AM or FM which needs an antenna please

[Roadster90]

Somewhere along the line I think I remember that either am or fm will receive or work without the antenna attached.....can someone set me straight - it is just to cold presently to go out and play to find out which....thanks,

Nic

[padgett]

Neither. It may be possible to get some reception just with a straight unshielded wire but it will be sub optimal.

Of course if close enough to a powerful station to fry small birds you could get reception on your teeth. However automotive radios are shielded to prevent picking up the 45-60,000 volt pulses of the plug wires and coils (passenger car plug wires are "resistive" to reduce the RFI).

[handmedownreatta]

longer antenna the better on am.fm has a certan length that works best.

[Guest steveskyhawk]

Years ago I was a very active skydiver. I never did this but I had friends that would climb a 50,000 watt AM radio antenna (at night) and jump off. They claimed they could hear the broadcast in their fillings.

They said they had to get on top of an out building and jump over to the superstructure to prevent them from being fried. I guess that was part of the fun.

Edited January 25, 2011 by steveskyhawk (see edit history)

[jonlabree]

Padgett, Sell him some land in FL.

[Guest Richard D]

For FM the best length is 31 inches, AM as long as it will go. After High School I worked at a FM radio station and back in the early 70s we would go off the air at 2 AM Sundays for antenna work. We were rated at 100,000 watts effective radiated power actual transmitter was 19.5 KW. All stations using the tower had to go down so the engineers would not get cooked. Can't imagine what a 50KW AM would do to a person.

[NTX5467]

Usually, AM is the most probable to have some kind of reception than FM. As mentioned, the alleged "optimal" mast length for FM is 31", but I'm not so sure just how "optimal" it might be compared to 29" or 30". As FM is "line of sight", then having a strong station might be more important than having the antenna exactly at 31" mast length.

AM just needs "something" hooked into the antenna socket, but generally longer is best for pulling in those long-distance stations, but it can work pretty well at the 31" mast length, too . . . from my observations "back when" antenna masts were adjustable.

Enjoy!

NTX5467

[wws944]

As I mentioned in the other thread, the reason they are 30" or so is because that is 1/4 wavelength at 100 mhz (3 meter wavelength). Since the FM broadcast band goes from 88 to 108 mhz, the length at either end of the band is less than optimal by a couple of inches.

On the AM broadcast band, which goes from 0.540 mhz (555 meters) to 1.750 mhz (171 meters), a 30" whip is 'electrically short' by a factor of 100. The electronics in the radio are tuned to make the best of it. But... the longer the better. Back before FM radios in cars became common, the antennas were often much longer than 30".

The other thing about AM, as Padgett noted, is that you want to get the antenna away from noise sources such as the ignition system on the engine, and also any computers.

Edited January 25, 2011 by wws944 (see edit history)

[Guest steveskyhawk]

Years ago I was a very active skydiver. I never did this but I had friends that would climb a 50,000 watt AM radio antenna (at night) and jump off. They claimed they could hear the broadcast in their fillings.

They said they had to get on top of an out building and jump over to the superstructure to prevent them from being fried. I guess that was part of the fun.

[padgett]

Back in the day low-power AM stations (500w) shut down at night because the "skip" started after sundown. OTOH there were a few in each region usually in the 5-10 kwatts that could stay on after sundown, and then there were the "clear channel" 50 kw stations that essentially had the wavelength all to themselves. WABC, WJR, WOR, WBAL and probably the most famous in the souf, WLAC which stayed with me in the late night hours on many Florida<>Texas runs after I lost the Big Ape.

There was a whole class of radio listeners back then known as BCB-DX and I remember straining into the headphones of a Trans-Oceanic in south Florida trying to pick up a "K" or "X" and sometimes a "C" station.

[Guest Richard D]

Padgett, back in 1971 I was amazed to pick up WABC 770 Kc loud and clear almost any night and sometimes CHUM from Toronto Canada on my first new car, A Plymouth Cricket.

Never heard of one? They were only sold 71, 72.

[chipcross]

"The electronics in the radio are tuned to make the best of it [the long AM wave length]"

Is this why after market radios have such lousy AM reception? I always have gotten great AM reception from my original equipment radios, but have never been able to pull in faint signals using after market ones (i.e., Pioneer, Clarion, Alpine, etc.)

Since I need an ipod input, I have to put up with a lot of static and pops from my AM (unless I am listening to one of those skydiving 50,000 watt blowtorches!)

[Guest steveskyhawk]

Chip and all,

Next time you drive up the 5 Freeway through Anaheim, Buena Park look to the east side of the road and you will see the KFI antenna. I had one guy tell me he was nervous about the height untill he saw an airplane going into or out of Fullerton Airport going below him. Getting on to and climbing the antenna was half the fun. I was quite contentent to jump out of aircraft. If I had known what it was like to get old I might have taken more chances.

[Guest Richard D]

I think most aftermarket models spend their research $ on FM and amplifier power, AM is an after thought.

[Thriller]

On the AM broadcast band, which goes from 0.540 mhz (555 meters) to 1.750 mhz (171 meters), a 30" whip is 'electrically short' by a factor of 100. The electronics in the radio are tuned to make the best of it. But... the longer the better. Back before FM radios in cars became common, the antennas were often much longer than 30".

It's been a long time, but shortly after buying it, I put a battery in the '61 Invicta to see how much of the stuff worked (car is still patiently awaiting restoration). I had the garage door open above it and I stopped putting the antenna up when it hit the garage door at about 7' high. Of course, that is just an AM radio. It's a shame there isn't much music on AM any more for when we warm up the tubes in the old cars.

[Guest whytshdo]

I recall having to "Trim" the radio for AM reception when I installed aftermarket radios. I haven't seen this on my radios lately which may be why tha AM reception is sub-par.