[TowerTalk] SWR variations with power
kiessig at gmail.com
Mon Nov 4 18:29:52 EST 2013
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.
There are no other toroids on the line until the antenna, which has a
combined 1:2 Unun + 1:1 Balun on a single core. See:
I think I found the design they're using in Sevick's book on baluns -- 6
trifilar turns for the unun, and 6 turns of coax for the balun, both on a
single core with permeability of 650.
The antenna manufacturer claims it's rated to 3500W continuous use and
tested well above that; I'm using 1000W max.
If it's a heating problem, it should be better at first and get worse after
a while. I'll watch for that.
(Love the 2K-FA; more off-list)
73, Rick ZL2HAM / ZM1G
From: Grant Saviers [mailto:grants2 at pacbell.net]
Sent: Tuesday, November 05, 2013 12:14 PM
To: Rick Kiessig; towertalk at contesting.com
Subject: Re: [TowerTalk] SWR variations with power
Any ferrite baluns, ununs, chokes on the feedline or at the antenna? At 2K
power levels you could be heating them up (or saturating something) and
changing the Z. Different frequencies will behave differently, also. The
time constant of the change may yield some clues and you can scan suspects
with an IR thermometer. (about $40 at Harbor Freight).
How do you like the 2K-FA? reply off list.
On 11/4/2013 1:03 PM, Rick Kiessig wrote:
> I have a power/SWR meter in my amplifier (an Expert 2K-FA), as well as
> an off-board Power Master.
> Both meters show that the SWR level fluctuates somewhat as the power
> level changes - getting higher with more power. for example, the range
> might bounce around between 1.2 and 1.4 as I talk on sideband at 1 kW,
> vs. close to 1.0 after the auto-tuner in the amp runs (generally with
> 20 to 50W input). It seems to behave a little differently on different
> Changes at the Power Master appear to be dampened somewhat compared to
> the amp's meter, but they are there.
> Is this normal? If not, what could cause that kind of thing? A poor
> connector or a weak coax segment? My station is fairly close to the
> tower and antenna. Could it be related to RFI intrusion?
> A TDR sweep using an AIM 4170 shows an impedance bump on the near end
> of the coax, perhaps from the Power Master coupler - but nothing else
> that looked unusual. A "regular" AIM impedance sweep also looks OK.
> If the SWR fluctuations aren't normal, suggestions for chasing them
> down would be appreciated. I've thought about putting a dummy load at
> various points in the coax run, but it would be a fair bit of work, so
> I'm hoping there's a better way.
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