note: first appeared in early 2015

In my day job I use often radios over very long distances.  To accomplish this the government has installed a series of repeaters in various locations around our six county area.  Repeaters make it possible for me to talk to folks hundreds of miles away as if they were sitting around the corner.  I wanted that same capability for any disaster or “grid-down” situation that I might have to personally deal with.  That led me on the journey to come up with my own repeater capable of operating in the field for long periods of time in rough conditions.  Here is that story…

Mission –

Provide the ability of handheld and mobile radios to communicate when line-of-sight was not possible.  Provide a dual band Ham radio unit that was field-going and sturdy/rugged to use when a repeater was not required.

Requirements & Limitations –

1. Must be capable of repeater operation on 2m and 70cm bands.
2. Must be a rugged, sturdy radio capable of operating in the field.
3. Must use 12vDC power.
4. Must be very conservative on power usage.
5. Should be easy to program, set-up and use.

Radio of choice – Yaesu FT-8800R (to read more about the radio read the 2-part article posted 3 days ago.).

Box of choice – Army Surplus 40mm ammo box in good shape, seal intact.

Notes before staring – I laid out the box contents first.  I kept moving them around until I had what I thought was the perfect location for each component.  I then used a felt-tipped pen to mark the location of mounting screws.  I did this to make sure that all the holes I had to drill made sense in relation to the box itself.

Step #1 –

I drilled a hole for the double-male 239 connector to fit in and pass through the box.  This gets the antenna lead from the radio to the outside of the box while making sure the seal stays “burr-free” and actually water tight.  The hole was slightly larger than the 239 barrel diameter.  The rubber grommet was purchased at Lowes.  I won’t put in the sizes of drilled hole, grommet, etc. since your 239 double-male barrel may be a different configuration or size than the one I used.  Take the 239 to Lowes and buy the right size grommet by trying it out right there in the store.  Then you will know the size of hole to drill based on the size of the grommet based on the exact pass-through you are using..

Same thing for the power leads, buy the smallest possible grommet to get the wires through, then drill the appropriate sized hole based on the grommet size.  I wasn’t trying to make it water tight, just “tight enough” to keep dust and rain out.  If I ever make the repeater a semi-permanent installation I will slather silicone seal all over the outside of the grommets.  Based on the “finished product” picture above, I drilled the holes on the back-upper-right-top corner.  It will be approximately the same height as the ventilation holes from the ground when the box is laying on its side for operations.  I drilled the holes from the outside towards the inside to make it easier with the drilling.

Step #2 –

I wanted to move some “cooling” air around in the box to keep the radio as cool as possible.  The radio itself has a cooling fan on the rear of the radio integrated with a heat-sink.  But I wanted to move air around and through the box itself to keep the operating temperature as low as possible.  Just in case the radio fan couldn’t handle sufficient air movement on its own, I wanted to give it a little help.  The fan is to draw air out of the box by drawing air from the outside through the three ventilation holes.

The fan is a 12vDC fan for computers that I found on Amazon.  I chose this particular fan based on the large intake area of the fan and the “flatness” of its construction.  The fan sits over the radio but to the rear of the box in relation to the radio’s built in heat-sink and fan.  In the event that the fan stops working I was trying to leverage the flow of air coming off the radio unimpeded.  So I drilled the three ventilation holes above the radio’s heat sink just in case.  I was thinking that natural air flow might move the hotter air out through the holes should the fan stop working.

I cut appropriate sized hole in the upper rear corner of the ammo can to match the outlet of the cooling fan. It required a square hole, so I drilled the center, cleaned it up with a saber saw, then finished it off with a flat diamond file.  Then I used another grommet on the squared-off hole before mounting the fan.  I wanted some cushion between the fan and the box to minimize any potential problem from vibration.  If you wanted to reverse the airflow to blow cooler outside air onto the radio heat sink you can reverse the power wires of the fan and it reverses the fan rotation.

Next I drilled the holes for the two screws that holds the cooling fan housing in-place.  But I did a trial run to ensure I knew exactly where the fan housing had to be placed to match up with the square hole for the fan’s square outlet/inlet.  Notice that I used a couple of rubber grommet again on the screws securing the fan housing.  But this time I didn’t worry about putting the grommets into the holes.  I used them as a “stand-off” to properly align the fan housing and absorb vibration from the fan.  But the grommets sealed the holes anyways due to the compression from tightening them.

