[Elecraft] OT antenna question

[Dale Putnam]

It would be very interesting to do that test... key for 10 sec.. if it is possible - on an early morning, that has frost or snow on the spacers...Look for the one that becomes dry or drippy.

Have a great day, 
 
 
--...   ...--
Dale - WC7S in Wy
 
 


> To: elecraft at mailman.qth.net
> From: k2vco.vic at gmail.com
> Date: Tue, 12 Jan 2016 23:24:12 +0200
> Subject: [Elecraft] OT antenna question
> 
> I'm having a problem which has me stumped. I'm going to describe my 
> complete antenna and feed system because something in it is misbehaving 
> and I don't know what!
> 
> My system works on all bands from 40 to 10 (or it should).
> 
> 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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