[TowerTalk] VHF/UHF Common Mode Choke
ea1ddo at hotmail.com
Sat Jun 29 05:41:16 EDT 2019
As you said, folded driven element is around 200 to 300 Ohm impedance.
To transform that to 50 Ohm is not a balun, it is a impedance transformer. You can transform 300 Ohm to 50 Ohm and still balanced.
What I am trying to say is these two steps are two different things; one is impedance transformation, and the other is balanced to unbalanced adaptation.
These two different functions can be done in different devices, or in only one.
95% V/UHF design for last 15 or more years are all 50 Ohm, including the folded dipoles.
We achieve that with the proper elements design. Adjusting the size and the position of the first director basically.
Some design gurus here in Europe:
73, Maximo - EA1DDO
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Glenn Pritchard
Sent: sábado, 29 de junio de 2019 3:34
Cc: towertalk at contesting.com
Subject: Re: [TowerTalk] VHF/UHF Common Mode Choke
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.
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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