This article will give you a rough idea on design considerations to build your own temporary use portable repeater.
On my workbench it is sometimes handy to have access to a full repeater for testing various radio configurations. I can also deploy it in the field to support an event if required.
Some time ago I landed on some Motorola Maxtrac 100 and 300 radios in the amateur band along with an Arrow Repeater Controller at an estate sale. Adding in an Astron SS18 power supply, some power distribution, cooling fans and some hand built cables, and a 2U road case for transport and durability, the parts list for the repeater was complete.
As you can see it is fairly compact. As pictured here it is on VHF at 5W, more than enough for bench testing related projects or for local event coverage along with a decent antenna like I keep in my RV.
Clockwise from upper-left corner you see the Astron SS-18 power supply, powerpole distribution, the cooling fans at the rear of the transmitter, the transmitter, the controller, and the receiver. This repeater is mains powered however the distribution strip has an input connector at one end so it could also be powered by a 12V power source if required. While the SS18 is overkill for this application, should I ever tear this kit apart I can use it for other things.
The cooling fans were one of the largest challenges when building up this kit. First off, the Maxtrac series radios are not designed for continuous duty use, like many radios so extra cooling is essential. Additionally Maxtracs have built-in circuitry to turn down the transmitter power if they have been transmitting for a an extended length of time no matter how much you have de-rated the TX power level.
A 12V fan can move a lot of air and is noisy both mechanically and can also be so electrically. I needed to find a balance of electrically quiet and mechanically quiet yet moving enough air to keep the transmitter happy and on the air. The fans that I settled on were removed from HP DL380 servers and are 1.5″ tall and 1″ thick and sound like jet engines at a full 12V, so it was simple then to turn the speed down – connect two in series. I get a balance of air flow and near silent operation in this way. Lastly they can throw a lot of electrical hash on the signal and so you’ll see a toroid with 15 windings (whatever I could fit on it really) in one corner which virtually eliminates the noise issue.
Shielding is key when building kits like this. When I received this controller it was simply a plastic enclosure as it was designed to be used with a couple of HTs in either in-band (one-way) or cross-band (two-way) repeat mode. The shielding I added in is MG Chemicals Super Shield and, while expensive, a little goes a long way. I use the silver spray version of this product for simplicity. Similarly, RF Cables are custom built RG-400 with mini UHF at one end and N at the other.
This kit is designed to be modular with any Maxtrac series radio, a key factor for ease of use and flexibility in future. It’s one of the key reasons I went in via the front panel port instead of cabling off the rear. Some Maxtrac use a 5 pin rear connector and some use a 16 – and mine had no connector at all. As with all of my dedicated repeater gear, the power output is noted on the radio along with the frequency in complete nomenclature along with a colour scheme – Red for TX, Blue for RX – which carries through all elements of this build to keep it as goof-proof as possible during set-up.
The same nomenclature and colour scheme are seen here on top of the input cavities of the duplexer.
This project used some lightweight cavities for its duplexer. Again, markings on every cavity are key to ease of use during set-up.
I hope this project, like others posted here, give you ideas for your own kit.