Excessive sensitivity / IMD prone?

[fog]

I just picked up a BCD536HP. I was really impressed at first; it appears quite sensitive and really pulls in the weak signals. It's plugged into a cheap discone I threw together in my attic. It sits beside a Yaesu ham rig with a high-gain vertical, and improbably pulls in weak signals better than that.

However, I'm beginning to think this is a bad thing. I've been playing with the 'Discover' feature and am finding signals all over the place that are plainly at the wrong place. In the 2-meter ham band, there's a mashup of a paging transmitter and NOAA weather radio (which is at least 20 miles away); clearly intermod. I've found my local EMS provider on at least 5 UHF frequencies in a 1 MHz span; they're only licensed for one of them, and none of my radios can hear the other 4. The EMS frequency is 5 Watts out and the transmitter is several miles away.

I live in the suburbs, in a bit of a valley. And I don't live near a radio tower or anything. No other radio I've owned has had this problem here. Curious if this is a known issue/defect, or if I'm just unlucky? I just realized it has an attenuator I can switch in, which I'll try, but it seems silly that it'd be necessary to achieve normal operation in what's not really a high-RF environment.

 

[jonwienke]

Actually the x36 receiver is one of the better ones for sensitivity and filtering out intermod and adjacent channel interference, so what you're hearing is probably real. All of it.

It's common for EMS stuff to be repeated on other systems for interoperability purposes. For example, PEMA (Pennsylvania Emergency Management Agency) repeats EMS stuff from West Virginia's statewide P25 SIRN system on an analog FM channel. And I hear some of the same EMS traffic on the Franklin County P25 digital system and Washington County analog freqs. Neither of those could possibly be intermod or images, because one channel is analog FM and the other is P25 digital.

The "pager tower" you're hearing in the ham band could be a D-STAR or some other digital repeater using a format the scanner doesn't recognize, or it could be a SKYWARN repeater rebroadcasting a NOAA weather bulletin. Some repeaters can mix digital and analog modes as well.

Bottom line is, don't assume the radio is faulty just because the signal doesn't make sense at first blush.

 

[dlwtrunked]

I just picked up a BCD536HP....
I live in the suburbs, in a bit of a valley. And I don't live near a radio tower or anything. No other radio I've owned has had this problem here. Curious if this is a known issue/defect, or if I'm just unlucky? I just realized it has an attenuator I can switch in, which I'll try, but it seems silly that it'd be necessary to achieve normal operation in what's not really a high-RF environment.

It is clear that you do live in a "high-RF environment" but do not know where the culprit transmitter it. You should do the following:

1. Make sure the NOAA transmitter is where you think it is. Use coordinates or exact locations from a NOAA site--not the "nearby big city".
2. Make sure there is no FM broadcaster near you. Some of the translator stations are often ignored and not known to most people. Even if low power, they can be close to you with a non-easily noted antenna. But you should be using an FM broadcast band block anyway--ALL scanner owners, unless they really live at least 10 miles from all FM broadcast stations, should even if they think they do not have a problem. I have seen FM broadcast stations cause trouble at 5 miles when they had a directional signal.
3. And very likely the problem: There could be a paging transmitter near you on VHF high band (152 MHz region). Antennas for these can be on a short tower, water towers, or even the roof of a house. If that is the case, a block for those can be bought from PAR.

 

[jonwienke]

Not so fast, there.

I live near a pager tower (less than a mile away) that spams the entire VHF band on my CCR mobile radio, but it has little noticeable effect on reception with my 436, even picking up MURS and other transmission that are within 1-2MHz of the pager TX frequency, while using an ST-2 outdoor antenna. I get 160MHz CC hits from Virginia (80+ miles away) occasionally.

Also, that wouldn't explain most of the OP's issues anyway. He's obviously not having desense issues, because his 536 is picking up weaker signals than his Yaesu. And if a pager tower was the problem, he would be hearing pager noise mixed with his weak signals.

Most of what the OP is describing is interop repeats--one system repeating traffic from a neighboring system. It's very common with EMS traffic.

 

[fog]

The "pager tower" you're hearing in the ham band could be a D-STAR or some other digital repeater using a format the scanner doesn't recognize, or it could be a SKYWARN repeater rebroadcasting a NOAA weather bulletin. Some repeaters can mix digital and analog modes as well.

I misspoke slightly and conflated two things.

