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Narrow Band for everyone???


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#1 russwbrill

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Posted 10 November 2019 - 04:40 PM

If all of GMRS went Narrow Band including Repeaters, would GMRS users benefit by getting more simplex/repeater pairs???

 

If yes, then why NOT???



#2 marcspaz

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Posted 10 November 2019 - 06:29 PM

The benefit of using a narrow channel is the lower noise along better sensitivity and range. The advantage of wideband is the capability to transfer higher data rates.  I would stay away from narrow band unless you want to forget about digital and hi-fidelity.

 

As far as repeater pairs go, CTCSS and DCS were created for a reason.  If you are using tone or digital code squelching, unless the repeaters are within miles of each other, you can have potentially hundreds of repeaters in place within a 500 mile radius and not interfere with another repeater.  The only exception I could potentially see there is if traffic went up 100 fold and all repeater owners started using 200' towers; which I don't see happening.

 

Additionally, there is a massive shortage of people willing to spend insane amounts of money for proper repeater equipment, site rentals and maintenance costs, just to allow other people to user their system for free. (Hard to imagine, I know.)  Opening up more pairs isn't really a need.


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#3 BoxCar

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Posted 10 November 2019 - 08:54 PM

Marc,

 

I'll disagree with you on range. I coordinated many part 90 channels that lost coverage area going to 12.5 kHz channels. Granted, many of these stations were way overpowered for their AOP so the loss didn't effect them. I mean we had one PS Coordinator that would slap a 40 km AOP for a town that was less than 9 km across and then complain (the term is properly object) to an adjacent whose coverage area stopped 10 km outside the city limits.


Old and wise infers you were once young and stupid


#4 WRAF213

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Posted 14 November 2019 - 08:06 PM

Wideband has a slight advantage (3dB) over narrowband when it comes to sensitivity. Cutting the bandwidth in half requires 6dB more power to get the same signal-to-noise ratio, but 3dB comes from having the noise power on the narrower receive IF filter. On business radio systems, this may not matter; but on GMRS, the users tend to be dispersed further as operating area isn't constrained by the license and minimal frequency coordination takes place. This effect also causes mobile flutter to be more pronounced. On modern commercial radios designed for narrowband use, the strong-signal voice quality doesn't take much of a hit when moving to narrowband channels.

 

We can't get any more pairs because FRS is already authorized for the 467 MHz interstitials. We'd need to go to 6.25 kHz ultra-narrowbanding (NXDN48 or dPMR) and use some odd channels (462.546875, 462.553125, 462.559375, ...), which won't happen anyways because digital voice isn't allowed. Adjacent ultra-narrowband channels would also interfere unless frequency accuracy is well-controlled, which raises equipment cost considerably (particularly for portables) and requires realignment during the equipment's service life. Additionally, wideband users would take interference from any of four ultra-narrowband channels, and narrowband users would take interference from any of two ultra-narrowband channels. The end result is even less voice capacity than before, unless everyone goes ultra-narrowband and maintains their equipment to high standards.

 

The repeater operator always has the option to narrowband should they want to, but there's no benefit unless there's significant adjacent-channel interference, all of the radios are properly configured for narrowband (travelers are probably not), and the radios used on the system have proper 12.5 kHz IF filters (the GMRS-V1 does not). A narrowband mandate would also screw up equipment certifications and cause the FCC to get a big headache over something that is not really a problem at all.


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#5 marcspaz

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Posted 14 November 2019 - 11:03 PM

Wideband has a slight advantage (3dB) over narrowband when it comes to sensitivity. Cutting the bandwidth in half requires 6dB more power to get the same signal-to-noise ratio, but 3dB comes from having the noise power on the narrower receive IF filter. On business radio systems, this may not matter; but on GMRS, the users tend to be dispersed further as operating area isn't constrained by the license and minimal frequency coordination takes place. This effect also causes mobile flutter to be more pronounced. On modern commercial radios designed for narrowband use, the strong-signal voice quality doesn't take much of a hit when moving to narrowband channels.

 

We can't get any more pairs because FRS is already authorized for the 467 MHz interstitials. We'd need to go to 6.25 kHz ultra-narrowbanding (NXDN48 or dPMR) and use some odd channels (462.546875, 462.553125, 462.559375, ...), which won't happen anyways because digital voice isn't allowed. Adjacent ultra-narrowband channels would also interfere unless frequency accuracy is well-controlled, which raises equipment cost considerably (particularly for portables) and requires realignment during the equipment's service life. Additionally, wideband users would take interference from any of four ultra-narrowband channels, and narrowband users would take interference from any of two ultra-narrowband channels. The end result is even less voice capacity than before, unless everyone goes ultra-narrowband and maintains their equipment to high standards.

 

The repeater operator always has the option to narrowband should they want to, but there's no benefit unless there's significant adjacent-channel interference, all of the radios are properly configured for narrowband (travelers are probably not), and the radios used on the system have proper 12.5 kHz IF filters (the GMRS-V1 does not). A narrowband mandate would also screw up equipment certifications and cause the FCC to get a big headache over something that is not really a problem at all.

 

 

 

I'm confused.  Are you referencing GMRS/FRS specifically?  How does wideband have a 3db advantage?  3db of what, exactly?  Carrier power? Modulation? Received signal strength?  Are you talking about a wideband FM receiver listening to a narrowband signal?

 

I ask because the tech stuff you wrote is nothing like what I have been taught and observe in practice in FM signals.  In AM, sure, you can make a preposterously wide peak to peak sign-wave and just flood the airwaves... but that still doesn't make it cleaner or better. Just more of it.  Hence the popularity of SSB in NLS radio services (MF/HF, for example).

