[TowerTalk] Any RG62 experts out there? (fwd from K8LV)

[Steve Maki]

My friend Eric K8LV seems unable to post to some of the reflectors
because of the new AOL front end he is using, so he asked me to relay
this. His e-mail address is evonvaltie@aol.com

--
Steve K8LX
********************************************************************
(fwd)

       I have discovered what may be a dirty little secret about RG62 coax. 
While building up some transformers for a new phased array design, I measured 
the VF of a brand new reel of ALPHA 9062 (RG62A/u) and found it be about 
.725. This varies considerably from the mfrs. published data, which specifies 
.82.

       Thinking that it was a defective reel, I then procurred a reel from 
another mfr. (JSC) and lo-and-behold: the same result!.

       This much of an error in the VF, if not properly accounted for in your 
design, can cause a considerable amount of reactance to appear/disappear 
where not wanted. Since this is the only readily available 90ohm coax 
available, it should be of interest to anyone who works seriously with the 
design of antenna feedsystems.

       I would like to hear from anyone who has any knowledge relating to 
this. I am attempting to contact the mfrs. for some input, but first I have 
to penetrate their "tech support" firewalls (always a daunting task.)

K8LV Eric von Valtier

---

Steve:        (pse fwd to towertalk)

       I just broke on through to the other side - of the firewall at JSC 
Cable. They admit that they have never actually measured and verified the VF 
of RG62A/u, despite the fact that they have sold millions of feet of it! The 
engineer that I spoke to is "looking into it" as we speak.

       My educated opinion goes as follows. Several years ago mfrs. of RG62 
changed the design of this cable, probably to allow use of a larger center 
conductor. That would make it a lot friendlier to handle in a world full of 
75ohm cable and hardware. 

       In this new design the center conductor is smaller than the hole in 
the dielectric and the difference in diameter is physically handled by 
spirally wrapping a thread of dielectric around the center conductor, which 
holds it centered in the dielectric.

       Now, however, the dielectric is not sold poly with an epsilon-sub-r of 
2+.  But the people who wrote the catalog really don't know anything about 
electronics, so they just used their old-wives wisdom: e=2.3 so Z-zero is 93 
ohms (based on the ratio of outer to inner diameter, as usual.) 
Unfortunately, we have a situation here with a sandwich dielectric where one 
layer is air with e=1.0.

       Worse, it appears that this discrepancy has been 100% duplicated by 
every mfr. of RG62 that I am aware of.

       Finally, I would (as a reality check) appreciate independent 
verification by anyone interested that VF=.73 +/- .1  for RG62.

Eric

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