| |
VK3ABK > COAX 01.01.06 07:34l 39 Lines 1891 Bytes #999 (0) @ WW
BID : 16382_VK3HEG
Read: DL1LCA GUEST OE7FMI
Subj: Coax dielectric measurement.
Path: DB0FHN<DB0MRW<DB0WUE<DK0WUE<GB7FCR<VK7AX<VK3HEG
Sent: 051231/0119Z @:VK3HEG.#WEV.VIC.AUS.OC #:16382 [Ballarat] $:16382_VK3HEG
From: VK3ABK@VK3HEG.#WEV.VIC.AUS.OC
To : COAX@WW
Hello all feeders.
A bulletin some time ago asked about the dielectric 'value' of satellite
coax cable. At the time, I thought the writer wanted to know about the
'velocity factor', as compared with RG58 coax. This was answered with some
added complication. A bit more than was wanted!
'Solid' dielectric as used in a majority of cables, has a VF of about 0.66.
That is, it causes a 34 percent delay in RF propogation. Other dielectrics,
or spacers in most cases, use a poly-foam, or a spiral of thin polythene.
This, causing a mainly air dielectric, results in a VF ranging from about
0.80 to 0.95 depending on the use and cost. 'Satellite' coax would be about
80 percent asuming a 'low microwave' quality, and cost effective consumer
applications.
A bulletin just arrived, from Jerry, K0CQ mentions an old way of finding the
dielectric VF of coax using a 'Grid-dip oscillator', (now called a Dip-meter
in the world of FETs) one of the essential tools for an RF constructor,
especially on VHF, but all frequencies are welcome! (I first used a 955
'acorn'tube in a 'grid dip oscillator' but now a TRIO dip-meter using a FET).
My method of testing unknown coax was to open circuit one end of the sample
(no cutting required) and soldering a small one turn short loop to the other
end as Jerry has described. Then, starting at the lowest frequency of the dip
meter tune for a dip which would then indicate quarter wave resonance. Then
by measuring the physical length of the coax and relating that to the
electrical length of a quarter wave at the dip meter frequency, a VF can be
calculated. More dips will be found as the dip-meter frequency is increased,
due to sub-multiples of the 'longest' quarter wave length. Use the first one.
This may also complicate the original question :-( , but it may help someone.
73. Dick. VK3ABK.
Read previous mail | Read next mail
| |
