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G0SYR > COAX 21.12.05 23:50l 62 Lines 1965 Bytes #999 (0) @ WW
BID : 7954G0SYR
Read: DL1LCA GUEST OE7FMI
Subj: Re: Dialectric value help
Path: DB0FHN<DB0THA<DB0HDF<DB0ERF<DB0FBB<DB0IUZ<DB0GOS<DB0RES<ON0AR<I0TVL<
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Sent: 051221/2144Z @:GB7CIP.#32.GBR.EU $:7954G0SYR
T:From: g0syr <g0syr@gb7cip.ampr.org>
T:Newsgroups: ampr.misc
T:Message-Id: <vshtqon6h8xt.4uji74z4mro5$.dlg@40tude.net>
Hi Andy and all,
I'll send a second copy of this post as I screwed up the topic
On Mon, 19 Dec 2005 10:01:00 +0000, g0ftd%gb7nna.#31.gbr.eu@gb7cip.ampr.org
wrote:
> GM0DHZ wrote:-
>
>> hi reader, can anyone tell me the dialectric value of Satellite coax, ie
>> RG 58 is .66.
>
> So you want to determine the velocity factor of some coax.
>
> Here's a practical way of working it out:
>
> - Cut a length of the cable to a free space 1/4 wavelength at your freq
> of interest.
>
> - Connect a 50 ohm resistor to far end
>
> - Plug other end to rig
>
> - Transmit and plot the swr, now cut a bit off and put replace resistor
>
> - Replot and do this several times
>
> When the length is eventually found that offers a 1:1 swr OR where
> the *DIP* in SWR is seen then measure length and you'll know that you've
> an electrical 1/4 wave length and as such you can determine the velocity
> factor.
>
> Also if the coax impedance is 50 ohm you'll see a 1:1 swr at the dip,
> if it's 75 ohm (more likely) it will only dip to about 1:5 swr.
>
>
> How does this work ?
>
> Because when the coax is at an electrical (not physical) 1/4 wave
> it acts as an rf short circuit. By stuffing a resistor at the end
> we see the resistor in series with the "short circuit" which makes
> measurment easier.
>
snip
Have you tried this method ?
Surely the quarter wave only looks like a short circuit when its
far end is left open circuit?
If you terminate a 50 ohm line with 50 ohms you will always measure
a 1:1 SWR regardless of its length no matter how much you cut off it,
there will be no dip to measure.
Your method 2 looks better, that should give you a rough idea of velocity
factor
--
73 de Bryan g0syr.ampr.org [ 44.131.244.60 ]
Amprnet mail g0syr@gb7cip.ampr.org
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