[Antennas] Rs twin lead

2002年4月19日 13:12:03 -0400

The RG-(**)/U type numbering system was initiated by the military during
WW-2, called the Joint, Army-Navy numbering system. Now-a-days the fact
that cable bears RG type numbers does not guarantee that it meets
military specs and some newer cables do not bear the RG type numbers.
The main thing to look for in buying co-ax cable is the percentage
shielding afforded by the outer conductor and whether or not the outer
covering is "non-contaminating." When co-ax was first developed, it was
discovered that the outer jacket of the cable, when exposed to the
weather (usually the hot sun) would give off a vapor that would be
absorbed into the dielectric (insulation between the inner and outer
conductor) and change its makeup so that after being in use a while
(months or years) it was no longer the original low-loss stuff that it
was intended to be. So the manufacturing process had to be changed to
assure "non-contamination." In the manufacture of some of the bargain
cables offered for sale, it seems, this precaution is overlooked or
skipped. Also some of the "similar to RG-(**)/U" designations are quite
misleading as to quality.
I would have to look at the "twin-ax" cable you saw at RS before I could
venture an opinion on it. I believe the cable like that that I saw had a
characteristic impedance of around 100 ohms, which might give SWRs in
the neighborhood of 1.4 or 1.5 to 1 with an ordinary dipole or even
slightly higher with an inverted vee.
Hope this info is useful to you.
73
Harvey/W4TG