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@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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