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G0FTD > COAX 19.12.05 15:16l 58 Lines 1696 Bytes #999 (0) @ WW
BID : 513121G0FTD
Read: GUEST DL1LCA OE7FMI
Subj: Re: Dialectric value help
Path: DB0FHN<DB0MRW<DB0SON<DB0SIF<DB0EA<DB0RES<ON0AR<IK1ZNW<VE3FJB<ON4HU<
GB7YFS<GB7SYP<GB7ESX<GB7NNA
Sent: 051219/1001z 75153@GB7NNA.#31.GBR.EU $:513121G0FTD [Witham, Esx]NNA V3.1
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.
METHOD 2:
Signal dip method (example).
Take a length of unknown coax and cut to a physical 1/4 wave.
If that was 150Mhz then that would be exactly 50cm.
Place it across the terminals (t-piece of whatever) of your rx antenna,
at the rig end or across antenna terminals it doesn't matter and leave
open ended at far end.
Whilst listening to a steady signal on 150Mhz (or chosen freq) cut bits
off coax until maximum signal attenuation is seen.
Make sure you are writing the results down each time!
If this occured when you have cut half of it off (25cm) then the
velocity factor would be 0.5 and pro rata.
- Andy -
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