[TowerTalk] VHF/UHF Common Mode Choke

jimlux jimlux at earthlink.net
Fri Jun 28 20:47:02 EDT 2019

On 6/28/19 11:50 AM, David Gilbert wrote:
> I've never been much interested in VHF/UHF, but with the current level 
> of solar flux and the summer static I'm thinking of giving it a try 
> using homebrew antennas based upon DK7ZB designs.  For 6m I'll probably 
> use a coaxial sleeve as a common mode choke (unless somebody can suggest 
> something better/simpler), but for 2m/70cm I'm planning to build a dual 
> band yagi with a common feedline and I'm not sure a sleeve would work. I 
> guess it might, since 1/4 wave and 3/4 wave (432/144 = 3.00) accomplish 
> pretty much the same thing, but I'd like some input from VHF/UHF hams 
> who would know better than I.
> And if a sleeve balun is indeed the way to go, what is the best way to 
> fabricate one at UHF?  As W8JI points out, coax jackets aren't 
> necessarily a low loss dielectric at UHF.
> Thanks much for any suggestions.

A few ferrite beads/toroids of the appropriate material will suppress 
current on the outside of the coax, which is what you want.

Use Jim's RFI cookbook, but rather than all those HF materials like #32, 
pick a material with good properties at VHF/UHF (and I'll bet those 
omnipresent ferrites used for EMI/EMC of computers are a likely choice).

Those little 75:300 ohm transformers for TV use are another possibility, 
but it's a 2:1 turns ratio inside - so maybe if your driven element on 
your yagi is a folded dipole it would work.  TV is 50MHz to 800 MHz, so 
covering 144 and 440 is easy.

Most Yagis have low feedpoint impedances in their native state - a 
folded driven element might be a direct match to 50 ohms.

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