[Antennaware] Hy-Gain TH-3 Mk4 Tribander problem

[Steve Harrison]

At 07:38 PM 12/25/2007 EST, RLVZ at aol.com wrote:
>I even ran the antenna on saw horses until the SWR rose to over 2:1 and
then 
>ran outside to check for hot spots!  Nothing was hot.

Most likely, some plastic insulator somewhere is changing characteristics
with power. But are you CERTAIN this only happens on 15m?? If so, it's
undoubtedly a trap. If so, it may also happen on 20m since the RF flows
through both traps for that band. However, on 20m, both the 15 and 10m
traps are supposed to look like near-dead-shorts so as to "connect" the
whole length of the element. Then on 15m, the outside traps isolate the
outer element ends.

Try this: remove either the director or the reflector while on the
sawhorses and see whether it still happens. If so, it's got to be a driven
element trap. However, your SWR will undoubtedly change without that other
element so you may have to temporarily shorten or lengthen the DE to get
the SWR back down for your test. That'll be a hassle.

You can buy a replacement Plastic Parts kit for the TH3-MK4 for right
around $36 shipped. It's listed on the Hy-Gain web site (now a part of MFJ,
who bought them, I guess) in their main price list. I just received mine a
cupla weeks back. I have a MK3 but suspect most of the parts are the same.
Haven't had a chance to install them yet. It includes new trap end caps,
new driven element insulators, and new end caps for the boom and elements.

I suspect, probably, the two black plastic insulators holding your driven
element may also have cracks but that shouldn't be band-specific.

Steve, K0XP