Tips to reduce Nextel interference on GRE-600?

[Beaconhunter]

I am using a GRE-600 in my car to monitor the various 800 Mhz trunked radio systems in the greater Seattle area. The received signal is totally wiped out whenever I drive near a cell tower, and there are so many towers these days that it seems like interference is the norm rather than the exception. I have turned the attenuator both on and off and this seems to have very little impact on the problem.

I previously have had Uniden 780 and 785 scanners in the car and these were also affected by the interference, but not nearly as much as the GRE unit is.

Any suggestions?

[DaveIN]

Attenuation and band pass filers are about your best bet, but the 500/600 are so sensitive to front end overload, I'm not sure how well it would help in that situation.

[N_Jay]

Don't just blame Nextel.

It could be lots of types of interference, some related to the receiver quality, and others not.

Some that can be filtered or attenuated, and others that can not.

[hertzian]

I'm going to assume you have a mobile antenna - and the built-in attenuators are just not enough.

RS used to have a variable 20db attenuator, but other options might be to purposely use a less-efficient antenna, coil up a long run of lossy coax such as RG-58 or perhaps a long run of RG-174 coiled up in the trunk.

I'd probably keep the good antenna, and choose a less efficient long run of feedline for the 800mhz stuff, which will still keep losses reasonable at UHF and below.

So the trick here is to find a working combination of built-in attenuator and length of lossy feedline to accomplish the goal. If you get the mix right, you might be able to selectively program the attenuator in for the 800mhz stuff - and along with the lossy coax get enough attenuation to make that band usable again, without adversely affecting the lower bands which won't need any attenuation programmed in, and the line loss not be too much of an issue.

I'm looking at mounting my GRE-made Pro-164 in the mobile too, and am pondering doing this as well in order to help tame the front-end at 800mhz.

[Beaconhunter]

Yes, I have a 800 Mhz mobile antenna that I use with this scanner. I normally use my ICOM ham rig to monitor conventional VHF/UHF channels so am using the GRE primarily for the trunked systems.

I have thought about using external attenuators, but have some doubts about how this would work. If the attenuator is effective in blocking out the strong signals generated by the nearby cell towers, won't it also block out the much weaker 800 Mhz radio signal, since they are fairly close in frequency?

I understand that the rebanding effort is an attempt to get the 800 Mhz public safety users a little further away from the frequencies used by Nextel (now Sprint). I wonder if this will make the interference issue go away for us scanner users, or if we will still have a problem.

I hear that rebanding is still a couple of years away here in the Seattle area.

[DiGiTaLD]

Several years back, I ran an 800 MHz ¼-wave antenna connected to an RS PRO-95 (a GRE-built radio) when mobile. That thing pulled in a grievous amount of intermod, not even being close to cell/Nextel sites. Switching to the stock duck eliminated the intermod problem, and using the mobile antenna didn't really yield any benefit in terms of improved received signals because of all the intermod it pulled in with them.

I know its a 600, and you probably have it nicely installed in your car, but that mobile antenna is probably too much for it. Probably the best you can do is use the built-in attenuator, but as mentioned above that may not be enough. When I take the 500 in the car with me, I just use the RS/GRE 800 rubber duck that I use on it everywhere else. It pulls in plenty of signal (and intermod) without the help of a mobile antenna, and I use the attenuator as needed. Its not bad out in the rural areas but in the city, forget it. Might as well just leave it on all the time.

Post-rebanding, some of the intermod situation may improve, but with a scanner with a wide-open front end, its always going to be a problem.

[SquierStrat]

Originally Posted by Beaconhunter

Yes, I have a 800 Mhz mobile antenna that I use with this scanner.

well that sounds like a large portion of your problem
if you didnt use an 800mhz specific antenna, you wouldnt get so much 800mhz interference/overload

[Septa3371CSX1]

I've noticed this problem on my RS Pro 106 (a GRE Radio) but not on my Pro 97 (another GRE radio). It seems like the 800 MHz antenna pulls in more interference than the regular rubber duck. The Pro 106 does seem to be a lot more sensitive in the 700 and 800 mHz range than the 97.

