it’s Tetra time .. so DMO yourself a favour and try it out ..

Time marches on, and it’s hardly news that our MotoTrbo DMR repeaters GB7MB and GB7UZ and the short lived TAC-9 have both passed into memory, but it’s okay, P25 continues in the Morecambe bay area, and although P25 is approaching what must be it’s seventh year or so of usage around Morecambe Bay, it was recently given an additional boost last year too with Dave G6CRV’s construction of the P25 – DMR repeater, GB7MP. Nice one Dave.

For several years I’ve been reading about Tetra, Many years back i purchased an old Dolphin Tetra HT just out of curiosity, but at that time P25 and even DMR overtook my Tetra interests back then, and the crappy old Dolphin HT stayed in the junk drawer, and i think i even threw it in the bin eventually.

Until a few months ago I’d been playing with SDR Tetra receivers, and had been doing so for a few years, Telive and Gnu Radio weren’t easily accessible for most, as for years it was the preserve of Linux users who love compiling stuff from source, not exactly a turn-on for most SDR users, but as i am pretty comfortable with Linux, starting as i did, messing with RedHat Linux and Solaris back around 1997/1998 after i upgraded from NT4, so it was easy, 
It has been interesting to see Tetra SDR decoding move over to windows and become somewhat mainstream, now, where it’s pretty much as easy as installing any Windows SDR program. and the looming ‘lockdown’ seemed like a good time to pick up a couple of bits of Tetra stuff to play with, so a couple of MTM800s were bought, an MTM800e and a MTP850S and a whole pack of paracetamols – for the inevitable headaches coming my way.

This is a Motorola MTM800E

Tetra sets are often a little low powered in comparison to other PMR sets, MTM800s are all around 3 or 4 watts RF out which is a little disappointing, but as radio amateurs we are quite capable of pushing a low power signal over a reasonable distance, negating losses, and and tweaking aerials etc, so not a huge hurdle, but one to bear in mind.

Typical Motorola build quality,  Physically it’s a standard DM4600 type body and uses the same mounting brackets and screws, power lead and fist mic. There’s no infernal speaker, so you will need a suitable connector, The DM4600 type connector do look similar on a computer screen, but thy are not,  they are different dimensions, so i picked a small box of used 16 pin connectors over on Epay, the rear connector is 20 pin and you do need the full 20 pin connector for the programming lead, (buy one instead) but a 16 pin connector will be just fine for speaker output. They have a nice chunky underside heatsink, a very familiar casing, and DM4600 mics will fit.  From all i gather though, most MotoTrbo IMPRES mics will work okay, but the navigation keys aren’t supported, sadly. Some Epay sellers sell an MTM800E version with the Amateur 70cms band codeplug programmed in already too, but to me, i see no point buying the more expensive one, because you will still need to edit the codeplug for UK Ham usage anyway.  We use our CCS7 ID, but there is no reason to yet. Most Tetra sets you will see are 380mhz to 430mhz, but with a simple codeplug hack, you can allow frequencies up to 440mhz to be programmed in to the CPS without it complaining then refusing, but the seller told me much above 435mhz and it’s going to be noticably less sensitive anyway, but it’s great at 430mhz and 431 mhz, which is where most Tetra users are anyway.

Most sets are 380mhz to 430mhz, but there are the rarer, (and pehaps slightly more useful) 410mhz to 470mhz versions, there are 800mhz variants too, but they are no use to us in Europe. Many sets will come with encryption in – so you may see ‘TEA’ mentioned. UK Airwave (emergency services) use TEA2, but that is of little or no interest to Amateurs as well as it being TEA2 being illegal to own, or have in your set, or something like that.

