/01/15 Design and build a tower for 3m parabolic dish.
Thunderbolt
30/09/14 LCD monitor for my Thunderbolt GPSDO.
PANFI
02/08/14 A home-made Noise Figure Meter.
Moon Tracker Update
09/04/14 New rotator and better position feedback sensors added.
1296MHz Antenna
09/02/14 Begin construction of a 1296MHz feed horn for the 3m dish.
1296MHz Activities
28/01/14 After success at 144MHz, operation at 1296MHz is planned.
Moon Tracker Update
11/09/13 Added a Background Sky Temperature calculation into the Moon Tracker.
Moon Tracker Update
28/08/13 Just Added some software limits to rotation of motors.
Roger Beep installed
09/08/13 Added a Roger Beep to my radio.
Andy's Website
06/05/13 Start build of website.

Contact

Andy, VK3ANX
Yarra Valley, Australia
Andy