PSR 500: Noise/Static Question ?

[BOBRR]

Hello,

Have been doing a little experimenting lately, and thought I'd post some findings.

Would be most appreciative for any thoughts on, or comments.

Do a lot of listening to an airport control tower on 118.5 AM, about 10 miles from house.

On my Uniden BCD 996 scanner, using a Scantenna in the attic, it comes in crystal clear.
It also comes in crystal clear using a disconne in my attic feeding a JRC NRD 545 shortwave set.

On my new GRE 500, it always comes in with loads of static.
This is so with the rubber duckie, or connected to the same Scantenna as the BCD 996.

Have tried it with the ATT on, but as a subjective guess, the S/N is pretty much the same.
So, what's happening, possibly ?

I read about an inter-mod problem with these GRE scanners, but really don't understand
inter-mod theory, or how it manifests itself in the 500.

What is inter-mod interference; what causes it, e does it show up, how does it manifest itself, etc. ?

Any way to minimize ?

Could this be the reason why ?
Would, or should it, be effected by having the ATT on or off ?

All in all, I'm pretty happy with the 500.
Haven't really done a side by side comparison on other frequencies, but most seem to be clear.

It's this one, unfortunately, that seems to have all the static, and of course, it's one of my favorites.

Thoughts ?

Thanks,
Bob

 

[mtindor]

Front end overload and intermod are two different things. The PSR-500/600 suffer from both of these from time to time, possibly more than some other scanners because of the increased sensitivity on VHF.

Front end overload is simply from a really strong signal on VHF that is swamping the receiver with so much signal that the receiver can't adequately do its job. It could be a TV station, an amateur radio repeater, a high power paging transmitter (most are high power, many hundreds of watts).

In the case of front end overload / desense you'll find that turning on ATT can sometimes help the situation since the attenuator attenuates enough of the signal to allow the receiver to become functional again. This can make a signal that sounds crappy/noisy with the ATT off sound better when the ATT is turned on.

Of course, the downside of having to turn on ATT is that you aren't going to be picking up weak signals that you may in some cases really want to hear.

The only way to try and resolve front end overload is to find the local transmitters that are causing the overload and then inserting a filter inline that is designed to decrease a signal on the transmitter's frequency to a reasonable level. This offending transmitter could be difficult to find, and depending on how close the offending transmitter's frequency is to the frequencies you want to hear, it can be very difficult to adequately filter out the offending signal without attenuating the surrounding signals that you want to hear.

The difference between using the ATT and using an inline filter designed to filter out the offending signal is that the ATT acts upon a frequency you _want_ to hear, decreasing the strength of all signals on that frequency. Whereas if you are able to put a filter inline, that filter may be narrow enough to block the unwanted signal without affecting the signal strength of incoming signals on the frequency you _want_ to hear.

I had a similar problem with TV channel 9 in my area. I bought a PAR filter from parelectroniccs.com that effectively cut down enough of the TV9 signal that it allowed the receiver to work properly again without attentuating the signals I did want to hear.

In $500 or less scanners, there is always the tradeoff between increased sensitivity and increased susceptibility to overload / intermod. More expensive radios (such as commercial radios) that are often cost hundreds or thousands of dollars more than a scanner have more filtering and additional circuitry to protect against frontend overload while allowing the radios to have increased sensitivity. In our scanners we don't get this better circuitry/filtering because it costs money and very few people would pay the additional cost for scanner manufacturers to add the needed circuitry.

Mike

 

[mtindor]

Intermod is a different beast - you should Google "intermod" for better explanations than I can give. But it is not uncommon to have a bunch of fairly strong signals in close proximity, such as a combination of an active amateur radio repeater, a paging transmitter, a television station, a radio station, and/or an NWS weather station.

From Wikipedia:

Intermodulation or intermodulation distortion (IMD), or intermod for short, is the result of two or more signals of different frequencies being mixed together, forming additional signals at frequencies that are not, in general, at harmonic frequencies (integer multiples) of either.

Intermod can often be the result of dirty transmitters that are emitting more signal than they should at frequencies other than their main [intended] frequency. Combine a few of these together and it can result in you hearing signals on your receiver at frequencies that are not the frequency of any of the involved transmitters.

Here is an explanation of intermod from the PAR website:

http://www.parelectronics.com/faq_scanner_filters.htm

Mike

 

[mtindor]

In the case of strong local signals causing desense / front end overload, ATT can "appear" to help when you turn it on because it decreases the signal of the offending transmitter enough that it allows the receiver in your scanner to work properly again - so the noise you may hear on un unattenuated frequency as a result of local strong signals overloading the front end can disappear when you turn the attenuator on. In reality, even though turning on ATT helps, it actually defeats effective scanning for many people who listen to weaker signals since turning the attenuator on adds considerable attenuation that effectively removes any weak signals that you may want ot hear.

In the case of intermod, turning on ATT helps alleviate the apparent intermod, again because it decreases the signal strength of the problem signals.

People seem to often recommend turning on the attenuator as if its some miracle cure. It is not. It may in some instances be useful for some people, but if you hvae a need to hear the weak signals and have a need for hte increased sensitivity, then turning on the attenuator defeats that completely.

Mike

 

[BOBRR]

For Mtindor From OP

Hi,

Just a quick thanks for such excellent explanations.
Appreciate all your time in writing it.

It really helps.

Best regards, and thanks again,
Bob

 

[mtindor]

You're welcome. I make no claim to being a know-it-all. I just know a little something about this simply because i've had similar problems that I had to find the answers to.

What you describe about ther still being the same noise on the frequency even after you turn the attenuator on would lead me to believe it is something other than strong signal overload. Why? Because your other scanners do not exhibit this problem. And I doubt your other scanners are significantly less sensitive... Remember, it still happens when you turn the attenuator on. If you had said that the noise disappears when you turn on the ATT, then I might be more inclined to believe it is front end overload. I don't remember seeing any actual db figure for how much attenuation the attenuator provides. I think typically they are 10 db - 25 db attenuators in scanners. Even at 10 db down, you should notice a significant improvement in the S/N of the signal if it was suffering from front end overload. Then again, that only applies if the signal you are hearing is relatively strong to begin with (S9+). If you turn on the attenuator, it knocks down the offending signal by likely at least 10 db but also knocks down the signal you are wanting to hear as well. If it knocks down the signal that you want to hear far enough that it is now a weak signal (barely moving the meter), then it's going to have noise on it as well that has nothing to do wtih overload.

Another thing to check is to see if it may actually be a 'birdie' frequency - i.e. some frequency that just happens to have an internally generated signal on it from circuits within the scanner. But if that were the case you would hear the birdie 24/7 and not just when another signal is present on the frequency.

Some people might recommend that you try to create your own tuned stub out of some additional coax and a T-fitting where the stub is on one end of the T-fitting and your antenna is on the other. This is probably useless because those stubs usually are not narrow enough and will more often than not attenuate signals across the band, if not across multiple bands.

If you have any way to make an audio clip of the signal and the noise both present simultaneously, it might help somebody diagnose more precisely what your problem is.

Mike

 

[bellguy]

I'm finding the same problem. After only using the 500 for only a week, when compared side by side on the same antenna with a Pro 43, listening to local tower frequencies, the 500 has a high static and low audio whereas the Pro 43 has much clearer and stronger audio. I just assumed the 500 had less sensitivity in the 118.0 to 136 segment. Being my first trunking scanner I thought it was one of the tradeoffs for all the other capabilities it offers.