[K3CAL] Chasing the Great Pumpkin, Repeater Update

[KB3WFV]

Hello All,

It has long been my belief, that a lack of knowledge, breeds misconceptions and misinformation, in turn breeding irrational behavior. The following information offers knowledge, intended to dispel misinformation, quell arguments, and prevent insults.

It is collected from, hundreds of, if not thousands of hours, of research, discussions, testing, frustrations, and tower site visits, by dozens of very smart people in both amateur radio and professional two way radio communications, from within the club, as well as outside. I truly appreciate all of there efforts, inputs assistance and transportation with this project. I hope you do to.

This information was not made up while sitting on the commode, or pulled from some self proclaimed expert on YouTube. Real, honest, methodical, thoughts, made by many, has been put in to this.

The Nutshell for those who will complain this email is too long: The kenwoods are on the air in Lusby and Sunderland. You won't ask, but we still need to raise more money to cover their purchase. NO, they are not linked. YES, there is still interference at the Sunderland tower site. YES, the cause has finally been identified. NO the source has not been found. NO it has not been fixed. NO we can not do anything else without the help of the county and or Motorola. YES a request for a meeting with the county has been submitted. NO we do not know when. NO additional questions can not be asked until after you read the rest of this lengthy email.

For those who wish to broaden their horizons, learning more about the problem and the hobby. Read On............

The 146.985 repeater in Sunderland continues to be plagued with, as you know, for several years now, a random interval interference problem. This problem can range from desensing the RX receive, to blocking weak signal stations, to holding the 146.985 in TX, re-transmitting a digital signal over amateur frequencies. The newest attribute to the interference, is locking up the remote repeater with a on air dead carrier transmission. Requiring a tower site visit to reset it.

The interference continues to occur even with the installation of the new Kenwood repeaters and the switch to IP network linking. Yes, the Kenwood NXR 1700 VHF 146.985 rpeater, is currently on the air in Sunderland, and Yes the Kenwood NXR 1800 UHF 444.950 repeater, is currently on the air in Lusby. NO, they are NOT linked, for reasons mentioned above.

These repeaters and their link is part of the county wide amateur emergency communications support. The amateur radio community is extremely fortunate to have wide area coverage repeaters located on not only one, but two, commercial county towers, in county shelters, connected to the same infrastructure, back up power, and grounding systems, along side of tens of thousands of dollars of multiple government transmitters. Connected to county provided antennas, the 146.985 and the 444.950 are the ONLY non government transmitters located in these shelters. Even the multi million dollar cell phone companies are banished to outside of the shelters.  It should be understood that amateurs are simply humble guest of the county in these shelters. Feeling Honored to be there.

The location of these repeaters was determined several years ago, again by several very smart people in both the amateur and professional radio world, during the planning stages of the county's upgrade to P25 Phase two. There location is intentional, their configuration on purpose, both of which cannot be changed without the collaboration of T-MARC for frequency coordination and collaboration of County and State Emergency management officials, in support of amateur radio emergency communications. Not to mention, finding a tower that is not already completely loaded, and the funds to install, what is currently free to use, commercial repeater antennas. Go price 300 feet of 1-1/4 hard line.

The fact that the repeaters are located in commercial tower sites with multiple government transmitters, contributes to the possibility of interference. It also makes it extremely difficult to locate the source, let alone the cause and repair of any interference. Why? the control of there systems and the access to the tower is limited to those agencies. It is not, nor has it ever been, nor will it ever be extended to amateurs. In other words amateurs can not legally key up a police or fire radio. Amateurs can not legally climb county towers. Actions that are sure to piss off the  host, risking amateur ejection from all tower sites.

To further understanding of why this interference is so hard to find, one must understand the County Public Safety Communications System and the common amateur radio repeater system........

The common amateur radio repeater system consist of an repeater radio capable of receiving and transmitting at the same time, a duplexer, consisting of filters for receive, and filters for transmit. Connecting all of it to a very high antenna. The RX signal 146.385, actually all signals are received by the antenna, travels hundreds of feet down the coax, to the RX filter side of the duplexer. The RX filter strips away all unwanted signals, allowing only the wanted 146.385 RX signal to pass.

The RX signal travels to the special repeating radio, where the signal is repeated or copied to be transmitted. The TX signal 146.985 returns to the TX side of the duplxer, filtering out all unwanted signals, allowing only the wanted 146.985 TX to pass. Travels up the same coax, to the same antenna, and Transmits. This is amazing! A single antenna, can receive and transmit at the same time. I have two ears and one mouth, I can't do that.

So how is this filter system getting interference? Let's look at the county system.......

Motorola professionals and self proclaimed experts. Please forgive my choice of creative license over technical accuracy, in hope of gaining general understanding.

The county public safety system, is a computer controlled,,, Digital,,, Trunked,,,, Simucast,,, Multicast,,, partial encryption, communication system for multiple agencies in Calvert County. One system, for everyone, Police, Fire, Ambulance, Public Works, Dog Catcher, etc. These agencies, refered to as Subscribers, are not assigned frequencies. Instead their radios are programmed with Talk Group ID's. Police officers may have been assigned Talk Group 100 and Fire Fighters may have been assigned Talk Group 200. The computer assigns these Talk Groups to a frequency of it's choosing when communications is needed.

The county uses Ten (10) 800 Mhz frequencies. Operating as digital P25 phase two. Similar to DMR, each frequency is assigned as a channel. Each channel has Two time slots. This configuration provides the county with at least one control channel, and Eighteen (18) voice channels.

