[RFI] Common mode choke on 4 Square feedlines without skewing phasing?

[Jim Brown]

Hi John,

As Gedas has observed, some of these specialty coax cables very from one 
mfr to another. I must plead a certain degree of ignorance -- until 
needing something with the specs of RG400 for these chokes, I'd never 
used any of these premium teflon cables.

Thinking about power requirements, each vertical in the array gets only 
1/4 of the TX power, which, if there were no feedline loss is 375W in a 
legal limit system. Anyone who does more than that gets no help from me. 
:)  Depending on length of the home run to the shack, my guess is that 
you're hitting the antenna with no more than 350W, and probably closer 
to 325W.

I'll leave the choice of coax up to the user, but it seems to me that 
virtually any practical coax should handle that much power with a lot of 
headroom.

Another piece of information. I just pulled out the data for the VNA 
measurements I did several years ago for the RG11 that I bought from 
Davis RF. VF converges to 0.8423 at VHF, and VF at 160M is 0.831. Loss 
on 160M is 0.167dB/100 ft. DX Engineering says their cable is 0.78. 
Assuming the controller in the geometric center of the array, it's 
94.5ft to each vertical. The lines to the verticals are specified as a 
quarter wave, which is 105' at 1830 with VF=0.78, and 111' with the 
Davis RF RG11.  This means that using Davis RF RG11 gives you 6 ft more 
coax to play with while still maintaining proper phasing, so it seems to 
me that there should be more than enough to rig to the antennas and the 
controller and still allow you to subtract enough to account for adding 
the chokes.

At that power level, the 10K ohm Rs you get from 18 turns on a 2.4-in 
o.d. #31 seems quite conservative for dissipation unless something 
breaks pretty badly. My spreadsheet says 4.8 ft to wind the choke, 
allowing a total of 8-in for leads.

Hope this helps.

73, Jim K9YC

On 1/25/2019 5:51 PM, John K9UWA wrote:
> Hi Jim
>
> Yesterday you mentioned a 75 ohm small diameter coax that had a small 
> bend radius to wind 80m and 160m 4 square chokes with. when I looked 
> that coax number up it came up as a 50 ohm cable so after looking 
> about I found some 75 ohm small diametercoax'es but none that had 
> small radius bending or ones that would handle 1KW PLUS for 160m and 
> 80m. My issue being that I have a K8UR style 4 square for 160m which 
> means I am pulling this vertically mounted InvertedV antenna out about 
> 95 feet from the supporting tower where both weight and mass being 
> that the station is located in Northern Indiana where we have Wind Ice 
> and Snow loads to think about. So yes I would love to put 4 nice 
> chokes at the feedpoint of these 4 antennas. We just need a coax 
> product number that will hold up in the above situation as well as 
> fitting onto hopefully a single FT240 mix 31 core. The RG302/U fits 
> all of this except the possible migration of the center conductor due 
> to bending radius?
>
> thanks
> 73
> John k9uwa
>
> > On 1/24/2019 5:38 AM, john at kk9a.com wrote:
> > > I use RG302/U for 75 ohm chokes.
> > 
> > Hi John,
> > 
> > I would be concerned about bend radius for tightly wound turns on the
> > 2.4-in o.d. toroids with the solid steel center of this cable. For other
> > applications it looks fine.
> > 
> > 73, Jim K9YC
> > 
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>
>
> John Goller, K9UWA & Jean Goller, N9PXF
> Antique Radio Restorations
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