By Per Magnusson, SA5BYZ

The receiver is mechanically based mostly on 3D-printed parts. The antenna is described in a previous article, so this section focuses on the mechanics of the receiver itself.

The 3D models can be downloaded via the following link:

Link to a zip-file with the files:

I printed most of the parts using PETG which is tough, reasonably heat resistant and relatively easy to print with. I used a 0.4 mm nozzle. Some of the parts (gaskets) are printed in flex material (TPU).

The main part of the box is the bottom:

It prints mostly without supports, but some may be helpful at the USB connector and perhaps at the power switch:

There are twelve holes where M2.5 heat-set inserts shall be, well, inserted. The holes are designed to fit IUB-M2.5-1 inserts, which are sold by Mouser under the part number 153-IUB-M2.5-1. I used a soldering iron set to 250 degrees or something like that to press down the inserts in a controlled manner. There are also dedicated tools to do this.

This is what it may look like with the inserts in place:

To fasten the box to the antenna, one has to glue clamps to the bottom side. These clamps will see some force pulling the layers apart, so it can be a good idea to print them with 3-4 perimeters. The newest version of the 3D files for the clamps has some added tiny holes (not shown here) around the nut-slots to ensure even more perimeters are formed where the strength is needed.

I used epoxy as glue and a simple fixture to keep the clamps aligned. There are two areas with recesses on the bottom where the clamps shall be glued. The recesses make the surface uneven, which makes the glue stick much better.

The fixture holding the clamps in place during gluing:

A piece of 16 mm electrical pipe of the same type used for the antenna should also be placed in the clamps to perfectly align them. Such a tube is also perfect for applying some weight during gluing to make the glue joints thin and strong.

When the glue has set, it is time to put the RF board in the box. In an attempt to reduce water ingress around the SMA connector, I designed a little gasket that can be printed in TPU or some other flex material:

To make this really effective, I think it needs some help of silicone as well.

The power switch also has a TPU gasket (which probably needs some silicone too to be waterproof):

Two ~2 cm long wires need to be soldered between the power switch and the USB board.

The USB board can be pressed towards the panel to compress the gaskets of the USB and audio connectors using a little excenter piece that rotates around an M2.5 screw:

The RF board (with its shield properly in place) is secured by the nut at the SMA connector and four 6 mm long M2.5 screws.

The 1500 mAh LiPo battery fits next to the RF board while the USB board sits in the other end of the box. Notice that the excenter has been rotated by about 30 degrees to push the board towards the panel to the right. The other screw secures it in position.

The USB board holds the battery in place and is secured by one 10-12 mm long M2.5 screw and one that is 6 mm long.

The Processor board plugs into the RF-board and is initially held in place only by the two M2.5 x 6 screws to the right:

The Teensy Micromod is locked in place by a countersunk M2.5×4 screw, preferably with a little Loctite or other compound to ensure it does not unscrew itself.

A 30 mm long 20-pole FFC cable with 0.5 mm pitch and contacts on the same side on both ends connects the processor board and the USB board. Flip down the locking flaps to secure the connection after the cable has been inserted.

The LCD rests on a 3D-printed standoff, which also helps form a shield around the Teensy.

Place this standoff under the LCD board and wrap copper tape from the top of the LCD board around the walls of the standoff. Solder the copper tape to the LCD board and place EMC gaskets with 2×2 mm cross section on the bottom surfaces:

To make it easier to put the LCD board correctly onto the connector of the processor board, one can print a little shroud and put it around the processor board socket. This prevents the pin header from fitting if it is misaligned:

Plug the LCD assembly into the connector on the processor board and secure it with four M2.5 x 12 mm screws.

Cut a 33 x 52 mm piece of 1 – 1.5 mm thick clear polycarbonate or other transparent plastic that is suitable as a window over the LCD. Glue this (using e.g. epoxy) to the inside of the top cover. There is a 34 x 53 mm recess in which this window fits. Take care to not get significant amounts of glue on the visible part of the window, while still forming a glue joint that is waterproof all around.

Let the glue set.

Insert the connector and cable of the membrane keyboard through the rectangular hole in the box front:

There is adhesive at the back of the keyboard, but I think this is too unreliable for this application, so I recommend gluing the keyboard with epoxy. It might be a good idea to place masking tape above and below the keyboard before applying the glue to get less messy glue squeeze-out results than I did.

It should be possible to tear off the masking tape before the glue sets to remove the excess glue. The keyboard needs to be kept under pressure while the epoxy sets.

Use epoxy to also glue the keyboard to the inside of the front panel. The glue joint should form waterproof seals around the LEDs and the encoder. (The photo below is from an earlier version of the front panel PCB which was not quite compatible with the new keyboard pinout, hence the misaligned keyboard connector and the patched ground connector to the pin header.)

Plug in the battery and connect the 127 mm long 24-pole FFC cable to the processor board. Remember to flip down the locking flap to secure the connection after inserting the cable.

Plug the FFC cable into the front board. It is very important to observe the polarity! See the photo below.

To help make the box more waterproof, a gasket should be printed using flex material:

Place the gasket between the two pieces of the box.

Insert six square M3 nuts in the slots at the screw holes in the top piece. A little tack-it or similar sticky material can be useful in helping keep the nuts in place.

Screw the two pieces together using six M3x16 screws.

The encoder needs a knob and one solution is to print one using flex material:

Push it onto the knob shaft.

Insert four square M3 nuts in the slots in the clamps on the bottom side. Again, it is a good idea to secure them using tack-it or similar.

The receiver is now ready to be mounted on the 16 mm antenna boom using M3 x 14 screws.

One optional feature could be added to the receiver, namely a button that is accessible by the thumb while holding the handle. The button can be seen to the upper left in the photo above. The purpose of this button is to allow quick and easy access to activating the AGC while direction finding. By optimizing the gain while pointing generally in the direction where the signal is the strongest, the audio-RSSI (see the description of the software) becomes maximally effective in pinpointing in which direction the signal is strongest. Without this extra button, one has to reach for the top of the receiver and press on button 1 to perform an AGC setting, or use the knob to adjust the gain manually.

The thumb-knob is built into a little housing made of 3D-printed flex material. The box consists of a thin lid with a channel for the cable and the box itself where a piece of perfboard with a tactile switch can be slid in:

I have not made any custom PCB for this. A small piece of perfboard is perfectly sufficient. The tactile switch I used was probably a PTS645SL43SMTR92 LFS, Digikey P/N CKN10880CT-ND. The cable was a very thin two-conductor thing from the junkbox. I drilled a hole in the side of the receiver box, pulled the cable through and sealed it with epoxy. Epoxy was also used to glue the button housing to the clamp at the bottom of the box.

The cable was soldered to the pin for button 1 and ground on the front panel board:

Bill of Materials

Here is a list of the parts that are required in addition to the 3D-printed ones:

The BOM is included as a PDF in the zip archive linked at the top of the article.