Step #3 –

I am concerned about moving air through the box so the radio doesn’t overheat.  But I also don’t want to run down the 12vDC battery that will be powering this repeater either. So I decided to install this temperature controlled rheostat to drive the fan. Since I am using DC to power the unit, the rheostat controller will continually pass on some minimal current to the fan.  So the fan will always be turning a little bit.  However, if the temperature is within the operating limits of the radio I don’t need the fan spinning like crazy, moving hurricane force winds through the box and eating up precious ampere hours from the battery.  So the rheostat will pass on more current as the heat rises and the fan will then spin faster.  Result – the fan doesn’t use any more juice than is needed conserving precious power for actual radio operations.  But as the temperature rises the fan spins faster pulling the hotter air out of the box through the ventilation holes located above the radio heat-sink.

I mounted the controller board close to the fan towards the rear of the box.  A picture a little later in the article will show its position.

To find the rheostat I did my search on Amazon for this little electronic beauty.  But what I noticed is these electronic parts come and go pretty quickly (i.e. what is available today may not be available next month).  So I am not quoting a specific part number of even manufacturer.  Just get online and search for a 12vDC temperature controlled rheostat, there will be plenty out there for you to choose from.  Or contact your favorite electronics site and they can direct you from there.

Step #4 – 

Picture of ventilation holes with metal mesh screening to keep little critters out.

Next I installed the radio housing bracket on the radio unit and placed it in the box towards the left side of the box.  Don’t crowd the cooling fan and keep the radio as far to the left in the box as possible.  Don’t forget that you will have several sets of wires (antenna & power) coming out of the backside of the radio unit so don’t crowd the front of the box either.  Using a felt-tipped pen mark the holes for the mounting bracket.  Then placing the lid into position mark the center of each ventilation hole; they should be directly in front of and above the radio’s heat-sink and fan.  Drill the holes for ventilation and the mounting bracket.

To pull in fresh (hopefully cooler air) that will pass over the radio’s heat sink then out of the box via the cooling fan, I placed three 7/8″ holes in the box lid above the radio’s heat-sink but on the “side” of the box, not the top.  This would pull air over the radio’s heat-sink across the top of the inside of the box drawing the warmer air outside via the fan.  Should the box’s cooling fan fail, the radio’s own cooling fan should pull air into the ventilation holes from the outside, or push it out, not sure which.  This may create a “positive-pressure” environment inside the box which in-turn moves air naturally out of the box’s cooling fan hole.

To reduce the derbies that might come in the box that I don’t want in there (i.e. dirt, sand, leaves, critters, etc.), I placed a metal screen mesh over the holes. Making a rough calculation on the reduction of air volume due to the screen material I decided on the three entrance holes in relation to the cooling powered exit hole. I might have to drill another hole, maybe two, but I thought this was a good guesstimate. I drilled them 7/8″ to give me enough room to secure the screen to the lid on the inside. I used a metal-to-metal glue to attach the screen material to the box lid.

I may put a thin layer of cotton gauze over the holes in addition to the screening to reduce the amount of fine dirt/sand that can enter the unit.  However, I would drill more holes if I did that to off-set the reduction in airflow from the gauze.

Step #5 –

Next comes installing all the various brackets for the radio components itself. First up was the radio housing bracket.  In Step #4 I talked about mounting the radio so the radio’s heat-sink was located next to the ventilation holes in the box’s lid.  This allows for cooling air movement even if the cooling fan fails.  But it leverages the cooling fan’s movement of air when all is functioning normally.

For practicality it is also very convenient to mount the radio with the rear of the radio exposed for access to the antenna, power, and programming ports.  Yes, this means you are doing a “remote head” installation.  And in my opinion works out well as an added bonus.  The radio’s control head is disconnected from the radio and that adds to keeping it just a bit cooler.

Go ahead and take the radio housing bracket off the radio and screw it to the box.

In the next part we will finish up the build.

 

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