And this (below) is a MTP850S

Actually, it’s a MTP850s fug (German emergency service special) the ‘proper’ MTP850S just has a little speaker grill on the top front corner, but anyway, the MTP850S is a very popular Tetra HT, i prefer these as the MotoTrbo DP3600 / DP4600 speaker mics fit and they’re a nice size and form factor too, they have plenty of cheap accessories and some have a high-power option too, which this set has, nearly a full watt of RF output extra, and is, of course, switchable like on any other Moto HT.  The FuG version which you see here is a slightly different version to the standard MTP850S, and it has an interesting feature which enables it to be used as a DMO repeater, Note: There is a plain vanilla MTP850 also (no S or other suffix) which does NOT have the big chunky milti-way MotoTrbo DP3600/4600 external mic connector, instead, it has a more conventional 2.5mm and 3.5mm jacks instead. Perhaps DMO repeater function is of limited use on a HT, but very interesting nonetheless a bit of fun to play with all the same, and it does work too. The usual (hidden) extended MS147 Hirose external antenna socket is still there, in the back of the HT under a rubber flap, and is a little more accessible (but not much) than the (Motorola-special) MS147 in the DP4800 / DP4801 DMR HTs, but i have yet to try connecting it to an external aerial, Of course Motorola sell a special, elongated MS147 adaptor for testing purposes which needs to be as long as the MS156, but the Motorola specific version you need is, er,  as expensive as the HT itself (cheers Moto)
Standard MS147s just about reach the MTP850’s ext aerial socket, and could, with a little bit of non-invasive and careful work, allow connection to outside aerials to the MTP850S when sat in the shack.

It goes without saying that you can’t hear the emergency services on these,  that’s not something we’re interested in here anyway, we’re interested in the tech, as radio Amateurs.  Of course TETRA is first and foremost a trunked system,  with little (or no) accessible Amateur Tetra TMO repeaters in the UK – that we’re aware of, there are several TMO ones on the continent, but as we’re up in Lancashire – we use DMO mode – (rig-to-rig or simplex)

(below) the older version of the popular MTM800 (not ‘e’ suffix)

Much the same as the MTM800E but an arguably less attractive control head and a few other refinements, It’s a GM380 styled Tetra mobile, and some older GM series RJ45 mics and bits fit. The rear programming cable is exactly the same on both MTM800 and MTM800e versions.

DMO repeaters are easily available, but there seems to be a lack of interest in Tetra in the UK Amateur radio scene, although pleasingly there seems to be many little pockets of Tetra users all across the UK however, but it’s a great pity that there is no MMDVM or PI-star type device for Tetra, which would really create a boom in UK Amateur Tetra use.

I notice there is some illegal use around 421mhz, but low powered sets and inefficient aerials on HTs mean they will go mostly undetected. Some UK sourced sets come with something called ‘National 430’  already programmed in, in addition to often the unofficial 421mhz pirate channels, but with a centre frequency of 430.000 mhz, this means that half of your TX signal falls outside of the Amateur band anyway, so that is not a wise choice.

Many LPD devices use 430.000 as a spot frequency too, and a depressingly large amount of other ‘license free’ plastic Epay crap pollutes these frequencies on 70cms too, so we decided to use a carefully chosen frequency that’s largely out of the way of most things, and seems pretty clear, for the most part, so 430.3125  is where we live on Tetra. We chose TG 10 (in Tetraspeak that’s GSSI 10) although perhaps ‘1’ would be more logical.

Local usage around Morecambe Bay is of course, low, only three of us are using it for occasional local chats, but hopefully a few more Amateurs will take the plunge, and as our antennas improve, maybe with some Tropo in the summer months too, it will provide a little more chance of some activity, and if in the coming months we are all allowed out of the house on non-essential trips to go up the hills and go out mobile, perhaps the reasonably cheap MTM800Es on Epay will all get bought by UK Amateurs.

Tetra gear seems a little less easy to find for sale, but do keep looking. There are a couple of good sites on Facebook where there’s no clique, and all information is shared in good humour, the Facebook grup below is only for licensed Amateurs interested in learning about the mode, and not for those strange blue-light fantasists and plastic paramedics who like to dress up in high viz gear and try to look a bit official. You know the type 🙂

I monitor 430.3125 Tetra TG 10 most of the time on a HB9CV up 4 storeys pointing South from Morecambe and it looks like there’s going to be plenty of time for us all to play radio whilst sat at home, where we can even wait for a Tetra rig arriving in the post !