Strange Interference

[emsflyer84]

Hey guys, so I hooked up a new folded dipole antenna on my roof to replace a home-made pvc all band antenna that worked fine but the range wasn’t great. The range on the folded dipole is better.

I’ve noticed something strange now though. On frequencies that are coming from further away and are a bit scratchy, there is a strange interference that I never had before. It sounds like a two-tone beep, high then low, about a second long, always accompanied by something that sounds like helicopter rotors spinning in the background. If the transmission is long enough, this cycle repeats itself. I can always hear the voices over these sounds. This only happens on analog VHF freq’s, not digital.

Here is the set up: Whistler TRX-1, 50’ RG6 to Centerfire folded dipole with 300 to 75 Ohm transformer. The antenna is mounted on the same pole on the roof as a wireless weather station.

Maybe the weather station is the culprit? But it never happened with the old antenna, and now it only happens on frequencies that seem far away, and only analog.

Any thoughts? Not a big deal, but the interference is strange, and I’d like to track it down if possible. Thanks!

 

[jim202]

Could possibly be as scada transmission. Problem is trying to track it down.

This type of transmission it used to control water systems water tank levels, power station control switching, water level info on rivers and many other uses where data info needs to be monitored.

 

[emsflyer84]

Thanks. When nothing is being received on my scanner, I don't hear anything, no interference. Not sure if that matters for your theory.

 

[vagrant]

Your description reminds me of pager signals around 152 MHz. Does it sound like the audio on this video? If so, one can purchase a filter to subdue that.

 

[bob550]

Thanks. When nothing is being received on my scanner, I don't hear anything, no interference. Not sure if that matters for your theory.

Did you open the squelch when nothing was being received? As @vagrant just posted, it does resemble a pager tone. They're known to bleed all over the VHF band if the signal is particularly strong.

 

[emsflyer84]

That’s it! It sounds very much the same. I don’t know enough about this stuff. Why would pager data be transmitted every time someone transmitted on a freq?

 

[Mike_G_D]

That’s it! It sounds very much the same. I don’t know enough about this stuff. Why would pager data be transmitted every time someone transmitted on a freq?

It's not like that, it just seems that way. Pager transmissions of this type are extremely powerfull and happen frequently. They, like other narrowband signals do have their own set frequencies but when they broadcast they can cause issues in receivers that are either very close to them or have mediocre receiver designs or some combination of both.

Without going into detail, consumer level receivers like scanners have notoriously weak designs in terms of rejecting strong in-band undesired signals. The GRE designs that the Whistlers use are especially notorious for being affected thusly. In many cases you won't hear the pager signals until an analog transmission is in progress and then you hear the pager junk being mixed in with the desired audio. Pretty common with GRE design-based receivers like the Whistlers. You can get special filters to notch out the pager interference if you know the exact frequency of the pager transmitter being used. That's usually not hard to find due to the strength and frequency of the transmissions. They are usually in the 152 and 158MHz ranges on the VHF band. Once you locate the exact frequency you can purchase a notch filter to help reduce the offending signal. PAR Electronics is a good source for these: PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts.

Cheaper variable notch filters available from Scanner Master can also work but they are broader and take some very patient fine tuning to adjust and it looks like the one I have used in the past is no longer available but you might find it used somewhere: Jim 96XI-1 Adjustable Notch Filter | Scanner Master.

Digital signals are just as badly affected but due to their nature you just don't hear the interference but it is still there. If bad enough the signal will just not demodulate ("mute") but you won't know why whereas with analog signals, assuming no CTCSS or DCS squelch, you may actually hear the offending interference.

-Mike

 

[emsflyer84]

Thanks for the explanation. How would you recommend tracking down that particular frequency of the pager transmission? I seem to be hearing the noise on several frequencies that I'm receiving on. If I'm getting this correct, there should be one frequency that, when I tune to it, there will be nothing but pager noise? Thanks again.

 

[Mike_G_D]

Thanks for the explanation. How would you recommend tracking down that particular frequency of the pager transmission? I seem to be hearing the noise on several frequencies that I'm receiving on. If I'm getting this correct, there should be one frequency that, when I tune to it, there will be nothing but pager noise? Thanks again.

