Hi Jim,
I have a setup with the tuner isolated from ground and a CCM chock made of 50'
of coax wound around a 4" PVC pipe. The chock is fairly wide band by NOT having
an even pitch on the winding. The choke insure that there is no CM current
entering the shack.
Yes, the tower is conducting (AL) and grounded (9 rods) and it is participating
in the radiation. How much, I don't know.The feed line is parallel to the
tower, about 12" outside.
I have the shack shielded by enclosing it in Al foil. Everything passing in or
out of the shack is grounded to the foil or decoupled to the foil. I tested
with a transistor radio (7 transistors :) and the reception inside the shack is
very poor.
I have read through your essay about "pin one problem. You are right but I have
fixed most of it and don't have any RFI/EMI problem except one (1) telephone.
That one went to the hip.
I had, earlier, a problem with my IC706. RF on the microphone doesn't do good.
Now it is gone, probably because I have been very ainal reducing RF inside my
shack.
I did put up some pictures on my facebook page (Hans Hammarquist). They are
"open" so anyone can check them out.
Anyhow Jim, thanks for all advice you are giving. Some of them I never thought
of but find them very useful.
Hans - N2JFS
-----Original Message-----
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk <towertalk@contesting.com>
Sent: Sun, Jun 19, 2016 2:49 pm
Subject: Re: [TowerTalk] Balanced wire feed line
On Sun,6/19/2016 9:36 AM, Hans Hammarquist via TowerTalk wrote:
> The problem is that the feed line works as part of the antenna and there is
> CM current along the feed line starting at the antenna.
Right.
> That current dies out once it reaches the shack and will cause no problem
> inside my shack.
Wrong. It only becomes small when it reaches the shack if it encounters
a high common mode impedance at the shack between it and other
conductors (like a ground, or connections to other equipment). That high
common mode impedance could take the form of an effective common mode
choke or an ordinary 2-winding transformer with low capacitance between
the windings. And it can still be large at the top and along the line.
> It's just like an extra wire from the antenna down towards my shack and it is
> radiating all the way down with an influence to the radiation pattern.
Right.
> I don't know how big of a problem it is in my case as I have nothing more
> than my tower next to it.
Your tower is another conductor, and is probably grounded. The common
mode current on the line will couple to it, and BOTH can radiate.
Remember, that tower acts like an antenna too -- it's a conductor with
some electrical length.
> I can, lively, imaging if it passes a TV, telephone or a HIFI setup on its
> way down.
This is radio -- the feedline doesn't have to "pass" these devices to
radiate energy that is received when wires connected to those devices
act as receiving antennas, because most of those devices have Pin One
Problems.
73, Jim K9YC
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