Transmissions coming through a different frequency

[Clats97]

Yes, a clean FM signal shouldn't be heard using an AM demodulator. It could be heard but very very weak. If it turned up just lately then it's probably a mix of two transmitters and then it could include a higher AM component. That Bell net are in the 141-143MHz range and +20MHz from 121MHz. If there's a new local transmitter at 2x20MHz at 161-163MHz then that could be the problem, or even a FM broadcast transmitter -20MHz at 100MHz. Then a broadcast trap filter would help.

Use one of the dongles and a spectrum view program like SDR# to see if the 160MHz range (or 100MHz) have a strong signal somewhere that can overload a scanner. You have to look when you hear that intermodulation issue. What's the model of that analog AM scanner? Any good quality scanner like Pro-2004/2005/2006 and BCT15x and BCD996 should not have these kind of problems. It probably are solved using the attenuator but you loose reception of weaker signals, but if the AM scanner has an IFX function then try that.

/Ubbe

The audio doesnt sound as good I can admit that. And there is a radio station around the corner, could it be that?

 

[Clats97]

is it a single frequency, or all of them ?

What do you mean?

 

[wtp]

does every frequency do it or just one ?

 

[Clats97]

does every frequency do it or just one ?

I'm not sure how to answer that. I monitor a trunked system and sometimes voice from the trunked system being monitored on my base scanner comes through to my analog airband scanner. Because trunked systems have constantly changing voice channels, I don't know if this is happening on a particular trunked frequency. I'm not watching that closely.

 

[nd5y]

Can you elaborate on this? Not sure I understand

That has nothing to do with what you are experiencing. If your 121.5 scanner was receiving an internally generated IF image on 121.5 it would do it all the time. What you are describing sounds like something mixing externally only when certain recievers are on certain frequencies.

 

[Clats97]

That has nothing to do with what you are experiencing. If your 121.5 scanner was receiving an internally generated IF image on 121.5 it would do it all the time. What you are describing sounds like something mixing externally only when certain recievers are on certain frequencies.

I see okay. So is there anything I can do about this or no?

 

[nd5y]

Probably all you can do is start turning off different radios or unplugging antennas when you hear something on 121.5 and try to figure out what combination is causing it.

 

[wtp]

the ws1010
IF Frequencies
1st IF.......................................................................10.7 MHz
2nd IF .......................................................................455 kHz
this might be the main culprit.
the other radios are triple conversion if i recall.
this is a double conversion radio.
the others are triple and are less prone to this.
i don't know how far away this radio could affect the others, but try that one first.

 

[wtp]

also what we need.
the radio station frequency of the one near you.
inter-modulation is still a possibility if it is really close.

 

[Ubbe]

I have too agree with lamarsy and wtp that it probably are a double IF problem as you only hear one transmission and not two that mixes.

When that WS1010 are set to receive 121.5MHz it could use an oscillator at 132.2MHz to mix with the frequency to produce a 10,7MHz signal that more easily can be amplified and filtered.

But that oscillator will also produce a 10,7MHz signal from a 142.9MHz signal and if the band filter for VHF air in the scanner isn't attenuating enough of a 142MHz signal it will be received and heard. The VHF air filter goes up to 137MHz and are probably passing 142MHz with very little attenuation. Modern scanners, and more costly old ones, use tripple IF system that also uses a first IF of something like 250MHz to avoid this double IF issue.

And as G7RUX explained a FM signal will be heard more loudly in AM mode if it isn't exactly on the frequency, like 142.920 that you have in your bell fleet system.

/Ubbe

 

[W4ELL]

the ws1010
IF Frequencies
1st IF.......................................................................10.7 MHz
2nd IF .......................................................................455 kHz
this might be the main culprit.
the other radios are triple conversion if i recall.
this is a double conversion radio.

Nail on the head.

Back in the 80s/90s, you could use a double conversion scanner to listen to part of the cell band because of this IF imaging. Just had to search the lower 900mhz range.

Explained here: Image Frequencies

 

[ecps92]

Nail on the head.

Back in the 80s/90s, you could use a double conversion scanner to listen to part of the cell band because of this IF imaging. Just had to search the lower 900mhz range.

Explained here: Image Frequencies

or to eliminate an image from 460 in the 483 T-Band by going to 504 to hear my local FD/PD's in T-Band