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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