GMRS Trave Frequency?

[nokones]

1 hour ago, SteveShannon said:

I wasn’t challenging you specifically but the notion in general.

Edited to add: 

Most people think of how far AM broadcasts carry compared to how far FM broadcasts carry, but the major difference between AM and FM broadcasts is frequency.  AM broadcasts are Medium Wave, 650 kHz to 1640 kHz.  At those frequencies the radio waves are much more likely to skip off the atmosphere.  FM broadcasts are VHF and much less susceptible to skip. Also, AM is much narrower in bandwidth which makes it easier to be detected at farther distances.

The difference in distance is not a function of modulation type. So, because they are using the same frequencies, AM CB and FM CB should not be greatly different in range.

The farz was not a big difference, but it was about a 1/4 mile to less than 1/2 mile better for FM.

[WRTC928]

12 hours ago, SteveShannon said:

I wasn’t challenging you specifically but the notion in general.

Edited to add: 

Most people think of how far AM broadcasts carry compared to how far FM broadcasts carry, but the major difference between AM and FM broadcasts is frequency.  AM broadcasts are Medium Wave, 650 kHz to 1640 kHz.  At those frequencies the radio waves are much more likely to skip off the atmosphere.  FM broadcasts are VHF and much less susceptible to skip. Also, AM is much narrower in bandwidth which makes it easier to be detected at farther distances.

The difference in distance is not a function of modulation type. So, because they are using the same frequencies, AM CB and FM CB should not be greatly different in range.

So, FM CB should be able to "shoot skip" about as effectively as AM? That's a new thought to me. 

[WRTC928]

12 hours ago, SteveShannon said:

I wasn’t challenging you specifically but the notion in general.

Edited to add: 

Most people think of how far AM broadcasts carry compared to how far FM broadcasts carry, but the major difference between AM and FM broadcasts is frequency.  AM broadcasts are Medium Wave, 650 kHz to 1640 kHz.  At those frequencies the radio waves are much more likely to skip off the atmosphere.  FM broadcasts are VHF and much less susceptible to skip. Also, AM is much narrower in bandwidth which makes it easier to be detected at farther distances.

The difference in distance is not a function of modulation type. So, because they are using the same frequencies, AM CB and FM CB should not be greatly different in range.

So, FM CB should be able to "shoot skip" about as effectively as AM? That's a new thought to me. 

[Davichko5650]

On 8/20/2025 at 5:03 PM, AdmiralCochrane said:

You can probably find videos on YouTube.  Pickup trucks with 50 and 80 kw generators in the back and antenna arrays that require another truck to haul.   Lots of stuff to make Randy smile.

Don't recall the Group name, but recently I did see some photos and a posting on a FB CB page with some very much massive set ups, with congrats to the "Key Down" winners, etc.

[Davichko5650]

10 minutes ago, WRTC928 said:

So, FM CB should be able to "shoot skip" about as effectively as AM? That's a new thought to me. 

I don't see why not. I talk on 10m FM simplex all over the states when the band is open, especially to the West Coast from MN, and have had a few contacts into Europe as well.

[WRUE951]

12 minutes ago, WRTC928 said:

So, FM CB should be able to "shoot skip" about as effectively as AM? That's a new thought to me. 

i think AM is going to be the winner in shooting skip over FM just because AM uses much less bandwidth which lessens the effects of the atmosphere and other conditions..   FM will be more clear in audio in shorter range but also be more acceptable to degradation over range because of its bandwidth characteristics...  AM transmission is defianlty better for longer distance transmission      

[SteveShannon]

9 minutes ago, WRUE951 said:

i think AM is going to be the winner in shooting skip over FM just because AM uses much less bandwidth which lessens the effects of the atmosphere and other conditions..   FM will be more clear in audio in shorter range but also be more acceptable to degradation over range because of its bandwidth characteristics...  AM transmission is defianlty better for longer distance transmission      

On CB, FM and AM are authorized to use exactly the same bandwidth:

95.973 CBRS authorized bandwidth.

Each CBRS transmitter type must be designed such that the occupied bandwidth does not exceed the authorized bandwidth for the emission type under test.

(a) AM and FM. The authorized bandwidth for emission types A3E and F3E is 8 kHz.

(b) SSB. The authorized bandwidth for emission types J3E, R3E, and H3E is 4 kHz.

[LeoG]

AM is easier to decode when it's all scrambled up from the atmosphere.  You can hear the voice even with lots of static.  In FM if the signal is garbled the decoding doesn't work well at all.

[WRUE951]

19 minutes ago, SteveShannon said:

On CB, FM and AM are authorized to use exactly the same bandwidth:

95.973 CBRS authorized bandwidth.

Each CBRS transmitter type must be designed such that the occupied bandwidth does not exceed the authorized bandwidth for the emission type under test.

(a) AM and FM. The authorized bandwidth for emission types A3E and F3E is 8 kHz.

(b) SSB. The authorized bandwidth for emission types J3E, R3E, and H3E is 4 kHz.

Authorized, yes.  But FM requires all of that bandwidth and AM doesn't.      

[AdmiralCochrane]

15 hours ago, SteveShannon said:

What would allow an RF signal travel further if it’s AM rather than FM?

You are correct, the signal doesn't travel any farther.  It is the inverse, an FM receiver will reject a stronger signal until it is strong enough to "lock on".

[SteveShannon]

33 minutes ago, WRUE951 said:

Authorized, yes.  But FM requires all of that bandwidth and AM doesn't.      

Well, no, narrowband FM is typically only 6.25 kHz and the few AM transmitters I’ve used do 6 kHz (up to 4 kHz per sideband on SSB on my ham radio). 
I would believe data sheets on modern am/fm CB radios if you want to do some research. 

[WRUE951]

18 minutes ago, SteveShannon said:

Well, no, narrowband FM is typically only 6.25 kHz and the few AM transmitters I’ve used do 6 kHz (up to 4 kHz per sideband on SSB on my ham radio). 
I would believe data sheets on modern am/fm CB radios if you want to do some research. 

in FM the modulation stays the same it uses all the bandwidth, In AM the amplitude of the carrier wave changes with the instantaneous amplitude of the modulationg signal.   AM essentiality uses less bandwidth with more emphasis on the modulating carrier   

[SteveShannon]

6 minutes ago, WRUE951 said:

in FM the modulation stays the same it uses all the bandwidth, In AM the amplitude of the carrier wave changes with the instantaneous amplitude of the modulationg signal.   AM essentiality uses less bandwidth with more emphasis on the modulating carrier   

Thanks for explaining AM to a forum full of radio people.

Data showing the actual bandwidth of a CB radio in FM vs. AM, please.