On Jan 12, 2016, at 5:27 PM, Guy Olinger K2AV <[email protected]>
wrote:
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 <
[email protected]> 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/
