On Sat, 5 Dec 2020 15:24:32 -0500, Ralph Mowery
wrote:

In article ,
says...

** But ( older) car radios had high impedance inputs on the AM band.
A short run of coax is a pure capacitor at AM frequencies.
The trim cap set the antenna coil to resonance while moving ferrite slugs did the actual tuning.

....

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.

Coax cable for AM car radios is AMC-62 modified for 125 ohms:
https://www.commscope.com/product-type/cables/coaxial-cables/automotive-cables/item469776350/
http://objects.eanixter.com/PD357800.PDF
http://www.tian-jie.com/CABLES_Automobile-Antenna-Cable-Series_255_266_list.htm
The input to the AM receiver looks like a big inductor with an
adjustable tuning capacitor in series. To the antenna, the tuning
capacitor and coax capacitance to ground (9.5 pf/ft) form a voltage
divider. The higher the coax capacitance, the less voltage will
across the big inductor. Therefore, short coax cables and low
capacitance coax cable are required. If one substituted common
RG-58a/u coax cable (50 ohms), the capacitance would be 30 pf/ft
resulting in a lower capacitor divider ratio (approx 1/3) or a -9.5 dB
voltage loss.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558