Channel bandwidth

[amcferrin90]

I'm fairly new to GMRS and still adapting my knowledge to work in that realm. I believe the official channel bandwidth for repeaters was 20 KHz. Can you use 12.5, should you? I know I can use 12.5 KHz to talk on a 20 KHz repeater but my receive audio may distort if the modulation exceeds 12.5.

I'm using Moto XPR7550's and XPR5550's. My 7550 will do the 12.5/20/25 since I have the entitlement. My 7550 will only do 12.5/25. I have my mobile programmed to 12.5 to stay compliant. IDK if it's a codeplug issue maybe.

 

[mmckenna]

I'm fairly new to GMRS and still adapting my knowledge to work in that realm. I believe the official channel bandwidth for repeaters was 20 KHz. Can you use 12.5, should you? I know I can use 12.5 KHz to talk on a 20 KHz repeater but my receive audio may distort if the modulation exceeds 12.5.

You need to match what the repeater uses.
While you may be able to sneak through some low deviation audio, it's going to sound like crap and you'll get complaints.
Also, CTCSS deviation is a real concern. The lower deviation may not let the repeater recognize the CTCSS tone and open the receiver.

 

[dlwtrunked]

I'm fairly new to GMRS and still adapting my knowledge to work in that realm. I believe the official channel bandwidth for repeaters was 20 KHz. Can you use 12.5, should you? I know I can use 12.5 KHz to talk on a 20 KHz repeater but my receive audio may distort if the modulation exceeds 12.5.

I'm using Moto XPR7550's and XPR5550's. My 7550 will do the 12.5/20/25 since I have the entitlement. My 7550 will only do 12.5/25. I have my mobile programmed to 12.5 to stay compliant. IDK if it's a codeplug issue maybe.

Make sure you understand the difference between channel spacing and channel bandwidth. I think you are talking about channel spacing (sometimes called step-size and usually the bandwidth is less that that to avoid interferenc). It is unclear which you are setting.

 

[alcahuete]

I'm fairly new to GMRS and still adapting my knowledge to work in that realm. I believe the official channel bandwidth for repeaters was 20 KHz. Can you use 12.5, should you? I know I can use 12.5 KHz to talk on a 20 KHz repeater but my receive audio may distort if the modulation exceeds 12.5.

I'm using Moto XPR7550's and XPR5550's. My 7550 will do the 12.5/20/25 since I have the entitlement. My 7550 will only do 12.5/25. I have my mobile programmed to 12.5 to stay compliant. IDK if it's a codeplug issue maybe.

I don't have any 7550 non 'e' versions, but my 7550e versions and 5550 ('e' and non) all do 12.5/20/25. You sure yours won't do 20?

 

[Project25_MASTR]

GMRS uses 25 kHz (5 kHz deviation for approximately 20 kHz of occupied bandwidth) which we refer to as wideband.
FRS uses 12.5 kHz (2.5 kHz deviation for approximately 11.25 kHz of occupied bandwidth) which we refer to as narrowband.

Once upon a time, in some areas the there was a midband option that preceded the 2013 narrowband mandate in some markets which was 20 kHz (4 kHz deviation for approximately 16 kHz of occupied bandwidth). It was only really adopted for business uses but is often confused with the fact a lot of amateur band plans use 20 kHz channel spacing with 5 kHz deviation (see the portion about the occupied bandwidth of a 5 kHz deviation signal).

 

[jeepsandradios]

As said. It depends. I have 2 of my repeaters setup with 12.5 khz as I use alot of midland gear and it works much better at 12.5. I know its not a good topic over on mygmrs but it is what it is. In the end use whats in use near you.

 

[amcferrin90]

Definitely not confusing the spacing vs bw. The spacing for repeaters is 25 KHz.

These are the choices for my HT:

These are the choices for my Mobile:

Both are XPR series, not the e models. Could be firmware. dang. The HT is R02.09.00.0001 and the mobile is R02.06.05.0002. I thought I updated the mobile to R02.09.00.0001. Frack. I need to borrow a cable again. Anyone have a list of firmware info? I did do a codeplug recover but it didn't solve anything and I'm not getting codeplug errors.

What I'm reading is I should be on 20 KHz, right?

Thanks.

 

[mmckenna]

What I'm reading is I should be on 20 KHz, right?

No. 25KHz is what is used on GMRS.

 

[amcferrin90]

No. 25KHz is what is used on GMRS.

Sorry not to beat the horse. This is where I'm struggling: FCC Part 95 BW

I did read on the myGMRS forum that people are using part 90 radios in part 95 service. Just making sure. Not like with all the bubble pack radios being used that anyone's going to hunt me down.

 

[mmckenna]

Sorry not to beat the horse. This is where I'm struggling: FCC Part 95 BW

I did read on the myGMRS forum that people are using part 90 radios in part 95 service. Just making sure. Not like with all the bubble pack radios being used that anyone's going to hunt me down.

There's a couple of things involved here. Deviation and Channel Bandwidth.

