I hadn't been home for a while and discovered that my 5-band trap dipole would not load (at least with my Heath single-banders which have fixed loading). Old ARRL Antenna Handbook design (32' per leg, 40m trap, 22' outer leg). Today it was warm enough (40's and sunny) so I lowered my trap dipole (a job in itself since vines had wrapped around the pulley rope and it wouldn't come down at first!) and found out why it wasn't working any more.
a) the egg insulator at the rope end had "flipped" and the wet braided rope was contacting the end of the wire, which is a high
voltage point. Maybe not a major effect but it can't help.
b) when constructing the traps I had missed a large solder splash across two turns. It wasn't a good solder joint but it couldn't
have helped either.
c) most importantly, one trap was entirely missing the 63 pf doorknob cap! The stumps of the wires were there but no cap
anywhere. A casualty of a couple of years swinging in the wind, I suppose. THAT makes a big difference especially on 40m where the trap is supposed to be resonant
I had a small 75 pf Jennings "W" series vacuum cap so I took a couple of turns off the trap and re-resonated it to around 7400, (which is in the 40m band once the 2-3 pf capacitance of the egg insulator loops is added).
It's fed with about 40 feet of 75 ohm (RG-59) coax which is supposed to be a good match, although I then connect it to the output of my HM-15 SWR meter which is a 50 ohm unit, and a 2 foot RG-58 from the Heath. So what SWR might I expect: 75/50 = 1.5:1? Or 1:1 if the antenna (supposedly 73 ohms) is matched to the 75 ohm line?
Anyway the indicated SWR on 75m was nearly 1:1, about 1.2:1 on 40m but 2.3-2.4:1 across 20m. At first I thought this meant that the 22' ends (past the traps) are the wrong length (especially since I had cut it for a trap with 63 pf and one of them is now 75 pf). But then again it doesn't seem to get higher at one end of the band as I might expect...
After perusing my '00 ARRL Handbook I came across this interesting chart which I think explains the problem on 20m:
(right-click on "View Image" for a larger view)
As you can see, with my antenna only about 25' above the ground (TV antenna tower at one end, tall tree with pulley at the other) the feedpoint impedance on 20m is going to be too high. Does this sound like a plausible explanation? I guess the only cure is to get it up higher!
-Charles