Great post John!! Yes, gets way complicated. Point is the factory knew what they were doing with these radios/antennas and best to follow the FSM, which spells out the antenna trimming for AM. I think 1400 KHz was selected because that one of the frequencies designated by the FCC for a ton of low power stations across the country (at night a garbled mass of signals) the idea being that a “weak” signal would be found at 1400 to tune at. I could be wrong (going from old memory) but I think the radio could be trimmed at other frequencies at the higher end of the dial the idea being to choose a weak signal not a local one. Cable is critical. Why the optional Delco AM/FM radio in 1963-64 was not offered with the rear mounted power antenna. Imperial waited until ’64 for AM/FM, but came with the power antenna because it was front fender mounted – short lead in wire. FM was particularly challenging in the mobile environment back in the day – signal strength constantly varies, bounces off buildings etc. Also, back in the 60’s (and even early 70’s) the FM band was not so crowded like today, best in class home systems were tuned for high fidelity (wide IF band) rather than highest selectivity (narrow IF band). My 1963 HH Scott tuner (tubes) only has 30db alternate channel selectivity but wonderful sound quality. Marantz and other high end tuners at the time were similar. My 1978 Kenwood tuner has selectable IF bands wide/narrow 30db/100db– really high end for the time. Car radios/speakers were lower fidelity – FM tuners could be more selective but limited somewhat by technology. Today almost all FM tuners use digital tuning – highly selective – necessary in today’s cars with crowded FM radio stations. Like John said, today’s AM car radios junk by comparison – probably one IC, mostly talk radio on local stations – nobody cares. In the home environment, we’re surrounded by RF interference from everything – WIFI, appliances, TV, etc. – no good for AM – and maybe not even healthy too. Back in the day there was hardly any RF around. FYI you can still get high performing AM band portable radios – CC Crane, Grundig/Eton, Sangean makes some – take it out to the country – no RF interference – huge difference. Often our old AM car radios will still work – but are performing at less than 50% of where they should be – only receive local signals etc. And old leaky capacitors not good for the tubes – premature wear. Remember that in our old letter cars these radios were still hydrid – front end (tuner) was tubes – only the audio amp was transistorized. The Toronto station is on 740 – remember it well from the 70’s – played classical music back then – will check it out. Iowa has some AM 60’s music stations – low power however – can’t get too far without significant signal degradation, but fun in the old Chryslers when close enough to receive them. well One or two up by Clear Lake- site of the terrible plane accident in ’59 that killed Buddy Holly, the Big Bopper and Richie Valens. Huge nostalgia theme up there. Carl From: John Grady Carl is 100 % correct ; and the cables are very important ; the AM band in such that the cable capacity to ground ( made minimal by that fine wire drawn into a tube loose .. it is not coax cable ) —can be made part of a resonant circuit in the input of the AM tuned antenna coil , why you adjust that trimmer to peak at 1400 . The AM band when tuned by capacitors is using like 350-370 uuF The capacity of the cable is on the order of tens of uuF . So we can live with that . But at FM frequencies that is almost a dead short to ground , the total tuning range of FM capacitors for tuning is only 10-30 uuF . 30 in the cable would kill it . So other approaches are used , —in cars generally FM is almost always essentially untuned “ low impedance” wideband input to amp and very sensitive FM front ends. To send high frequency like FM over longer distance , the cable requires “ matching “ the impedance of the coax ( like RG-6 catv ) 50- 75 ohms or like 300 ohm twin lead and a matched 300 ohm antenna too . ( the old folded dipole ) . But then you may need an amplifier at antenna or an antenna with known matched ohms to avoid losing signals . Note that as Carl says , the wavelength of FM is like 30” almost the same as the cable length in the car from radio to antenna . AM is closer to 500 - 800 feet wavelength , 10 feet of cable is ok then, does not even look like a cable to the AM , just a capacitor that can be tuned out . This stuff gets way more complicated , ( band width front end design etc) but