Source of Intermod?

[n5pwp]

I'm about 5 miles north of a local airport. I'm monitoring a local P25 site and during the time I'm on the site I notice a pulse (~ -0.7 on display) that pops up in different locations on the spectrum. From time to time I see the control channel spike fade up and down and sometimes go away. I was wondering, since the pulse showing up seems to be a frequent occurrence, if this is the airport radar? My yagi is pointed in a southerly direction towards the airport. It is a broad pulse. Anyone else experience this and confirmed this is what the radar signal looks like on a spectrum sweep?

 

[Voyager]

What frequencies?

It could be some other electronics in your house or even the PC power supply.

 

[n5pwp]

It's around 859.9375 MHz. The spectrum shows 1.2 MHz either side of the 0 or central frequency. I guess I need to log at what times it happens and how long it occurs. I was trying to track down why the control channel signal would fade out and back in. I suppose it could be an indicator of propagation? Although the width of the interference (~400Khz) kind of caught my attention.

 

[AggieCon]

Sounds like the voice channels from the site you are monitoring or other systems and sites.

 

[n5pwp]

It seems to show up when the Yagi is pointed towards the airport. I have been monitoring the Spring simulcast site all day and nary a ripple has shown up.

Sent from my SHIELD Tablet K1 using Tapatalk

 

[AggieCon]

What does it look like in SDR#?

Post some pictures or video if you can.

 

[n5pwp]

Well, I'm not seeing it today. Radar should still be there if that was what was doing it. I see some weak digital P25 signals (NAC: 14) But I see no 'blobs' popping up. Maybe it was something coming in from a distance away. Next time I see it on the OP25 machine I'll fire up GQRX and see if I can see it on there.

 

[RFI-EMI-GUY]

Making a guess that it could be the Secondary surveillance radar on 1030 MHz, I did a backwards intermod calculation and indeed a frequency of 1031.775 MHz "might" create an intermod hit 750 KHz below 859.9375 MHz. So if you have the means to tune in the SSR, you might see if it coincides with the IM hit. More likely it is not, because the interrogator frequency tolerance is usually 1030 +/- 200 KHz. Otherwise, there are plenty of in band IM 850 MHz possibilities that could be more likely. I would do a scan of the 800 MHz band and identify any strong emitters that match the duty cycle of your IM hit.