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:
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Useful Links
- Loop Fed Array
- NEC tools