As I wrote, look around 152 and 158 MHz. That's where they usually are in the VHF-HI band. Also, as I wrote, these are very strong signals that overwhelm receivers of this class, low cost consumer level gear. GRE designs, as the Whistler is based on, are notoriously bad at rejecting strong in-band undesired signals. If you want to really understand it, study up on superheterodyne reciever design and inherent potential interference issues such as internal receiver intermodulation and IF image interference.

It only seems like the signal is "everywhere" but, in reality, that is just the nature of the cost saving compromises that were made in the design and construction of your receiver.

-Mike

 

[Ubbe]

My Whistler TRX-2 (and probably all GRE scanners) has one internal filter that covers the whole 75-200Mhz range and my local pocsag pager transmitter at 169MHz, that isn't particulary strong, can be heard on top of all transmissions in that frequency band. I've never heard that type of interference, that has an impact on all frequencies in VHF, on any other scanner brand or 2-way receiver or SDR dongle so it's pretty much a genuine GRE problem in the frontend of the scanner.

/Ubbe

 

[emsflyer84]

Thanks for all the info guys. Are there other scanners out there that do a better job at filtering? Just curious at this point. Thanks.

 

[kruser]

Thanks for all the info guys. Are there other scanners out there that do a better job at filtering? Just curious at this point. Thanks.

While it's still possible, pretty much all the newer Uniden models that are still being made will handle this type of problem as will several of their older models.
I find this to be more of an issue with GRE designed scanners including those sold Under the Whistler and RadioShack brand names.
The Uniden's seem to be mostly immune to this problem.
I can't speak for Uniden's newest SDS models. The front end design in them is 8 MHz wide with very poor filtering. Uniden added some form of filtering after user complaints but I find them a very poor choice for VHF scanning if you are in an area with high power paging transmitters as I am.

GRE's design is known to have poor filtering of the FM broadcast band and an overly sensitive (as if a pre-amp is always turned on) front end. Either of these problems or both combined can make for the problem you see.

To find your offending signal, try searching between 152 to 153 and 158 to 159.
Most common VHF paging will be in those ranges. There are other paging signals as well used privately but they are usually not a problem as they don't transmit nearly 24x7 as most consumer paging transmitters do.
If you have a second scanner, leave one sitting on one of the VHF frequencies you hear the interference on and then use the other for searching.
That can make it easy to make sure you found the culprit on your search radio.
You should be able to tell when you find the strongest paging signals that are probably the cause of your problem, It will be a very strong signal.
A good test may be to try using an indoor antenna when searching for the offender. The paging signals are usually plenty strong enough to be heard with just the indoor antenna that came with the scanner.

You also don't need to get down to the exact frequency of the paging signal, just figure out if you hear it between 152-153 or 158-159. The paging filters sold by PAR typically notch out the entire 1 MHz wide swath of frequencies for 152 or 158. If you have bad luck, you may find strong paging signals in both ranges but I've noticed lately that most paging services are now just using one of the ranges in a given area. Possibly to free up 1 MHz for others to use for two way services we can only hope. You only need to 152 or the 158 MHz filter and not both unless you do find strong paging signals in both ranges.

My area here had tons of paging in both VHF ranges a few years back.
As paging demand started dropping off big time being replaced by cell phones and text messaging, paging companies started shutting down paging sites.
As they shut them down here, we had only a handful left in 152 and the 158 range.
They consolidated what was left and placed them all in the 152 range. The 158 is now free of paging signals here but I can still faintly h
ear 158 paging from distant cities.

Good Luck!

 

[emsflyer84]

Great info! I actually have a second TRX-1 with a Diamond RH77CA, maybe I’ll play around with it in those frequency ranges snd see what I can find.

Again, I only hear this interference when I’m getting signals that are weak to begin with. Fir example, if I’m listening to one of our regional fire dispatch centers, dispatch could be clear as day, but the mobile units which are faint to begin with, come through with the interference on the same frequency.

