[Elecraft] HI CUR on 40

[Jim Brown]

On 10/31/2020 6:07 PM, David Olean wrote:
> I would put the ferrite at the end of the coax before it goes into the 
> rig if there were rf coming down the coax. Common mode energy can and 
> will get on the outside of the coax. I am not an expert on this,
I AM an expert on this. :) The most important place for a choke in any 
antenna system is at the feedpoint. It's function is to prevent the 
feedline from becoming part of the antenna by adding a very high common 
mode impedance to the common mode circuit. The only good reasons for a 
choke elsewhere along the line are

1) to prevent the feedline from becoming a parasitic element of another 
antenna; for example, I choke the coax feedlines to my high (125 ft) 
dipoles for 80 and 40 so that they do not become parasitic to my three 
160M verticals. e

2) to prevent coupling shield current by a mechanism quantified as the 
"transfer impedance" of the coax shield. It is the ratio of the 
differential voltage generated inside the coax as a result of shield 
current. Its unit is Ohms, lower is better, and the lower limit is the 
RF resistance of the shield at the frequency of interest. But the 
quality, density, and uniformity of the shield makes an additive 
contribution -- a poor quality shield increases the transfer impedance.

Both of these measures minimize inter-station interference, both in-band 
and cross-band. W6GJB's contesting trailer has antennas for 15, 20, 40, 
and 80 supported on a single pneumatic mast, with their feedpoints 
within about 2 ft of each other. Because they are very effectively 
choked, and because we have dual stubs on the outputs of the power amps 
for the two stations, we can operate on adjacent bands at 500W 
(K3/KPA500) with minimal interference from the lower frequency station's 
harmonics to the higher frequency station. One day, Glen rebuilt the 
connection at the antenna and accidentally left out the choke, and the 
interstation interference was terrible.

A choke at the rig is only a band-aid for failure to do proper grounding 
and bonding in the shack and throughout the house. See N0AX's ARRL book 
on the topic, on which I collaborated, or the slides for my tutorial 
talk at Pacificon, Visalia, and to various ham clubs.
http://k9yc.com/GroundingAndAudio.pdf

The half-wave section of line you described in an earlier post may take 
care of the feedpoint for one band (and it may not be as effective as a 
well-designed choke), but it's not a solution for other bands. Properly 
designed chokes can cover multiple adjacent bands. See 
http://k9yc.com/2018Cookbook.pdf

73, Jim K9YC