noob question. narrowband

[sagacious]

alright, so lets say i have two portable radios. (conventional, VHF). one programmed in narrowband and one is wideband and they are on the same channel. I know from my bench that they will talk to each other. but do they operate seamlessly with no loss of efficiency between one or the other? is there any decrease in range, or audio quality between the two? am I gaining anything by keeping "like bandwidths" together? I guess im asking because I know theres a part of the picture im missing.

i.e. if i send two guys that work together out in the field with equally programmed radios, except one is wideband and one is narrow. where will the problem occur?

 

[gmclam]

Channel width

Unfortunately the term "narrow band" does not have an absolute meaning in terms of numbers. That is because channels in different bands have different "widths".

Let's take FM broadcast from 88 to 108 MHz. Each channel is 200 kHz wide. Some scanner makers would call that wide band FM (WFM). Compare that to a channel in the 400 MHz public safety band, such as 460.025 MHz. A channel there is 25 kHz wide. That's certainly narrower than FM broadcast, but compared to the "new" narrowbanded channels, it is wide.

Move on to a VHF high band channel such as 153.890 MHz, and its width has been 15 kHz. That is narrower yet, but not as narrow as the government wants to get.

So the answer to part of your question depends on the specific band/frequencies you are using. What does your license state is your channel/emission width? And possibly the brand of radio (what one manufacturer calls one thing, another calls something else).

If one radio is transmitting 10 kHz wide and the other is receiving 20 kHz wide, the audio will sound about 50% undermodulated. Of course a radio set to 10 kHz wide and receiving a 20 kHz wide signal will sound LOUD and probably overmodulated and distorted.

 

[kb2vxa]

OK let's get down to basics and stick with the question. With the wide band one transmitting the narrow band one may sound loud and somewhat distorted as the FM deviates out of the receiver's IF passband. On the other hand with the narrow band one transmitting it may sound somewhat low on the wide band one but no distortion. Es claro?

 

[sagacious]

If one radio is transmitting 10 kHz wide and the other is receiving 20 kHz wide, the audio will sound about 50% undermodulated. Of course a radio set to 10 kHz wide and receiving a 20 kHz wide signal will sound LOUD and probably overmodulated and distorted.

"OK let's get down to basics and stick with the question. With the wide band one transmitting the narrow band one may sound loud and somewhat distorted as the FM deviates out of the receiver's IF passband. On the other hand with the narrow band one transmitting it may sound somewhat low on the wide band one but no distortion. Es claro?"

Yes, es claro. these two statements clarify and gave me the answer i was looking for. these things make sense when i hear them i just dont always come to the idea right off the bat on my own (yet)...as im a new rf guy learning the ropes. thanks guys.

 

[sagacious]

one more question on that.. what if im using PL encode/decode. if my signal is under/over modulating (either way) will my tone not be seen correctly by the reciever? If that were the case i would have to guess... the reciever may still open but no audio would pass?

 

[gmclam]

one more question on that.. what if im using PL encode/decode. if my signal is under/over modulating (either way) will my tone not be seen correctly by the reciever?

Again, it depends on the specific parameters of the equipment you are using. What is the modulation depth on the transmit side? And what channel width do you have the receiver set to? Then, what is the required CT signal level at the receiver for the squelch to open?

If that were the case i would have to guess... the reciever may still open but no audio would pass?

You say 'the receiver may open' and I wonder what you mean by that.

Since we're talking FM, the RF signal level will affect quieting. Now if your receiver is looking for a wider signal than is being transmitted, you will also not get as much quieting. If the CT is not strong enough, regardless of RF signal level, the squelch will not open.

 

[dougr1252]

I've seen this problem with scanners. Unidens seem to default to narrow FM, but if you try to monitor a channel with loud audio _and_ PL, the audio can cut out even though the RF signal is strong. The solution is to program as FM (5 KHz deviation) instead of NFM (2-3 KHz). "Real" radios may have better ability to deal with this problem, but you would need to experiment.

one more question on that.. what if im using PL encode/decode. if my signal is under/over modulating (either way) will my tone not be seen correctly by the reciever? If that were the case i would have to guess... the reciever may still open but no audio would pass?

 

[Murstech]

Hmm??
This gives me an idea for an experiment.

Build a pc board with circuitry that will install in place of the IF filter in a normal wideband receiver.
This circuitry has its own discriminator or circuit to detect the deviation of the incoming signal. The circuit will then select the appropriate filter and volume setting according to the deviation it has detected.

I know my brother in law doesnt talk loudly enough into his mike and I have trouble hearting him.
But sometimes he talks loud enough. A circuit that is as least able to detect his deviation and adjust the volume would be nice. I could try an audio derived alc type circuit after the discrimitator in the receiver I guess? But if I had a dedicated discriminator to detect deviation it might work alot better.

 

[ramal121]

Another thing to consider crossing wide band to narrow band is compandering. A narrowband signal, due to reduced deviation, will have more background noise/static (signal to noise ratio). Narrowband channels have an option to enable compandering to help fight off the background noise and is is commonly used.. Wideband channels usually don't have this option available. If a compandered radio transmits to one that is not, the audio will sound tinny. Turn that around and now the audio is muddy and muffled.