[TowerTalk] TH7 SWR trouble ... Coax balun question

Fri, 5 May 2000 11:42:01 +0300

Gerald,

Thanks, this seems like a clue, however if I remember correctly the TH7
manual states that a coax balun is an option, what they don't say is what
kind it should be!  I am happy to hear that perhaps the work to be done can
be done on the tower, not after taking this thing down.  The previous owner
of the antenna had a "bundle" coax balun essentially hanging of the feed
point curved aluminum parts such that the coil was right at the feedpoint
and he states that proper functioning was achieved.  My solution has some
(half a foot) RG-213 between the feedpoint and the 6 turn, 4 inch coil (from
K1TTT archive)taped to the boom.  If it really is the case that the TH7 feed
is a complex combination of feedlines I am beginning to see where the
problem is.  It is funny that having the coax taped to the boom with the TH6
works just fine with the coax balun and the BN-86 works too. 

What I think I will try to do is and see what works (in order)
1) The feedline between the feedpoint and the coil is taped the boom, I will
lift it of.
2) Move the balun coil very close to the feedpoint by coiling that half foot
on the form
3) Shorten the "pigtails" from the coax to a minimum
4) Try a BN-XXXX balun ( I have some old (rusty :(  ) BN86 baluns, need new
BN4000 for real use.

Option 4 means that I may have to put a similar balun to all of the beams on
the tower (now TH7 + 2 TH6, next month 3 TH6) in order to maintain proper
phasing accuracy of the stack.

As you can see I *really* don't want to use commercial baluns, I have always
found coax baluns to be more robust and there is one less wire-terminal
connection to worry about.

Your comments would be very much appreciated!!!!

Markku
OH2RA

-----Original Message-----
From: EXT TexasRF@aol.com [mailto:TexasRF@aol.com]
Sent: 05 May, 2000 4:22
To: towertalk@contesting.com
Subject: Re: [TowerTalk] TH7 SWR trouble ... Coax balun question

I guess Hygain products have been out of site so long we have forgotten some

of the deatails of the TH7DXX. This antenna was designed to be used with the

BN86 balun (BN4000 is also ok) and the conductors supplied with with the 
antenna. The wires are spaced a few inches apart as they leave the balun and

connect to the driven element. The spacing and wire diameter is such that a 
short transmission line of a few hundred ohms is formed by these wires. If 
you run the numbers through a transmission line program (or Smith chart) you

will see a surprising shift in the feedpoint resistance especially on 10 
meters. As K7LXC sez: do what the manufacturer says! The coax choke baluns 
and other nice baluns may work well in most cases but the TH7 is not one of 
them. Check the owners manual for details.

73 de Gerald, K5GW/Texas Towers

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