 

Every 3db of loss is power cut in half compared to the reference power, or 3db gain is double the reference power.  And to change a received signal by 1 s-unit, you would have to quadruple your power or reduce it to one quarter of the starting power.  That is a gain or loss of about 12db compared to the reference power.  So, I'm having trouble tracking how you are determining loss or gain based on this understanding.

 

As far as signal to noise goes, narrowband is the lower noise bandwidth, not the higher.  The reason we use wideband in line-of-sight radio is because wideband carries more data... in this case a voice signal, thus better audio fidelity inside of the short rang capability of the frequency and power being used.  Due to being a line-of-sight service, there is no benefit to using narrowband.  Maybe there is a misconception that wideband has better signal to noise ratio because there is more voice data for a wideband receiver to work with, but there is actually more noise too, reducing the heard distance.

 

Narrowband usually has a far greater range of reception because narrower filters can be used, cutting out unwanted band noise. The transmitted energy is also concentrated on a smaller portion of the spectrum.



#6 marcspaz

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Posted 14 November 2019 - 11:15 PM

I have no idea who this company is... but this article supports my comments above.

 

https://www.mwrf.com...deband-channels

 

 

This is one of the several reasons why planners of the emerging 5G wireless communications network are looking to millimeter-wave frequencies and their available bandwidths in support of high-speed data communications links, both for terrestrial links and those incorporating low-earth-orbit (LEO) satellites. Transmitting and receiving more voice, video, and data over wider-bandwidth frequency channels comes at a cost, however, since wider sections of frequency spectrum also contain greater numbers of noise sources and higher levels of noise (see figure).

 

22B_F1_1.gif

 

Narrowband signals occupy much less frequency spectrum and require less transmit power for a given application than wideband signals, while UWB signals are short pulses that send information while briefly occupying a large portion of the traditional communications frequency spectrum.  ...

 

... As a result, typically higher transmit signal power is needed in a wideband channel to overcome the noise level—as well as other factors, such as signal propagation losses—so that a significant signal level will appear at the receiver and meet the receiver’s minimum signal-to-noise-ratio (SNR) performance requirements for reception and processing.



#7 WRAF213

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Posted 14 November 2019 - 11:27 PM

You get more fidelity at a given power level using a wider deviation. You need more power (a net change of 3dB in this case) to reproduce a transmitted signal with the same fidelity. So if you've got a receiver trying to receive your buddy down in the valley and you're in a theoretically perfect hilltop site, you'll have -130dBm of thermal noise coming in. If everything is wideband, your buddy needs to be received with some signal-to-noise ratio to achieve some level of fidelity. Let's arbitrarily set that threshold -122dBm to achieve a good-enough signal. If your buddy switches to a narrowband radio and you switch the repeater to a narrowband configuration, you'll have a narrower IF filter and thus less thermal noise within the IF passband: -133dBm. Passing the input signal at the same signal-to-noise ratio (which would be received at -125dBm) will not reproduce the input signal at the same degree of fidelity -- it'll sound worse. You're trying to do the same thing with less RF, it just doesn't work like that, or we'd have been 6.25 kHz compliant for a long time. The signal-to-noise ratio needs to improve by 6dB each time the bandwidth is halved to produce the same fidelity, and we gained 3dB from the narrower IF filter. To get the same fidelity, we'd need a received signal at -118dBm. The easy way out of the problem is to lowpass the audio so less noise is audible, and that's one of the reasons you'll hear a lot of people say narrowband sounds like junk. The audio loses fidelity in the process, but has less high-pitched hiss from FM noise.

 

The problem is with reproducing the transmitted frequency range. More and more RF power is needed to allow the higher pitches to be demodulated with an audio SNR above a fixed threshold as IF bandwidth goes down. It's a property of FM.


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#8 marcspaz

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Posted 14 November 2019 - 11:33 PM

You get more fidelity at a given power level using a wider deviation. You need more power (a net change of 3dB in this case) to reproduce a transmitted signal with the same fidelity. So if you've got a receiver trying to receive your buddy down in the valley and you're in a theoretically perfect hilltop site, you'll have -130dBm of thermal noise coming in. If everything is wideband, your buddy needs to be received with some signal-to-noise ratio to achieve some level of fidelity. Let's arbitrarily set that threshold -122dBm to achieve a good-enough signal. If your buddy switches to a narrowband radio and you switch the repeater to a narrowband configuration, you'll have a narrower IF filter and thus less thermal noise within the IF passband: -133dBm. Passing the input signal at the same signal-to-noise ratio (which would be received at -125dBm) will not reproduce the input signal at the same degree of fidelity -- it'll sound worse. You're trying to do the same thing with less RF, it just doesn't work like that, or we'd have been 6.25 kHz compliant for a long time. The signal-to-noise ratio needs to improve by 6dB each time the bandwidth is halved to produce the same fidelity, and we gained 3dB from the narrower IF filter. To get the same fidelity, we'd need a received signal at -118dBm. The easy way out of the problem is to lowpass the audio so less noise is audible, and that's one of the reasons you'll hear a lot of people say narrowband sounds like junk. The audio loses fidelity in the process, but has less high-pitched hiss from FM noise.

 

The problem is with reproducing the transmitted frequency range. More and more RF power is needed to allow the higher pitches to be demodulated with an audio SNR above a fixed threshold as IF bandwidth goes down. It's a property of FM.

 

 

I got you.  I agree that the audio fidelity is lower in narrowband because there is simply less data. 

 

I never tested how much power increase would be needed for the same fidelity.  That would be a fun field experiment with a couple of radios.  My son just got his ham ticket.  I'll have to see if I can get him to hop in the truck and help me run that test.






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