[TowerTalk] VHF/UHF Common Mode Choke

Wes wes_n7ws at triconet.org
Sat Jun 29 09:41:26 EDT 2019

Back when I was on EME, we didn't have the knowledge or material to use ferrite 
chokes, but that might be the way to go now.  On my one balanced-line fed array, 
I manufactured a coaxial sleeve balun using copper tubing and pipe. Nominally a 
single band device I imagine at 3/4 wavelength it would work well.

But I had access to a machine shop then :-)

I would think about solid sleeves rather than clamp-on if your feedline allows it.

Wes  N7WS

On 6/28/2019 7:47 PM, David Gilbert wrote:
> The DK7ZB designs I plan to use give a 50 ohm impedance at the center of a 
> single split driven element.  A small amount of gain is sacrificed to 
> accomplish that.  I modeled the designs in EZNEC+ and they appear to be spot 
> on.  In addition, quite a few VHF/UHF enthusiasts in Europe have built 
> antennas per his designs and verified the 50 ohm feedpoint impedance.
> So all I really need is a common mode choke, and I will probably use the 
> clamp-on ferrites that K9YC suggests.
> Thanks es 73,
> Dave   AB7E
> On 6/28/2019 7:33 PM, Glenn Pritchard wrote:
>> 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. 

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