The computer controlled Control Channel uses two time slots, consuming the use of one entire channel. It directs the user's radio, called a subscriber, aka police officer, or firefighter, to it's chosen channel or frequency and time slot.

A voice channel, like all channels, has two time slots. But will use only one time slot for a single conversation, consuming only half of a channel.  A second, separate conversation can be held on the second time slot, consuming the remaining half of the channel. Thus a voise channel can carry two separate conversations at the same time.

The control channel transmits constantly, at all times. It is the beacon or mother ship that tells all of the other radios what to do, when to do it, and where to do it. The control channel can even assign it's self to a different channel and often does. It may be on channel one, one day, channel two another day or, if feeling particularly froggy on any given day, it can jump it's self to several different channels on the same day. It's that smart.

When a police officer uses his or her radio for contact with dispatch. He/she keys the radio, the control channel sees the Talk Group ID, assigns a channel and time slot. Tells the dispatch and all of the other radios to use the same channel in that time slot. It could be whatever channel or time slot the control channel decides to use at that moment. It could be channel 7, time slot one. OR it could be channel 10, time slot 2.

Often a conversation started on one channel will finish on a different channel. Forexample, am officer may ask a question on the system assigned channel 6. time slot one. Getting the answers later, on system assigned channel 4, time slot 2. The computer decides, using the control channel to manipulate subscribers calls, assigning them to system channels and slots, managing the resources of the system.

When a subscriber keys the systenm they have no idea what channel or frequency they are using. The control channel assigns the frequency to the talk group at that moment. All they know is that their buddies are on the same channel at that time and they can talk to them.

It is believed that there are fifteen (15) county towers. ALL of the towers carry ALL ten (10) 800 Mhz frequencies. Any channel with it's individual frequency, when transmitting, is SIMULCAST on EVERY tower at the same exact time.   SIMULCAST is the retransmission of the same frequency, at the same time, from multiple locations. If channel four on 800Mhz A, is transmitting, then ALL 15 towers are transmitting 800 Mhz A, at the same time. Remeber that the system can carry 18 different conversations at the same time. So at any given time one or more frequencies may be transmitting on all of the towers, at the same time.

 The system uses GPS to coordinate time delay between the towers. Sending a signal from Barstow to Solomons, takes longer than sending the signal to Prince Frederick. To ensure that the messages are transmitted at the same time, the system manages the traffic placing specific, measured, delay for each tower, in the message.  This ensures that every tower say the same thing at the same time.

The system also MULTICAST the Fire Main, or Dispatch Channel. MULTICAST is the transmission of the same information on two or more DIFFERENT frequencies. In the case of Fire Main, or Dispatch Channel, messages sent to SIMULCAST on 800 Mhz are also sent MULTICAST on VHF. It is believed that there are two or more VHF frequencies in Calvert County. The VHF is used to accommodate older analog scanners, pagers, and fire department alert systems. It is an announce only system. Simply rebroadcasting the original transmissions from the fire main talk group on the 800 Mhz P25 system. It is often, perhaps due to it's ease of listing on any VHF receiver, incorrectly blamed for interference with the 146.985.

In addition to the VHF patch discussed above. The 800 mhz system is also patched through to the Maryland State, Maryland First system on 700 mhz. This accommodates the full service Marylandd State police in Calvert County.

ALL transmission on the 800 Mhz P25 system are Digital. Only the police, and ambulance medical information communication calls to the hospital are encrypted.

Now that it is understood how lucky amateurs are to be on these tower sites. How sophisticated the P25 system is. How wrong it is to point blame at any one agency, as they all use the same frequencies at any given time. And how the 146.985 could be part of the problem. Then it is easy to understand why there is resistance to point the finger at the host who graciously accomodates us.

All that could be done, until now. Was testing. Changing configurations of the 146.985. Then wait for the Great Pumpkin, the majuc unknown combination to bring about interference. Many times it was believed that a solution had been found. Only to be deflated with the next burst of digital noise. Persistence and a lot, a whole lot of collaborative effort has uncovered the cause.

It appears that the interference is the result of a 3rd order inter modulation. The IMD is caused by the mix of 146.985 with not one, but two 800 Mhz frequencies. Resulting in a signal being generated on 146.385. DEAD on the receive frequency of the 146.985 repeater.

It is very concerning to realize that this IMD is not simply caused by a harmonic of one frequency mixing wwith a second frequency. Instead,  It is the mix of THREE fundamental frequencies, 800 mhz A in TX, 800 mhz B in TX and 146.985 in TX. This IMD has been repeatedly duplicated, recorded and documented. NO! out of respect to our host and their vendors, the actual 800 Mhz frequencies will not be shared at this time. Don't ask.

The cause can be a faulty transmitter, including ours. A faulty antenna, including ours, or some other thing on or near the tower, such as a rusty bolt somewhere on the 400 foot structure.

Trouble shooting this unusual three fundamental frequency IMD, will require the use of specialized equipment, and access to other transmitters in the shelter or antennas on the tower. All of which is under the control of the County or Motorola.

A meeting request has been sent to the county. We are awaiting confirmation.

Want to know more? Check out these links......

Finding PIM
https://www.youtube.com/watch?v=aRgqcVFmPRk


Equipment for hunting PIM and Demo
https://www.youtube.com/watch?v=HPVZS9nSj_Y

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Brian
KB3WFV