[Elecraft] OT antenna question

[Don Wilhelm]

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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