Tom Osborne wrote:
> Hi Gang
> In recently putting my TH-3 together, I used LOTS of Ox-Gard on
> the joints when I put it together. Not noticing till AFTER the
> antenna was up, I see where the tube says it needs to be worked
> in with a wire brush. Needless to say, I didn't do that. Now
> the antenna seems to be acting erratic. Do I need to take it
> back down and clean the joints and wire-brush the stuff in to
> make it work right. I put it on kinda thick and then just put
> the joints together. In 40 years of antenna building, this is
> the first time I have used an anti-oxidizer on an antenna. Seems
> like it's more trouble than it is worth. Thanks and 73
> Tom W7WHY
>
Hi Tom,
Using some kind of goop is definitely worth the effort. Specially if you
live in a humid environment. How the stuff works is the subject of a
previous post, June-Aug timeframe. Look for GOOP in the title.
Too much paste is a common problem. When too much is applied it forms a
semi-liquid layer between the surfaces being connected. This prevents the
complete tightening of the connection by the clamp. Later the stuff dries
a bit and loses volume (shrinks) and causes the connection to become
loose.
I've played with this phenomenon in the shop while building antennas and
found that it is pretty easy to determine when too much has been applied.
Put one side of the joint in a bench vise and grab the other side with
both hands and twist it with all your might. If the mated pieces move you
either have a clamping mechanism problem or you have too much Goo in the
joint.
Here's the how and why I prep and assemble these kind of joints the way I
do.
.
1) Clean the mating surfaces of both tubes with either 320 grit aluminum
oxide wet or dry sandpaper or fine Scotchbrite. I use sandpaper first to
remove heavily corroded material.
The Scotchbrite is prefered for final prep. This is because its structure
allows for movement of contaminated material away from the working
abrasive surface, into the sponge-like body of the Scotchbrite pad. With
any type of sandpaper, the surface contaminants that you are trying to
remove are just repeatedly smeared all over the surface you are trying to
clean. Unless, you are very diligent about moving to a new, clean, fresh,
uncontaminated areas of the sandpaper, every few strokes. Everyone has
seen how quickly the junk loads up sandpaper. Why just keep smearing it
all over everything, getting nowhere, when the Scotchbrite allows it to
move away from the abrading surface, into the body of the pad.
Aluminum alloys are some of the most active materials for chemical and
mechanical bonding, just check them on your handy dandy galvanic chart.
What makes them highly reactive (negatively for corrosion resistance)
also makes them very good for accomplishing the good electrical bonds we
desire. The bottom line is that we need to somehow get down to just plain
simple clean bare aluminum, without any junk, to make our connections.
2) Wipe all dust from the mating surfaces with a CLEAN dry cloth.
3) Take a small piece of Scotchbrite that is soaked in the joint compound
and vigourosly scrub the mating surfaces. Leave a thin film of compound
on the surfaces, not heavy dark (so you can't see the aluminum), but wet
and appearing about 50% dark streaks of the compound.
Note: All compounds are made up of some solid particles and a liquid
(mostly petroleum based) carrier. The solids settle to the bottom of the
container when not used so they need to be thoroughly mixed before use.
4) Assemble and tighten the connection.
5) Check it by twisting, as mentioned above.
Evaluating the twist check:
If you can twist the connection and it moves endlessly, you have a
mechanical problem with the clamping mechanism.
1) It is either, you (or somebody else) didn't make the slots in the
outer tube wide enough and they are bottoming out preventing full
clamping pressure. A common problem with homebrew antennas with slots cut
by a hacksaw.
2) The clamps are not the proper size or are not properly oriented for
the connection.
Hose clamps that are too big form a huge bridge over the tubing at the
screw body part of the clamp. The large screw housing forces a large part
of the available contact area to not be in contact and hence, result in
most of the clamp pressure not acting to tighten the joint. It just
distorts it in other directions that don't result in locking the joint.
Note: Use the smallest SAE hose clamp that will fit the joint. The body
of the screw housing will be closest to the tubing size and therefore
result in the most uniform clamping pressure.
That is why they made all of the different sizes.
My closest electrical analogy would be to say that putting a #12 hose
clamp on a 1/2" dia tube is like trying to excite your 15 meter antenna
with the transmitter set on 20 meters. None of us would intentionally do
that, but I'll bet that hundreds of us have put the way too big clamp on
the way to small tube hoping that no one would notice. Of couse no one
did, till we had to go out there and try and fix the thing.
No comments from the riveteers allowed here. This is about conventional
clamped, Gooped joints. The rivets have their own set of different unique
challenges.
If using SS worm drive clamps, The screw housing should always be
directly over one of the slots in the outer tube. This will result in the
maximum pressure exerted to pull the outer piece down onto the inner one.
Again, if the slots aren't wide enough, a Godzilla clamp will not make it
tight.
If you rotate the connection a few times and then it locks up, you do not
have a problem with the clamping mechanism, but have just put too much
Goo in the joint.
The multiple rotations have caused the excess paste to flow from the
joint until the mating surfaces came together, with the relatively small
amount of paste trapped in the connection.
Retighten the clamp, make a mental note of how much to leave in there
next time, and move on to the next part of the assembly.
73, Kurt
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