[Elecraft] Re: Best way to secure RF ground from 2nd floor- a related question

[Solosko, Robert B (Bob)]

 Hi All,

	I have a related question: how do I tune the counterpoises?

	My shack is also on the 2nd floor and I had major problems with
RF in the shack on 15m, 17m and 20m. My antenna is a 100' dipole
connected with ladder line to a 4:1 current balun that's just outside
the window where my shack is. Then I have 5' of RG-8 running through the
window to my KAT100 inside. I also have the braid from some old RG-8 as
a ground wire running from the KAT100 through the window to an 8' ground
rod. I had RF in the shack problems with this arrangement that was
mostly resolved by connecting 3 counterpoises to the ground side of the
balun, one for each band. It's not perfect, but it's a major
improvement. The counterpoises were cut approximately for each band - so
the question is how do you tune the counterpoises, since the effects of
RF in the shack at this point are somewhat subtle. 

	I just purchased an MFJ 259B antenna analyzer (mine actually
seems to work out of the box!), but I don't see any obvious way to use
it to tune the counterpoises, since the counterpoises are just heavy
insulated wire, and not coax. Can the antenna analyzer be used to tune
the counterpoises? If not is there some other way to do so without
needing other specialized equipment?

	Thanks.

Bob W1SRB

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ron D'Eau Claire
Sent: Wednesday, February 20, 2008 2:24 PM
To: elecraft at mailman.qth.net
Subject: RE: [Elecraft] Re: Best way to secure RF ground from 2nd floor


-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Larry Dodson
Sent: Wednesday, February 20, 2008 10:26 AM
To: elecraft at mailman.qth.net
Subject: [Elecraft] Re: Best way to secure RF ground from 2nd floor


I recently tried this on my multi band offest center dipole.  300 ohn
twin feed to near ground; box there contailing 4:1 balum which connects
via RG8 to rig in second floor room.  In the box ( one foot above
ground) I connected outer sheild of coxial to horizontal copper earth
pipes below ground.  Result is that I get good SWR across 80M band and
no RF in shack.
I am getting good signal reports, but have not had the opportunity to
check _______________________________________________

That's a setup that often works well. The balun decouples the twin lead
from the coax, so there's little RF current flowing on the outside of
the shield (lots of RF INSIDE, but the RF doesn't penetrate the shield).
The coax is far enough from the antenna itself that it's not picking up
too much RF on the outside by induction. 

So, even if a high impedance voltage loop happens to appear at the rig
end, the high RF voltages are safely shielded inside the coaxial line. 

The dangers of such an arrangement, as your comment suggests you're
aware,
are:

1) The behavior of the balun can be quite unpredictable with potentially
high losses when the SWR on the line is high, as it probably is on most
bands. 

2) The coaxial line will have relatively high losses, also due to a high
SWR. Your 300 ohm ribbon will also show losses but they are much lower
because of its higher impedance. For example, if you terminate the 300
ohm line in a 2000 ohm load it will see an SWR of about 7:1. Terminate
the 50 ohm coax in the same 2000 ohm load, and it will see an SWR of
40:1!

The ideal solution is a remote antenna tuner, in your case where the
balun is located. Then the 50 ohm coax would always have a very low SWR.


Ron AC7AC

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