What are these commercial paging systems used for?

 

[kruser]

What are these commercial paging systems used for?

Back in the popular paging days, pagers were carried by everyone but they started out more for the medical industry for doctors and such. Then they became "a thing" and were carried by teenagers and then a preferred use by drug dealers.

Today, it's kind of like the old days and they are used very heavily by the medical industry again. Doctors and all sorts of uses in hospitals.
Call phones pretty much wiped out the paging industry except for the select few.

They are also used by a lot of maintenance and network technicians responsible for watching building control and automation systems for proper operation and alarms that may be broadcast.
Some users are not allowed to have cell phones inside secure areas but they are allowed the use of pagers (1-way) so they can do their jobs.
I think the health industry is probably the largest user base of paging these days with automated readings and alarms of plant and building control systems being the next highest use of paging.
Law Enforcement is also a fairly large user. Fire also but a lot of fire uses pagers on their own licensed frequencies.

I'd say in the past 8 years or so, paging use has dropped over 1,000 percent and has now leveled out at what will probably be a normal level for several years to support those that still rely upon them.
Encryption is something that's also available with commercial paging today. I'd imagine it would be mostly used in health care and probably law enforcement.

In my area, we had 8 to 10 VHF paging transmitters in the area. All were on the air most of the time.
Today, we have two VHF transmitters running in this area. They both run a fairly high duty cycle but one on 152.240 is on the air about 80% of the available minutes on any given day. That alone is plenty to annoy the heck out of a scanner user that has issues like you are hearing!

 

[emsflyer84]

Thanks! With a hospital and nursing home not far from my house, I’m not surprised I am hearing this.

 

[tvengr]

Great info! I actually have a second TRX-1 with a Diamond RH77CA, maybe I’ll play around with it in those frequency ranges snd see what I can find.

When everything was on VHF, I had a paging transmitter getting into every scanner in our newsroom. I had to insert a notch filter tuned to the offending frequency in the antenna line to eliminate the interference. The interference you describe sounds like a paging transmitter. Check around 152 MHz.

 

[vagrant]

I figured it might be that as I am familiar with the pager noise on 152 MHz. It affects me significantly. I run a 152 filter on my main receive antenna at home, one in the vehicle and another for my VHF/UHF amateur radio. I use various PAR brand pager freq filters that I previously linked. Dale will tune it to the frequency you need for maximum results. Thus, it would be wise to identify the exact frequency before ordering. You will probably need to use a rubber ducky antenna and even turn on attenuation so you can really narrow it down. Alternatively, an inexpensive RTL-SDR V3 dongle and a free program like SDR sharp will allow you to view signals on your computer and really narrow it down to the exact frequency. I have several paging transmitters nearby, so identifying the exact one was critical.

For my location, it is so bad that when it transmits, I can see it across the noise floor on VHF. The whole noise floor span shifts when it transmits. I also see it up in UHF as well. I am using an SDRplay RSP2 device with a 10 MHz span. With the filter inline the signal is about -54 dBm. It was around -20 dBm without (That level is significant). Right now one is working well enough for me. I am still considering adding another with a proper jumper between the two in order to really crush it.

Read this for more background on the issues you're experiencing and why.

 

[emsflyer84]

Thanks, I’ll work on narrowing down the frequency tomorrow and talking with PAR.

 

[Ubbe]

With the filter inline the signal is about -54 dBm. It was around -20 dBm without (That level is significant).

With that kind of level you have to be careful to not let the signal travel on cables and into your receivers. Even the coax might need to be decoupled by ferrite cores or it will work as a receive antenna that then radiates directly into your receivers. But you probably already have considered those things.

/Ubbe

 

[emsflyer84]

Ok guys, I think I found the offending freq using a spectrum sweep. The scanner stopped on 152.6000 and it was a constant broadcast of that data noise. That’s the only one I could find. On the screen it was showing FM while receiving this pager noise. What does that mean?

Would the next step be to email PAR and see if they can do a band pass filter for that particular frequency?

I really appreciate all the help on this guys.