[DickH]

Originally Posted by Beaconhunter

Yes, I have a 800 Mhz mobile antenna that I use with this scanner. I normally use my ICOM ham rig to monitor conventional VHF/UHF channels so am using the GRE primarily for the trunked systems.

I have thought about using external attenuators, but have some doubts about how this would work. If the attenuator is effective in blocking out the strong signals generated by the nearby cell towers, won't it also block out the much weaker 800 Mhz radio signal, since they are fairly close in frequency?

I understand that the rebanding effort is an attempt to get the 800 Mhz public safety users a little further away from the frequencies used by Nextel (now Sprint). I wonder if this will make the interference issue go away for us scanner users, or if we will still have a problem.

I hear that rebanding is still a couple of years away here in the Seattle area.

Have you tried removing the antenna in a noisy area? If you can still hear the signal without interference, that's the best solution. If that works, get a co-ax switch, then just switch the antenna in or out for noisy areas.

[hertzian]

Originally Posted by Beaconhunter

I have thought about using external attenuators, but have some doubts about how this would work. If the attenuator is effective in blocking out the strong signals generated by the nearby cell towers, won't it also block out the much weaker 800 Mhz radio signal, since they are fairly close in frequency?

Well as we've discovered, GRE radios are already artificially sensitive - at least to my experience on 800mhz. So much so that when trunking edacs on my GRE / Pro-164, it is only when the attenuator is inline that my confirmed weak trunking systems are truly just as weak on other radios like Unidens, Icoms, etc. So running with the attenuator is what I think should be considered the norm as compared to most other radios. Now add additional problems like a high-rf environment, and perhaps yet another 10db or so should be added by some other means. In other words, starting out with 20db of attenuation should be thought of as getting back to a real world "0db attenuation"

The real fix is to get GRE to tame that front end, but it may be a marketing decision to run this way really hot. Great for the sticks with the oem duck, but not so great for those of us in the rf-jungle.

(Note that I am frustrated by the fact that my conventional 800mhz channels that are just mind-blowingly strong without the attenuator are not affected so much at all by similar desense/overload! I wonder if you have any conventional systems that you could compare the problem with the trunking systems. At least on my 164, the scanner seems to have differing levels of rf amplification depending on whether I'm trunking or doing conventional - argghh.)

What could be complicating the issue are the power-cord and audio-cable antennas just dumping a lot of common-mode rf back into the decks. Since it seems to also affect your older Unidens, maybe try a test running without any audio cabling, or perhaps running with a dc-pack NOT connected to the vehicle to see if this changes the intensity for better or worse.

[Beaconhunter]

Thanks for all the great tips. I was out driving today, and when experiencing the interference problem, just reached under the dash and disconnnected the coax from the GRE-600 as suggested by DickH. I was surprised to find that this solved the problem! The signal was a little scratchy but perfectly readable. However, when I drove into some areas, I had weak or no signal without the antenna.

I next tried reconnecting the coax, but removed the 800 Mhz antenna from its NMO mount, leaving just the mount itself to act as the antenna. This gave me a signal that sounded roughly the same as when the coax was disconnected from the radio (I was expecting it to be a little stronger). Still not acceptable in weak signal areas.

I then installed a straight 1/4 wave 800 Mhz whip on the NMO mount instead of the 3 db gain antenna I was previously using. This gave me a good signal, but the interference problem was back - but not as bad as when I was using the previous gain antenna. I turned the GRE attenuator on, and this seemed to give me an acceptable compromise between signal strength and interference, but it still is not perfect.

I have an old NMO mount 1/4 wave VHF whip. I am going to try cutting this down to about a half-inch or so to see how it works. I also want to try connecting a BNC connector to the back of the radio that just has a short length of wire dangling from it to see how this works.

