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July 2024 - G7FEK Antenna
I happened upon information about this antenna quite by accident. I don't remember seeing it in any ham magazines. It promised an 80m Antenna for small gardens (plus some other bands) - and what is not to like about that?
At first glance, you might think it perhaps a variation on the offset feed dipole, such as the "Carolina Windom". It is none of those things. Mike G7FEK has a excellent construction guide with a lot of detail.
Mike G7FEK explains better than I can in his article but you can regard it as two inverted "L" antennas with a common feed,
I filed the idea away for a rainy day. My doublet already covers all of the above bands.
Until Mike MW7EYC - a fellow Chepstow Club member expressed the desire to get on 80m - which his 66ft Half Wave End Fed Long Wave antenna did not support.
A chance therefore to "play aerials" at someone else's expense!
"The G7FEK" will fit in approximately the same space as Mike's end fed. The difference being that while the end fed is a high impedance feed, the G7FEK is a low impedance feed so effective grounds and counterpoises are important.
At the time that we installed it, Mike had no room for the elevated counterpoise that is usually recommended. We made do with an earth spike and a dozen or so counterpoise wires laying in the ground.
The 450 ohm twin feeder shown in the drawing came down near Mike's shack so his coax cable run is fairly short. A common mode choke before it entered the building was used to tame RF traveling in on the screen of the coax.
After minor twiddling of horizontal wire lengths we managed to see results on the antenna analyser. Unfortunately, I can't find the file for that test.
We didn't yet have 20m performance and in Mike G7FEK's article, he provides a modification which we then added (a quarter wave vertical section).
The SWR sweep below shows that 20m is now coming in nicely.
We now have three bands with an acceptable SWR but the 15m response is a little low in frequency. The shorter of the two inverted "L" elements covers 40m - and as a consequence 15m as well (we would hope).
I left it for Mike MW7EYC to adjust that element length for a best compromise between 40m and 15m.
I later started to wonder if a similar mod would work for 10m as did the 20m mod. I set about modeling it and while I was at it, I increased the distance between the two vertical sections. 450 ohm twin feeder is obviously a convenience to employ - but was there an interaction between the two inverted "L" elements?
That seems to be the case and and I settled for a model that looked like this with 200mm spacing....
The simulation of first the 20m mod below.
The 15m response seems to be a lot better in simulation and one of the factors here is that it is defined as being higher than Mike actually managed to get in his garden.
Below is both the 20m mod and the 10m mod.
That's where the adjustments ended as Mike was happy to leave it without the 10m mod added.
He intends some garden layout changes in the spring and then perhaps installing an elevated counterpoise. Currently he is having fun on 80, 40 and 20 metres.
I didn't capture his 80m WSPR tests but he was really doing well with world wide results.
Here is a day of 20m tests.
... and here a day of 40m WSPR tests.
Towards the end of the year I had some trees cleared in my own garden and I now have room to retain my doublet and install a G7FEK antenna a little way distant. I will be able to do some comparisons and actual testing of that greater spacing of the vertical sections.
More later then ....