[TowerTalk] VHF/UHF Common Mode Choke

[David Gilbert]

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.
>>>
>>
>> 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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