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Re: Topband: RG-179 Coax

To: topband@contesting.com
Subject: Re: Topband: RG-179 Coax
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 14 Feb 2020 15:04:24 -0800
List-post: <mailto:topband@contesting.com>
On 2/14/2020 12:59 PM, CUTTER DAVID wrote:
However, in my experience cmc causes erroneous swr readings on antenna analysers I have used.  I would say it's almost useless to measure swr until cmc is eliminated.

The proper location for the first common mode choke is at the feedpoint. When it is inserted, it 1) adds to the length of the line; and 2) isolates the feedline from the antenna so that it cannot detune the antenna; and 3) may shift the APPARENT resonance of the antenna by virtue of this isolation, AND by whatever impedance transformation along the feedline that occurs by the addition of the choke.

This is all predicted by the fundamentals of how transmission lines work. Consider any real antenna -- it's feedpoint Z will be affected by height, surrounding objects, etc., AND will be different at every frequency. VERY few antennas have a feedpoint Z that is precisely equal to the feedline Zo over the range of operating frequencies (i.e., the width of a band), so any change made to the feedline can change the apparent resonance.

Take the simple example of a dipole resonant on the band. It's feedpoint Z at resonance will vary depending on its height above ground, soil conductivity, and surrounding objects. Specifically, the soil acts as a passive element of the antenna -- the reflection from it changes the feedpoint Z just like any other passive element, and HOW it changes it depends on the strength of the reflection and the phase shift corresponding to 2X the antenna's height. There are plots in ON4UN's "Low Band DXing" that show how feedpoint Z oscillates around its nominal value with height for certain ground conductivity values, and modeling software like NEC can predict it. In general, dipoles that are electrically low tend to be a better match to 50 ohm feedline, while those that are electrically high tend closer to 75 ohm coax. My high dipoles (hung at about 40m high from tall trees) for 80 and 40M measure in the range of 85 ohms at resonance. The measurement is done with a vector analyzer in the shack and exported to SimSmith, where the electrical length of the feedline is subtracted from the measured data. The feedline length is obtained by a TDR.

Similarly, feedpoint Z of vertical antennas will be affected by the radial/counterpoise system, ground losses, and surrounding objects.

73, Jim K9YC
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