AM radio/antenna replacement

[joe_peters]

Just for grinz and giggles I pulled my OEM 90 radio AM/FM/cassette off the shelf and couldn't find any trimmer screw on it.

But I too remember those, had to tune the radio to a station close to 1400 Mhz or something. Those screws were sometimes under the knobs and sometimes right on the faceplate.

 

[heyhar]

Somewhere in the late '60s, when FM became more popular in cars, they went to the one piece antennas, which were optimized for FM reception. In the AM/FM radios destined for use in cars with the fixed mast antennas, the AM portion was probably trimmed to make use of the 30-32" antennas. This length was chosen as it is the correct size to catch the typical FM bandwidth. Time was, the longer the better for AM, 32" for FM.

 

[orange]

XERF, KOA, WWL. Rear seat speaker and a fader - heck, we even invented 'Cool"!

:cheers:

-Good Times-

Regards,
Orange

 

[joe_peters]

4-track tape players, walk into the store and pick the songs you wanted, record them, walk out and start cruising!

 

[heyhar]

Hell, my Uncle Bob's '66 Impala convertible had a reverb in it!

 

[wgregt]

Did the cost hanger trick again. Got same results: AM technically works, but very faint. Has to be maxed out on volume to be able to only get it to a listenable level.

I'm thinking maybe the AM amp inside the unit (if there is such a thing) is shot.

 

[kastein]

the reason one works and the other doesn't is because the vastly different frequencies involved (and thus, wavelengths.) AM is in the MF frequency band (300kHz to 3MHz, AM broadcast is 540 to 1700kHz iirc, channels are 10kHz wide) while FM is in the VHF frequency band (30MHz to 300MHz, FM broadcast is 87.5 to 108.0MHz, channels are 200kHz wide.) So a true resonant-length quarter wave AM antenna would be on the order of 50 meters long... and that's for the highest frequency in use, the lowest would be more like 140 meters long! In contrast, FM resonant-length quarter wave antennae are about 70 centimeters long (again, for the highest frequency in use.) Many radios actually have an AM "loopstick" ferrite core and coil antenna inside their case and only use the external antenna for FM reception. I'm not sure whether most stock radios use this method or not.