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Re: [TowerTalk] rf burns

Subject: Re: [TowerTalk] rf burns
From: Jim Brown <>
Date: Mon, 06 Feb 2012 22:18:57 -0800
List-post: <">>
On 2/6/2012 8:49 PM, Tommy Alderman wrote:
> I think I really don't understand this? How can a loop antenna be
> unbalanced? What about the length of the feed line in the shack being a
> voltage max? Why isn't it true that all he would have to do is change the
> length of his 450 ohm 'window line'?

Tommy, you're confusing common mode and differential mode voltages and 
currents. Both can be at play in any antenna, including this one.  RF in 
the shack is the result of high common mode current, which in turn is 
caused by imbalance in the complete system -- that is, the antenna, the 
feedline, any matching network, and the rig.  The DIFFERENTIAL voltage 
and current varies along the line because it is a transmission line, but 
that voltage is  BETWEEN the conductors and that current is flowing up 
one side and returning on the other.  It has maxima and minima that 
relate to its velocity of propagation as a transmission line, but it is 
excited  by the transmitter and the signal propagates up to the antenna, 
and depending on the match, there are standing waves with maxima and 

The COMMON MODE voltage and current treats both conductors of the 
feedline as a long wire antenna, and the current is everywhere in phase 
along the line. That voltage and current will exhibit the maximia and 
minima of an ANTENNA, based on a velocity of propagation that is nearly 
1. That common mode current is the DIFFERENCE between the current 
flowing on the two sides of the antenna at the point where the feedline 
is connected, usually the result of imbalances in the antenna, and also 
any imbalance at the transmitter end. That COMMON MODE voltage is ALONG 
the antenna -- in the shack, it shows up as a voltage between both sides 
of the antenna and everything else in the shack (like the operator) and 
the equipment we touch.

When we change the grounding in the shack, we are simply changing the 
current and voltage distribution along that long wire antenna, which 
includes the intentional antenna, the feedline, and whatever we connect 
at the shack.

When we add a common mode choke in series with the feedline, we are 
adding a high impedance in series with the common mode circuit, but, 
depending on the impedance of what we use to wind the choke, its only 
effect on the differential mode voltage and current is the result of the 
additional length of line needed to wind the choke, and any small 
differences between the impedance of the line around the choke and the 
rest of the line.

For coax, there's NO effect on differential mode other than to make the 
line longer, but the bifilar choke I described is roughly 100 ohms, so 
it puts a small bump in the differential circuit.  That bump doesn't 
matter much, because the length of that added line is quite small as a 
fraction of a wavelength, and it's no more lossy that the line we're 
adding it to.

Hope this helps,  FWIW, I think you and I are about the same age, and 
it's taken me a long time and a helluva lot of study to get my head 
around this stuff too. :)

73, Jim Brown K9YC


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