Squelch and Attentuation

[johnnymitch74]

Hey everybody,

I'm using a Yaesu VR-500 scanner with a homemade ground-plane antenna (indoors) tuned to the 161MHz railroad band with 25 feet of RG-58 cable. I just have a few questions about problems I'm having.

1. My squelch seems to be very "poppy." As in, it often opens to nothing but static. Is this normal? Are squelches normally "leaky" like this, or are they generally all-or-nothing?

2. Since my scanner has an attenuator, I do not know if it would be more helpful to have the attenuator on, and use little squelch, or high squelch with the attenuator off to get rid of this white noise popping. Which would result in the most signals heard?

3. I'm using a pl-259 to BNC adapter to connect the cable to my scanner. Could this account for any problems?

Thanks a lot!

 

[RKG]

Squelch (on an FM receiver) and attenuation are two different functions designed to serve two different purposes.

On an AM receiver, the absence of signal yields essentially no sound. On an FM receiver, the absence of signal yields loud white noise. A squelch circuit is designed to mute the audio amplifier circuit until the FM detector circuit detects the presence of RF energy on the tuned frequency at or above a given level.

(Crudely, most FM squelch circuits take the "no signal" white noise, convert it to DC voltage, invert the voltage (i.e., high noise equals low voltage and low noise equals high voltage), and then use the voltage to trip a zener-type gate on either power to the AF amp or the AF line to the speaker.)

If you have a variable squelch control, what it does is to set the trip level at or above which the squelch circuit unmutes audio. If you set it too low, the circuit may unmute on stray signals, which sounds like what you're hearing. If you set it too high, weak but otherwise valid and readable signals will fail to unmute audio and be missed.

To a large extent, RF squelch becomes less significant if you are running a PL or DPL gate on audio unmuting (except insofar as too low a squelch setting will slow down scan and, as above, too high a setting results in missed signals altogether.)

Attenuation is a means of reducing the level of all signals that an antenna would otherwise pass to the FM front end. It is used where signals may be encountered of such a high strength that they would result in front end overload and distortion.

 

[johnnymitch74]

Ah okay thank you for that explanation. So do you think it would be more beneficial to use the RF squelch rather than the standard tone squelch (i think that's what it's called)?

 

[RKG]

Ah okay thank you for that explanation. So do you think it would be more beneficial to use the RF squelch rather than the standard tone squelch (i think that's what it's called)?

You use both. (As I observed earlier, they serve different functions.)

The RF squelch pot should be set to a value just above the ambient noise floor in your neighborhood for the band in question. For want of better data and instruments, do the following:

Program a freq in CSQ (i.e., no tone gating) in the relevant band.

Open the squelch pot all the way; you'll hear rushing noise.

Close the squelch pot until the noise goes away (i.e., audio is gated), plus a wee bit more.

Using tone gating to monitor any channel that employs tone (and today virtually all do) has a number of advantages. It eliminates unmuting on co-channel transmissions in which you're not interested. It prevents squelch breaks caused by collateral sources. And it probably speeds up scanning in many cases.

 

[gmclam]

Ah okay thank you for that explanation. So do you think it would be more beneficial to use the RF squelch rather than the standard tone squelch (i think that's what it's called)?

Using the attenuator will reduce your receiver's sensitivity. The best answer for you to your question depends a little on what you're trying to listen to. I wonder what band you are monitoring that has so much popping. And if you know the source of the popping. If so, it would be better to mitigate that than to reduce what your receiver is picking up (unless all you want to hear is strong signals anyway).

 

[johnnymitch74]

okay i think i found my problem. I took the antenna outside, and all became well. i 1/4 wave guess ground plane antennas are just not good indoors. thanks for the help everybody.