Strange 24 hr drops

In the past week, I have seen my reception of a control channel drop to nearly nothing four times just after 1am. In three of the four (the fourth still in progress), the control channel went back to normal nearly exactly 24 hours later.

Alma 1am Mar 21 - 1 am Mar 22
Alma 1am Mar 24 - 1 am Mar 25
Wykoff 1am Mar 26 - 1am Mar 27 (not as low as Alma, 18:00 - 20:00 were relatively good at just over 50%)
Alma 1am Mar 27 - ???

Has anybody else noticed this? What could be causing it?
1) Weather. (seems very unlikely to begin and end at 1am repeatedly)
2) Switching to alternate control channel that I am not handling. (maybe?)
3) Decreased signal power. (why?)
4) Increased interference. (why?)
5) Something else.

With better logging, I should be able to tell the difference between #3 and #4, I think.

johnmoe1 said:

In the past week, I have seen my reception of a control channel drop to nearly nothing four times just after 1am. In three of the four (the fourth still in progress), the control channel went back to normal nearly exactly 24 hours later.

Alma 1am Mar 21 - 1 am Mar 22
Alma 1am Mar 24 - 1 am Mar 25
Wykoff 1am Mar 26 - 1am Mar 27 (not as low as Alma, 18:00 - 20:00 were relatively good at just over 50%)
Alma 1am Mar 27 - ???

Has anybody else noticed this? What could be causing it?
1) Weather. (seems very unlikely to begin and end at 1am repeatedly)
2) Switching to alternate control channel that I am not handling. (maybe?)
3) Decreased signal power. (why?)
4) Increased interference. (why?)
5) Something else.

With better logging, I should be able to tell the difference between #3 and #4, I think.

Click to expand...

Did the site revert to an alternate control channel?

Sent from my SM-G930V using Tapatalk

dwgelle said:

Did the site revert to an alternate control channel?

Click to expand...

My guess is no. Adjacent sites are still saying Alma is there at 857.9625 and I seem to still be able to mostly find the frame sync/NID that starts each packet.

I guess I posted too soon. I am getting Alma again.

On many trunked systems is normal to roll the control channels every 24 hours. Seems like 1am was/is a popular setting. The old SmartNet system I had at work was originally set up by Motorola to roll at 1am every morning. This spreads the load out on the repeaters/PA's.

I suspect that is what is happening. Could very likely be that the source of your programming information isn't listing all the possible control channels. For this to be happening at 1am consistently really suggests that this is the cause.

I have never noticed our MN Armer system roll their control channels. I have seen them on their designated alternate CC for short times or have been replaced for some reason. I'm thinking there may have been a site update during that time. Just a thought.

4/1 is freeze date for any changes prior to the next system update taking place 2nd half of April so could have been updates or sone type of system work. Since the last system update I have not seen CC changes that often. I monitor Hennepin 2-001 and they used to change off the main control fairly frequently compared to other sites.

I have seen deep signal drops up north here but id say most were weather related... Anywhere from minutes to maybe a day long at most... Nothing with the timing you have seen though...

Looks to me like there is a pattern - every third day at 1:00 AM for Alma. I'd be guessing at some type of scheduled "thing", possibly maintenance.

I drove to Lake City (on Mar 30) and confirmed that Alma was broadcasting on the normal control channel as expected (so no to the alternate control channel theory). One of these days when the signal looks good in Rochester, the weather is good and I have nothing else to do, I will repeat the trip and compare signal strengths and where I lose the signal.

It also skipped a day on the every three day Rochester outage schedule (roughly % of control channel messages decoded per hour):

(I don't expect that they intend for the Alma site to be available in Rochester, but I still find it interesting that something changes at 1am.)

Could be the site controllers rolling over especially if it is for a short period of time and on a regular basis like that pattern is showing. For an example on the WISCOM system the TSNI's change every 24 hours from A to B to A etc.. Based on the traffic of your site it might be at 1am or it might be at midnight or it could even be a completely different time then that one listed. I know of a couple of trunked systems I have worked on in the past that based on system usage might roll at 2am 3am 4am and even a few during the day.

n9upc said:

Could be the site controllers rolling over especially if it is for a short period of time and on a regular basis like that pattern is showing. For an example on the WISCOM system the TSNI's change every 24 hours from A to B to A etc.. Based on the traffic of your site it might be at 1am or it might be at midnight or it could even be a completely different time then that one listed. I know of a couple of trunked systems I have worked on in the past that based on system usage might roll at 2am 3am 4am and even a few during the day.

