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

Jim Brown jim at audiosystemsgroup.com
Wed Jan 23 15:45:53 EST 2019


On 1/23/2019 5:10 AM, Dave Cole (NK7Z) wrote:
> If you are using the DX Engineering box shown at:
> https://static.dxengineering.com/global/images/instructions/dxe-tfs4-80b-p.pdf

Looking at the instructions, I see that the cables to the individual 
antennas are RG11, so a 50 ohm choke would likely disturb the match, and 
thus the phasing. A possible solution would be to wind the chokes with a 
75 ohm coax.

My measured data is for RG400, which has a PTFE jacket, and RG400 was 
chosen because it has a stranded copper center to accommodate the tight 
bend radius. The fly in the ointment is that resonance in the choke 
depends somewhat on the diectric properties of the outer jacket. The 
good news is that on 80M, the circuit Q of the choke resonance is low 
enough that guidelines for RG400 would likely be "close enough" for a 
good 0.195" 75 ohm cable like LMR200-75, which specs a minimum bend 
radius of 0.5-in. That would NOT be true for 160M, where the circuit Q 
is much higher, and the resonance much narrower.

Taking all of this into consideration, I would suggest a choke wound 
with 19 turns of LMR-200-75 on the big 4-in o.d. toroids. That works out 
to be about 6.5-ft by the time connectors are added (measure as you 
build the chokes). LMR-200-75 has VF of 0.83; if the RG11 you're using 
also has a foam dielectric, the cables would be shortened by the same 
length as what you're adding with the choke.

73, Jim K9YC




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