[TowerTalk] Potting Baluns
K8RI-on-TowerTalk at tm.net
Tue Feb 5 13:48:31 EST 2008
Just a thought here:
When putting something together like a balun do you want to seal it, weather
proof it, make it rugged, reliable?
The odds are the answer is "all of the above". If it is then it's likely the
job is going to be *permanent*! IOW the parts are not going to be reusable.
Only one method is the most reliable and that is potting, but if potting
then the capacity of the balun and the ability to get rid of heat have to
be taken into account. If the balun is going to be inside a container such
as a PVC sleeve/tube like the old KLM and many of today's the RTV, be it
regular or non acetic is unlikely to cure properly or if it does it'll take
months to do so. remember this stuff will cure in the tube eventually.
One thing that should be pointed out right at the start: RTVs are not
designed to be used as potting compounds. But that requires defining Room
Temperature Silicones or RTVs. Originally they were the _one_part_ materials
that smelled like Acetic Acid and later the non-corosive types were
developed. *Outside* industry the term RTV has been applied to anything the
cures at room temperature. Last I knew there were no two part RTVs. Those
are sealants and potting compounds. Another variant that came along is the
CONDUCTIVE RTV . the only color in which I've ever seen the conductive
*stuff* is black. That doesn't mean all black RTVs are conductive as they
So, were it me, and it's not, I'd use either a two part Silicone potting
compound which is likely to be quite expensive, or an epoxy of a fairly low
viscosity. West Systems (TM) makes a very nice two part epoxy that should
do the trick, it's inexpensive (expensive and inexpensive are relative terms
here),easy to use, and readily available at most boating supply stores.
Like many epoxies it is exothermic and it does get HOT.<:-)). This approach
has a number of advantages over the RTVs. It is STRONG (If you want even
stronger you can mix in what are called "mill fibers"), It is water AND
moisture proof. RTVs are water proof, but not moisture proof. Also the cure
time is a known factor that can be varied a bit to suit conditions. I
prefer Vinyl Ester Resin as it has a lower viscosity than the epoxy and I
have a lot of it. Unfortunately it only comes in gallons and it has a very
short shelf life compared to the epoxy family. It also uses a rather nasty
catalyst that makes it expensive to ship.
It may take a little practice to do a good job of potting, but the results
are worth the time. Also, any potting job should be considered *Permanent*.
IOW the parts are unlikely to be reuseable but there's nothing in a balun
that should be considered expensive. OTOH I do realize some of us have to
pinch pennies by necessity where even PL-259s and SO-239s get reused. (been
there and done that)
The nice thing is by rolling your own, you can build a balun with ratings
high enough to withstand considerabley higher voltages and currents handled
by the regularly available "store bought" stuff.
73, good luck, and happy potting,
BTW I retired as a project manager from one of the major corporations that
produces a series of RTVs.
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