I haven't figured out the paging transmitter; it's occasionally at 144.??? MHz. The audio is unmistakably POCSAG. I suppose I can't rule out that there's a ham paging transmitter, but I suspect it's the 152.6 paging transmitter coming through. It's infrequent enough that I haven't been able to listen to it and confirm that it's the same signal.

The other signal that pops up often is 145.885, and it is my local fire department (154.205) and NOAA weather radio (162.525) mixed together. 145.885 isn't a valid channel with the 15 kHz spacing scheme here, but 2x 154.205 - 162.525 = 145.885, leaving me pretty sure I'm hearing a third-order intermod product. I measure 1.7 miles to the fire department transmitter, so it's indeed quite strong. The NOAA transmitter is 34 miles away. My other radios, including ones with a higher-gain antenna, do not hear this product.

I initially assumed the other EMS frequencies were valid. They do multicast on VHF and UHF, so it wouldn't be shocking if they had another UHF frequency. But they probably wouldn't transmit their EMS dispatches on the neighboring town's DPW frequency. Something still doesn't make sense, because the 'real' signal is a 5 Watt repeater which is scratchy on an HT here, so it's odd that it's popping up all over the band. Other radios don't hear the other frequencies, so I still think it's some sort of phantom signal, but I don't quite understand what is happening with this one.

The FM block is a good idea. I'll also check out the PAR filter. But I'm still a bit dubious about the fact that no other radio I own has these problems. (I also got the scanner for $380 from Amazon; the price jumped back to $500 the next day. I'm slightly skeptical that someone was cleaning out a lot of bad ones or something.)

 

[Ubbe]

I hope you're not using your yeasu rig while discovering. When a strong signal enters the antenna it acts as an oscillator and mixes with all kinds of signals, even weak ones, so they multiply on other frequencies.

You have tried your discone with other scanners so there isn't a fault with the antenna or coax?

When you hear a NOAA signal on the wrong frequency and you lock the scanner on that channel, is the signal there for ever or only occasionally? The offending transmitter should be in sync with the wrong NOAA frequency and could possible be found with another scanner, as it must be a strong signal to have that effect.

/Ubbe

 

[SCPD]

Actually the x36 receiver is one of the better ones for sensitivity and filtering out intermod and adjacent channel interference, so what you're hearing is probably real. All of it.

That is funny ... my 536HP is one of the worst receivers that I own. However .. to be fair my Whistler 1095 is about the same.

In comparison .. my Kenwood V71A, Icom R7000 and Pro 2001 scanner make the 536HP look like junk. Even my Homepatrol 1 is better than the 536HP in this regard.

 

[fog]

I hope you're not using your yeasu rig while discovering. When a strong signal enters the antenna it acts as an oscillator and mixes with all kinds of signals, even weak ones, so they multiply on other frequencies.

You have tried your discone with other scanners so there isn't a fault with the antenna or coax?

When you hear a NOAA signal on the wrong frequency and you lock the scanner on that channel, is the signal there for ever or only occasionally? The offending transmitter should be in sync with the wrong NOAA frequency and could possible be found with another scanner, as it must be a strong signal to have that effect.

/Ubbe

Huh. I actually hadn't thought to try swapping the antennas. To my surprise, things like other NOAA stations (using them as an always-on signal to check strength) tend to be about 2 S-units stronger on the little discone than the antenna with 6 dBi gain on 2 meters. Wasn't expecting that. Not sure why yet -- whether it's traps that hurt out-of-band performance or if something's wrong with the setup. The intermod problems didn't immediately follow, but they're intermittent enough that it'll take me a bit to rule it out.

To your other questions -- no, I'm not transmitting while experiencing these problems. I gave a more detailed explanation of the NOAA signal in another comment, but I figured out that it's the product of my local FD and the NWS transmitter. The NWS transmitter is obviously always on, but the intermod only happens when the fire department repeater is transmitting. Neither's right on top of me, though.

That said, my goal in this post wasn't necessarily to sort out my intermod problems (though the replies have been most helpful in that regard!), as much as to see if others have had this problem. Seems like it's not a common problem based on the replies.

 

[Ubbe]

Get a variable attenuater between scanner and coax and see what happens with a little attenuation, with both antennas. Maybe you only need to attenuate a few dBs to get back a clean signal. The attenuator can be found on the net or in stores selling tv/sat equippment and costs something like $10 but you'll also need suitable connectors or adapters.

I have a 1-6 splitter to my scanners and when I crank up the signal the TRX-2 is the first to croak.

/Ubbe