[TowerTalk] Choke on feed point of dipole

[Jim Brown]

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