[AMRadio] new antenna and matchbox a bust.


Jim candela jcandela at prodigy.net
Mon Jan 29 20:06:35 EST 2007



Brett,

   It sounds like you got through this OK, and the results are good for you.
I have a couple of comments however:

* The EF Johnson Matchbox's have in my experience limited tuning range when
the load is reactive, and even if the load is resistive, they don't like
anything under a few hundred ohms or so. Feeding a resonant antenna with 450
ohm feedline, and a High Z antenna (like a folded dipole), and they work
great.

* Others are all upset at you using dual coax feed. You and I realize that
this is a compromise. One advantage here is there is no HOT wires coming in
the shack this way, and you can have a balanced feed all the way to your
tuner. This approach is more lightning friendly too. To keep losses down
however, I'd keep the length at a minimum, and the SWR less than 10:1 on
those lines. A Coax with Teflon insulation and lower capacity per foot might
be lower loss. Your tuner will have to compensate for the ~ 30pf / foot of
the Rg-214.

The W9INN balun is a commercial variation on this where you have coax from
tuner to balun, and the balun is outside. The coax runs at high SWR like
what you are doing. As a suggestion, I'd take a impedance bridge outside
where your 450 ohm line comes down, and see what the line looks like on your
favorite frequencies. If the SWR is high, add or subtract from the feedline
a little at a time until the SWR is < say 3:1. This should cut the loss in
the Rg-214 to negligible.

Folks go berserk when they hear about high SWR with coax. They need to be
reminded that this is 80 meters, and not 2 meters.

Regards,
Jim
JKO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Brett gazdzinski
Sent: Monday, January 29, 2007 11:19 AM
To: 'Discussion of AM Radio in the Amateur Service'
Subject: RE: [AMRadio] new antenna and matchbox a bust.



Today I put up about 50 feet each side, up about 35 feet,
fed with 40 feet of open wire line.
Coming into the house I used 2 RG214 runs, with the shields
tied together at the ends, into the Johnson antenna tuner.

A worse working thing I ever put up.
It seems to work real well on receive, but the lowest
swr I was able to get on 80 or 40 was 3 to 1
and even then it acted crazy.

I also tried connection both to the single wire
terminal on the tuner, no go there, worse then ever.

I put my dummy load on the balanced antenna terminals
and it would not tune to a low swr at all.

I wonder how things would work if I left the antenna
at 100 feet and feed it with coax, and used the
heathkit antenna tuner to tune to 80 and 40 meters?

Brett
N2DTS



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