AM vs. FM Power?

[K6GBW]

Had a conversation with a friend recently about the new FM CB radios when the question of power came up. If my memory serves, with AM radio a 4 watt dead key will get your about 12-16 watts at full modulation, but frequency modulation a 4 watt signal is...just 4 watts. Am I wrong on this?

 

[smittie]

I watched a YouTube video that demonstrated this to be the case. The meter jumped to 8 and 10 watts on AM but remained steady on FM.

 

[AB5ID]

The peak output power of a 100% modulated AM signal is 4 times the carrier power. As far as AM vs FM goes, I don't know which mode a modulation has an advantage and how big the advange would be.

 

[prcguy]

The peak output power of a 100% modulated AM signal is 4 times the carrier power. As far as AM vs FM goes, I don't know which mode a modulation has an advantage and how big the advange would be.

Yes on this. A properly working AM transmitter will increase 6dB (4X) from dead carrier to 100% modulation. You would need a peak reading wattmeter to measure this.

 

[wtp]

so, it is like the AM signal goes "up and down" and the FM goes "side to side"...
i wonder if there is a term for this.

i gotta have some fun, and still give the answer.

 

[K6GBW]

Thanks guys. I'm pretty sure I knew this but wanted to confirm. So the FM CB will actually have slightly less power. Seems like FM would be nice for caravaning and off roading when you are relatively close, but AM for longer distance.

 

[sloop]

AM is amplitude modulation (up and down)
FM is frequency modulation (side to side)

 

[prcguy]

Thanks guys. I'm pretty sure I knew this but wanted to confirm. So the FM CB will actually have slightly less power. Seems like FM would be nice for caravaning and off roading when you are relatively close, but AM for longer distance.

In my experience FM mode in most radios puts out the same average power as the peak power in AM or SSB. On paper there would be a weak signal or distance advantage for SSB over AM or FM but I have never experienced that. FM seems to work down to the weakest levels where SSB fades out.

 

[AB5ID]

Informal comparison on YouTube

 

[n7lrg]

Also to note if 2 FM stations are talking at the same time you will only hear the stronger station. The term is called Capture Effect.

 

[K6GBW]

Good stuff to know. I was pretty sure that a CB on AM at full modulation was actually more like 12-16 watts and that makes them much more usable when you look at it as a 12-16 watt radio. But when in FM at 4 watts, especially with inefficient antennas, it's gonna be tougher. I can see a use case for FM if you are off road or in a caravan because you're relatively close and having a cleaner and easier to understand signal might be desirable. But when things go further I'd definitely switch to AM or even better SSB.

I love military history and its an interesting side note, during WWII the Germans used tank radios around 27 MHz that were 6 watts AM. The Allied forces used radios that were also on 27ish MHz with similar power but FM. One wonders which actually performed better in the field.

 

[smittie]

For distance I switch to GMRS and 50 watts with the potential for repeaters.

 

[prcguy]

Good stuff to know. I was pretty sure that a CB on AM at full modulation was actually more like 12-16 watts and that makes them much more usable when you look at it as a 12-16 watt radio. But when in FM at 4 watts, especially with inefficient antennas, it's gonna be tougher. I can see a use case for FM if you are off road or in a caravan because you're relatively close and having a cleaner and easier to understand signal might be desirable. But when things go further I'd definitely switch to AM or even better SSB.

I love military history and its an interesting side note, during WWII the Germans used tank radios around 27 MHz that were 6 watts AM. The Allied forces used radios that were also on 27ish MHz with similar power but FM. One wonders which actually performed better in the field.

I guess you could look at who won. Or you could just assume the AMers are Nazis.

 

[slowmover]

Story I read was that Amateurs in North Carolina were picking up Rommels Afrika Corps (tanks), and relaying that to Pentagon.

FM has its place, I just haven’t run across use. Yet. Tried scanning it earlier today having heard locals across AM.