Ghosts, Birdies, Interference, & Overloading...distinguishing?

[wannabescannist]

How can you tell whether the signal you think you're receiving is the actual signal or is one of the things listed in the thread subject?

My guesses, and please correct me if wrong:
Ghosts: no way to tell
Birdies: if the manual specifies that particular frequency
Interference: static sound that changes with position of scanner and/or antenna
Overloading: if the signal disappears when attenuating

There's a certain signal I hear on lots of frequencies (vhf & uhf) and am uncertain if whether it's actually multiple signals or one strong signal that's showing up on different frequencies.

 

[ka3jjz]

Without a little more information - like where you are, and what you are hearing - it's not possible to diagnose this fully. But in general, you can easily test for a few of these without expensive equipment or procedures;

a. To test for a birdie, stop on the desired frequency - you could hear nothing, or a tone, or any combination thereof - disconnect the antenna. If it's still there, it's very likely to be a birdie. There may well be others not listed in the manual - there is some variation from unit to unit

b. Interference is much too wide a topic to just be dismissed by changing the position of the antenna or scanner, although sometimes that is one way to get rid of it. You're suggesting that some outside source - say a light dimmer, a common cause of interference - can be attenuated by moving, and this is correct. But there are other kinds of interference, and in some cases, it can be related to...

c. Overloading. The symptoms are rather wide ranging - for example if you have a nearby FM station that bleeds onto the civil air band, sometimes the interference is likely being caused by overloading (if you can see the FM station's tower, youi're way too close, hi). Another symptom (when in the FM portion of the bands) can be that you actually lose sensitivity in one or more bands. This is a very loose description of what is referred to as 'desense', and is often directly related to overloading. A simple test would be to turn on the attenuator - if it has little or no effect on the problem, desense due to some overloading signal(s) is a likely problem.

As you can see diagnosis is difficult, and can vary from place to place. A careful evaluation is needed to make a correct guess as to what is causing the problem.

As to 'ghosts' - be more specific. That's a term I haven't heard in many years, and is usually a synonym for Images - something that most modern scanners do a pretty good job of rejecting - although it's certainly possible to encounter them.

73s Mike

 

[wannabescannist]

Thanks for the explanation, Mike. When I say ghosts, I guess I mean a signal that's popping up at frequency Y on the scanner, when the actual frequency at the transmitter is X (and the difference between X and Y is significant).

 

[zz0468]

Thanks for the explanation, Mike. When I say ghosts, I guess I mean a signal that's popping up at frequency Y on the scanner, when the actual frequency at the transmitter is X (and the difference between X and Y is significant).

The term "ghosts" is generally referring to multiple images on a tv receiver caused by multipath. You're talking about what would best be described as a spurious response. There's two likely possibilites for a cause of that - it could be an IF image, or it could be intermod. There are also other potential causes, but those are the most likely ones.

Going back to your original question, how can you tell... If it's an intermod, it could be generated in your receiver, or externally somewhere else. If it's external, it's a very real signal to the receiver, and there's not much that *YOU* as a scanner listener can do about it. If it's internal, there is a quick and easy way to tell. Add some attenuation. If you add 6 db of attenuation and get, say, 15 or 20 db or more (varies!) of apparent reduction in the signal, chances are it's internally generated intermod.

 

[w0fg]

Ghosts? ....oh, you mean intermod & images. .I thought you were talkin' about Bruce Willis. "I hear dead people." Now, if your scanner starts channeling Jim Morrison...that's a ghost!