Wide, Narrow, SSB, FM

[Seven-Delta-FortyOne]

My understanding is that SSB, compared to AM or FM, will generally travel farther, because the signal, and therefore power, is compressed into a narrower bandwidth.

However, FM wide will generally give you better range than FM narrow.

Am I correct?



Delta

 

[zz0468]

Receiver threshold sensitivity, and therefore range, is a function of noise figure and IF bandwidth. Narrower bandwidth gives a better receiver threshold, given the same noise figure. So, theoretically, narrow FM should have slightly better range.

Anecdotal evidence of poor range with narrow FM is probably the result of the IF bandwidth not being optimum for the transmitted bandwidth.

 

[Seven-Delta-FortyOne]

So if you have a rig that will do both, you're better off switching to narrow?

I have a GMRS mobile, and I set it for wide, because I thought that would increase range a bit.

Also, on 2 meter of 70 cm, if I set the rig for narrow, but the person I'm talking to has it set to wide, will receive audio on their end be distorted/



Delta

 

[kb5udf]

Narrow band FM has decreased range

VHF/UHF Narrowbanding FAQs

No, you will not improve range on GMRS by going narrow band. The opposite is true. See the above
FCC document on the subject and note the section on "coverage."

 

[12dbsinad]

Depends on the radio. I've seen older units turn deaf when switched to narrow. Receive sensitivity goes to the dogs. Thus, would definitely decrease your effective range. Newer radios seem to be a lot better.

 

[Seven-Delta-FortyOne]

Thanks to all who replied.

I'm slightly less confused now.


Delta

 

[Project25_MASTR]

From what I remember, the official FCC documentation states narrow FM results in a 3dB signal reduction. Some independents say more towards 6 dB of signal reduction.

It is worth noting that ssb is more similar to am as far as modulation goes than it is FM.

 

[zz0468]

I'll have to make some measurements on some real radios and see... I did some calculations, the theoretical difference between an 11K0 receiver and a 20K0 showed about a 2.6 db improvement for narrow.

I'm not sure I understand what the issue would be with reduced range, unless it's a circumstance where transmitter deviation is reduced but receiver IF BW isn't. With FM, you want the entire IF passband filled with modulation, or you will achieve less than optimal performance.

In the systems that I have narrow banded, there have been no noticeable reductions in coverage.

 

[Project25_MASTR]

Everyone locally has done nothing but complain about poor performance since narrow banding.

 

[zz0468]

Everyone locally has done nothing but complain about poor performance since narrow banding.

If the receivers aren't getting narrow banded, that would be quite understandable. I narrow banded a number of VHF stations, and didn't notice any performance degradation at all. But I made sure the receivers were narrow banded as well. For a brief period of time, there were some repeaters that had transmitters narrow banded, and the receivers had to be left wide until all transmitters were done, and performance suffered substantially.

 

[WA0CBW]

One thing we noticed when narrow banding was that some of the earlier radios that were narrow band capable didn't perform as well when narrow banded. Maybe because the manufacturer didn't really narrow band the IF and only narrow banded the transmitter. Some manufacturers had a problem with defective IF filters. Latter radios seemed to perform better when narrow banded.
BB

 

[Seven-Delta-FortyOne]

So what I'm getting, in a nutshell, is that if you narrow-band, both transmitters and receivers need to have it done, and then the best is that it will have as good coverage as wide.

So if you have the choice, like with GMRS or VHF/UHF Ham, the best option is to stick with wide.

Also, if the transmitter is narrow, but the receiver is wide, audio will still come through, howbeit degraded and with, more than likely, decreased coverage.

Is this correct?



Delta

 

[zz0468]

So what I'm getting, in a nutshell, is that if you narrow-band, both transmitters and receivers need to have it done...

Absolutely, both TX and RX need to be done, or there WILL be severe performance loss.

and then the best is that it will have as good coverage as wide.

Apparently, and according to participants in this thread, that would depend on who you're talking to. Theoretically, reducing bandwidth (while maintaining equal noise figure), there should be a slight improvement in performance. But there are plenty of people out there declaring substantial loss in coverage. In my personal experience, where both TX and RX were properly narrowbanded, I believe performance is about equal. I have not made measurements to compare.

So if you have the choice, like with GMRS or VHF/UHF Ham, the best option is to stick with wide.

If there is a choice, yes. The advantage with wider bandwidth is a better signal to noise ratio when the receiver is saturated (full quieting). The choice has nothing to do with weak signal performance. This is why FM broadcast uses 75 KHz deviation. Old analog FM microwave radios used something like 200 KHz deviation per multiplexed channel, all for S/N ratio purposes. But FM broadcast and microwave operate on the assumption that received signal levels are well into saturation, with 40 db fade margins or more.

Also, if the transmitter is narrow, but the receiver is wide, audio will still come through, howbeit degraded and with, more than likely, decreased coverage.

Is this correct?

