Actually this is not true. AM frequencies carry much further and around objects better than FM frequencies which are higher in frequency and tend to travel in a straight line. There is no way FM stations can come close to the distance an AM station can broadcast. In the old days of AM dominance it was possible to tune in a good station like WABC in New York, CKLW in Ontario across the river from Detroit, or KOMA in Oklahoma City plus many others to numerous to list and listen to that station for a day or more as you drove across several states. I could listen to CKLW in the middle of upstate NY and in Huntsville, Al. When I lived in El Paso in the mid 60s I listened to KOMA instead of the local El Paso stations.

The broadcast range in the AM band of frequencies is so large that at nighttime the FCC requires the stations in the US to change their antenna pattern and reduce broadcast power so they do not reach as far in the East West direction as they do in the North South direction. This reduces interference problems with stations in other cities hundreds of miles away.

FM became popular due to better sound fidelity since audio frequencies were not limited to a 10 KHZ bandwidth. They could produce a great base but couldn't handle the higher music frequencies. Sort of like today's aftermarket high output car stereo installations.

Check the antenna connections to the radio, grounds for good contact and any spark related noise but a lot of the problem may be due to recent radio design practices which do not pay much attention to the AM input. This is common among all new radio products no matter the manufacturer.

Bill

 

I said AM is line of sight, You saying it isn't?

 

Sorry Bill, FM is line of sight at VHF frequencies. AM is lower frequency and gives much greater ranger but at the expense of quality.

 

All radio waves are line of sight. However, the low frequencies of the AM band bounce around objects and bend in the earth's atmosphere to vastly increase their earth bound range in comparison to a signal in the FM frequency band. The AM band is in the 530 KHZ to 1.7 MHz band while the FM band is in the 88 to 108 MHz band. The lower the frequency the longer the range. CKLW was at 800 KHZ (Radio 80) and I believe WABC New York was at 770 KHZ.

FM radio has a greater audio frequency bandwidth and the modulation method (the frequency is varied) is less susceptible to electrical noise than AM. AM really stands for amplitude modulation which means the signal level is varied and is easily affected by electrical noise pulses in the atmosphere from lightning, electric motors, spark plugs, etc.

Here is an example of how far the signals could reach from the Wolfman Jack biography:
"WHO'S THIS ON THE WOLFMAN TELEPHONE?" XERB was also the original call sign of a border blaster station in Rosarito Beach, Baja California in Mexico, which was branded as The Mighty 1090 in Hollywood, CA. The station boasted "50,000 watts of Soul Power." Some late nights, that signal could be heard all the way to upstate New York and parts of Canada. That station continues to broadcast today with the call sign XEPRS.

Try getting a 50K watt FM station to reach that far.

Bill

 

And try talking to your wingman on UHF. Its sometimes hard when he's a mile a way