|[AMRadio] RE: new antenna and matchbox a bust.|
k4kyv at charter.net
Tue Jan 30 17:45:59 EST 2007
> Today I put up about 50 feet each side, up about 35 feet,
> fed with 40 feet of open wire line.
> Coming into the house I used 2 RG214 runs, with the shields
> tied together at the ends, into the Johnson antenna tuner.
Not sure exactly what you are doing with the RG214, but it should be on the
side of the tuner that goes back to the transmitter, not the side that goes
to the antenna.
With your open wire feeders and dipole, 50ft + 40 ft = 90 ft. total length
for each leg of the dipole + feeders. 30ft is approximately 1/8 wavelength
at 75m. so the total length is ~3/8 wavelength. An odd number of
wavelengths is about the worst possible length to choose for a resonant
feeder system, since it is midway between a current loop and a voltage loop
at the input end of the dipole/resonant feeder. That is the most highly
reactive length you can possibly achieve, and very difficult to tune, and
oddball things like the ant tuner capacitor arcing over even at low power,
low efficiency, impossibility to get low SWR, etc. Add about 30 more feet
of open wire line, and the total length will be 120 ft. (a half
wavelength), and the end of the feedline where it attaches to the tuner will
be high-Z @ low reactance. It should feed very well with parallel tuned
circuit configuration at the tuner.
If you cannot alter the length of the feeders, try putting an identical
inductor or variable capacitor in series with each leg of the feedline at
the tuner, to add some capacitive or inductive loading. Experiment with the
capacitance or inductance. That may give you a load that the tuner can cope
with. Remove the loading capacitance or inductance to work other bands. On
40m and higher in frequency, your lengths should work OK.
I had the same problem feeding a dipole 65' per leg plus 130' of feedline on
160. That made a total of 185' per leg, which for 160 is almost exactly
what you have on 80m. I could arc over a BC-610 plate tank capacitor,
spaced at 7500 volts breakdown, with 100 watts of carrier modulated 100%. I
could get 1:1 swr by tapping the feeders down on the coil, but the tuner
still arced over, and the efficiency sucked. I added 60 more ft. of
feedline, and it loaded up like a charm with parallel tuning, and a full KW
DC input modulated at 150% on positive peaks, would not arc over the
capacitor with the same spacing.
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