Topband: 160M shunt fed choke

[Jim Brown]

On 5/28/2020 12:20 PM, Raymond Benny wrote:
> One way to tell if a choke is needed I would think, is to add say, a random
> lenght of coax, 20 - 40 ft, to the shack end of you coax and see if your
> SWR changes. If so, then a then a choke could help.
> 
> Jim, does this sound like a reasonable test?

No, I don't think so. The reasons for the choke are 1) to minimize noise 
pickup on the coax from coupling to the antenna, and from there to your 
RX; and 2) to keep TX RF off the coax shield. With the robust radial 
system Tony describes, it seems unlikely that the coax shield would see 
more than 1/60 of the TX current, or that the coax shield would 
significantly change the feedpoint Z, or that it would contribute 
audibly to RX noise.

If, however, the antenna was not ideally matched to the feedline, 
changing the feedline length could change the Z at the transmitter. But 
this would be a transmission line effect, not a common mode issue.

73, Jim K9YC