Transmissions coming through a different frequency

[Clats97]

Hi, I have multiple scanners and dongles running all at the same time, and I've noticed something weird that's been happening lately. I have an analog only scanner that scans the airband frequencies from the nearby airport. One of those frequencies is the guard frequency, 121.500 AM. I have noticed that sometimes when I hear audio coming through my base scanner, there's the EXACT same audio coming through the analog scanner, but it's showing that it's coming through 121.500. Thats impossible. The system I monitor on the base scanner is VHF FM, I don't understand how it could come through as airband AM.

Anyone know what might be going on?

 

[a417]

Common antenna?

 

[Clats97]

Common antenna?

No

 

[nd5y]

If you are hearing the same signal on 121.5 on two scanners in the same location at the same time there is nothing weird about that. Did you mistype one of the frequencies?

 

[Clats97]

If you are hearing the same signal on 121.5 on two scanners in the same location at the same time there is nothing weird about that. Did you mistype one of the frequencies?

How is hearing something that is VHF FM 145.xxx normal to hear on 121.500 AM? Those are 2 different frequencies and modulations. No the frequencies aren't mistyped. I'm monitoring a trunked smartzone system but occasionally a transmission comes through the analog scanner on 121.500

 

[nd5y]

How is hearing something that is VHF FM 145.xxx normal to hear on 121.500 AM? Those are 2 different frequencies and modulations. No the frequencies aren't mistyped. I'm monitoring a trunked smartzone system but occasionally a transmission comes through the analog scanner on 121.500

You didn't say anything about other frequencies in the first post, just 121.5.
Are you saying there is a trunked system on 145 MHz and you are hearing that on 121.5?

If you have several receivers or their antennas in close proximity the local oscillator in one or more of them could be strong enough to cause mixing, birdies, intermod, etc. to other receivers.

 

[oracavon]

I've seen that happen when I have some of my scanners sitting close to each other. The local oscillator in one of them can broadcast what it's receiving (after demodulating it) to another nearby nearby scanner, which then converts it to the same audio, regardless of what it's tuned to. Try separating them a few feet and see if that eliminates the issue.

 

[Clats97]

You didn't say anything about other frequencies in the first post, just 121.5.
Are you saying there is a trunked system on 145 MHz and you are hearing that on 121.5?

If you have several receivers or their antennas in close proximity the local oscillator in one or more of them could be strong enough to cause mixing, birdies, intermod, etc. to other receivers.

Correct, trunked system on VHF 14X.xxx and the occasional transmission comes through as 121.500 AM.

If looking up the system will help you it's called "bell fleetnet zone 2"

 

[Clats97]

I've seen that happen when I have some of my scanners sitting close to each other. The local oscillator in one of them can broadcast what it's receiving (after demodulating it) to another nearby nearby scanner, which then converts it to the same audio, regardless of what it's tuned to. Try separating them a few feet and see if that eliminates the issue.

Well they're as far apart as they can be one o's on the far side of my room one is on my bedside table, not much I can do there lol

 

[oracavon]

Well they're as far apart as they can be one o's on the far side of my room one is on my bedside table, not much I can do there lol

Ah, well, in that case there's something else going on that I'm not familiar with. Sorry.

 

[Clats97]

Ah, well, in that case there's something else going on that I'm not familiar with. Sorry.

No worries! Thanks for weighing in

 

[ai8o]

Hi, I have multiple scanners and dongles running all at the same time, and I've noticed something weird that's been happening lately. I have an analog only scanner that scans the airband frequencies from the nearby airport. One of those frequencies is the guard frequency, 121.500 AM. I have noticed that sometimes when I hear audio coming through my base scanner, there's the EXACT same audio coming through the analog scanner, but it's showing that it's coming through 121.500. Thats impossible. The system I monitor on the base scanner is VHF FM, I don't understand how it could come through as airband AM.

Anyone know what might be going on?

Somebody at the tower or Approach control is accidently selecting 121.5 and the frequency they are supposed to be operating on.
At smaller airports sometimes several frequencies are are selected simultaneously, so one controller can handle several functions at once.

This is very simmilar to what happens at ARTCC when they are short staffed.
A controller may be operating two or more sectors.

 

[merlin]

I think nd5y pegged that. also possible IF from one radio, bleeding into the same IF on another radio.

 

[wtp]

is it a single frequency, or all of them ?

 

[paulears]

I read the OP that a random non air and signal received on one receiver, came through the other receiver that was monitoring an air band frequency, 121.5. He didn’t correct or explain, but I’m guessing it’s local oscillator output leaking directly into the other radio. Distance between them will prove that one. Move them apart, and see if it stops. Two radios using the same receiver design make weirdnesses fairly common. Bring back metal boxes.

 

[Ubbe]

it's showing that it's coming through 121.500. Thats impossible. The system I monitor on the base scanner is VHF FM, I don't understand how it could come through as airband AM.

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

 

[G7RUX]

If an AM receiver is receiving an FM signal but away from its filter centre frequency then the skirt of the filter can and does produce some amplitude variations in sympathy with the frequency variations; it is a “slope detector” and could quite conceivably be causing what the OP is hearing.

 

[lamarrsy]

It could very well be a classic case of the “double-IF” image in presence of a strong signal :
if the scanner IF frequency is 10,7MHz, then the original signal would be at 121.500 + (2 X 10.7) = 142.900MHz

 

[Clats97]

Somebody at the tower or Approach control is accidently selecting 121.5 and the frequency they are supposed to be operating on.
At smaller airports sometimes several frequencies are are selected simultaneously, so one controller can handle several functions at once.

This is very simmilar to what happens at ARTCC when they are short staffed.
A controller may be operating two or more sectors.

In not sure I understand this. Maybe I didn't explain myself well. I'm going to try again here.

So, I have a homepatrol 2 scanner, a 436hp, and an sds100, and a Whistler ws1010 as well as multiple SDR dongles. The homepatrol 2 is connected to a discone and monitors my province's system. Non stop traffic all day. It's called bell fleet net zone 2.

I monitor that on my homepatrol.

I also have a Whistler analog scanner running monitoring the airport.


Sometimes, I will hear a transmission from the provincial fleet net system which is mainly EMS, and it will come through 121.500 as well as through the homepatrol. The homepatrol is monitoring a smartzone VHF system in the 140mhz range.

The Whistler that picks up audio from that system is monitoring airband only.

 

[Clats97]

It could very well be a classic case of the “double-IF” image in presence of a strong signal :
if the scanner IF frequency is 10,7MHz, then the original signal would be at 121.500 + (2 X 10.7) = 142.900MHz

Can you elaborate on this? Not sure I understand