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

[Jim Brown]

Hi Tony,

There's a VERY serious flaw in your plan. That flooded Commscope RG6, 
which I use extensively for RX antennas, does NOT do well with sharp 
bends. The center conductor, which copper-clad steel, shorts to the shield.

There's another flaw.  I suspect you're looking at SWR to define 
bandwidth. That's a poor indicator for a directional antenna, especially 
one that depends on coax lines to provide the required phase 
relationships. That's because the phase varies with frequency AND the 
length of the line, so when you triple the length of the line, you 
narrow the bandwidth over which the antenna exhibits it's desired 
directionality.

73, Jim

On 1/29/2019 8:18 AM, N2TK, Tony wrote:
> Jim,
> I would like to continue on with this 4-sq subject but for 80M. For many years I have been using an elevated feed (10' high) with elevated radials. I have been using Commscope flooded RG6 wit 100 beads on the end of the coax at the feedpoint. I want to bury the feedlines. I made up 3/4 wave feedlines with big clamp ferrite cores with several #31 cores. I tried these feedlines. The only thing I noticed was a very slight narrowing of the bandwidth. In the spring when the ground thaws here in up-state NY I will bury the lines. Based on your latest updates it seems I should change the big clamp to 4" cores. I would like to use the RG6 as it would be a continuous run. I am sensitive to what you said about sharp bends. The plan would be to make up some kind of fixture like in your Fig 8 so I can ensure the turns don't overlap and the bends are not too sharp.  Can you think of any negative aspects of doing it this way over making up some RG400 chokes?
> Tnx for any feedback
> N2TK, Tony
>
> -----Original Message-----
> From: RFI <rfi-bounces at contesting.com> On Behalf Of Jim Brown
> Sent: Tuesday, January 29, 2019 1:55 AM
> To: john at johnjeanantiqueradio.com; Rfi List <rfi at contesting.com>
> Subject: Re: [RFI] Common mode choke on 4 Square feedlines without skewing phasing?
>
> 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
>> Visit our Web Site at:
>> _http://www.JohnJeanAntiqueRadio.com_
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>
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