Jeff is exactly right about resistance, but a note of caution about
cheap coax made for CATV. It tends to be made with a copper clad steel
center and Al shield. That depends on skin effect, so RF resistance is
reasonably low at VHF and UHF, where it is designed to be used, but RF
resistance is higher at low HF.
Remember also that coax with a foam dielectric will have a Vf closer to
0.84 than to 0.66 (solid dielectric), so the stubs will be have to be
about 27% longer, which also increases their resistance, which reduces
the depth of the null.
I think skimping on coax is a bad idea. The cheapest stuff I would use
for stubs is a decent grade of RG8X that has a copper center and copper
braid shield. The Wireman and Davis RF sell some decent cables in this
class, but they all seem to have a foam dielectric. Wireman CQ-117 would
be my choice.
Another point in support of Jeff's "use what you have" philosophy. About
five years ago, I helped N6RZ's widow dispose of his gear. I took a
bunch of his used coax to an NCCC meeting, and everyone turned up their
noses at it. So I took it back home, made stubs out of it, and measured
them for attenuation. With the exception of one piece of coax that
showed signs of having been wet, all measured like new coax, even though
most of it was probably at least 20 years old. This was not junk coax
-- most was Times, Belden, etc.
73, Jim K9YC
On Tue,7/26/2016 6:38 PM, Jeff AC0C wrote:
Works fine. Attenuation depth depends on the shield & conductor
resistance. RG11/6 tends to have a bit more resistance so the
attenuation null is not as deep. But you should get at least 23 dB
out of even the worst coax.
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