VHF squelch - technical questions

[Playtime]

My VHF radio and masthead antenna function together quite well. However, the VSWR of the antenna is greater than 2 and water was present in the coax under the deck, so I have been preparing to replace the antenna/coax.

When conducting tests of the new antenna (a Metz) with the radio before fitting to the masthead, I found that I couldn't remove the VHF 'hiss' with the squelch control. Further tests using a second VHF set and another Metz antenna (thanks SaltyJohn!) have proved that it is the radio at fault and not the new antenna/coax set up. The radio is a NASA SX35 DSC unit - about 5 years old.

Questions -

1. Why does the old antenna (VSWR >2) still work well with the radio while the new antenna (VSWR approx. 1.2) produces continuous hiss?

2. Is it possible to 'tweak' the radio so it works (properly) with the new antenna?

 

[GrahamM376]

Suggest you give NASA a ring, I've found them very helpful when tech info needed.

 

[Playtime]

I will call NASA but they are closed right now. In the meantime I'm interested if our technical experts have any thoughts.

 

[misterg]

...When conducting tests of the new antenna (a Metz) with the radio before fitting to the masthead, I found that I couldn't remove the VHF 'hiss' with the squelch control.

...

1. Why does the old antenna (VSWR >2) still work well with the radio while the new antenna (VSWR approx. 1.2) produces continuous hiss?

Were you testing the new antenna anywhere near the set? If so, it may be feedback of stray emissions from the set to the antenna is causing problems because of their proximity. Different radio sets will react differently.

It could always be a problem with the set, with the wet coax mopping up most of the incoming signal on the old antenna, and making it easier to 'squelch' out.

Andy

 

[Playtime]

Were you testing the new antenna anywhere near the set? If so, it may be feedback of stray emissions from the set to the antenna is causing problems because of their proximity. Different radio sets will react differently.

It could always be a problem with the set, with the wet coax mopping up most of the incoming signal on the old antenna, and making it easier to 'squelch' out.

Andy

I don't think proximity of the antenna to the radio is the problem. I think it is more to do with the sensitivity of the receiver. The new antenna is sending a stronger signal (noise included) to the receiver and the squelch control can't reduce the sensitivity sufficiently to remove the noise (vhf hiss).

I suspect that there is an adjustment that could be made to the cut-off point (sensitivity) of the squelch control but would welcome an informed opinion from an expert!

 

[William_H]

Squelch

The squelch circuit in a receiver is a circuit which detects if there is a received signal and opens the pathway for the audio to be heard. The squelch control is adjusted so that the weakest possible signal will still open the audio while the background noise is not enough to open the squelch so is not heard.
Some modern radios seem to have a non adjustable squelch or an auto detect arrangement.
It follows that to hear the weakest signal you should adjust the squelch control so that noise can be heard.
I get iritated at people who refer to the noise as "squelch" yet I fear it may become a defacto name for noise.
Anyway at VHF and using Frequency modulation most of the noise will be generated in the receiver unlike HF AM where most of the noise will be coming from atmospheric noise down the antenna.
It is unlikely that your old radio has a problem with the squelch circuit although possible. Any really bad local interference could produce enough noise to open the squelch at all settings.

Regarding the antenna. Radio works on a huge range of actual received power. The perception of strength is logarythmic. Your transmitter generates 25 watts of power. A bad antenna might only radiate 1% of the power and likewise receive only 1% of the available voltage for the receiver yet you might never be able to tell the difference. That is until you get to the extreme of range.
So all we can do is use instruments to ensure the antenna and cable are working at their best and trust that this will give us the best range. A simple talk check may be reassuring but not necessarily accurate test. So fix the cable and ensure you have a good VSRW then look at th radio. It may be usable with bad squelch but make sure it is not some on board device causing interference. (turn everything off)
good luck olewill

 

[Playtime]

It is unlikely that your old radio has a problem with the squelch circuit although possible. Any really bad local interference could produce enough noise to open the squelch at all settings.

I still believe that it is a receiver sensitiviity issue. With the new antenna the radio is receiving a stronger signal, including the noise. For some reason the squelch control is incapable of 'desensitising' the radio sufficiently to remove the noise.

The question is why and can a simple adjustment correct the problem?