I'd agree with K9YC and N6RK that heating of the choke toroids is not
the issue... but your remark about "RFI from 6m" raises a warning flag.
A yagi on a tower should never be a particularly tough common-mode
problem. The antenna and its surroundings are normally quite symmetrical
and the driven element is center-fed, so there shouldn't be a
particularly strong source of common-mode current on the feedline (not
like there is in asymmetrical antennas like an end-fed or an OCFD, for
example). Even quite ineffective common-mode chokes are generally "good
enough" to prevent common-mode problems in most Yagi installations.
The combined 2:1 unun and balun seems to be the one already used on
hundreds of existing SteppIR yagis. It isn't a good common-mode choke
at all, but the SteppIR experience proves that it's normally "good
enough for Yagi work".
So it seems very wrong that you should need <two> of Jim's large,
high-impedance chokes to suppress that problem. That seems to indicate a
pathologically high level of common-mode current coming back down the
outside of the feedline. In other words, you need to be looking for a
FAULT.
First, check the shield connection to your PL259 - this is THE most
prolific source of common-mode problems.
Maybe you also need to check the wiring in the driven element housing
unit, because balun/transformer configurations are notoriously easy to
mis-wire. The wrong way can look almost the same as the right way, and
can even give identical DC test results... but when RF is applied, the
wrong way can be spectacularly wrong!
73 from Ian GM3SEK
>-----Original Message-----
>From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
>Rick Kiessig
>Sent: 04 November 2013 23:30
>To: 'Grant Saviers'; towertalk@contesting.com
>Subject: Re: [TowerTalk] SWR variations with power
>
>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:
>
>http://www.ultrabeam.it/site/index.php?option=com_content&view=articl
>e&id=73
>&Itemid=73&lang=en
>
>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
>
>
>-----Original Message-----
>From: Grant Saviers [mailto:grants2@pacbell.net]
>Sent: Tuesday, November 05, 2013 12:14 PM
>To: Rick Kiessig; towertalk@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.
>
>Grant KZ1W
>
>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
>bands.
>>
>>
>>
>> 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.
>>
>>
>>
>> Regards,
>>
>> Rick
>>
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>
>_______________________________________________
>
>
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|