Dipping My Toe in DMR

Because I’m always looking for an excuse to buy a new radio, I’ve been looking at the various VHF-and-up digital modes in use on the ham band. There are a multitude of them in use.

Going by New England Repeater Directory’s 2 meter page, here are the number of repeaters listed for each digital mode:

  • P25: 4
  • NXDN: 9
  • D-STAR: 23
  • C4FM: 24
  • DMR: 49

(To be clear, that is just on the 2-meter ham band in New England.)

Why DMR?

DMR drew my interest because it’s the most widely deployed in the region (NEDECN, etc.), by far. (More than the next two combined!) In part, this is probably because groups like NEDECN and EWARN, as well as the the international DMR-MARC group. They link up repeaters over the Internet (primarily, at least), making it a wide-area network. (This linking isn’t unique to DMR. It’s not even really relevant to DMR per se; it just happens that all or nearly all of the local DMR repeaters are linked. D-STAR repeaters are similarly configured from what I’ve seen.)

DMR is also interesting to me because it’s a two-slot TDMA system. Put more simply: in digitizing the user’s voice, it captures 60ms segments of voice from the microphone, converts them to digital format, and then transmits them in 30ms of airtime. Therefore, a single frequency can carry two conversations simultaneously. They’re dubbed slots, and many of the aforementioned ham repeaters will carry a local talkgroup on one slot and a larger-area one on the other. Users select which they want as if they were changing channels. (Another fun aspect of this: battery life is said to be improved about 40%, because the transmitter runs half as much.)

The other thing I think DMR has going for it is that it’s an open standard (ETSI) with a lot of commercial gear from a number of manufacturers. That serves to drive down cost a bit. DMR isn’t entirely unique here, but it seems to be the cheapest path.

As with some of the other digital modes, DMR supports things like one-to-one calls (though the operators of the networked ham repeaters ask that it not be used there) and sending of text messages / data.

So, I decided to jump in, and ordered a radio off eBay:

It’s a Motorola XPR 5550, VHF. Interestingly, it also has a dedicated GPS receiver built in.

I’m now onto my next challenge: obtaining the programming software (CPS) for the thing. Because it’s a commercial radio versus something targeted at hams, it’s not easily programmed. (FCC Part 90 regulations intentional forbid allowing users to enter their own frequencies, for example, since in many cases that would wreak havoc.) For some reason, Motorola insists on selling the software through Motorola Online (MOL), which requires that you request an account and wait days for it to be manually provisioned. They also, frustratingly, charge close to $300 for it.

The software is floating out there for illicit download, but I want to do this right. Also, this being a newer commercial rig, it only supports “narrowband” 12.5 kHz channel spacing out of the box. Ham radio has not narrowbanded, so this is undesirable. Motorola offers a free “entitlement” to enable legacy 25 kHz channel spacing, but getting it apparently requires an active CPS subscription.

So for now, this really slick digital radio sits on my desk, monitoring the pre-programmed 146.52 2-meter calling frequency, in analog FM. I can’t wait to get this to do a bit more!

Towerspotting: Georgetown, MA

Being a radio nerd who’s trying to spend more time outdoors, I’ve figured out a good way to kill two birds with one stone: hiking up hills with radio towers on them. Since the towers tend to be atop hills or mountains overlooking the surrounding community, they often have pretty scenic views.

Today was an almost completely cloudless day, so I decided to cross Baldpate Hill in Georgetown, MA off my list. (Note that the cover image on that page is not from Baldpate! I was disappointed.) I never did find the “fire road” on Spofford Street; I’m pretty sure it does not exist. The only way I’m aware of to get where I went—plainly the same place on the map as that site depicts—is via an access road on Baldpate Street just before Baldpate Hospital.

I expected a fairly scenic summit with panoramic views, but it was nothing of the sort. Baldpate has dense foliage, and lots of poison ivy that discourages you from venturing off-trail. (And by trail, I mean the dirt road.) Really all there is to see are a few towers and some plants.

