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Re: [TowerTalk] SWR variations with power

To: towertalk@contesting.com
Subject: Re: [TowerTalk] SWR variations with power
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 04 Nov 2013 16:00:59 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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 shack.

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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