getting back to what to do , they used to sell like a thin wire in scotch tape you stick to windshield , very short wire to radio , for FM ; I think some GM have that . Or shortest wire you can to AM mast pulled to 39” . Yes it works — if not right , but not as well . I bought some 70’s era Motorola and Sparkomatic FM to AM converters trying to keep 300 F radio . Sort of work , but nowhere near modern FM . And as Carl alludes to — AM radio performance today in a new car is a joke compared to our tube radios when working right . They are well designed —there will be solid stations end to end if radio is perfect at night . Tubes are almost never ever bad , (!!) but corrosion in tube sockets is common . wiggle back and forth or even better toothpick and silicone grease on each pin put back in . Loose antenna connection / corrosion in antenna base also kills it . Check with ohmmeter —- pin at end of cable to mast should be almost zero ohms. Plus today heavy interference in AM band is the rule , automatic volume control in our radio hears that noise , thinks it is a station and cuts sensitivity way back ... Try engine off in the country some night . Like it was ... Other stuff goes wrong —-too long for here , worst is mice piss from on top through vent holes ruins the PC card and some fine wire coils ; not uncommon if stored . In Boston we listened to WMEX ( Arnie Woo Woo Ginsberg , )but it faded later in night , but WPTR Troy NY , 50 kw clear channel would start coming in , even better than WMEX . In 57 dodge , essentially same radio . I Hung around Big Burger Ranch , every one there ( 40 cars?) has PTR on by 11pm . Was just like American Graffitti very single night ! Just too many chevy guys.. smile. There is a station in Toronto , some guys’ personal thing , about 720 AM plays great 60’s music . Sky bounce might let you hear that at night , generally 200-500 mikes away ! Otherwise AM is lame now. The 30” length (actually closer to 31”) is a compromise for the middle of the FM frequency band (98Mhz in the USA) where that length matches one-fourth of the wavelength. Lower frequencies like 88 MHz would match to an antenna a couple inches longer and higher frequencies like 108 MHz would match to an antenna a couple inches shorter. The radios have sufficient sensitivity that a precise match isn’t necessary. I used to design and built my own yagi FM antennas in high school for “DXing” which is the arguably nerdy hobby of picking up distant radio stations. FM signals are at a high enough frequency (located just above channel 6 on the old analog TV lower band) that the signals tend not to bounce off the atmosphere and are restricted to somewhat “line of sight” from the transmitters, with some exceptions due to weather conditions. Those exceptions are called tropospheric ducting or tropospheric propagation, when the FM and old analog TV signals would sometimes strangely travel for hundreds of miles for a few minutes or hours. AM signals, on the other hand, are at a much lower frequency (KHZ vs. Mhz) with very long wavelengths that will bounce off the atmosphere at night, every night, allowing one to hear stations from Mexico and Cuba that are allowed to transmit at up to 10 times the power of US stations. Back in the day folks would listen to AM stations from across the country at night. Hence, some stations were “clear channel” at night so they could be heard many hundreds of miles away. In Chicago, we could receive stations from Canada, Mexico, Cuba, the Caribbean, and from Boston (WBZ), New York (WABC), New Orleans, Dallas, and many other locations. I liked KAAY 1090 in Little Rock, a great rock ‘n roll station in the early 1970’s. The AM car radios of the day were highly tuned RF amped circuits with exceptional sensitivity and selectivity for long distance reception. All, of course, before the days of digital tuners. Anyway, when Chrysler introduced the AM/FM radio for their cars, a Delco model of very high quality (January of 1963 for Chrysler) the antenna was front fender mounted (no power option), standard on New Yorker Salon and optional on other models. It may have included a fixed length antenna of 31” (can’t remember offhand). The AM only radios were matched to the antenna via an antenna trimmer adjustment for maximum reception, with the antenna set a 40”, regardless of front or rear mounting. Carl Bilter From: Bob Podstawski FM antenna should be 30” for max reception. Cables are shielded and are of no consequence in my understanding
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