Vic,
Your antenna has a very low feedpoint impedance on 40 meters. The 30
feet of open wire line plus the 3 feet of 450 ohm is close to a quarter
wavelength on 40 meters. That feedline length should transform the low
impedance of the dipole to a higher impedance, so the choice of a 4:1
balun *should* be OK.
However, check it - measure the impedance on the coax side of the balun
to see what its value really may be. If the measured impedance is quite
low, replace the balun with a 1:1 current mode choke - or add about 16
feet (1/8 wavelength on 40 meters) to the parallel feedline and see what
happens to the impedance.
OTOH, you indicate that the SWR changes when you drive it with power.
That is a sign that something is breaking down in your antenna or
feedline. That could be a loose connection or some leakage point.
Leakage across a spacer in your open wire line is one possibility, or
oxidation at any junction in the antenna system. The clamps of the
copper wire to the aluminum dipole is one suspect area. Dissimilar
metals will oxidize unless preventative measures are taken.
73,
Don W3FPR
The antenna is a full-size 20m rotary dipole. It is all aluminum
tubing,
no traps or stubs. Just a dipole. I am feeding it with about 30
feet of
"true ladder line," which is open wire line made of #16 insulated wire
spaced about 3-1/4" with black PVC spacers every 18" or so, except
near the
antenna and the rotor where I've added extra ones so that the spacing
doesn't change when the antenna rotates.
The line comes into the shack and is connected to a static drain,
which is
a box with two 10-megohm high voltage resistors to ground and a
couple of
spark gaps. Then a piece of 450-ohm window line about 3 feet long
connects
it to a pair of large air variable capacitors in series with each
leg which
knock out some of the reactance on 40m to make it possible to tune
more
easily. Then a very short piece of window line connects to a big
5kW DX
Engineering 4:1 balun, spec'ed for tuner service, and finally via a
piece
of RG-213 18" long, to a T-network tuner.
My K3 drives a TL922 amp and I have an SWR meter in line.
Now here is my problem: it works OK on all bands except 40 meters.
On 40,
it tunes up fine with low power, but when I run more than a couple of
hundred watts, after perhaps 10 seconds of key-down, the SWR starts to
climb. I have watched it go to 4:1 before I stop sending for fear of
destroying something.
The SWR rises both on the meter in the tuner and the extra one I
have in
line.
Classic symptoms of something heating up. But what?
- The tuner components are all cold.
- The coax to the balun and its connectors are cold.
- The balun itself is just barely perceptibly warmer (I have to
touch the
core to tell).
- The window line, the static drain resistors, the air capacitors
and all
the connections in the shack are cold.
I know the SWR is astronomical on 40 meters, so currents and
voltages are
high. But nothing in the shack seems to be heating up. Any more
ideas of
where to look?
--
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
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