In a message dated 9/18/01 9:57:59 AM Pacific Daylight Time,
john-k1rc@juno.com writes:<<
Hi all,
I'm in the process of cleaning up/refurbishing a tribander for the winter
season. I cleaned out all the traps, used new h/w where needed etc.
Put the thing back together and in the air. It worked great for about a
week till we had some rain.... SWR on all bands shot up to over 3:1.
********** This goes unnoticed by those who don't have a full time SWR bridge
in the circuit. A pattern change can occur like reduced F/B without a
noticeable SWR change. This doesn't happen with quads and a properly solder
joint (1 per element)
Took the ant. back down and went thru it again. I noticed the "old"
Noalox in the tubing joints was somewhat caked on contained alot of
moisture.
****So have others!
When I took the ant down, I put the 259B on it and found the resonant
freq on all bands about >800kc high on all bands, which is about 100kc
higher than when up in place and the SWR went crazy.
********That's a fair change. Perhaps a clamp wasn't tight enough.
I started cleaning the gray gunk from the joints with great difficulty.
*********A high power W6 I know had to use a file to get Penatrox off. He
"RF cooked the Penatrox." I've had Penatrox joints go bad also 6 blocks from
salt water back in 1958. Perhaps the zinc conductive particles in it change
if the Penatrox grease doesn't keep the moisture off them.
Didn't really know what to use to clean up the joints and went rummaging
around the garage... I ended up using Ether (AKA engine starting fluid).
This stuff on a paper towel, easily cleaned up all the tubing joints like
new...
**********I have a series of steel brushes and also brass shot gun barrel
brushes I mounted on the end of a 12" rod rotated by a drill motor. The
places that sell nuts&bolts sometimes have these steel brushes.
Used the paper towel wrapped around a round dowel to clean the inside of
the larger tubing. The tubing came out really clean/shiney and with no
residue.
I did a dry-fit of the driven element with proper dimensions and the 259B
said the Resonant freq was right on where I wanted it. The res. freq
should drop a bit when the antenna is in the air.
Now that I've put you all to sleep, here are a couple of questions:
1. Why did the moisture mix with the Noalox?
********There was nothing to stop it.
2. Did the moisture cause an "insulating" barrier causing the SWR
problems?
********Do you have better explanation? You had a "capacitive joint."
3. What does everyone use to clean out the tubing joints? (Any cleaning
solution in a pinch?)
4. What are the disadvantages of "dry-fitting" tubing joints and using
the antenna as is? I have no problem taking the antenna down every couple of
years for maint.
*******Sounds like a good plan for yagis. It creates a "Saw Tooth
Performance & Efficiency Curve" where a quads is a straight line and higher.
Any constructive guidance would be appreciated
************Use the flee Market Aluminum solder on the joints. I rotate the
elements in the lathe after a thorough cleaning, rotate it slowly, heat with
a BurnsOMatic torch and at the right time apply the solder. If you do it
right it flows perfectly. Tubing bigger than 3/4" may need 2 torches. No
pattern SWR change for 9 years and it still Kicks Butt. I'd suggest
completely cleaning the all the joints before a contest. If rivets were used
to join elements you have to drill them out and re-rivet.
I've had some good luck with just plain Silicone grease. It keeps the
air and moisture off the surface (the other geases seem to fail here) which
are the 2 main elements of corrosion along with dissimilar metals (even
aluminum to aluminum. The joints always come apart easily. It stops
aluminum oxide forming with SS clamps and bolts for years even with some salt
water spray. Has anyone else got any data on the reliability of Silicone
grease.
As far as conductive grease is concerned the only place where conductivity is
needed is at the joint thin rim of diameter change. RF travels on the
outside and makes the jump at the joint. Conductivity inside the joint is of
little value ecept for DC currents--perhaps static electricity.
Ease of disassembly years later and no Aluminum Oxide is the main goal if you
grease the joints. Silicone grease is inexpensive, never gets hard, never
washes off and is hard to beat with a tight clamp. I'd grease the SS clamp
screw and surface also. The RF has to jump from the tubing to the clamp and
back to the aluminum again--for each clamp joint after joint after joint.
Would you believe it creates "Jumpy RF?" I have a picture of the "corrosion
pattern" of the SS clamp slots on the tubing and it went deep after being
together for years. K7GCO
73 es DX
John K1RC
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