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Wide FM vs Narrow FM and distance

poltergeisty

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Not sure what subforum to put this in, but I guess it relates to my MURS radios.

I was testing the distance of my MURS radios primarily with frequency 154.600 WFM. For the most part I got a good signal no matter how far I walked from the house. But once I started dropping out I tried frequency 151.820 NFM and I was able to break the squelch at home where on frequency 154.600 WFM I couldn't. So I was wondering if NFM has better range over WFM.? I thought WFM would have better range since it uses a wider bandwidth for voice. But it was in fact the NFM frequency that seemed like it had better range.

Thoughts?
 

prcguy

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With everything else equal and looking at only band width, it takes more power to transmit the same information in a wider band width than it does in a narrower band width. On the receive side every time you cut the band width in half you gain a 3dB advantage in noise floor. So narrower is better for signal to noise ratio.

If you look at the way wide and narrow band is implemented, sometimes a radio is design is compromised by including both wide and narrow IF filters, etc, and using the same detector. So some radios can perform slightly better in wide band mode.
 

nd5y

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It could be a lot of reasons.
It is more likely a result of the ~3 MHz difference in frequency than changing the mode from FM to NFM.
The receiver might be less affected by wideband noise or other signals.
The low quality CCR receiver might actually work better on 151 than 154.
The transmitter power might not be the same at 151 than 154.
The antenna might work better at 151 than 154.
Theoretically you should have a better signal to noise ratio with wide FM but if the signal drops completely out that isn't the reason.
 
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poltergeisty

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Theoretically you should have a better signal to noise ratio with wide FM but if the signal drops completely out that isn't the reason.

Kinda figured that.

I guess I need to add that I was testing range by myself. What I did was feed the received audio from the MURS channels on my scanner via an external antenna into my laptop and on there I was running Team Speak. I then run a local VPN in my router and make a direct connection to my router via the OpenVPN App on my smartphone and once I established a local connection, fire up the Team Speak App on the phone. Now I can hear myself speak over the radio into my Bluetooth ear piece.

Simplified version: MURS is monitored via scanner, scanner audio goes to computer and into Team Speak. I hear the audio via the Team Speak App on my smartphone.

So as you mention, I guess it could have something to do with the scanner and/or antenna as well. But knowing that I should have a better S/N ratio with wide band, I guess I'll continue to use WFM as my main channel. From all five MURS channels, two are WFM and the other three a NFM.
 

nd5y

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Its the opposite, you have better S/N ratio in narrow band. I doubt the antenna has much to do with your case.
I thought on FM when you cut the deviation in half and don't change the filter bandwidth you reduce the SNR by 6 dB or so. Isn't that why all these communications consultants were telling people that narrowbanding will cut your system's range in half?

I would like to see an engineer with the proper test equipment check out a single-chip CCR and see exactly what the difference is between wide & narrow.
 

prcguy

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I've field tested several radios going from +/- 5KHz deviation to "narrow" and the results are different between different brand and model radio. Using them at extreme fringe, weak signal conditions can show one radio in wide band to have a slight advantage and another radio can work a little better in narrow band. I think it has something to do with some radios using the same demodulator optimized for wide band even though they might switch to narrow IF filters in narrow mode.

There is the modulation index advantage of +/- 5KHz deviation vs +/- 2.5KHz deviation in that the recovered audio can sound better and appear to be less noisy once quieting sets in, but when using the radios out to the limits of working or not with weak signal, a properly designed narrow band radio should have a slight advantage over a wider band width radio at the outer limits of reception. I've done similar tests comparing FM to SSB on 2m where SSB should have a distinct advantage and some radios work as well and are more pleasant to use in FM mode in weak signal conditions over SSB due to the way the radio was designed. I found this true with the Yaesu FT-857 and FT-897 series. In the end a good high end VHF/UHF SSB radio will out perform a similar FM radio in weak signal conditions.

I thought on FM when you cut the deviation in half and don't change the filter bandwidth you reduce the SNR by 6 dB or so. Isn't that why all these communications consultants were telling people that narrowbanding will cut your system's range in half?

