On 1/12/2026 1:28 PM, Tom Hellem wrote:
Measured it with an swr analyzer. The dip in the swr curve , approximately
2.3:1, went from 14.1 mhz to 13.1 mhz.
The feedpoint Z of a vertical dipole approximates free space, because it
doesn't see a reflection from the earth. Thus, it is best fed with 75
ohm coax. Remember that most indicators of SWR assume 50 ohms, but the
actual SWR is determined by the match between the feedline and the
antenna. This will cause the minimum reading to shift a bit, but not
nearly as much as what you're seeing, so I suspect the parasitic effect.
It should be possible to model this in NEC by including the feedline and
the choke. For this purpose, I'd use a 5K resistor as a load 1% down the
line from the choke.
73, Jim K9YC
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