Minimum usable signal strength

[gatekeep]

Ok, so my brain isn't functioning this morning...What is the typical UHF minimum readable signal strength in dBm? For some reason I'm thinking its around -87 to -91 dBm? I know most receivers are sensitive enough for a -107 dBm signal but its not usually readable that low.

 

[nd5y]

It depends on the receiver sensitivity and what you consider readable.
In the past I had my own service monitor at work and checked some of my hand helds. If I remember right my old VX-6R would receive weak and noisy at -120 dBm and the analog cellphones I worked on were about the same. I think the passing spec on the phones was -112 dBm at 12 dB SINAD

 

[zz0468]

There are two common specifications found for FM communications receivers, 20 db quieting, and 12 db sinad. One microvolt is -107 dbm, and that should be plenty strong enough. A specification of 0.25 microvolt for 20 db quieting, or 12 db sinad would be consered decent, and it's not uncommon to find commercial gear that performs a whole lot better than that.

There are other factors involved, though, noise being one of them. Scanners have really poor local oscillators, in the form of a programmable synthesizer. These synthesizers generate a lot of phase noise, which creates the symptom of received signals never achieving full quieting. When this occurs, some people might perceive that even what should be strong signals are not as readable as they should be, and feeds the perception that commercial receivers are a lot more sensitive than scanners are.

Other factors that affect a users perceptions of sensitivity would include receiver IF bandwidth. This is becoming more noticed as narrow banding takes place. A transmitters occupied bandwidth really needs to fill a receiver's IF bandwidth, or less than optimal audio recover will result. Over modulate, and a signal might be full quieting until someone talks. Under modulate, and even a slight amount of hiss in the receiver will have the same recovered volume that the recovered voice modulation has.

It all adds up to how sensitive a user percieves his receiver to be.

 

[gatekeep]

Thanks. We're talking about a commercial /\/\ transceiver.

I'm using RadioMobile to visualize Rx/Tx range around me, and had set the Rx threshold at 1 microvolt (-107 dBm), but I wanted to be sure, because for some reason I thought a clear usable signal would need be higher then that.

 

[zz0468]

That leads to the question... It's a commercial radio, is this for a commercial customer? Because the parameters that are acceptable to an amateur application are not necessarily acceptable to a commercial user.

For starters, use the manufacturer specified receiver sensitivity. It's going to be closer to .15 microvolt, not 1 microvolt. But if it's a commercial application, you need to factor in some real world losses that a radio is going to find between it, and the transmitter it's trying to listen to.

It's a complicated process, and Radio Mobile is NOT a user friendly program to try to figure that stuff out with. I've compared it with very expensive commercial software and found that, while it has the tools to run some pretty accurate coverage plots, it's no where close to intuitive, and unless you get all the details right, the plots can come out pretty meaningless. Especially if you're looking for specific received signal levels.

It does ok in defining the shape of the coverage area, but it can do a lousy job predicting actual signal levels. That takes repeated plots, field measurements, tweaking to the parameters in the software, more plots, more field measurements... and so on.

 

[prcguy]

As zz mentioned an amateur vs commercial user will have different quieting baselines to consider.

In my experience most radios made in the last 20yrs will easily meet a .2uv or -119dBm for 12dB SINAD spec. At that 12dB SINAD level the signal is moderately noisy but perfectly readable and probably ok for amateur use.

A commercial customer expects landline telephone quality and Motorola radios usually have very good limiting so another 10dB of signal will get you much closer to full quieting (now at -111dBm) and 10 more dB will usually guarantee full quieting with any modern radio and now we're at 2.0uv.

With that said I think the OPs original level of 1uv or -107dBm into the radio is a good compromise for general system planning since many radios these days are incredibly sensitive and will pass .15uv/12dB SINAD and 20dB above that is around .55uv. or -112.7dBm.
prcguy

 

[gatekeep]

The /\/\ spec is .25uV for 12db SINAD. And while it's a commercial radio, I'm doing the calculations for a amateur system. Based on prcguy's post, assuming the radio is properly aligned, is it safe to assume that a signal at -107dBm (1uV) should be full quieting?

 

[zz0468]

The /\/\ spec is .25uV for 12db SINAD. And while it's a commercial radio, I'm doing the calculations for a amateur system. Based on prcguy's post, assuming the radio is properly aligned, is it safe to assume that a signal at -107dBm (1uV) should be full quieting?

It'll be mostly quieting, depending on the effectiveness of the receiver's limiters. If you're talking a about a commercial (or modern amateur) grade receiver, 1uV is just about the point where it starts getting hissy. If you're talking about a scanner, 1uV would still be pretty noisy, for several reasons.

 

[gatekeep]

It'll be mostly quieting, depending on the effectiveness of the receiver's limiters. If you're talking a about a commercial (or modern amateur) grade receiver, 1uV is just about the point where it starts getting hissy. If you're talking about a scanner, 1uV would still be pretty noisy, for several reasons.

Ok thanks, zz0468 & prcguy. Yea I know the front end on nearly all scanners (because I haven't played with EVERY scanner), is quite terrible and has both poor selectivity and poor sensitivity.