[CQ-Contest] SO2R stubs.

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

On Mon, 12 Nov 2001 17:23:23 +0000 Tom Osborne <w7why at harborside.com>
> 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
> instead of plastic?  Thanks
> 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.

Dave Hachadorian, K6LL
Yuma, AZ
K6LL at juno.com

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