I did pass this guy walking up the trail:

Backlit spider’s web

This Jack in the Pulpit blends in well, emulating poison ivy’s distinctive leaves a lot more closely than I realized:

Jack-in-the-Pulpit plant

Walking up the access road, the first thing you come to is a grassy clearing with a couple large water towers:

Water towers atop Baldpate Hill

At the top of the hill, you come to a fire tower:

A fire tower atop Baldpate Hill converted into a cell tower.

At least, it was a fire tower. It’s now fenced in and loaded up with cellular antennas. I should have grabbed a picture of the feedline; though I knew they used low-loss coax due to the high frequency and low-power handsets they’re trying to receive, I guess I never really appreciated multi-inch-thick coax until I saw it in person.

Between the “fire tower” and the water towers lies the communications tower. It’s about 250′ tall, and posted as ASR 1004094.

ASR 1004094, an 80m tall tower atop Baldpate Hill in Georgetown.

The tower is listed as being owned by New Hampshire Public Broadcasting, which is odd because, as far as I can tell, they have no presence on the tower. It looks like it’s just cellular, land-mobile, and microwave.

Some licenses near the top of the tower list a height above average terrain (HAAT) of about 450′, so the hilltop itself must be about 200′ above average terrain. With the tree cover, though, there’s nothing really to see. I do wonder what the views from the fire tower would be like if it were open.

Playing with the excellent Radio Mobile Online, here are projected coverage maps from the site. There are, tragically, no ham repeaters up there to the best of my knowledge, but here’s what coverage would look like.

Both maps are based on being almost at the top of the tower (80 meters), 30W out of the transmitter, 1.5 dB loss, and 6 dBi antenna gain, for about 85W ERP. Modeled for 70% reliability.

446 MHz:

And here’s 146 MHz:

There are, in fact, no 2m of 70cm ham repeaters in Georgetown at all, according to the New England Repeater Directory.

Much of the stuff actually on the tower appears to be T-band and 900 MHz SMR/LMR, though a lot of it is licensed for 300W – 1kW ERP. Other tenants, according to the FCC database, include Georgetown and Boxford public safety, Northeast CMED, an ambulance provider, and a few others.

BTW: I failed to bring my “real” camera along, but was pleasantly surprised by what the iPhone camera could do. I used Photoshop to lighten up the shadows on the tower photos, but otherwise these are straight out of a cameraphone.

Big Antennas

I’m on a local ham radio mailing list. (Though I’m actually not sure how this came to be.)

So a few neat links. The first is a yagi for 160 meters. Antenna size is inversely proportional to frequency, and 160 meters is the lowest ham radio frequency out there. The notion of a 160 meter yagi is almost obscene. You can just barely make out that there are two people working in the antenna.

There are also photos as a neat site called Artificial Owl of the former Russian Woodpecker, a Cold War-era “over-the-horizon radar” meant at detecting incoming missiles. It’s rumored to have transmitted as much as 10 million Watts (ERP), spewing interference all over the HF/shortwave bands the world over.

Strange Antenna Challenge

You know those times when you decide to let yourself surf aimlessly? And an hour later, you have absolutely no idea how you got to where you did?

I found the K0S Strange Antenna Contest page from 2003, where some ham radio operators started using, well, strange things as antennas. Who’d think that a ladder works well? (No no, not ladder line, but an actual ladder.) In fact, after working some people off of a ladder, they got an even better idea, and stood several ladders up, using them to support a pair of extension ladders laid horizontally, forming a ladder dipole, with impressive results. Sadly, they report that combining two shopping carts to make a dipole did not get them any contacts, nor did a basketball hoop.

This has me wondering what else would work… An aluminum chain link fence? A railing? Train tracks? Power lines? (Kidding on that one. Please do not try to attach anything to power lines.) Curtain rods? A couple of cars? A section of guardrail? A metal lamppost?

