[TowerTalk] SWR variations with power

Jim Brown jim at audiosystemsgroup.com
Mon Nov 4 19:00:59 EST 2013

On 11/4/2013 3:29 PM, Rick Kiessig wrote:
> The feedline goes through fourteen 2.4-inch #31 toroids as it leaves the amp
> (three turns through seven toroids, then the same thing again). Might seem
> like a lot, but that's what it took to get RFI from 6m at 1 kW down to the
> point where it didn't cause problems in the shack.

It's rather unlikely that those chokes would heat perceptibly, and 
certainly not enough to change SWR unless for some reason they are 
seeing a higher than normal common mode voltage due to serious imbalance 
in the antenna (for example, an off-center feed).  BTW -- while a choke 
(or chokes) at the entry point may help keep RF out of the shack, chokes 
are FAR more effective at the antenna feedpoint in decoupling the 
feedline from the antenna. This can matter a LOT when the concern is 
keeping RF noise picked up on the feedline from coupling to the antenna.

There are measurements of the common mode impedance of practical chokes 
like those you're using, as well as specific recommendations by  band in 
http://k9yc.com/RFI-Ham.pdf  That document, and a Power Point 
presentation on Coax Ferrite Chokes shows how to derive the equivalent 
circuit of a choke you've measured, and that circuit can be imported 
into an NEC model (treating the feedline as a single wire with a choke 
in it). Computing currents for such a model yields dissipation in the 
chokes (taking duty cycle and peak to average ratios into account). Duty 
cycle also covered in the stuff on the website.

Another important point about RFI in the shack -- for lightning 
protection, all the antenna feedlines need to be bonded together at a 
common entry point, and that common point to a serious earth connection, 
and to all other grounds (earths), including power system, telco, and 
your shack operating desk, where all of the gear should have their 
chassis bonded together, and all of this with short, fat copper.  If 
you've done that properly, it's unlikely that a feedline will couple to 
gear enough to cause difficulty unless it's actually radiating into the 

Although I tend to agree with N6RK that the errors are likely to be in 
the metering, components like RF capacitors with high values of 
temperature coefficient can change value enough in high power circuits 
to change the match to the power amp. I saw that in my Ten Tec antenna 
tuners, which, as I recall, use N750 and/or N1500 caps.

73, Jim K9YC

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