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[TowerTalk] TH7 Advice found & appreciated!!

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Subject: [TowerTalk] TH7 Advice found & appreciated!!
From: (Kurt Andress)
Date: Fri, 06 Aug 1999 00:50:10 -0700

Thanks to all that responded to my request for help with the TH7
As expected, I got some quality help, and it is greatly appreciated!
I didn't expect to be posting a report till tomorrow, but sometimes you
get lucky. I'll take lucky over good any day!

Had a big day today, 7:30am - dark, but think I got what I was after.
The antenna did two complete trips down, up, down, up the tower. The
last trip up was some real fun as the wind gusts were around 35 mph by
Putting up an AB477 mast in these conditions is like being a "cat in a
room full of rocking chairs."  Fun Stuff!

Tom Whiteside, N5TW, won the "best advice, for my problem" lottery (it's
like that with these things), He'd already been unlucky enough to
already have been there!

The matching system was installed reversed on the antenna. I never
really looked at it, as the antenna was previously assembled and arrived
in several big pieces.

Here's what I learned:
There are two things here that might be useful for other TH7 users.

I wanted to replace the BN86 balun with a teflon coax/bead choke balun
to  avoid any problems, as many responses had indicated with the BN86. I
had several choke baluns that I have used over the years and had no
problems with them (all homebrew).
Stew, K3ND & Gerald, K5GW were very helpful in providing advice on how
to approach the choke balun retrofit.
The drama over this exercise exists because the entire TH7 matching
system, right up to the connections to the BN86, are part of the match.
If what one connects to the system, changes the original physical
relationships, the match is changed.

After reversing the matching system, I went thru several iterations with
the choke balun, and compared it with the BN86 (which, BTW, was not
bad), to arrive at what I think is equivalent ,and makes me happy!

Beware, You need to read #2 to keep this discussion calibrated.

After several iterations with the choke balun vs BN86 with the antenna
at 13', I found what I thought was an equivalent match. Then I raised
the antenna to abt 33' and saw that it was resonant way high in the
bands (for me. I'm a cw oriented guy), and the SWR at the bottom of the
bands was not within the Hygain spec's, nor acceptable to me:

I brought the antenna down again and worked some more on the choke
balun. I had tried several variations of places to connect the thing to
the TH7 system and found only one place and configuration that worked ,
here today, for me.

I made the pigtails on the teflon coax. from the bead balun as short as
they possibly could be made, and connected them to the exact place that
the BN86 balun terminals connect to the system (the bottom ends of the
curved aluminum straps). This produced almost identical swr readings as
the BN86.

Somewhere along the way, while chasing the balun replacement iterations,
I made a change in the driven element lengths. Only msde one change at a
The new De lengths were provided to me by Tony, N2TK, and later
confirmed by Roger, WB0DGF. These were the "DX" settings for the Th7. 

Length of second section outboard from the boom:
My manual: FWD DE = 40"         REAR De = 31"
"DX Settings: FWD DE = 42"      REAR De =33"

I tried these with both the BN86 and the final choke balun
configurations, and still had a heart attack over how high in the bands
the antenna was resonant, and how high the swr was at the bottom of each
band compared to the top end:

28.000 swr = 1.8        F0 = 28.770     swr = 1.15      
21.000 swr = 1.65       F0 = 21.270     swr = 1.05
14.000 swr = 1.25       F0 = 14.180     swr = 1.05

These are not bad numbers, just not what I would prefer.

So, following one of my more annoying traits, I stepped "outside the
box" provided by the factory, and the "Prime Directive" and set the
driven elements to even longer dimensions!
I had noticed, in my previous rebuild that these sections contained a
rather large overlap. That indicated that the factory intended for this
section to have the ability to be changed.

I set section #2, out from the boom to:

FWD DE = 44"            REAR DE = 35"

And here's what I see, antenna @ 48' over very poor sandy soil:
14.00 Mhz       swr = 1.15
14.09 Mhz       swr = 1.0       F0
14.35 Mhz       swr = 1.55

21.0 Mhz        swr = 1.5
21.23 Mhz       swr = 1.05      F0
21.45 Mhz       swr = 1.4

28.00 Mhz       swr = 1.35
28.500 Mhz      swr = 1.3       F0
29.70 Mh        swr = 1.55

Those numbers make me a very happy guy,and I send thanks to those who
provided advice and support.

There may be others out there that wondered if antenna dimensions,
outside "the box" might work. They do here.
YMMV! Gotta go find it out for yourself! It only takes time and effort!
And the willingness to gain nothing out of the effort, except an

BTW, F/B on this antenna is still FB! S7-9 signals on the front, are S0
off the back! Many I can't hear at all, so I have no idea what the
figure is.

Could not have done this in one day without the help from TT folks,

Not done yet, still need to tune the tower tomorrow am, Kinda hard to
get it right in the dark!

Kinda tired around here about now, but will sleep well!

73, Kurt, K7NV

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