Re: Best way to secure RF ground from 2nd floor

[Larry Dodson-2]

I recently tried this on my multi band offset center dipole and would
be interested in any comments.

300 ohm twin feed from antenna to near ground level; box there
containing 4:1 balun which connects via RG8 to rig in second floor
room.  In the box ( one foot above ground) I connected outer shield of
coaxial to horizontal copper earth pipes below ground.

I was getting RF in shack and poor SWR on 80M band. The result now 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 on the
amount of power that is actually getting out of the antenna.

Larry (G0IKE) K2 #2424
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Larry Dodson-2]

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
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[AC7AC]

CONTENTS DELETED

The author has deleted this message.

[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: [hidden email]
[mailto:[hidden email]] On Behalf Of Ron D'Eau Claire
Sent: Wednesday, February 20, 2008 2:24 PM
To: [hidden email]
Subject: RE: [Elecraft] Re: Best way to secure RF ground from 2nd floor


-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Larry Dodson
Sent: Wednesday, February 20, 2008 10:26 AM
To: [hidden email]
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

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Don Wilhelm-4]

Bob,

The best way to tune the counterpoises is to use an RF current probe and
tune for maximum current in the counterpoise wire.  Several designs for
RF current probes have been published in QST and other journals in the past.

An old method (which still works) is to put an RF ammeter in series with
the counterpoise wire to tune it.

You are correct, cutting a wire to some formula will not necessarily
yield optimum performance - the counterpoise wires are usually mounted
low and close to earth and other objects - that proximity will detune
them, sometimes drastically.

To use the MFJ259, I would think the easiest would be to split the wire
in half and measure its resonant frequency (with both ends connected to
nothing) - it would be a half wave dipole on double the frequency -
prune for zero reactance and reconnect the cut made in the middle of the
wire.

73,
Don W3FPR


Solosko, Robert B (Bob) wrote:

>  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
>
>  

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[AC7AC]

CONTENTS DELETED

The author has deleted this message.