Your link shows: Main channels. The authorized bandwidth is 20 kHz for GMRS transmitters operating on any of the 462 MHz main channels (see § 95.1763(a)) or any of the 467 MHz main channels (see § 95.1763(c)).

Bandwidth is the total occupied bandwidth for the signal. The emission used is 20K0F3E. This is standard wide band FM analog. That translates into 5KHz deviation.
This uses Carson's Rule, which is:
BANDWIDTH = 2 X (PEAK DEVIATION + HIGHEST MODULATING FREQUENCY)
For GMRS, you'll be at 20KHz. But you don't want to cram 20KHz of bandwidth in a 20KHz channel width. You want a bit of space to allow for the transmitter being slightly off frequency, over deviation by a bit, etc. If you ran 20Kz bandwidth in a 20KHz channel, some could easily spill out of the allowed bandwidth. That's why they use 25KHz channels. Just a bit of "slop" room around the edges.

Here's what you need:

Federal Register :: Request Access

(a) Main channels. The peak frequency deviation for emissions to be transmitted on the main channels must not exceed ± 5 kHz.

5KHz deviation, using Carson's rule will give you the 20KHz emission of 20K0F3E. But you need that extra slop room I mentioned above. So, it goes in a 25KHz channel.

4KHz deviation translates into 16K0F3E, which will fit in a 20KHz wide channel.

Narrow FM uses 2.5KHz, and that translates into 11K0F3E, which will fit in a 12.5KHz channel.


You could certainly run 20KHz channel width on your radios, but it's going to sound a bit quite for other users, and your PL tones may be a bit underdeviated for the repeater.

So, a long way of saying: Run 25KHz channels on your radio. 20KHz channels are not used in GMRS.

 

[mmckenna]

For what it's worth, you are asking a good question. They don't make this easy.

Like I started to say above, there's a couple of ways to define this, and it's super easy to get yourself confused. There's the maths to figure this out, but once you've been doing it for a while, you just know.

Wide FM used in 2 way radio is 25KHz channel spacing (sometimes called Channel Width or Channel Bandwidth). It really comes down to deviation of the transmitter. 5KHz deviation is what is used for 25KHz channels. The 5KHz deviation gives you the 20K0F3E, which fits nicely inside the 25KHz channels without spilling out and impacting adjacent channels.

The 20KHz channel spacing is only used in a few very specific cases, and for most hobbyists or even a lot of professional radio guys, they'll never run into it. The NPSAC channels used in 800MHz sometimes use the 4KHz deviation, which fits in the 20KHz channels. It's also used on some slow speed data stuff. The 4KHz deviation works out to the 16K0F3E emission, which fits nicely inside the 20KHz channels.

Narrow FM is more common once you get outside the hobby radio services. Just about everything on VHF and UHF is going to be 2.5KHz deviation, or the 11K0F3E, which fits nicely inside the 12.5KHz channels without spilling out.

25KHz is what you want. The radio takes care of the rest. Getting off into the weeds on this stuff leads to confusion, headaches and a loss of hair. Making sure you are doing the right thing is good, but I think you're confusing the terminology (and the radio manufacturers don't help much…).

 

[amcferrin90]

Thank you for the explanation. That makes sense to me actually but needed to learn the breakdown.

 

[sallen07]

Sorry been tied up and not able to respond until this morning.

@mmckenna I was (am?) confused too, because 95.1773 doesn't appear to be consistent:

(a) Main channels. The authorized bandwidth is 20 kHz for GMRS transmitters operating on any of the 462 MHz main channels (see § 95.1763(a)) or any of the 467 MHz main channels (see § 95.1763(c)).

(b) Interstitial channels. The authorized bandwidth is 20 kHz for GMRS transmitters operating on any of the 462 MHz interstitial channels (see § 95.1763(b)) and is 12.5 kHz for GMRS transmitters operating on any of the 467 MHz interstitial channels (see § 95.1763(d)).

To me that looks like, "use 12.5 for narrow and 20 for wide". But the actual bandwidth for narrow is *not* 12.5, correct?

 

[mmckenna]

Federal Register :: Request Access

§ 95.1775 GMRS modulation requirements.
Each GMRS transmitter type must be designed to satisfy the modulation requirements in this section. Operation of GMRS stations must also be in compliance with these requirements.

(a) Main channels. The peak frequency deviation for emissions to be transmitted on the main channels must not exceed ± 5 kHz.
(b) 462 MHz interstitial channels. The peak frequency deviation for emissions to be transmitted on the 462 MHz interstitial channels must not exceed ± 5 kHz.
(c) 467 MHz interstitial channels. The peak frequency deviation for emissions to be transmitted on the 467 MHz interstitial channels must not exceed ± 2.5 kHz, and the highest audio frequency contributing substantially to modulation must not exceed 3.125 kHz.


This is confusing, I agree.
However, 95.1775 covers it.
5KHz deviation for the main channels
5KHz deviation for the 462 interstitials
2.5KHz deviation for the 467 interstitials

Understanding deviation and assigned channel width is key. FCC never makes it easy.

 

[sallen07]

Thank you. If I had any radios set up to transmit on GMRS I would make sure they were set to 25KHz wideband and not 20.