On Wed, 02 Dec 2009 22:28:14 -0500, "Michael A. Terrell"
wrote:

Whatever. I heard it from some Delco engineers who designed their
car radios in the '70s, and saw it on plenty of car radio antenna cables
in the '60s & '70s.

Sigh. I never bothered to look. I just found a stainless car whip
antenna with coax cable attached. The number on the cable is:
5PT3-30C LCN
Google couldn't find anything. Both ends are soldered so I can't
inspect the guts. The OD is 5.0mm, which is at one end of the range
of RG-62a/u jacket diameters of 5.1 to 6.2mm. I would measure the
PF/ft, but my destroyed my LCR meter is busted. It probably is
RG-62/u but I'm still not convinced.

The Delco engineers explanation was that a whip
antenna for AM was used as a voltage probe, and 93 ohm coax had the
lowest loss available.

Well, let's see. The cable is 4.5ft long.
ohms pf/ft pF
RG-62/u 93 13 58.5
RG-59/u 75 17 76.5
RG-58/u 50 30 135.
Yeah, I can see the reason. The input tuning stage would never tune
with that much capacitance.

That made it easier to match to the tuned RF
input.

Well, it could be matched to almost anything. However, the low coax
capacitance would give the trimmer in the radio a much wider tuning
range.

It was also cheap cable with a polypropylene tube to hold the
braid and surround the zigzag center conductor. Compare that to a low
loss teflon which can cost several dollars a foot.

They didn't PTFE when Bill Lear invented the first "Motorola" car
radio.
http://en.wikipedia.org/wiki/Bill_Lear
Also, at 1MHz, the difference in cable losses between various
dielectrics is zilch at 4.5 ft. They could have used aluminum foil
wrapped garden hose, with a coat hanger as center conductor, and
gotten the same cable loss.






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