Unknown coax
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
Hi Dave,
How much you looking for as I still have some RG 400 but my supply is getting low. I like the 400 for the flexibility and have used Harbour Industries which is a good product. It is available on fleabay and here is a link. Check it out.
Randy WB8ART
-----Original Message-----
From: mvus-list [mailto:mvus-list-bounces@febo.com] On Behalf Of Dave Sublette via mvus-list
Sent: Friday, September 22, 2017 9:04 PM
To: Mid-America Microwave Society; David Feldman via Microwave
Cc: Midwest VHF/UHF Society Mailing List
Subject: [mvus-list] Unknown coax
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
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