I would like to see an engineer with the proper test equipment check out a single-chip CCR and see exactly what the difference is between wide & narrow.
 

dem1

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This topic was batted around last year in a qrz.com forum, and was best summed up by K7JEM in posts 32 & 42:
"The bandwidth of a normal FM signal is around 16KHz. A narrow signal is around 11KHz... If you reduce the deviation by half (5 KHz to 2.5 KHz) your output audio drops by 6dB. If you didn't change the RX bandwidth, you would have a 6dB worse SNR, no question about that. If you narrow the RX filter to 11KHz, you [reduce noise by] 1.6dB due to the narrower bandwidth. That means, just based on these numbers, there would be a 4.4 dB degradation in going from "standard" to narrow. In reality, most of the narrow filters are narrower than 11KHz, maybe around 8-9KHz or so of effective bandwidth. This improves narrow to the point that it is only about 3dB worse than normal FM, with somewhat higher distortion, and less effective mobile flutter rejection. In reality, these things can be measured, but they don't really impact range that much."
Or see this article for a deeper, technical explanation about the complexities of FM modulation, bandwidth, and delivered audio quality.

In a post to an earlier thread, I demonstrated how switching my Whistler TRX-1 from FM to NFM while listening to a National Weather Service (NWS) FM broadcast produced slight distortion but also slightly lower noise. When the signal level was reduced to the point that it was barely audible, I could copy the broadcast using NFM but not FM. So prcguy is correct that narrowing one's receiver bandwidth can help with signals on the outer limits. But if NWS had then reduced it transmitted audio from +/- 5 KHz to +/- 2.5 KHz, the audio would have been 6 db weaker and would not have been heard at all.

prcguy is also correct that results differ among brands and models. While my TRX-1 uses separate IF passband filters for FM and NFM, my ham rigs do not. They simply boost the NFM audio, with no impact on SNR or adjacent channel selectivity.
 

nd5y

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So as you mention, I guess it could have something to do with the scanner and/or antenna as well. But knowing that I should have a better S/N ratio with wide band, I guess I'll continue to use WFM as my main channel. From all five MURS channels, two are WFM and the other three a NFM.
If the software and radio allows it you could set 154.57 or 154.6 to both modes and see if switching modes on the same frequency has any effect.
 

poltergeisty

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If the software and radio allows it you could set 154.57 or 154.6 to both modes and see if switching modes on the same frequency has any effect.
The radios I have don't allow me to change modulation scheme. But I could test in my UV82. Not exactly legal per say, but I'm curios to really see if there's a range difference between WFM and NFM.

Question: I see the acronym CCR used a lot. What does that mean? My guess is Cheap Chinese Radio? HAHA
 

RRR

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Regardless of what all the "theories" and "this is supposed to happen..." may be, it has been absolutely proven by many agencies and users, that when the exact same W/B freqs, on the same antennas, at the exact same locations, range was lost when NB was implemented (forced)

The only way to get the range you had before back, was to go to a digital mode, and even then, on the fringes it is "all or nothing"
 

Ubbe

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Most squelch designs operates by detecting the amount of noise in the signal. When using a filter for medium FM and then switch to a narrow the amount of noise will change. All radios and scanners that handle both narrow and normal channel widths have seperate settings for the squelch adjustment for the different filters.

When testing maximum range for different bandwidths you cannot go by when the squelch opens as that depends of how it have been adjusted for the different filter settings. Open squelch fully and listen to how well you can hear any kind of modulation, or measure the s/n value, if the transmitter can send a test tone to measure from.

/Ubbe
 

poltergeisty

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When testing maximum range for different bandwidths you cannot go by when the squelch opens as that depends of how it have been adjusted for the different filter settings. Open squelch fully and listen to how well you can hear any kind of modulation, or measure the s/n value, if the transmitter can send a test tone to measure from.

That does sound prudent and more scientific pertaining to a constant. Ideally, I should just tap the audio output on another MURS handheld instead of basing my conclusions on the circuitry of my scanner and its internal electronics and what not. But in order for me to do that, and since it seems I can't use the same frequency with different modulation bandwidths, I would have to wire up two outputs from two of my radios (I own four) to the audio input of my computer. One radio monitoring a WFM frequency and another radio monitoring a NFM frequency. But that voids the frequency constant. I guess it would be better than nothing and the MURS frequencies are only a few MHz apart. Don't know if that's really enough to add a variable to the testing concerning range with different modulation bandwidths.
 

stingray327

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I was just wondering if the radio you have gives you the choice of choosing the narrow band or wide band how do you determine which one to choose?
Also same thing how do you select the different settings from +/- 2.5 KHz to 50 KHz. I don't understand this.
 

poltergeisty

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I was just wondering if the radio you have gives you the choice of choosing the narrow band or wide band how do you determine which one to choose?
Also same thing how do you select the different settings from +/- 2.5 KHz to 50 KHz. I don't understand this.

The software for this radio doesn't allow you to change the modulation bandwidth.
 
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