[MAMS] Unknown coax

[Dave Sublette]

Good evening microwave folks,

Confession time:  I still fool around with HF.  I built a rotary dipole for 40 meters this week — full size.

I’ve been having a discussion on towertalk about feeding it with some sort of an RF choke to keep RF off the shield while transmitting and suppressing common mode noise for receiving — all pretty much straightforward, standard practice.

I seem to have opened a Pandora’s box.  Opinions are flying and options are being discussed.  What it boils down to is a two section choke consisting of a couple turns of RG142 through two mix 31 beads and a repeat of that 10 inches away on the same piece of coax looks to be a very good approach and I would like to try it.  

Finding the coax is another story.  Some folks say that RG 400 is better because it has a stranded center conductor whereas the RG142 is solid.  I would agree.  Both are teflon, double shielded and I suspect are used extensively in the microwave world. The stuff I am finding is mainly on eBay and comes from China.  I am dubious about the quality.

So I am soliciting your opinions about where to find surplus RG 400.

On the other hand — digging around in my microwave stuff, I have found some coax that might be a substitute.  It is double shielded, but the center insulating material is clear  I doubt that it is teflon.  One end has what I think is an SMC connector.  The other end is a cutoff.  It is white and the OD is 0.125 inches, smaller than RG142 or 400.  One end had a table on it that says, “ASU 1-3”.  I’m hoping that someone has seen this stuff and knows what type of coax it is.  

The HF guys are claiming they can use these RF chokes at a power level of 1.5 KW in the shack on all frequencies in the HF range.  I won’t be running any more than 100 watts, so I’m not concerned about the power rating.

Another option would be for me to wind one of the chokes and test it.  I have a sig gen and spectrum analyzer.  I’m not sure at this moment just how to go about testing it.  The key thing to look for is a high impedance between the ends of the shield at HF frequencies.  I know what the performance is of the RG142/400 chokes.  If I could come close, I would use this “unknown” cable.

Sorry to ramble, but I had to describe the situation.

Dave, K4TO