Yes. I often find that the VF varies from published specs on RG-62 and
other cables as well. I always cut measure with an antenna analyzer or
Grid dip meter to make sue the pasing lines or whatever I am making
really do what want them to do.
K4TO
Steve Maki wrote:
>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
>
>List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
>Trylon Titan towers, coax, hardline and more. Also check out our self
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>
>-----
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>
>
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
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