[Antennas] Re: Common-Mode choke at 430 MHz.

[Dave Kelley]

At 04:01 AM 7/18/2002 -0400, you wrote:
>Am building a couple of 8 element 430 MHz Yagi on 1 inch PVC boom.
>
>The driven element is a dipole with direct feed RG-58A. I thought a
>common-mode balun wound with the RG-58 around the 1 inch OD PVC boom would
>be neat so I computed the reactance needed was .4 micro-Henry, which is five
>turns on the boom. During my research I was could not find the use of
>common-mode baluns on frequencies higher than 6 meters. I have three
>questions:
>
>1                     Will a common-mode choke work on 430 MHz?

Yes, you can use a common-mode choke at 430 MHz, but not the design
you propose.  The free space wavelength at 430 MHz is around 70 cm (a
little over 2 feet), so the choke you're proposing would not act as a "pure"
inductor.  Inductors are usually designed with the assumption that the
the coiled wire is no more than a small fraction of a wavelength long.  That
assumption is violated in your case.

Another way to make a common-mode choke is to place ferrite beads
along the outside of the coax near the feed point.  I'm not sure what bead
"mix" (material type) or number of beads you should use, but you could
probably get some advice from Amidon or one of the other ferrite bead
manufacturers.  There may be other alternative designs that would be
effective at 430 MHz.

>2                   Will the capacity coupling between turns of the balun at
>430 MHz make the choke ineffective and if so would winding the balun by
>spacing the turns the diameter of the RG-58 help?

Capacitive coupling will almost certainly be a problem.  That's one of 
the main reasons why a coil-type choke won't work well at this frequency.

>3                   How would one check the common-mode currents on the yagi
>models?


I'm not sure.  Perhaps someone else on the list has some ideas?

73,
Dave NB4J