Control Channel "Spikes" on BCD396XT

[Kumba]

I'm really not sure what's causing this kind of interference. I've mentioned it here before, in other threads, but actually recorded some of it this time. It's on a new BCD396XT I just bought (so no discriminator output), and I've heard the exact same thing on my older BCD396T.

I'm fairly close to one of my county's antennas, but across from my neighborhood is a large antenna with several cellular and microwave (I believe) antennas. Not 100% sure if they're the issue, as I've looked that tower up on AntennaSearch, and most of it's frequencies aren't anywhere near my county's trunked frequencies. I think the lowest frequency range I could find for one repeater/antenna on that tower is ~1GZ (but I'd have to double check), and the highest was either 6GHz or 10GHz.

So I'm at a bit of a loss as to why this area seems to be in a dead zone. Thoughts?

WAV file uploaded at:
drop.io qatui0j
(The spikes are visible in waveform mode, and sound like a hammer hitting something).

The frequency being heard is 868.2625MHz. Site ID is A42E.

 

[kilowa22]

You would have to listen to the system away from your current location to see if the audio peaks are in the TRS or caused by RFI. Does it affect the rate of reception and decoding of the trunked system?

 

[Kumba]

You would have to listen to the system away from your current location to see if the audio peaks are in the TRS or caused by RFI. Does it affect the rate of reception and decoding of the trunked system?

Oh, I've picked up the same pulses outside in my car, sitting in my driveway, with the scanner jacked in to a trunk-mounted Larsen tri-band (which theoretically, should get better reception than a rubber duckie or my expandable Maldol). My house lies within the shadow of the aforementioned tower, and I have noticed that the signal does seem to be better the further away from that tower I get (i.e., driving away in my car).

But the info from AntennaSearch & the FCC's own database suggests that isn't the cause (since it doesn't look like it's carrying anything using a frequency near what my county's radio system is using).

Managed to find a good price on a desktop discone on eBay, so I'm going to try to win that and see if it helps. If I get the interference signal in stronger, that hints at something not being right, or that tower's information is out-of-date.

 

[Thunderknight]

But the info from AntennaSearch & the FCC's own database suggests that isn't the cause (since it doesn't look like it's carrying anything using a frequency near what my county's radio system is using).

Some services are not site-license based and are instead market or geographic-area licensed and may not show up on your search. Federal users would not either.

6 and 10 GHz would be microwave...probably in dish form.

Is there any chance you know someone with a spectrum analyzer?

Since you said FCC, I'm going to assume you are in the US. The possible good news there is that the frequency you mention, 868.2625 MHz is pre-rebanded. Eventually that frequency will change due to rebanding. This may move it further from whatever is causing your problem.

If it is a strong signal from the nearby tower, a "better" local omni antenna may only make it worse. You may need a yagi (directional) antenna pointed away from the problem site to try to block some of the signal. Hopefully the desired site (I assume that is the County one) is in a different direction.

Edit: Have you tried the attenuator feature on your scanner?

 

[Kumba]

Some services are not site-license based and are instead market or geographic-area licensed and may not show up on your search. Federal users would not either.

6 and 10 GHz would be microwave...probably in dish form.

Is there any chance you know someone with a spectrum analyzer?

Since you said FCC, I'm going to assume you are in the US. The possible good news there is that the frequency you mention, 868.2625 MHz is pre-rebanded. Eventually that frequency will change due to rebanding. This may move it further from whatever is causing your problem.

If it is a strong signal from the nearby tower, a "better" local omni antenna may only make it worse. You may need a yagi (directional) antenna pointed away from the problem site to try to block some of the signal. Hopefully the desired site (I assume that is the County one) is in a different direction.

Edit: Have you tried the attenuator feature on your scanner?

Nope, don't know anyone w/ a spectrum analyzer. If the price isn't too bad, I might consider hunting one down on eBay or such.

Rebanding is definitely going to affect things, I just don't know when. I believe the system will move more towards the 850MHz range, when I last checked on things. I did discover a few new county antennas that are licensed for that range, so I assume the transition will be very soon, and maybe the newer antennas are designed to cover some dead spots in the county.

As for the attenuator, on the BCD396T, it seemed to help at times. I have a Maldol W100RX antenna on that, and the ATT plus extending the Maldol to its full ~2ft length and cocking it at an angle (requires balancing the scanner, too), usually helped quite a bit. On the BCD396XT, though, the attenuator seems to make the signal even worse. I had to cut that off and put a Diamond SRH77CA on that just to even have squelch release from time to time.

I do keep both near my main computer, but getting up and walking around my house doesn't seem to change things much. Best reception I've gotten so far is by placing the thing up on the refrigerator.

 

[slicerwizard]

Not 100% sure if they're the issue, as I've looked that tower up on AntennaSearch, and most of it's frequencies aren't anywhere near my county's trunked frequencies.

It doesn't matter. I get the exact same AGC pulsing on my 396T from nearby 800 MHz signals, even though I'm monitoring VHF traffic. You need to use an antenna that is deaf on the interfering band. You can also use polarization to kill the interference.

 

[Kumba]

It doesn't matter. I get the exact same AGC pulsing on my 396T from nearby 800 MHz signals, even though I'm monitoring VHF traffic. You need to use an antenna that is deaf on the interfering band. You can also use polarization to kill the interference.

How does one determine the interfering band? That's what I've had a hard time doing. I don't even know 100% if the source of interference is even from the tower in question.

As for polarization, how does one go about that? I'm only using portable antennas. I haven't gotten around yet to setting up an outdoor antenna (too many other things to fix in this house first).

 

[slicerwizard]

How does one determine the interfering band? That's what I've had a hard time doing. I don't even know 100% if the source of interference is even from the tower in question.

You can start by sweeping DC to daylight with your scanner minus its antenna. Anything it picks up has to be very strong.

As for polarization, how does one go about that? I'm only using portable antennas.

You can lay the scanner/antenna on its side or on an angle.

 

[Kumba]

You can start by sweeping DC to daylight with your scanner minus its antenna. Anything it picks up has to be very strong.



You can lay the scanner/antenna on its side or on an angle.

Ack, totally forgot about this thread.

The Scanner minus its antenna seems like it can still pick uo the local public safety signal. At least, I can listen to the control channel audio pretty well. But there is a more pronounced amount of "static wind" that comes across.

With the antenna, there's less, but it seems to be enough to make the decoding of the data difficult for the device. It usually registers between 1-3 bars with the antenna, and 0 bars without (even though I can hear decent control channel audio).


Laying the unit on its side sometimes seems like it affects things. But this seems really inconsistent, so I'm not for certain if polarization really matters.

 

[slicerwizard]

And I should add this important info... Check your scanners' audio AGC settings; disabling analog AGC will probably eliminate the surging audio in both of them.