Sort of. The decreased coverage isn't a result of weaker carriers, it's the result of much lower demodulated audio. As signals get to be weak, an FM receiver starts to noise up. If the modulation from the transmitter fills up the receive IF passband properly, the signal can noise up considerably before it becomes unintelligible (threshold).

A narrow transmitter into a wide receiver, however, will still noise up, but at some point, the noise now equals or exceeds the level or the recovered audio, and therefor it's unintelligible, long before the received carrier is too weak to demodulate, hence the perceived reduction in range.

 

[prcguy]

Looking at the performance of wide band vs narrow band by calculating the system noise reduction and improved performance from narrowbanding a receiver is only part of the story and peculiarities between brands of radios will effect the outcome more than the calculated difference.

I've done a lot of wide band to narrow band mods over the years and about 25yrs before the FCC narrow band mandate. I used to run lots of commercial UHF splinter channel repeater systems and changed out the 15 to 18KHz IF filters in the repeaters and radios to roughly 10Khz versions, then realigned the radios.

I had no perceivable change in performance but was running about 3KHz voice deviation, which is a little wider than the current spec and did this to avoid complaints and interference from the mainline 25KHz channel users. I believe some of the performance complaints depend on the brand of radio used and that most radios have narrow band added as an afterthought and the radio is not optimized for narrow use.

When you simply reduce transmitter deviation and add a narrow IF filter, the transmit audio pre-emphasis and de-emphasis in the recievers has not been changed, which can affect performance and the receiver discriminator output is less, causing you to turn up the volume on a radio that was once wide band and is now converted to narrow band. I think if you take an older wide band radio and test system performance, then compare to a new radio truly designed for narrow band, you should find the range is about the same.

I've also seen big differences in receiver limiting points where say a Motorola Spectra will be full quieting at maybe 5uv and a Bendix King mobile will not fully quiet until about 20uv, but they both pass a 12dB SINAD spec at .25uv. Using both radios in the field shows the Spectra seems to be receiving much better with less picket fencing and noise on weak signals compared to the BK even though they both have the same sensitivity spec. The limiting problem in the BK or similar radios may be worse when narrow banded.

It would be interesting to gather facts on systems that appear to work the same wide vs narrow compared to systems that have complaints of reduced range after narrowbanding. You might find a common thread of radio brands or models that get the complaints.
prcguy

 

[nd5y]

Am I correct in assuming that noise from external sources like alternator whine and power supply hum will be twice as bad? Let's say you have alternator whine that moudulates the transmitter at1 kHz deviation. If you narrowband everything and the whine stays at the same level, then its audio level should be twice as loud in a narrowband receiver, or sound like the same thing at 2 kHz deviation in a wideband receiver, right?

 

[zz0468]

Am I correct in assuming that noise from external sources like alternator whine and power supply hum will be twice as bad?

Stop assuming. What happens would depend on the mechanism thats causing the noise.

 

[prcguy]

I would say if a transmitter is modulated with anything like alternator whine and its the same deviation in a wide or narrow band system, it would appear louder compared to the voice in a narrow band system but not necessarily twice as loud.

This is based on the modulated whine being less than required to engage the deviation limiter in the transmitter, so the actual deviation level is actually the same in both cases. It will also depend on how the added voice modulation runs the combination of voice and whine into the deviation limiter.

When I was narrow banding radios I remember setting up an oscilloscope at the discriminator output of a stock wide band radio and after it had narrow filters there was less audio voltage from the discriminator, maybe about 20 to 25% less. This was going from usually a 15KHz wide filter to a 10KHz wide filter and I have no idea if the skirts were the same.

At the speaker output you need a voltage change of about 3X to have an audible difference of about 2X, so my narrow banding would not have produced enough change at the discriminator for a 2X change in volume from the speaker.
prcguy

Am I correct in assuming that noise from external sources like alternator whine and power supply hum will be twice as bad? Let's say you have alternator whine that moudulates the transmitter at1 kHz deviation. If you narrowband everything and the whine stays at the same level, then its audio level should be twice as loud in a narrowband receiver, or sound like the same thing at 2 kHz deviation in a wideband receiver, right?

 

[zz0468]

A lot would depend on how the noise is getting into the transmitter. If its getting in at some low level audio stage, wouldn't the deviation be narrow band, just like normal audio? If its getting into a vco, then yes, you could expect the noise to be received louder.

 

[prcguy]

I've worked on radios where the mic audio gain seems to stay about the same and the limiting point is changed with the dev limiter pot, and other radios seem like the dev pot affects mic audio gain and the limiting point together. So it would depend on how the radio is designed as to transmitted alternator whine changing or staying the same between wide and narrow band.

Just my opinion.
prcguy

A lot would depend on how the noise is getting into the transmitter. If its getting in at some low level audio stage, wouldn't the deviation be narrow band, just like normal audio? If its getting into a vco, then yes, you could expect the noise to be received louder.

 

[zz0468]

Agree... it's going to depend on the radio, and the noise source. But it won't automatically be worse in NB.