On Tue, 15 Dec 2020 23:30:39 -0800 (PST), Michael Terrell
wrote:
On Saturday, December 5, 2020 at 3:24:42 PM UTC-5, Ralph Mowery wrote:
I don't recall the impedance of the car antenna coax, but it was special
low capacitance/high impedance . Most common coax for radios and TVs
are usually either close to 50 or 70 ohms and have about 2 to 3 times
the capacitance per foot as the car coax.
GM/Delco used RG62, which is 93 Ohms. I asked an EE from Delco about
this about 45 years ago.
See my previous posting in this thread.
https://groups.google.com/g/sci.electronics.repair/c/PNT_pCc-a3A/m/XEJLOtNnCAAJ
The AMC-62 coax is 62 ohm RG-62/u modified for American Motors Corp by
replacing the small center wire with an even smaller gauge wire. I
couldn't find the gauge but I do recall taking apart a broken antenna
and finding what looked like 40AWG. Broken center wires were probably
common:
https://www.jeepforum.com/forum/f176/radio-antenna-went-bad-how-replace-184449/
However, that was tolerated because it improved the sensitivity of the
AM radio.
A lot of aftermarket car radio antennas in thee '70s used rg58, which
lowered the sensitivity of the radio. Some had a series capacitor at
the car radio end, to lower the capacitance across the input, but
that became a voltage divider.
Yep. I remember those, but at the time, I didn't understand the
implications. For a time, I was working for a 2way radio shop
installing disguise antennas in law enforcement vehicles. It was
common to replace the AM/FM antenna with a "disguise" 1/4 wave
stainless whip antenna and RG-58c/u coax cable. I soon discovered
that the replacement AM/FM antennas were the same as the "disguise"
antennas, except for the connector. When I started seeing Motorola
car radio connector to PL-259 adapters, and replacement antenna kits
shipped without connectors, it became obvious why they were using
RG-59/u. Somewhat later, car antennas with built in diplexers became
available. These had ports for AM, FM, VHF, and sometimes UHF radios.
With the high power 2-way radios of the day, they tended to blow up
AM/FM car radios, and sometimes burn out the diplexer. They are still
sold today:
http://uscomm.atwebpages.com/Embedded%20Antenna%20systems.htm
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558