20 kHz / 25 kHz confusion (16K0F3E)

[xhci]

I know this has somewhat been beaten to death, but I am still confused.

It seems that my Wouxun GMRS radios (KG-S88G) comply with the 16K0F3E emission standard based on the FCC test documents, which would suggest 20 kHz channels (and 4 kHz deviation??). However, the spec sheet lists them as 5 kHz deviation.

Part 95E also mentions 20 kHz bandwidth, and 25 kHz spacing (I’m assuming for legacy 25 kHz bandwidth radios before the standards changed).

The reason I’m asking - western states have been adopting GMRS/FRS emergency channels in some places, and I’d like to know how to program some backcountry part 90 radios to be “most compliant” for emergency purposes.

So which is it? 20 kHz or 25 kHz bandwidth for GMRS?

edit: I also think I’m mixing up some terminology here - I think I meant 16K0F3E = 16 kHz bandwidth vs 20 kHz for 20K0F3E. Its very confusing!

 

[Project25_MASTR]

Just don't overthink it. The emission designator essentially represents the widest occupied bandwidth. The reality is 4.0 kHz versus 5.0 kHz doesn't really make a difference as under most normal operating circumstances, a user won't deviate above 4.0 kHz even when operating in 5.0 kHz deviation.

20 kHz occupied is pretty common to see on 25 kHz channels (allows for 2.5 kHz of guard space on either side).

 

[xhci]

Just don't overthink it. The emission designator essentially represents the widest occupied bandwidth. The reality is 4.0 kHz versus 5.0 kHz doesn't really make a difference as under most normal operating circumstances, a user won't deviate above 4.0 kHz even when operating in 5.0 kHz deviation.

20 kHz occupied is pretty common to see on 25 kHz channels (allows for 2.5 kHz of guard space on either side).

Interesting - thanks! Now just out of curiosity... if a radio is said to use emission standard 16K0F3E, would that imply it's 4 kHz deviation, and 20K0F3E mean that it's 5 kHz? I gather that in practice it doesn't make much difference

 

[nd5y]

To figure the approximate FM bandwidth you need to know the peak deviation, the highest modulation audio frequency and Carson's bandwidth rule.
Most radios limit the voice audio to the range of 300-3000 Hz. I don't know how much this varies.
Add the deviation and highest audio frequency (in the same units) and multiply x2.
If the max deviation is 5 kHz and the max audio frequency is 3 kHz then 5+3=8 and 8x2=16 so the theoretical maximum bandwidth (on audio peaks) is 16 kHz.

 

[xhci]

Super interesting! Thanks for sharing this.

 

[AK9R]

Carson's Rule addresses these questions.

The occupied bandwidth (BW) is 2 times the sum of the highest audio frequency of modulation (fM) and the FM deviation (fD).

For radio communications, fM is usually accepted to be 3000 Hz.

So, for +/- 5 kHz deviation, you have BW = 2 (3000 + 5000) = 16 kHz.

+/- 4 kHz deviation would be BW = 2 (3000 + 4000) = 14 kHz

+/- 2.5 kHz deviation (aka narrowband) would be BW = 2 (3000 + 2500) = 11 kHz

Do not confuse transmitted bandwidth with channel spacing. Bandwidth is a physical property of the transmitted signal. Channel spacing is an administrative decision made by regulating bodies. You normally want your channel spacing to be greater than the bandwidth, but there are exceptions.

 

[xhci]

Understood. I was also getting tripped up by the 20/25kHz bandwidth option in Motorola CPS, which I think are probably just 5 kHz deviation, and getting confused by wideband channel spacing regulations.

 

[Project25_MASTR]

Understood. I was also getting tripped up by the 20/25kHz bandwidth option in Motorola CPS, which I think are probably just 5 kHz deviation, and getting confused by wideband channel spacing regulations.

Depending on what radio you have, the CPS might go by channel width or deviation to define wide/narrow operation.

 

[xhci]

Depending on what radio you have, the CPS might go by channel width or deviation to define wide/narrow operation.

Mine is by channel width with 12.5/20/25 kHz options.

 

[xhci]

This seems to confirm everything everyone has said (from the maintenance manual of my radio)