417.8 Repeating or Simulcasting RFD CMD 1

[retiredFD07]

This is really weird. My previous notes for this freq are as follows:
AirCo repeating 122.975
2003 Fire storms use w/ 167.1
2003 Fire storms use, Running Springs area with 168.1

Now I'm hearing on this freq, with a PL of 97.4, what appears to be a simulcast of Riverside City FD Command 1 (460.575).
To add a little confusion to the situation, RFD CMD 1 uses a PL of 23.0, but CMD 2 (460.6) uses a PL of 97.4.

Is anyone else hearing this, and does anyone know why this freq is repeating RFD 1?

<>< Mike

[tk353]

I have been listening since 7 pm. I have not heard any traffic on 417-8000.

[CQPSK]

It seems unlikely RFD would be there since that is in the Federal band. Sounds like your scanner is picking up an image frequency. Here is some pretty good reference material on the issue.

http://www.scrrba.org/PDFs/ScannerInfo.pdf

[CQPSK]

460.575 - 417.800 = 42.775. 42.775 / 4 = 10.69375 or 10.7, so your scanner is running an intermediate frequency amplifier of 10.7 Mhz (typical in most scanners) which means you're hearing a 4th order image. I bet if you cracked open the case of your scanner and looked at your circuit board you'd find a 10.7 Mhz crystal.

This is a pretty common problem with hobby grade "scanners" that do a lot of mixing to allow for their "broad-bandedness". I still remember hearing 800 Mhz AMPS cellular calls images in the 400 Mhz range in the early 90's on my BearCat. Your "professional grade" receivers and commercial radios won't have this problem, but naturally cost a lot more.

[tk353]

If I can remember the old scanner manuals printed a list of " birdies" referring to this.

[zz0468]

460.575 - 417.800 = 42.775. 42.775 / 4 = 10.69375 or 10.7, so your scanner is running an intermediate frequency amplifier of 10.7 Mhz (typical in most scanners) which means you're hearing a 4th order image.

What would be the mechanism that causes a 4th harmonic (42.775) to get divided down to near 10.7? The math works, but frequencies don't get divided down without special hardware (divide-by-n counters, etc). There is no mechanism in the receiver to do that math function. Mixing (adding and subtracting) and multiplying, sure. But mixers don't know division.

I bet if you cracked open the case of your scanner and looked at your circuit board you'd find a 10.7 Mhz crystal.

Actually, what you'd find is a 10.7 MHz filter. Might be a crystal filter, but probably ceramic. The injection top the mixer will be 10.7 MHz off from the desired frequency, and the downconverted signal will be AT 10.7 MHz. If it's a double conversion receiver, you might find a crystal for 10.245 to mix down to a 455 KHz second IF.

Of course, not knowing what his scanner is, we have no idea what the LO injection scheme is. Some scanners are pretty wild when it comes to selecting a frequency to listen to. The synthesizers are noisy and full of spurs, and on some scanners are pretty limited in range requiring a multiplier or mixing scheme for the upper bands. And with the barn-door wide front end, the only thing keeping it from getting overrun by images is an image reject mixer, which will give you, at best, 20 db if image reduction.

This is a pretty common problem with hobby grade "scanners" that do a lot of mixing to allow for their "broad-bandedness". I still remember hearing 800 Mhz AMPS cellular calls images in the 400 Mhz range in the early 90's on my BearCat. Your "professional grade" receivers and commercial radios won't have this problem, but naturally cost a lot more.

Yep. In the end, between a wide front end, poor quality synthesizers with lots of spurs and phase noise, LO multiplier chains for the upper bands, and multiple levels of conversion, it boils down to receivers that are prone to hearing things where you'd least expect them. I'm holding out for a scanner that uses high side injection, and upconverts to the 1st IF, and puts all the images and birdies into the microwave spectrum where they won't bother anyone. But I'm not holding my breath.

[CQPSK]

Not knowing whether he has a single or dual stage mixer, it's hard to tell. I don't think his receiver is dividing down to land at 417.800 by design, but it's a harmonic image.

One thing I know for sure is that RFD is not transmitting on 417.800 Mhz, so if he's hearing it, it's got to be a harmonic or a mix. I do find the 97.4 PL very odd. Makes me wonder if 460.600 is somehow involved in the mix, being only 25 khz away and transmitting it's 97.4 pl. RFD just began simulcasting and upped their power levels as far as I could tell, so if he's close to one of their sites that could contribute to what he's hearing. CND 1 & CMD 2 are usually patched together for dispatches as well, which might be playing a role.

[cvrules90]

If I can remember the old scanner manuals printed a list of " birdies" referring to this.

I was going to suggest birdie or bleeding over.

[zz0468]

I was going to suggest birdie or bleeding over.

A "birdie" is a spurious response generated and received by the receiver itself. With complex local oscillator injection schemes, and multiple IF stages, it becomes possible for a signal generated in the radio for one purpose to leak into another stage, get converted, and wind up within the IF bandpass somewhere. Once that happens, the receiver doesn't know it from a legitimate signal, and so it passes on through the rest of the receiver unmolested by filters and things.

You can tell it's a "birdie" by the fact that it's still there when you remove the antenna.

[zz0468]

Not knowing whether he has a single or dual stage mixer, it's hard to tell. I don't think his receiver is dividing down to land at 417.800 by design, but it's a harmonic image.

It would be interesting to see what the exact LO chain and mixing scheme is being used. I just tried to look up the IF frequencies used in the BCD536HP, just for kicks, and it's not listed in the specifications. Same is true for a lot of scanners.

One thing I know for sure is that RFD is not transmitting on 417.800 Mhz, so if he's hearing it, it's got to be a harmonic or a mix. I do find the 97.4 PL very odd. Makes me wonder if 460.600 is somehow involved in the mix, being only 25 khz away and transmitting it's 97.4 pl. RFD just began simulcasting and upped their power levels as far as I could tell, so if he's close to one of their sites that could contribute to what he's hearing.

That's a possibility. Hearing a recording of the audio might be useful, because there might be additional voices buried in there that the OP isn't paying attention to.

CND 1 & CMD 2 are usually patched together for dispatches as well, which might be playing a role.

Yes. That can make tracking the source of a mix very challenging, because you may think you recognize the source frequency based on what' you're hearing, and you're not only on the wrong frequency, but the wrong band. Good times...!