Is this overloading?

[DMBFireman]

I just moved into a new house and I bought 75 ft. of LMR-400 cable and hooked it up to an antenna temporarily up on the roof to test out my reception. As I figured I was able to receive some frequencies that I couldn't at my old apartment and it's working great except for on outlying 800 MHz trunked systems. I understand with this low loss cable you can get too good of a signal and it will overload the scanner however I thought that the overloading would most likely be from nearby systems. The problem I am having is poor reception of systems that are miles away from me...like 20 miles or more. The two 800 MHz systems that are closest to me come in fine. The other farther away systems I can't hear unless I turn the Attenuator on. I'm just confused because I figured if it was overloading it would happen on the closest trunked systems too since their signal strength is the strongest (confirmed on my signal meter on the BCT-15). I also noticed that less than half a mile from me (I can see it from my front yard) is a cellular tower...could that be causing the issue? Is there something besides attenuation I can do to fix the reception of outlying trunked systems? The other freqs I listened to in the Air band, 150MHz range, and 450 MHz range all appear fine...it's the 800 MHz freqs that are trouble and only for systems distant from my location. Thanks for the help!

Brandon

 

[zguy1243]

It sounds like the scanner is getting overloaded to me in the 800Mhz range. Here are some things you can do for the 800Mhz listening:

Provided the more distant 800Mhz system that you like to listen to is not in the same direction as the cell tower you can use yagi style antenna and possibly null the high power trash from the tower out by pointing in another direction. A antenna with a good F/B ratio is needed. Of course this would require using two antennas, one for 800Mhz and your other antenna for other bands of interest. You can use a simple antenna selector to choose between the antennas if this works.

Most scanners will have problems with weak signals in the presence of strong signals due to the lack of selectivity of the scanner. You would probably not have this issue with a higher end receiver such as a Icom.

I would do some scanning in the 800Mhz range with antenna off the radio. This will tell you what the super strong offending signal is or get you close to it. Most of the stuff spewing from the cell tower will be very broad banded so the scanner may have issue stopping on the signals in a search. You have luck by tuning around with the squelch off with VFO and see what you can find.

Get us some more info and we can go further ....

Jody

 

[W2NJS]

800 mHz trunked systems are usually closely engineered to cover only their intended areas and run at lower power than the old analog systems that covered the same areas, and as a result you'll find many trunked digital systems cut off their coverage when you get into the areas that they are NOT designed to serve. I've experienced this myself with a TRS in the next county to me where my receiver was virtually useless (constant packet loss) on an inside antenna but works pretty with the roof antenna. Again, I'm NOT in the intended service area and the closest transmitter is probably a couple of miles from me. Remember, too, that when you take a directional transmitting antenna with a lot of gain in one direction you can end up with almost nothing in the other direction, and if you're in the "other" direction you won't hear much of anything. Your bad reception might very well be helped, as suggested, by using a yagi antenna; they're small and cheap and you can get a lot of gain in very little space.

 

[DMBFireman]

thanks for the replies...

W2NJS, I get what you are saying and I realize that the coverage area is limited for 800 MHz trunked systems. The problem I'm experiencing is not from being outside that area...I realize that I'm outside of their areas. I can't even receive the control channel until I turn on the attenuator and then I receive the control channel fine...granted some of the longer distance systems still sound a little scratchy but I can track them.

Jody, I did some quick testing without the antenna as you suggested and found that most of what I could pick up without the antenna was on 900 MHz. In fact I actually heard 2 sides of a conversation which actually sounded like a HAM conversation on 927.875 but I didn't know many HAM's transmitted on 900 MHz? I also picked up some weak 800 MHz stuff but I was able to narrow those down to being transmissions from my local public safety trunked systems. I'll do some more experimenting but that's what I was able to come up with so far.

Brandon

 

[zz0468]

800 mHz trunked systems are usually closely engineered to cover only their intended areas and run at lower power than the old analog systems that covered the same areas, and as a result you'll find many trunked digital systems cut off their coverage when you get into the areas that they are NOT designed to serve.

This is VERY true, and something many people fail to consider when attempting to listen to distant systems. Having done quite a bit of system design, I can say for certain one of the major goals in designing coverage of a system is to insure that it DOESN'T cover much outside the political boundary it was designed to serve.

The other thing to keep in mind is, a wire open receiver front end, like in a scanner, can get crunched by stuff you're not listening to, or that the scanner isn't even capable of receiving. Cellular is going to be blocked, for example, but a nearby cellsite operating at 880 MHz could swamp the front end of a receiver trying to hear on 868. You'll never pick up a signal by searching, but the receiver sure is aware something is there.

Adding attenuation to better receive a distant station is a pretty good sign that there's some overload taking place.

 

[n5ims]

If the closer systems are not simulcast, but the one(s) you're having problems with are, this could be the issue. Simulcast systems have multiple towers that transmit synchronized signals to increase coverage within the designed area. If you're outside of this area, the signals may not be synchronized and arrive out of phase and cause you to see less signal strength since the signal from one tower is canceling out the signal from the other.

The attenuator may reduce the total strength so that the weaker tower is no longer canceling out the signal, at least not as much to allow you to receive the system. Directional antennas pointed at a single tower is often the best solution if this is your issue and the attenuator doesn't help.

 

[DMBFireman]

That's some great additional information guys! Thankfully VHF and the air bands are not having any issues that I was able to tell during my initial listening and those are what I will primarily be listening to but occasionally I may want to jump on one of the trunked systems when something major is happening. My plan for this antenna system is the single antenna mounted on the chimney with the LMR-400 feeding a multi-coupler. At that point I will be hooking up the multi-coupler to my various scanners and will be scanning military air and VHF fire department channels with 4-6 scanners (I'm getting an 8 port multi-coupler). Occasionally I might tune to an 800 MHz trunked system and it looks like I'll be having to use the attenuator on those systems. I'm hoping to permanently install the antenna and run the cable tomorrow to at least hook up one scanner so I can listen again...the multi-coupler will have to wait till next months budget!

Brandon