I’m experimenting with a 160m AM portable station. The transceiver is a homebrew project and runs around 20 to 50 watts of AM carrier depending on battery capacity. If I get enthusiastic I might add CW down the track. Why AM Portable? Why not SSB? Mainly because there’s a healthy AM group here in VK3 and so this rig will be used to call into local AM nets, but I will use it for the occasional novelty activation too.
Now to my antenna challenge.
I am using a 160m inverted-L. Because my Tx is homebrew I do not have the luxury of an inbuilt magic auto tuner/mismatch problem solver! On two outings I’ve tried to throw the weight as high as possible for the vertical section but I have not managed to get it higher than about 8 metres. That is low for a 160m inverted -L. The driven element (vertival /horizontal wire) is a 1/4 wave on 1850 or about 42m.
I modeled it in EZNEC and due to the short vertical section the feedpoint impedance was something like 10 to 12 ohms. Really low.
So I made up a simple L-match consisting of the usual 20 turn air cored tapped inductor wound on a PVC pipe length, and a dual gang variable capacitor with another 900pF (s.m.) in parallel, giving a tuning range of about 900 to 1800pF. The L was in series, the C shunt on the transmitter side.
Here’s what I found in the park today. Threw the wire about 8m into a tree, the rest pretty horizontal to a second tree. Ground was a clip onto the top galvanised wire of a mile long fence, far better than my experiments with a counterpoise. Lots of receiver noise, started hearing the guys in the Net, so far so good.
Couldn’t get the homebrew L-match to improve SWR at all. As soon as I added any L whatsoever, matching was worse and receiver noise dropped.
But, connecting just the dual gang variable cap (with parallel C, a total C of 1.8nF) in series (between the coax inner and driven element) seemed to work, got the SWR right down, and good reports from the other stations I worked.
Antenna afficionados, does that sound right? Seems a LOT of series C, but it appears to have worked. I note that many inverted-L users use longer wires and series C to tune out extra reactance to match 50 ohms. But my wire is only a 1/4 wavelength.
Inverted-L antenna theory is not simple and I am loath to spend a lot of energy to dig into it. I think the two factors in my case are, a) that my wire is 1/4 wave long and b) that the vertical portion is short.
Any comments appreciated.