Another coax choice for 75 ohms is RG179, TFE + SCCS conductor + silver
shield. Slightly less diameter than RG400. I've not had SCCS creep
problems in dozens of QRO chokes I've made with 50 ohm SCCS RG142, but
if buying new 50 ohm cable the stranded RG400 is the better choice.
Belden says 1250 watts at 50MHz for the 75 ohm RG179, so for a 160m or
80m 4 sq probably has lots of headroom. 179 is available on ebay.
Grant KZ1W
On 1/23/2019 12:45 PM, Jim Brown wrote:
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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