The FX-4CR transceiver is a very compact radio transceiver for amateur radio, manufactured in China by BG2FX.
I’ve had a lot of fun using mine from mountain tops around Australia and overseas. When taking a radio on a SOTA mountain activation, I like to keep things compact, lightweight and simple. One thing this radio is missing is an integrated battery pack which means you have to take a separate battery pack and associated wiring.
I made a 3D printed battery pack that flange mounts to the bottom of the radio to produce an all in one package. The radio has a built in microphone on the front panel, so you don’t really need the plug in and use the fist microphone. The RIT button on the radio front panel can be programmed as a Push to Talk button to use in conjunction with the inbuilt microphone. I’ve had very good audio reports using the radio in this configuration.
The 3D printed battery box is pretty simple, it is affixed to the end of the radio using 12mm M2.5 cap head screws that replace the existing screws holding the radio endplate. The box is still a bit of work in progress. On the down side, a few connectors are covered up by the battery box, in the future, I might extend the connectors to the side of the box.
One of the criteria for the battery pack was safety while travelling. For this reason, the 18650 Lipo cells inside the pack are not soldered in but inserted into a battery tray. So, when I am travelling, say in a plane, the cells can be removed from the pack and individually carried in a cell storage box. The cells also include built in short circuit protection for additional safety.
The internal battery tray was made from a two cell holder and a single cell holder glued together so that they would fit in the space. Note that more cells could be packed in if the battery tray was not used. The cell holders are wired in series and connected to an XT60 female connector that can protrude through the radio end of the case. A 3 cell balance connector is glued to the side of the box which is connected to each cell. The cells can be balanced charged through this connector using a compact 3S charger made for model planes.
Initially, the balance/charge socket was on the end of the battery pack, but this was later changed to the side of the box to keep the overall length down and protect the socket somewhat. The combined radio and battery box still fits inside the original case.
Clik here to view.
Clik here to view.
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This is the radio in operation, hand held in this instance where space is restricted on a summit. In this configuration, the antenna is a Comet Toybox multiband loaded whip. For this antenna to work successfully, a counterpoise is needed that is matched to the band. I found that a 4 metre length of wire is a fair compromise. For 10 metres, I tuned the counterpoise and had to raise it slightly off the ground at the far end for a match of less than 1.5:1. Of course, the telescopic section of the antenna must be carefully tuned as well. It’s best to sort this out before you go mountain topping as it takes some time to get right!
Clik here to view.
I have uploaded the battery box STL file to thingiverse. Just search for FX-4CR, although it is still work in progress.
