Chasing Nextel 800MHz Police Radio Interference Before Rebanding.

Public safety is ensconced below 860MHz while Sprint/Nextel blasts away with wideband CDMA above 862MHz. I was tasked with finding why every fifth or so radio transmission to the police station was missed. This immediately raised some ideas, since the trunking system was 5 channel, of the type that used a low-speed control channel (LTR).

First Action

While on the phone, I asked to listen to some transmissions. Crystal clear. However, due to the quieting effect of FM, we know that there is a rather non-linear relationship between C/N and SNR. That is, the FM improvement factor once you’re over the signal level where popping and cracking stops puts you quickly to the 35dB maximum SNR typical of FM communication radios. That is, the link could be weak but you’ll only intermittently see problems.

Before I left for the police station, I popped open Radio Mobile Deluxe for a “radio link” path simulation.

desired path repeater to base station
desired path repeater to base station

From that photo, there are two things that leap out. The over 40 km path length despite the mild terrain leads to only 0.2 Fresnel zone clearance instead of the desired >= 0.6. Since this system had only one central transmitter for the county, and the police station antenna was on a tower with a good gain omni antenna, there wasn’t much to do. The omni antenna had to stay to allow for backup simplex (radio to radio) communications in case the main tower went down–they weren’t going to buy a second base radio or swap yagi to omni antennas in that case.

The deficient design of the county-wide 800MHz system caused a variety of problems due to having the single central transmitter on a tall hill and a very tall tower. You just can’t be 40 km from a transmitter and get a solid signal when safety of life transmissions are concerned. Yes, ham radio operators go over twice that distance on VHF/UHF repeaters, but two key distinctions are:

  1. Ham repeater frequencies are generally afforded far more co-channel and adjacent channel protection than commercial frequencies
  2. Ham radio operators will tolerate far more static, fading, repeating transmissions than police officers in hot pursuit

In fact, given terrain in other parts of the county, some places only 20 km from the tower had problems with reception. Yes, there was a receiver voting system, but in a trunking system, you have to hear the base station before you can initiate voice communications! You can’t say “man down” on a trunked system even next to the voting receiver if you can’t hear the base transmitter. Exception is for the emergency button, the protocol designers were smart enough to allow the emergency unit ID to go through even one-way.

On-Site Observations

It was time to get into the field. On the way, I set a conventional receiver to each of the channels and listened for the 10-second periodic “kerchunk” of each repeater. As I got into range, I noticed the repeaters were roughly the same signal strength by ear. No other complaints had come in besides the usual. I did a few test transmissions at the police station from their radio, tested their radio with my Aeroflex COM-120C service monitor, checked the SWR with my Bird 43 wattmeter, all was well.

Aeroflex COM-120C service monitor
Aeroflex COM-120C service monitor. Photo copyright Test Equipment Connection

Next, I put the COM-120C into spectrum analyzer mode, the resolution bandwidth and update rate weren’t as good as an HP, but it did the job. I checked that the 5 repeater channels were approximately equal in strength, allowing for fading and the like. The received signal  amplitude was in line with the path loss predictions, keeping in mind the confidence interval in the Longley-Rice propagation model. I tried listening with the COM-120C receiver, but the receiver sensitivity of the COM-120C wasn’t nearly as good as a regular radio, and it’s hard to make fine-grained SNR estimates on a kerchunk. So I reprogrammed the police base station to listen directly to each of the channels. Then I could notice a clear SNR deficiency on only one channel, despite the spectrum analyzer showing nearly equal signal strengths and equal background noise level within the sensitivity of the spectrum analyzer. I don’t recall anymore the spectrum analyzer intrinsic noise floor, but it was on the order of -100dBm for the resolution bandwidth used.

This was not a lab-grade spectrum analyzer, so you went from either seeing very rounded-off traces at decent update times, or fine traces at too slow update times. I couldn’t be sure if I saw something or not. Nextel’s iDEN signals fill the channel with a fairly uniform spectrum, so you can’t zero in on a carrier like you can with analog transmissions and wait for the analyzer to update.

The Catch

There was one good test left to try–the “desense” test–using a special T-connector called an isotee that passed straight through between antenna and radio, and the T port had the center pin removed so that it provided controlled (at least at each frequency) isolation such that a signal generator connects to the isolated port, and the radio is connected to the antenna via the other two ports.

isotee diagram for desense check
isotee diagram for desense check

Four channels were normal, perhaps less than a dB desense. The fifth channel had perhaps 10-20 dB of degradation, fluttery time-varying. Changing the signal generator frequency slightly did not generate beat notes. Tuning the radio/sig gen to one 25kHz channel either side didn’t have the interference. The interference was fluttery but constant otherwise, no duty cycle.

A case of long-distance Nextel iDEN co-channel interference!

interference path Nextel to public safety
interference path Nextel to public safety

Pick up the phone

Nextel despite purchasing up SMR licenses like crazy (thanks) in those days did not put sites at the license coordinates necessarily. They took advantage of the SMR regulation allowing different site location as long as the license footprint was not exceeded. This is where the cell phone DXing came in. I had a Wilson antenna adapter for my Nextel i850. I dialed # , * , Menu, Right to go into field test mode, giving me current frequency and signal strength in dBm, among other parameters.

Wilson Antenna coupler, allowing use of external antenna
Wilson Antenna coupler slid over Motorola i850 antenna, allowing use of external antenna for far greater range (photo copyright Wilson Antenna)

There was not cell service locally available at ground level, but with an external antenna, the Nextel signal was there. I couldn’t force my phone to connect to a particular channel, but it was just enough with the Nextel phone showing similar signal strength on the Nextel signal as what I hypothesized the non-directly observable interference to be, that it was time to call Nextel.

Phone a friend at Nextel field engineering

I had a previous line to the relevant Nextel field engineering staff, and they were a bit surprised that a Nextel tower over 90 km away could be a bother to a police radio, but I noted the weak desired signal, and the part of the interference path that went over water, conducive to thermal inversion layers–superrefraction that can make radio waves travel unexpectedly long distances. He put in a channel change request. Next time I went through I did the desense test again and all 5 channels were good. Hopefully someone at Nextel did a license search to give a wider co-channel geographic exclusion zone for public safety licensees, it would be a simple FCC license search away.