[TowerTalk] VHF/UHF Common Mode Choke

Glenn Pritchard gpritchard7000 at gmail.com
Fri Jun 28 22:33:46 EDT 2019


A folded driven element is inherently 200/300 ohms, if you take the
Sinclair or Comprod antennas there is a 96 ohm transference within the
element when they talk about open dipole (feed point open) or closed dipole
as in the phasing. I made a 7 element yagi with a folded dipole driven
element from a 210-C1, designed the antenna around the driven element.
With a yagi, without this arrangement you have to take the inherent 300
ohms down to 53 ohms, hence the balun.
I did a LOT of work with similar antennas when I was with CNCP
Telecommunications and Unitel here in Canada.

Glenn, VA7UO

On Fri, Jun 28, 2019 at 5:47 PM jimlux <jimlux at earthlink.net> wrote:

> 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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