I poked around the site some more, to see if they did it in subsequent years. And they did. 2004, for example, saw my joke about using two cars come to fruition. (Okay, so they beat me to it by four years.) 2005 saw someone use a bronze statue, and, the next year, he was at it again with railroad tracks, albeit not full ones, but some sort of art exhibit / monument. (Aside: I’m pretty certain that trying to hook up a bunch of wires to train tracks may arouse a bit of suspicion by the police?) 2006 also saw a pair of exercise machines being used, with a comment about how they weren’t very effective, but the apt comment, “On the other hand, we did in fact make two contacts with a pair of exercise machines standing only a few inches above the earth!” And, confusing everything I know about antennas, someone used a tree. And a football stadium (which includes a commentary about how the university police were initially slightly suspicious about someone getting out of their car and hooking wires up to the stadium for some reason). 2007 saw a bridge as an antenna.

And 2008? Well, see, here’s the best thing. The 2008 Challenge is this weekend!

Of course, as a Technician-class license, I don’t have many HF privileges… The Technician license was (before all license classes saw it eliminated) the only class that didn’t require a Morse code exam, so it’s somewhat ironic that almost all of the new HF privileges Techs were given are in the CW portions of various bands. I do get 28.3-28.5 MHz now, allowing SSB on HF…

Time to hit the books, I think. (I think mine–and that one–might be outdated, actually. Looks like the question pool got revised in 2007.) There are always sample exams online, and the feedback can be helpful. Study a bit and take an exam a day, and then review your answers. (Theoretically, actually, you could just learn the answers to each question without understanding the concepts, though that’s really missing the spirit and point of ham radio.)


There were a couple people screaming profanities outside my window. After about ten minutes one of them shouted, “Come drink with us” at some girl, and I looked out and saw that they had beer bottles.

Sufficiently irritated at this point, I called Campus Police. The dispatcher said, “I just sent someone up that way for this,” so I turned on my radio.

Sadly, the officers had the wrong location and missed them. But I was observing the whole thing through my window listening to them. I had to find a way to call back without divulging that I was listening to them, so I just stated that I’d seen the officers look in the wrong spot and leave. She asked me for a better description of their attire, in addition to my exact description of their location. I watched a couple cops slowly approach, observed the people I’d called about, and swooped in.

At this point I expected the students to stop yelling, apologize, and be asked to move back into their room. Instead, one of the students, upon seeing the police, threw his bottle down, screamed, and took off, with the second kid following him. The officers initially chased them into the woods.

One officer stopped chasing them as they got into the woods. I was slightly disappointed to see them get away, except that all I really wanted in the first place was for them to stop yelling profanities and derogatory comments at people. So I was happy.

But I was even happier when another officer radioed in that he had seen the kids come out of the woods and was picking up the chase.

At this point my batteries died, so I’m not yet sure how this ended. But suffice it to say that it ended up significantly more interesting than I’d initially expected. And the drunken jerks are gone.

As an aside, the school’s 100W repeater (into a high-gain antenna, it seems) seems to overload my poor little VX-2. I tried enabling its attenuator feature, but it still didn’t seem to cut it. I then tried removing the antenna, but then the signal was too weak. (Although still audible.) My IC-W32 holds up much better to the overwhelming nearby signal, but its batteries don’t. And my ASTRO Saber is entirely unphased by the signal, but its batteries died the other day and I hadn’t recharged them…

Digital Radio

There are a few different technologies commonly used in two-way radio. One is digital voice, a la Motorola ASTRO / Project 25’s CAI (IMBE), which is a 9600bps (9.6kbps) digital stream. Another interesting technology is trunking: a city might have 12 talkgroups (think “virtual channels”), but only 4 frequencies. One frequency is designated as a “control channel,” which is a digital stream announcing system status. When you want to transmit, your radio will go out to the controller and get assigned one of the frequencies, and the system will then announce that you’re transmitting on one of them, and all radios in your group will switch over and listen. This allows much greater spectrum utilization: rather than needing a new frequency for every group that might want their own channel, you just need to license enough frequencies for however many simultaneous conversations you expect.

