Ham DMR Digital Kicked Up a Notch

The new Ham Digital Mode of DMR has taken off in the last 5+ years and lots of folks are just getting into it in recently. The repeaters are great and some of the High Site one’s can give awesome coverage. This is even better when you have multiple repeaters with overlapping coverage and is connected to the same C-Bridge or Brandmiester system. We have 6 repeaters in Atlanta connected this way thru a common C-Bridge.

The largest issue is for folks running RT3’s, on UHF, where the vast majority are using a RT3 in the car with the rubber duck antenna. The issue is signal strength into the repeater from the portable. The RT3’s have pretty good receivers with some better than others but all work well. The 4 watt transmitters with a rubber duck inside of a car is less than ideal. Yes there are 25 to 45 watt Mobiles out there that power from the 12VDC car system and feed to an outside antenna, some of which have gain at the 440Mhz frequency range. These give very good coverage for those that have them and cost 3-5 times what one of the HT’s cost, which as of this writing can be bought on EBay and other places for $100 or less.

This very attractive entry cost for a single radio with new technology is inviting to new and older hams. So you have one of the new RT3’s and have used it mobile with mixed results. The most common mode is inside the car using the rubber duck that came with it, which come in small to big sizes. There are a couple of ways to improve on this without cutting holes or tapping into 12VDC power.

The first is an external antenna either mag mount, gutter clip (not many cars have the gutters on the roof anymore) or a trunk lid clamp mount from Diamond, Comet or others. The most common HT’s have a Female SMA connector on the top of the HT. This is a somewhat delicate connector used in UHF to Microwave frequencies with a near perfect impedance match for the radio. Adding this simple accessory can get your radio to an outside antenna with gain and give you something like 8-12 watts ERP with the gain antenna.

The picture below shows the basic use of the external mag mount antenna with an SMA connector and what could be simpler. Mount antenna on roof and connect to antenna port on HT.

You can take this to a next step (Picture below) by adding an external speaker/microphone to the RT3. This adds ease of operation, as you do not have to hold the radio with antenna cable coming out of the top of it while you talk. Set it in the drink cup holder and talk away. This picture also shows a battery eliminator for the RT3.

One thing that occurs in this process is the battery will drain over a period of time and most of the low end radios do not give you a low battery notification, they just shut down in mid transmission. There are two ways to get around this. Buy a second battery for the radio and carry it with you or get a battery eliminator. I always have bought an extra battery for every RT3 I have ever purchased.

One thing about external speaker mics for these low end RT3’s, get the manufacturers version and not an aftermarket one for 1/3 the cost of the original or one that has the same pin spacing that fits the side of the radio if forced into the plug. You likely will get motor boating on your signal because of RF feedback into the mic from the antenna. An external antenna can sometimes mitigate this issue.

Ok these options can give you a step up into better coverage from your RT3 in your car. The next step can give you the external antenna and a power amplifier to boost your power to 50 watts PEP. There are some amplifiers out on the EBay market from China that can take your 4Watt RT3 and boost it to 50 watts PEP. The DMR mode transmits from your RT3 only half the time because the other half of the time is used for synchronization to the repeater and for radio’s that Roam to check for signals from adjacent repeaters that may be stronger to switch to.

Some radio’s have SMA Male connectors on the top of the RT3 and when using a SMA Male to SMA Male connector cable will require a SMA Female to SMA Female connector adapter (shown below on the right) to use the cable. These are available at most hamfests and can be bought on line, as Google is your friend. If you have an antenna that has a PL-259 connector you can get an SMA Male to SO-239 jumper cable (shown on the left) to make the safe connection.

NOTE: Do Not use a BNC or SO-239 to SMA Male adapter to connect to the top of the RT3 or to the input of the amplifier as the first bump on the adapter will tear the threads off the SMA Female connector on the radio or amplifier.

Using the combination of RT3, Amplifier, External Speaker Mic gives you a very good High Powered Mobile vs. buying a 25-45 watt mobile. This way you still have the portable to take with you.

The links listed below show Amplifiers that take 4Watts DMR input and output 25 or 50 Watts PEP when connected to 12VDC. These amplifiers will work on DMR, D-Star, and regular FM. They also have a Receive preamplifier with a gain of 4db. They use Pin Diode switching and not a relay like some others use. The Pin Diode switching enables the amplifier to switch with the transmitted RF from the RT3. The amplifiers have an input SMA connector and an output connector of an SO-239. You will need to get a 4-5 foot jumper cable with SMA male connectors on each end to use the amplifier. These jumpers are available from several places including HRO. The amplifiers are priced in the low $100 range with some under $100. They come from China and take 4-5 weeks to arrive when ordered. Mine took 5 weeks. You have to be patient on this. Yes it says DRM but they are DMR friendly.

These amplifiers are small and do not get hot when keyed down for a long time. They will come with a 4-5ft power cord. I installed Power Pole connectors on mine and use a cigarette lighter plug with the power plug built in. My car is the first leased car I have ever had so no drilling holes.

Enjoy these notes on how to improve mobile operation of DMR RT3’s.

Author Note: I have been building Ham Repeaters in the Atlanta area since 1971. For the last 35 years I was in Land Mobile communications working for Dial Call, Nextel, Sprint and retired from Ericsson US. I have both D-Star and DMR as well as Analog FM equipment. I have partial ownership of 2 DMR Repeaters in the Atlanta area and I really like the DMR mode of operation.

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