In addition to the comments already, PVC material is notorious for breaking
down under high voltage RF. Avoid at all costs. Use Pol-ethylene "rain for
rent" tubing. Works well in high Voltage RF.
Mel, K6KBE
From: Guy Olinger K2AV <k2av....@gmail.com>
To: Vic Rosenthal 4X6GP/K2VCO <k2vco....@gmail.com>
Cc: Elecraft Reflector <elecraft@mailman.qth.net>
Sent: Tuesday, January 12, 2016 3:27 PM
Subject: Re: [Elecraft] OT antenna question
Hi Vic,
Your story does suggest trouble at very high current points. Just a list of
things below I've heard or seen to stir up a new idea or two. No opinion on
which if any makes any sense in your situation.
---------
Insect nests in tubular spacers on open wire.
Spacers have carbon tracks.
Material inside the balun housing is much hotter than touchable material.
Stranded wire in open wire has been waterlogged, corroded and the remaining
conductor material at a current max along the line is heating up.
Wire inside insulation is nearly all broken, or is broken, and contact is
miscellaneous and highly resistive.
Insulation on the wire is significantly compromised by ultra violet or
critter nibbling, letting in water to the stranded conductor. Advice, has
been to use bare solid #12 copper or larger for open wire that is carrying
large standing waves, to handle the current maximums.
Check heat all around the entire core. I have burned up some number of
cores before I understood the materials and engineering. On one all the
damage was on a spot that comprised only 15 degrees of the circumference.
Electrical connections in aluminum elements made of telescoping tubing go
highly resistive as water is boiled out of the joints.
Connections made of dissimilar metals/materials go bad and become more
resistive as water is boiled out of the joint.
Tuner rotary switches going bad.
The rolling contact on on a variable coil losing its tension (numerous ways
for this to happen depending on construction), contact is very small and
becomes worse as it heats up. Fixed by repairing mechanism for maintaining
contact.
Parted conductors at conductor joints due to metal erosion at the contact
point.
--------
Your troubles would have me taking down the dipole, rebuilding it with all
mating surfaces wire-brushed and reassembled with clear silicone dielectric
grease, completely new hardware likewise treated, with all balanced
conductors replaced new, using bare solid #12 outdoors, and brand new runs
indoors. Old wire to recycling.
Hope this has kicked off a new idea for you. 73 and good luck. Guy K2AV
On Tue, Jan 12, 2016 at 4:24 PM, Vic Rosenthal 4X6GP/K2VCO <
k2vco....@gmail.com> wrote:
> 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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