I’ve been thinking that it’d be interesting to merge the two technologies. Technologies like Speex will let you process audio at exceptionally low bitrates, seemingly as low as 3.4kbps. (And they have some neat technology, like variable bit-rate encoding and even further drops in data transfer in between words, dropping quite low for when it’s just background noise.) So I think it’d be neat to start a “data network” at 32 kbps, which could be done with relatively low bandwidth. You could keep one frequency, and yet fit as many as 7 or 8 simultaneous conversations on it. (And you can take its VBR support one step further, and have it scale to fit system capacity: on a system with minimal activity, allow 8-16 kbps, but when the system is starting to fill up, drop down to 4 kbps.) HE-AAC (also known as AACPlus) looks promising too, although it’s a proprietary technology.

And since it’s now a 100% data network, you can do what I’ve always thought mobile radio systems, especially those used by public safety agencies, ought to do: put a GPS unit in each radio, and have them embed GPS coordinates in each transmission, as well as periodically announcing their coordinates in the background.

The net result is insanely efficient (radio) bandwidth usage. For example, Boston PD has 16 frequencies licensed, but it’s rare for more than 2 or 3 to be in use at any given moment. They could get more efficient by switching to a trunking system, maybe with 5 frequencies (plus a control channel). Of course with an established system, there’s really no incentive to, but I digress. But if they could get entirely usable digital audio at 3-8 kbps, they could actually move to a single frequency and support multiple, simultaneous conversations.

Another neat side-effect is that linking the systems would get quite easy: the entire system, with multiple conversations, could even fit over a single dial-up modem link. And you can have better “emergency” support, although most trunking systems seem to do it anyway: public safety radios carry an “orange button,” which will signal an emergency to the radio system. Analog systems do this by basically making the radios “obnoxious”–they’ll just keep transmitting a distress signal over and over, increasing the odds that they get through. With an all-digital system, they can just send packets indicating an emergency, and have the network make way for them, going so far as to terminate existing conversations if needed.

Oh, and another novel benefit is power management. If I’m on a handheld radio and I’m standing twenty feet away from the tower, I can dial my power down as low as it goes and still make it in fine. But if I’m a few miles away, I need to be using the highest power I can to make sure I’m making it. Of course, no one in the field fiddles with power settings. (In fact, most radios don’t make this something the user can do.) But if you just exchange a bit of signal strength info in the data flowing between radios, you can make this automatic. As I talk to the tower, it’ll be periodically confirming that it’s hearing me. But when it does that, rather than just using a boolean, “Yup, got it,” it can send me signal strength data, and my radio can dial down power until it’s at a, “I’m still getting 100% of your packets but I wouldn’t go any lower in power…” point. The net result is longer battery life. (And potentially, less interference to distant users of the same frequency.) As a really obscure benefit, if you’re transmitting this information, and also embedding GPS coordinates in your transmissions, the system could silently log all of this and generate coverage maps, which would get more and more detailed over time.


So my building here is one in a “set” of three dormitories. There’s a walkway, and another building on the other side of it. (And the third is to their side.) As I came back from class, I noticed a rope running from a room on the floor above mine across to the room pretty much opposite mine. It was extremely nice out today, so even those of us not creating improvised clotheslines (?) had our windows open.

We took a partial interest in whatever they were up to, but mostly about our business, just periodically looking to see what they were up to. We could also hear everything they were saying. So I sat at my desk working on something or other, when I heard a “roger beep.” I instinctively knew that it was from an FRS radio. It’s one of those silly noises they make at the end of a transmission. (As compared to things like MODAT [.wav] or MDC1200 [.wav], which are useful for ANI.) I’ve only ever heard it on FRS, so I “just knew.”