Click to expand...

TSNI?

I've got it half figured out. The thing that changes every morning at 1am is: Mayo Foundation - Clinic Trunking System, Rochester, Minnesota - Scanner Frequencies . When it is transmitting on 857.73750, I can't get Alma on 857.9625. Somehow, I am getting "echoes" of signals shifted in frequency, and one of the echoes from the Mayo system happens to line up with Alma. I'm not exactly sure yet what I changed that made these echoes start appearing or how to make them go away...

johnmoe1 said:

I've got it half figured out. The thing that changes every morning at 1am is: Mayo Foundation - Clinic Trunking System, Rochester, Minnesota - Scanner Frequencies . When it is transmitting on 857.73750, I can't get Alma on 857.9625. Somehow, I am getting "echoes" of signals shifted in frequency, and one of the echoes from the Mayo system happens to line up with Alma. I'm not exactly sure yet what I changed that made these echoes start appearing or how to make them go away...

Click to expand...

If I follow what you're saying, what you're seeing is an "image." It's a function of superheterodyne receivers, including SDR where the baseband is shifted up through the "mixing" of a local oscillator's signal. The words in quotes are specific terms, you can google them.

Suggest using a different baseband frequency in your SDR, that also should shift the image away - but it may bring other images into play as well.

sixtytwo said:

If I follow what you're saying, what you're seeing is an "image." It's a function of superheterodyne receivers, including SDR where the baseband is shifted up through the "mixing" of a local oscillator's signal. The words in quotes are specific terms, you can google them.

Suggest using a different baseband frequency in your SDR, that also should shift the image away - but it may bring other images into play as well.

Click to expand...

I thought/think image refers to the negative frequency showing up (for example) because of i/q imbalance. I am seeing something like 12 copies (see picture). In at least some cases, the solution seems to be to remove a certain ferrite bead from the board: https://nuand.com/forums/viewtopic.php?f=4&t=3793&start=10#p6751 . I am going to try messing around with settings before I resort to a soldering iron.

My guess is sixtytwo is on point.

You spectrum analyzer/receiver is likely a Super heterodyne receiver. On test equipment, there is a switch that will change the oscillator frequency from high/low side injection. When you are watching the offending "carrier, toggle the switch. If the carrier stays on the display at the same frequency and signal strength, it is a legit signal. If it moves and/or changes in strength, it is a false carrier generated/mixed internally within receiver.(IE Image)

I & Q is a baseband thing, what you've described (and I've seen it on my SDR as well) is in the RF realm.

SDR does use superheterodyne techniques - no ADC can handle the super ridiculous sampling rate involved to capture a 800 MHz signal. Well, nothing commercially available anyways - we can rule out space and military gear...

Here's an overly quick and simplified description, using really round numbers.

Let's say you have a signal at 100 MHz you want to hear. You need to step that down to a lower frequency for further amplification. Let's say 10 MHz. You do that by mixing that 100 MHz signal with a locally-generated signal of either 90 or 110 MHz with an appropriately named device called a mixer. While the mixer actually multiplies the signals together, effectively what it does is step the desired frequency down (or up) by the difference between the signals.

The mixer gives you the signal that you want at 10 MHz (100 - 90), but it also will give you other signals as well, such as 110 MHz and many others as well.

Why? Well a common reason is that our local oscillator isn't just producing a single frequency, but all sorts of harmonics and sub-harmonics, so when those frequencies hit the mixer, you get more than what you expected.

As I mentioned before, SDR does use the superheterodyne approach, so you may have success by altering the Local Oscillator (LO) frequency, and/or sample rate. Some combinator of those two will likely move the interfering Mayo system to some other place where it's no longer an issue. Or, you may create new problems. Normally images in RF aren't a big issue since you'll use really narrow filters for both RF and IF, but with SDR those are very specifically not used. Hence, images.

Such is engineering...