I don't know if this is the right place, but I'll give it a try.
I bought a BC-796D digital scanner because Wake County, NC is going to be moving to their new system fairly soon. There are some agencies on there now, and the system seems to work well. I did notice an issue with my scanner though.
When listening to analog trunked stuff, I find that sometimes the audio is total crap. I mean lots of static to the point of the signal disappearing, even though the signal meter is full strength. I'm using a high quality 800 antenna for this thing when this happens.
when I go to the crappy scanner antenna, it all clears up. The signal strength is not nearly as strong, but the static doesn't occur NEARLY as much as with the better antenna.
I know this is a simulcast system so I guess it might have something to do with it. I'm not an expert on trunked systems, so I thought I'd seek some advice here.
BC-796D Digital Scanner
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When I am down there visiting the parents, I noticed that Durham/Raliegh/Greensboro has a high concentration of 800 radio's. Not just public safety, but everyone.
My initial feeling without being down there, is either the antenna has a high gain on it bringing in unwanted cochannel interfearence, or there is something not right with it.
With it all clearing up with the stock antenna, I would try something different. Could just be as simple as a bad antenna, connector or wire.
My initial feeling without being down there, is either the antenna has a high gain on it bringing in unwanted cochannel interfearence, or there is something not right with it.
With it all clearing up with the stock antenna, I would try something different. Could just be as simple as a bad antenna, connector or wire.
Lowband radio. The original and non-complicated wide area interoperable communications system
Edited feb 27-04
My mistake on the audio upgrade, it is for the rad shack pro-96 and not the uniden so your problem could be intermod since is is also on analog channels.
You probably need the upgrade to the digital card, you can download it from Unidens web page. Also I'd check out yahoo's BC796 group for more info. Everybody who has downloaded the audio upgrade is pretty pleased with it, without it there were numerous complaints similar to yours.
http://groups.yahoo.com/group/BC796D/
mike
My mistake on the audio upgrade, it is for the rad shack pro-96 and not the uniden so your problem could be intermod since is is also on analog channels.
You probably need the upgrade to the digital card, you can download it from Unidens web page. Also I'd check out yahoo's BC796 group for more info. Everybody who has downloaded the audio upgrade is pretty pleased with it, without it there were numerous complaints similar to yours.
http://groups.yahoo.com/group/BC796D/
mike
Last edited by mike m on Fri Feb 27, 2004 10:54 am, edited 1 time in total.
Checked the SWR on the antenna and that is good. In ref. to the digital card, that seems to be working fine. Very clear when on P25. My problem is occuring on analog signals. I went back to the lower gain glassmount today and it was much better in terms of static even though the signal was acutally worse.
Really weird. I'm going to try the good antenna with the attenuator on and see what kind of luck I have. I've heard that these 796's are succeptable to intermod.
thanks for the posts
Really weird. I'm going to try the good antenna with the attenuator on and see what kind of luck I have. I've heard that these 796's are succeptable to intermod.
thanks for the posts
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radiomidwest
- Posts: 31
- Joined: Wed Mar 19, 2003 4:45 am
796
It's most likely Iden system adjacent channel interference
This is common with scanners in metro areas - the lower gain scanner mounted antennas provide better performance than an external one. This is due to the plethora of 800mhz signals in your area flooding the front end of your scanner. A scanner being a broad band receiver doesn't have the selectivity that a commercial radio has.
Re: 796
Gee, thanks for your insightradiomidwest wrote:It's most likely Iden system adjacent channel interference
RX desense
I agree with BDB - you are probably getting desensed by other signals in the area.
If the signal of interest is of lower power than nearby signal not of interest, the interfering signal will be the dominant signal in the front end amp of the scanner, and will set the gain of that stage. The poor signal of interest will be of very low signal level.
The "signal meter" of most radios simply measures the gain of the front end of the system - the higher the gain, the lower the signal level displayed. So when this monster signal maxs the system out, you will have an indicated high level even though the signal of interest is weak.
You can check this by introducing some attenuation to the big antenna - either by partially unplugging it from the scanner, or by putting an attenuator inline with the scanner. Then adjust the attenuation and see if the signal quality improves.
If it does, then you can try to find the offending signal in your area and notch it out - you really need a spectrum analyzer for that. You can try to find the strongest signal then create a "tuned stub" by making a length of coax such that the coax is on-half wavelength (or a multiple thereof) of that signal long. Short the end of the coax, and tee it into the antenna. The stub will be a short at the tuned frequency, and will move away from being a short as you move away from the frequency. The problem is getting the selectivity - the longer the coax the narrower the frequency notch, but as length increases, so does attenuation, which reduces the depth of the notch.
Or, if you can identify the offending frequencies you can try to get a notch filter can for that frequency.
Or, if the interfering frequency is far enough away from the frequencies of interest, you might be able to buy an appropriate helical filter from the likes of Digi-key, and put that inline on the antenna to pass only the frequencies of interest.
If the signal of interest is of lower power than nearby signal not of interest, the interfering signal will be the dominant signal in the front end amp of the scanner, and will set the gain of that stage. The poor signal of interest will be of very low signal level.
The "signal meter" of most radios simply measures the gain of the front end of the system - the higher the gain, the lower the signal level displayed. So when this monster signal maxs the system out, you will have an indicated high level even though the signal of interest is weak.
You can check this by introducing some attenuation to the big antenna - either by partially unplugging it from the scanner, or by putting an attenuator inline with the scanner. Then adjust the attenuation and see if the signal quality improves.
If it does, then you can try to find the offending signal in your area and notch it out - you really need a spectrum analyzer for that. You can try to find the strongest signal then create a "tuned stub" by making a length of coax such that the coax is on-half wavelength (or a multiple thereof) of that signal long. Short the end of the coax, and tee it into the antenna. The stub will be a short at the tuned frequency, and will move away from being a short as you move away from the frequency. The problem is getting the selectivity - the longer the coax the narrower the frequency notch, but as length increases, so does attenuation, which reduces the depth of the notch.
Or, if you can identify the offending frequencies you can try to get a notch filter can for that frequency.
Or, if the interfering frequency is far enough away from the frequencies of interest, you might be able to buy an appropriate helical filter from the likes of Digi-key, and put that inline on the antenna to pass only the frequencies of interest.
This is my opinion, not Aeroflex's.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
I use the attenuator route myself - I have a Narda 0-10db step attenuator coupled with a yagi with decent rejection. I can monitor systems with a scanner reliably 20 - 30 miles away. This is with me being 1/4 mile from an iden site, 1/2 mile from a cell site, 1 mile from an AM broadcaster, and about a mile from downtown and the multitudes of transmitters there. Essentially I live in RF hell . For a handheld scanner I made a 1/4 wave helical that is just a tad larger than the connector itself. People are shocked at how well it works in a moderate to strong signal area.