Strange Hampshire Co. SIRN site issue

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lebrunmn

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I’ve noticed a strange issue when monitoring the two SIRN sites in Hampshire County. I’m wondering if anyone has any theories about the cause…

With my external antenna, I can monitor both Hampshire County SIRN sites: Romney and Ridge—almost equally well. The Ridge site does come in slightly stronger. I routinely monitor both sites, since the traffic on each isn’t 100% identical—no surprise there. I have both sites programmed into all of my scanners as separate systems, and they share the same talkgroup scan lists.

Every now and again, when a mobile unit is in a “bad spot,” the transmission on one site might come in a bit digital-sounding. When that happens, I usually just hit the scan button, and go to the other site to hear that transmission. That solution works about 95% of the time, since the traffic on both sites is almost identical. So, with two scanners, you can usually hear identical transmissions on both the Romney and Ridge sites at the same time. Lately though, I’ve noticed that—100% of the time—when the transmission hits the Romney 460.6125 Mhz voice channel and the Ridge 460.6000 Mhz voice channel at the same time, the Romney 460.6125 Mhz voice channel seems to have some type of impairment and no voice is heard—even though the signal is strong. The Ridge 460.6000 Mhz voice channel comes across just fine.

I’ve been able to replicate the issue on all my scanners, and it’s strange that I never noticed it before—wondering if it’s something that’s just happened lately. I’ve also noticed that the voice channels aren’t exactly on frequency: the listed 460.6125 Mhz freq actually measures at 460.611875 Mhz, and the 460.6000 Mhz freq actually measures at 460.599375 Mhz. Whatever the actually freqs are, they’re both exactly 12.5 Khz apart—so I’m guessing that has to have something to do with the issue. Still, the voice channels are designated as 8K10F1E—so in theory, the max bandwidth of 8.1 Khz should not interfere with a freq that’s 12.5 Khz away. Or is my thinking incorrect? I’m just not technically savvy enough to know.

I’ve done quite a bit of research on the issue—as you can probably tell—but I’m hoping someone more knowledgeable with trunking systems and their programming may have a theory. What bothers me the most about this issue is that when it happens, I’ll often hear the first responder mention that they couldn’t copy anything. This is a small place, and I actually know some of those folks—it makes me worry for them.

For now, my work-around is simply to monitor both sites on one scanner—as I always have—but I take another digital scanner and just park it on 460.6125 Mhz, programmed as a standalone freq—not a trunked system. Whenever the trunked 460.6125 Mhz voice channel “chokes,” the standalone scanner, sitting on the identical, non-trunked freq, receives the transmission just fine.

It is a puzzlement… and you're a real trooper for reading this far.
 

mike_s104

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I'm able to monitor the Ridge site from Berkeley Co. using a yagi. I've noticed some distortion on some transmissions on that site. I'll try to catch what voice channel it is.
 

lebrunmn

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Thanks Mike--your reply gives me an idea... From my home, the Ridge site, which is slightly stronger, always comes through just fine--it's the Romney site that has the issue. But I wonder if I were nearer the Romney tower, if the Ridge voice channel would be impaired. Looks like I get to go into town this week for an experiment--whoo-hoo!
 

mike_s104

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Thanks Mike--your reply gives me an idea... From my home, the Ridge site, which is slightly stronger, always comes through just fine--it's the Romney site that has the issue. But I wonder if I were nearer the Romney tower, if the Ridge voice channel would be impaired. Looks like I get to go into town this week for an experiment--whoo-hoo!
They probably use different voice freqs.
 

lebrunmn

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Yes, you're correct, of course--different sites will use voice freqs. What I'm trying to do is see if I can get a different result by moving to another location. This is what I'm getting now--see attached--it's only when these two sites match up with these two voice channels.
Ridge-Romney issue.jpg
 

lebrunmn

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Nope. Listening to a fire call now and the problem still exists. I also haven’t had a chance to set up in another location, closer to the Romney site, to see if I can get a different result. Hope to do that in the next few weeks. Will post any results I get.
 

lebrunmn

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It was just too cold and windy to work outside today, so I decided to set up nearer to the Romney site to see if I could replicate the issue, but in reverse. It's exactly as I thought it would be:

81189

This has to be some kind of a bandwidth issue with the scanners, even though I've set up the Romney and Ridge towers as separate, NFM systems. Maybe if I attenuate the nearer site, that might help...
 

lebrunmn

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OK, so it's a crappy video, I know, but it's better than those stupid PowerPoint slides I originally posted...

Here's the strange behavior live & in color: The G5 is monitoring the Ridge site on voice channel 460.6000MHz and the PSR-500 is monitoring the Romney site on voice channel 460.6125MHz. Notice how the PSR-500 is just "stuttering" and flashing, not able to lock onto the transmission, but the G5 is receiving just fine. Both of these sites always come in 5 by 5, (and the proximity of the two radios to each other never causes interference). I should be able to hear both voice channels from both towers without any issue--like the other 99.9% of transmissions.

I understand this is probably just a scanner issue, and that radio site affiliation probably keeps most actual system users from running into this problem. But what happens to an actual user whose radio has affiliated with the other site? I'm guessing they hear nothing. Cannot help but feel that the bandwidth that separates these two voice channels has something to do with this.

Sorry about the background noise--my wife was watching TV!

 

lebrunmn

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Mystery solved--at least to my modest satisfaction. This "strange issue" is nothing more sinister than good ol' simulcast distortion. Neither my G5 nor my recently-purchased KNG-P400 exhibit this behavior. Not surprisingly, both have real I/Q demodulation, so they aren't affected by the simulcast issue.

The only strange issue now is how I keep answering my own posts. While I do enjoy talking to myself, I'm honestly hoping that if someone ever experiences the same issue, this thread may prove helpful.

And as a final aside, that KNG-P400 is one sweet radio...
 
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