# [CQ-Contest] SO2R stubs.

David O Hachadorian k6ll at juno.com
Mon Nov 12 16:53:38 EST 2001

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On Mon, 12 Nov 2001 17:23:23 +0000 Tom Osborne <w7why at harborside.com>
writes:
>
>
> I've been compiling some data to build the stubs here and was
> wondering, what is the formula for determining the stub length.
> Is it just cut and try or what.
> I tried 234/F and for 40 meters came up with 21.75 feet.  Most of
> the posts say 23 feet open or shorted work best.  Where did the
> 23 foot figure come from.  I have a bunch of RG-6 CTV coax here
> that I'd like to use, but it has foam instead of plastic
> dielectric.  How to figure the length of stubs using the foam
> Tom W7WHY

quarter wave in coax (ft) = (246/F) x (velocity factor)

where F is freq in MHz

velocity factor:
RG 213/8/58 = .66
RG 6 = .75
RG 8X, foam RG 8 = .78
9913 = .89

Some of the foam dialectrics may vary from nominal VF.

I think the best way to put a quarter wave stub exactly on freq is to
temporarily connect a 1 inch piece of wire from the shell to the center
pin of the stub's PL-259. Arrange this wire into an "L" shape, making
a little loop with the center pin and shell of the pl-259. Lightly couple
a dip meter into this loop. This works on an open stub, which is handy,
because you'll be cutting little pieces off to bring it up to frequency.