If all else fails, I may have to go the antenna switch suggested by DickH and simply disconnect the antenna when in an interference area. I hate the thought of having to manually switch back and forth however...I have enough "driving distractions" as it is..:-)

[N8IAA]

Have you tried using the RS 800Mhz antenna on the scanner? Also, tighten the squelch by turning the squelch knob clockwise.

Larry

[SOFA_KING]

Unplugging the antenna?

Originally Posted by SquierStrat

well that sounds like a large portion of your problem
if you didnt use an 800mhz specific antenna, you wouldnt get so much 800mhz interference/overload

This is the only intelligent answer on this thread. Everything else, like disconnecting the antenna, or running more and more attenuation, is a joke. If you plan on listening to any other band, you will be out of luck. There is a better solution! And it won't limit you range on anything!!!

I have tried many antennas, but this weekend I wanted to scan some of the low band skip on 6m and 10m, in addition to my usual VHF/UHF/800 stuff, when I took a trip through West Palm Beach and down into Ft. Lauderdale on I95. This is RF H3LL along I95 with many cell and RF towers, and I have made this trip many times using a VHF whip that worked fine for VHF and UHF, but bad on low band and had tons of interference on 800. The big county 800 systems would get wiped out in many spots and were generally always noisy...and I have the attenuator programmed on for all of my 800 stuff! So I put a 53.3" whip on my NMO mount and could not believe how well this worked on 800! Not only was I getting all the great 6m and 10m skip (I also discovered my KIA had VERY LOW ignition noise on low band ), but my VHF high band was also great as this works out to a resonant 3/4 wave on high band. But I expected 800 to bomb, and it did just the opposite. Not only could I pickup the systems well outside of each county (and clearly), but I had ZERO noise problems on 800 and ZERO dropouts. In-county reception was great! Plus I heard much more areas of each county that I had not heard before...like maybe the multi-site trunking was working better too. ??? Best mobile 800 reception I have ever heard with my GRE RS PRO-197. My UHF suffered a little, but I still heard all the local stuff just fine...just not two counties away. Even my CB reception improved so much that I had to attenuate those channels (9 & 19) so as not to get locked up on skip (heard the close traffic just fine). So this has been my best solution to hear all the bands...a straight unloaded (no coil) 53.3" whip. Larson sells the thin whips and has an NMO base too, but I like my old gutted Antenna Specialists UHF black base with the fiber contact plate and chrome spring the best. Maxrad makes a slightly longer thin whip if you want to tune it right to 6m (57"). I calculated the 53.3" to work well enough on 6m and act as a 3/4 wave on 166 MHz, and it worked perfect. 10m was strong too, so I was hearing all my friends up in NY loud and clear as well as all over the country. I also heard plenty around the country on 52.525. What a great antenna!

Give one a try. No sacrafice on any band except maybe UHF a little.

Phil

[bneilson]

Originally Posted by Beaconhunter

...I understand that the rebanding effort is an attempt to get the 800 Mhz public safety users a little further away from the frequencies used by Nextel (now Sprint). I wonder if this will make the interference issue go away for us scanner users, or if we will still have a problem.

I hear that rebanding is still a couple of years away here in the Seattle area...

We have finished re-banding in my area (Salt Lake City, Utah) and I am still getting the same issues described. Prior to rebanding the really bad locations would totally wipe out the PDs radios. Now after rebanding they are just fine. My GRE however is still just as impacted when I am close to a site.

[SOFA_KING]

Originally Posted by bneilson

We have finished re-banding in my area (Salt Lake City, Utah) and I am still getting the same issues described. Prior to rebanding the really bad locations would totally wipe out the PDs radios. Now after rebanding they are just fine. My GRE however is still just as impacted when I am close to a site.

After rebanding you can use a low pass filter to eliminate all cell interference...that is as soon as someone makes one cheap enough. You could get one today, but it will cost ya. The point of rebanding was to allow the use of such filters by putting the cell traffic up in their own section of the band.

Phil