Of course, merely thinking, “They must be using FRS” because of a sound I overheard wasn’t geeky enough. So without missing a beat, I picked up my ASTRO Saber and switched to the “Zone” I’d created for the FRS band, and threw it into scan mode. A couple minutes later they transmitted again, and the scan stopped on Channel 2. Sadly, they didn’t discuss the actual purpose of the wire, only the difficulties they were facing on one of the ends.

It was tempting to radio back, “What exactly are you guys doing?,” but I didn’t want to blow my cover just yet. And besides, this thing puts out five Watts, ten times the power allowed on the band. (And the deviation/bandwidth is probably wrong, and it’s not type-certified…) Oh, and I think I probably have it set up to send an MDC PTT-ID.

It really concerns me how my mind works sometimes.

Awesome Radios

So the Project 25 protocol, with its IMBE CAI digital voice, is being rolled out in huge numbers. Motorola’s ASTRO is the most commonly-used, although several other providers make radios that do the P25 standard, too.

They’re calling it interoperability. When you and the next town over both buy $3,000 radios doing a crappy 9600bps digital audio protocol, your radios can talk to each other! I’m really not sure who fell for this, since the existing strategy, analog voice, worked 100% of the time. They’re switching people over to a new, proprietary (kind of: the voice codec is proprietary, but it’s part of an open standard) digital system so that everyone’s radios can talk to each other. This has never made sense to me.

The other big problem is that there are multiple bands out there. In New Hampshire, VHF is most common. You’ll find police, fire, and ambulances between 152 and 161 MHz. (More or less: they expand a bit on both sides.) Massachusetts is big on UHF, with the police usually being between 460 and 490 MHz. But it’s not quite that standard–some municipalities do their own thing. The 800 MHz band is becoming big, too, especially with trunked radio systems. You find that a lot in big cities. (Indeed, I think Manchester and Nashua are on 800, though I don’t monitor either so I wouldn’t swear to it.) And now there’s a 700 MHz band coming out that’s slowly being introduced.

As a ham, we have allocations on VHF and UHF that are both very common: 144-148 MHz and 420 or 430 through 450 MHz. Since both are commonly used, a lot of ham radios will do both bands. So my ham radios merrily transmit on both VHF and UHF. But, for whatever reason, this never caught on with “commercial” radios, like the ones public safety agencies use. If you want your police cars to monitor VHF and UHF, you put two radios in them. God forbid you work for something like a huge city, or a regional task force, where you may need to communicate on VHF, UHF, and 800 MHz. That’s three radios everyone will need to carry.

Vertex (the “commercial” branch of Yaesu, a ham company) made a dual-band radio. Once. The FTH-2070 came out in the 80s (?) and did VHF and UHF in one radio. It was huge, but for people who needed both bands (lots), it was a huge boost. For some reason, no one ever made a radio to follow in its footsteps, and the radio hasn’t been made for several years.

So it’s actually a grand mess. You have four different bands that a public safety agency may be on. They might be using analog voice, or they might using digital. (And this isn’t even counting wacked-out proprietary standards like OpenSky / EDACS / SmartNet, which are all additional technologies that need their own radios.)

Finally, someone saw the light. Thales has announced The Liberty, a handheld that supports P25 digital (and, of course, analog voice), and covers all four major bands. Pricing is rumored to be around $5,000, but you have to keep in mind that it’s doing the job of four radios that would probably cost $2,500 each. It’s SDR-based, front-panel programmable, and supports several different encryption protocols as well.

Thales previous filled a bit of a niche market. It looks like they do a lot of government stuff, and I seem to recall them being bigger overseas (but maybe I’m confusing them with another company). But this radio has a lot of high-end radio afficionados over here drooling. They not only did a dual-band radio, but they did a multi-band radio. It’s got a big color screen. (What the functionality is remains to be seen.) I think a lot of people, not just me, are hoping that this will lead to more competition, which will lead to more innovation.


I’m a long-term radio geek, and I’ve realized that the technology interests me more than actually using it. Having worked with lots and lots of radios (I realized that I have three sitting on my desk, all of which I have used in the past 30 minutes), I’ve concluded that I’d like to start a radio company. Our motto would be, “Our radios don’t suck.”

