[CQ-Contest] Coax Stubs for SO2R

[Jim Brown]

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.