Use squelch to determine vhf reception quality?

[joss]

Is there a reasonbly easy way to determine the amount of interference being picked up on VHF? I am wondering if I can use the fact that the squelch is only about half-way round to deduce that there isn't much interference. Is this valid? I ask becasue I am having trouble getting my AIS receiver to work reliably and don' t know what the problem is. Have checked the obvious stuff like quality of comnnections and have turned everything off, connecting the AIS receiver direct to the battery, but still it only periodically picks up a signal. Hopefully, when I get out of Port Solent Marina it will work OK, and then the problem will be solved.

 

[john_morris_uk]

The simple answer is no.

If you want the more technical answer:

The way that a squelch operates depends on the design of the VHF. Some squelches are 'noise operated' and their function depends on the characteristics of an FM detector. (In zero carrier conditions the output is a lot of white noise). When the receiver detects a carrier (which quietens the output of the detector), the audio is allowed through to the speaker.

The other common way in which a squelch is designed is to operate off the Automatic Gain Line (AGC) line of the receiver. The AGC voltage will rise as soon as a carrier is detected - this turns the audio stages on and allows audio through to the speaker. In the latter system, the squelch control determines at what AGC level the audio will be turned on.

Neither system will allow you to easily determine local noise by the amount of 'squelch' you need.

 

[shmoo]

Assuming you are using a separate antenna for the AIS then you could try running it temporarily on the main VHF antenna (i.e. the one at the top of the mast)
Where I am at Woodbridge at the top of the Deben the AIS gets nothing on the pushpit antenna but when I connect it to the boat's main VHF antenna I reach 7 M down the Deben and further 5M out to sea as well as 5 M overland to Felixstowe

 

[William_H]

As John Morris says squelch is the control and system which silences the set. It is unlikely you could calculate interference noise from this control. However if you open the squelch ie so noise is heard you can sometimes ascertain the nature of the interference by the sound it makes. There is often much less backgroundnoise when you disconnect the antenna cable. This is a good trick to do when you know it is working ok for future reference as it can indicate a bad antenna.
If you suspect interference from your boat try turning other things off while listening for a change in noise level or nature. Likewise if you move the boat note any changes of loudness at a constant volume setting.
Or try a mast top antenna if you don't already have one. good luck olewilll

 

[steverow]

The only way you can calculate the level of interference to an FM radio receiver is by doing a SINAD test.
This is a signal to noise and distortion test.
This is usually done by using a calibrated RF signal generator and audio distortion meter, and the level of the RF generator noted when the noise level drops to 10dB, thus giving your SINAD value in uV. This is basically a test of how sensitive your RX is for a given amount of noise. A reasonable set should give about 0.5 microvolts for 10db. Any value above 1 microvolt should be investigated as a fault.
More commonly these days the test is carried out in a combined tranceiver test set.

This is engineering stuff and is really only in the province of professional radio engineers with the right test equipment.
The squelch, which can operate in any one of three ways depending on the the circuit of the radio, is of no value whatsoever in this.

Steve.

 

[joss]

Thanks for all responses.

Still doesn't bloody work though!.....

Jo

 

[ColinR]

I dont pick up anything on my AIS reciever in harbour (Dartmouth, admitedly a bit landlocked) but it works fine at sea. My antenna is pushpit mounted. I would try it at sea before you do anything too complicated.

Colin