Metal case for duplex hotspot

posted in: DMR, Ham radio | 2

Unlike the “Jumbospot” most duplex MMDVM hotspots are not sold in a case.  You simply get the “modem” board and normally need to buy a Raspberry Pi and case separately.

 

I’ve tried making my own case from laser cut perspex, but it ended up being quite bulky, and I prefer to put RF devices into metal boxes, so I started to look for a suitable case which could be modified and eventually found this one.

As links to online vendors go dead very quickly, I’m not going to post any links, but if you search for   Aluminium case Raspberry Pi 3   you should find cases which look like this .

 

 

 

 

My initial concern with using a metal case, was that the WiFi would not work very well, but somehow the 2.4GHz WiFi signals must be able to leak out of the case, because I found that the WiFi connectivity was not a problem at all, and it even worked where I thought my WiFi router signals were quite weak.

 

There are 2 main problems with fitting the hotspot into the case…

  1. The antennas
    With this box, I was able to drill holes in the top which were just big enough to insert antennas that normally come with the hotspot board.
  2. Securing Raspberry Pi to the case and still being able to secure the hotspot board to the raspberry pi.

 

With this box. The Raspberry Pi board is only secured by one screw,  which attached to the post in the lower right corner of this photo.

Additionally the end of the RPi which has the USB sockets rests on a shelf which is part of the end cap, and the USB sockets are a reasonably tight fit, which overall seems to retain the RPi board well enough.

 

 

What the case does not have however, is any way to access the bolts that I use to secure the hotspot board to the RPi.

There are several different ways to secure the hotspot to the RPi, and I normally use 2.5mm bolts (about 30mm long), with plastic or metal tubes between the hotspot and the RPi on the side of the hotspot furtherest from the connector.

The other way to do this is to use threaded pillars, and small screws, however I couldn’t find any suitable pillars which didn’t bump into the USB power socket on the RPi, but I had some thin metal tubing.

So my solution was to drill 3 access holes in the base of the box, though which I could insert the hotspot securing bolts, after the RPi had been bolted in place.

Note. Its not possible to access the securing screw if you try to pre-assemble the RPI and hotspot together, because the hotspot gets in the way.

I guess it could be possible to cut the corner off the hotspot board, to allow access to the securing screw, but I don’t mind having some holes in the base of the case, as it helps a bit with ventilation 😉

 

Here is the case, with the RPI fitted and secured by the single screw, which can be seen in the upper left corner of the board in this photo.

I’ve also fitted heat sinks to my RPi 3B because my shack often gets to 30+ degrees C in the summer, and a little extra cooling does no harm.

 

 

With the RPi in place, the 3 bolts that secure the hotspot can be inserted though the access holes in the base of the case

 

And then fed through the metal tubes, before being secured by a nut

(In this photo I removed one end of the case so that the tube could be seen)

 

 

To stop the nuts working loose, I put some clear nail varnish on them, because its quick drying, and cheaper and easier to get hold of than threadlock.

 

 

 

You’ll have noticed that my hotspot seems to have 3 extra wires which go down though into the RPi.

These are the wires which I use to control the hotspot using a 2 position switch, and connect to a RPi GPIO pin which is accessible on the hotspot board.

At the moment, I don’t have a small switch which is suitable for this case, but I didn’t want to unsolder the wires, so I’ve simply pushed them out the way to stop them shorting onto anything.

 

 

And this is what the final case looks like.

 

Note. I’m using right angle adaptors on the antennas. This is for several reasons…

I think that having both the Tx and Rx antennas in the same polarisation , in very close proximity to each other is not a good design idea, and I always use right angle adaptors so that I can orient the antennas away from each other.

And… I find its handy to be able to stand the case on its end, and have the antennas as “bunnies ears”,

And.. I can almost completely fold the antennas away, but unscrewing then a bit and rotating them to be along the length of the case.

Unfortunately the antennas are slightly longer than the case, but only by a few millimetres.

 

 

My next upgrade will be to refit the switch, as I think I will be able to remove the end cap at the SD card end of the RPi and fit a small toggle switch, which I can then use to easily switch between either it being a Simplex or Duplex hotspot, or use it as a network switch between BM and DMR MARC, or possibly use it as a frequency switch for instances where I’m in proximity with another hotspot on the same frequency.

 

I’m also hoping to make a display that I will fit to this case, but that’s a far bigger project

2 Responses

  1. Filipe Miranda
    |

    Hello, amazing project! I will try to replicate this for my hotspot. What did you use around the holes for the antennas? I see some sort of black rubber ring. Where do we get that?

  2. Roger Clark
    |

    I did not use any rubber around the holes for the antennas, I think its just a dark shadow when I took the photo.

    The difficult thing is to drill the holes for the antennas. The Rx antenna is quite easy to drill, because the center of the SMA connector is aligned with the air holes in the top of the case.

    However the Tx antenna, is far more difficult to get in exactly the correct location, because you need to drill half way between 2 of the existing holes, and its almost impossible to do this, except perhaps if you have access to a accurate drill press machine and can clamp the case into a vice.

    But it does not matter very much if the hole is a little off-center, as you can just drill the whole a bit larger and it still is OK.

    I normally use 90 deg SMA adaptors, for the antennas, so the holes do not need to be as large as if you want to screw the antennas directly onto the board

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