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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|