One of my radios is a ham radio, which is front-panel programmable (FPP), meaning that you can punch in frequencies on the keypad. This is pretty common with ham radios. By contrast, land-mobile radios (things that, say, a police officer would carry) very rarely have FPP capability; in fact, the FCC frowns on certifying radios with that capability, except for certain federal agencies that need to be able to reprogram their radios in the field. However, it’s often offered as a software add-on. But even using the ham radio, it’s really hard to use. Part of the problem is that the radio’s probably a decade old, and the print on the keypad has worn off. So I’m guessing at what buttons do.

There are very few radios with a graphic LCD. Dot-matrix LCDs almost seem cutting-edge in the radio world. By contrast, try to find a cell phone that doesn’t have a big color LCD on it. I have an old Garmin GPS III, and still admire that screen. I think it’s four shades of gray, and fairly high resolution. It’s a nice graphic LCD. It’s so much easier to use, and introduces stuff like the ability to “arrow” around a screen, as opposed to trying to use obscure key combinations. I’d actually love to see something like a 2″ by 2″ e-ink display (which, in addition to looking amazing, would reduce power usage), but it’d be a pain since it’s slow to redraw.

Motorola’s MDC1200 technology is practically ubiquitious in the public safety industry, transmitting a 1200 bps data burst containing a four-digit identifier. This could be so easily improved. Put a little $20 GPS chip in it, and have it transmit GPS coordinates on each transmission. (You could also include stuff like battery level, if on a portable, and information on received signal strength. The latter would be useful to run in the background and plot a map of the radio system’s reach.)

Programming is always a pain. Some of Motorola’s radios are programmed in ways that are so obscure that they border on comical. (I think the goal there is security.) I want to write an XML file for my radio. Put a USB port on the side of the radio. Let me hook it up to a computer, or just plug a thumb drive in and reprogram from that. But consider bigger problems, though. Boston PD switched to an “improved” channel lineup last year. Apparently they worked for weeks to pull radios in at the end of a shift, load up the new set of data, but leave the radios set to old configuration, until all the radios had the new programming in them. And then, at a quiet time one day, they broadcast a message telling officers how to switch to the new configuration. Over-the-air programming is possible, but it’s generally used in some specific situations. (OTACS, Motorola’s Over The Air Channel Steering, to direct a radio to switch to a particular channel, and OTAR, Over the Air Rekeying, to send new encryption keys to the radios.) Why not let the system send out bursts of programming data when the radio system is idle, loading up new programming data in the background, until they’re ready? Obviously, all of these programming things need some security constraints, but that’s trivial to implement.

I’m pretty confident that software-defined radio is going to become ubiquitous in the next decade, but no one’s really making use of it yet, except for uber-geeks in labs. APCO’s Project 25 digital voice (IMBE) has emerged as a standard in digital voice, but it’s meant to be made obsolete in the future by a “Phase II” implementation. Various other technologies have come and gone, such as Motorola’s VSELP. And there exist myriad trunking protocols for larger networks. I want to embrace SDR and use it in everything, “future-proofing” radios. (Of course companies have an incentive to not future-proof their hardware, forcing people to upgrade… But you can still make your money on selling software upgrades!)

Oh, and put an SD slot on the darn thing. Record the audio it receives, letting people play back transmissions they miss. Or host applications. (Or, permit programming!)

Lost in Translation

Check out this radio‘s description. It’s got excess value and pettiness! And a phrase-lock-loop. And “LCD aphellotropic lights of showing screen,facile operation in dark” has got to mean “backlit screen.” (Facile is Spanish for “easy,” no your mom jokes.) Oh, and you can’t forget the Auto-charger rabbet. And it comes with a chargeable battery.

Anyone else confused? (Bonus points: I tried to copy-and-paste a quote, and it carried over the HTML tags, which included <font face=”Wingdings”> at one point….?!)