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Re: [TowerTalk] Fabricated common mode chokes and their impedances

Subject: Re: [TowerTalk] Fabricated common mode chokes and their impedances
From: K8RI on TT <>
Date: Mon, 29 Aug 2011 16:26:01 -0400
List-post: <">>
On 8/29/2011 10:01 AM, Tod - ID wrote:
> How high is the SWR on your feed line Roger?

I think it hits around or between 5:1 and 8:1 at the highest point.
> If I was using a feed line that
> had 3:1 or higher I expect I would want to use something other than RG58,
> RG59 or RG8X. Davis BuryFlex certainly will do the job but I think Teflon
> dielectric coax will work too. Have you tried RG400U as the coax for the
> chokes? That is what I use here in Idaho,
Most of my feed lines were LMR-400 and the minimum bend radius would 
allow it to work. It also stays put where ever you bend it and is fairly 
slippery for feeding through the cores.  I do not use the UF or Ultra 
Flex versions because of the rubber like jacket that does not slip well 
and deteriorates  rapidly out in the elements...probably UV sensitive.

I've since gone to mostly LMR-600 except to the center fed slopers where 
I use Davis BuryFlex for 75 meters. I did use CNT240 for the feed line 
to the 40 meter center fed sloper but it did not stand up well to the 
movement caused by the wind. I've found CNT240 to be low loss, but rigid 
and easily broken.   Right now I'm using Davis BuryFlex, but would like 
to try RG8X on 40 (foil plus braid) as it's very flexible, light, and 
has low wind resistance. The voltages are too high for 8X and CNT240 on 
75 and 160 at the band edges..
I hope to get a chance to try remoting balanced line tuners at the tower 
and using 450 ohm window line from them to the antennas.  I'd like to 
avoid using 4:1 baluns as a shortcut to running open wire from the tower 
to the antennas.

The problem with that approach is I lose a lot of ability to raise and 
lower antennas easily and I have to climb around the weather proof boxes 
containing the tuners when going up the tower. OR at leas some one has 
to climb around them.  Maybe it's time to take the big 45G down and 
replace it with  a couple of heavy duty crank ups.


Roger (K8RI)

> I have plans to run measurements on 5 and 6 cores but maybe not until I
> return to Idaho in late September. The experiments that I have run clearly
> show that an extra turn produces more 'bang' than an extra core. That agrees
> with the theory of course.
> 73, Tod, K0TO
> -----Original Message-----
> From:
> [] On Behalf Of K8RI on TT
> Sent: Monday, August 29, 2011 3:11 AM
> To:
> Subject: Re: [TowerTalk] Fabricated common mode chokes and their impedances
> On 8/28/2011 6:51 PM, Tod wrote:
>> This probably has a limited audience but I do know that there are some
>> readers on this list interested in fabrication of high impedance common
> mode
>> chokes for their stations.
>> I built and measured 18 of this type of choke during the past week. There
> is
>> a write-up on my web site which shows how they were fabricated, how they
>> were measured and the measurement results. I used Fair-Rite 2.4 inch
>> diameter type 31 toroidal cores and small diameter [RG 59 size} coax. I
>> measured the magnitude of the impedance for each of the chokes on the 160,
>> 80, 40, 20, 15 and 10 meter bands. There are also attenuation values for
>> each choke [specific to the 50 ohm measurement circuit]. The results are
>> shown as a series of graphs for two cores, three cores and four cores with
>> 3, 4, 5, 6, 7 and 8 turns on the core bundle.
> I use 5 and 6 turns of Davis BuryFlex on 5 and 6 cores of the same
> type.  8X and 59 will not handle the high SWR when operating the entire
> 75 meter band at the legal limit.
> 73
> Roger (K8RI)
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