What is this that I'm hearing? How can find its source?

[hgw_xx7]

So I got this Radioshack PRO-404 scanner because I am a total noob at radio stuff and I wanted something to play with. I found this signal on 437.425 MHz, it's constantly going at all hours of the day and it's not coming from my apartment, I walked to a market about a mile away and I could hear it almost all the way there.

Here's a recording:
YouTube - ‪Weird RFI on 437.425 MHz‬‏

It doesn't sound like data because it's so repetitive. It sounds like someone recording a machine but why would anyone do that? What the heck is this?

I started looking into radio direction finding and I found some instructions on how to make a direction finding antenna. This one (http://theleggios.net/wb2hol/projects/rdf/tape_bm.htm), specifically, looks like it would do what I want. It looks like it would be easy to build, but other pages seem to suggest that this type of antenna only works well on certain frequencies. All the terminology on the page is unfamiliar to me, so before I spend 1000 hours on wikipedia (I can't find any good central source of information about this stuff), could someone tell me if this antenna would work or if there's a better solution?

edit: I'm in San Francisco, if that helps at all.

 

[W8RMH]

It is a control channel, probably a ghost transmission from the San Mateo UHF system, 482.4250.
Tune that frequency and see if you can hear it. It should be stronger too.
(437.4250 is in the amateur radio 70 cm band).

 

[instrumentphilip]

That definitely sounds like a control channel / frequency for a Trunked System. If I would have to guess I would guess a Motorola trunked system. Google "trunked system control frequency"

 

[ST-Bob]

The audio on the youtube video is pretty low but it does sound like a control channel. I suspect, as mentioned previously, that it's a "ghost" of a nearby UHF trunked system being picked up at some multiple of the scanner's IF frequency offset from its actual frequency. Strong signals from nearby frequencies are often picked up as intermodulation products with the local IF. It could be either above or below the frequency you're hearing it on by some multiple of 10.7 MHz (or whatever IF frequency the PRO-404 uses.

 

[hgw_xx7]

Thanks! I played around with my scanner and I found the same thing on 416.025, 417.275, 438.675, 482.312, and 489.450. Nothing on 482.4250 though.

 

[krokus]

Wirelessly posted (BlackBerry8530/5.0.0.973 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/105)

I agree with the control channel likelihood. I would guess that it is 21.4 MHz from the frequency you have dialed in, and is being received as an image.

 

[nanZor]

The 404 is a pretty standard dual-conversion receiver, with the first IF at 10.7 and a 2nd IF at 455khz. It was kind of a backwards step to the 80's and 90's.

What you are hearing are "images" of that 1st IF. Multiply 10.7 * 2 = 21.4. Add that to 437.425 and you'll come up with 458.825. I'll bet that the REAL frequency you are hearing is near 458.825.

When monitoring the VHF airband on the 404, you can sometimes hear an image in the amateur 2-meter band, although you won't be able to make out any voices since the modulation is different - FM for the amateur band, and AM for the airband.

While the 404 has some pretty amazing audio for it's size, (which could be further improved with some high-end rolloff), the image rejection isn't the greatest with the older dual-conversion at 10.7 mhz IF, and makes it susceptible to this issue - just like many of the older scanners.

Filters can help here, but being a handheld, it isn't pretty.

Still, if the image problem isn't too bad, the 404 has probably the best audio out there - if only they would roll it off at the high end. BUT, this is one scanner that can actually be HEARD in an emergency!

I played around with my scanner and I found the same thing on 416.025, 417.275, 438.675, 482.312, and 489.450. Nothing on 482.4250 though.

Oops. I just read this and forgot if the 404 uses upconvert or downconvert on UHF. So either 416.025 or 437.425 is the real frequency. ST-bob is right - check either 21.4 mhz above or below on uhf - I forgot which direction for conversion they are using on uhf on the 404....