Radiomaster Boxer Folding Antenna

A low-profile, folding antenna for the Radiomaster Boxer that you can leave installed when you throw the radio in your bag! The antenna bolts on using the screw holes for the carry handle and can be installed without any disassembly of the transmitter. The bracket that connects to the transmitter doesn't extend past the module bay, so it shouldn't interfere with any modules and doesn't affect how the radio sits on its back.

There are two versions available: One for the standard Boxer which requires the carry strap to be removed, and one for the Boxer Max that bolts on top of the metal folding stand/carry handle. I have not tested the non-stand version so please leave a comment if you print it!

Bill of Materials

You will need the following to assemble the antenna:

• 1x Happymodel 90mm 2.4GHz ELRS antenna
• 1x Right angle RP-SMA crimp connector for RG-174 or similarly small coax cable (I used Digikey Part#CONREVSMA012-G-ND)
• 4x M3x14 socket head cap screws
• 2x M3x10 socket head cap screws
• 2x M3 nylock nuts
• 2x M3 washers (optional but recommended)

Assembly

Step 1: Prepare the Antenna

The antenna has be inserted mid print, but stripping the insulation is much easier before the antenna is trapped in the part.

First, cut the antenna to ~85mm in length as measured from the top of the T-housing to the end of the wire.

Strip ~10mm of the outer insulation off the long leg of the antenna. The best way I found to do this was to gently squish the wire against a cutting mat with a very sharp blade, then rotate the antenna while keeping the knife stationary.

Push back the braided shielding and strip ~1mm off the inner insulation. Tin the exposed inner conductor wires with a little bit of solder.

Step 2: Print the Parts

.3MF files are available for both parts with my preferred settings and the pause included at the right layer. I recommend 0.15mm layer height for both parts.

The transmitter bracket will need supports in the counterbores for the screws, but should print fine without any other supports from PLA or PETG. The Antenna Case needs supports under the overhangs near the pivot.

The Antenna Case print needs to be paused before printing above 7.3mm to allow the previously prepared antenna to be inserted. When inserting the antenna make sure to push it all the way down along its length and tape the loose end to the bed. There are some retaining features in the channel that will help keep the wire in place during printing.

Resume printing and forever seal the antenna in the case. Once the print completes the antenna is ready for assembly!

Step 3: Assembly

Feed the antenna through the hole on the transmitter bracket. Next, you'll need to snap the antenna case into the transmitter bracket. The antenna case has to flex slightly for this so there will be a little resistance. To avoid crushing the cable as you slide the parts together, push the antenna cable into the slot near where the antenna exits as shown below.

Insert the M3x10 screws, nylock nuts, and washers as shown below.  Pull the antenna cable into the small channel on the antenna case to allow the socket head cap screw to pass through on the antenna side. Getting the washers lined up can be a little frustrating, but having a small screwdriver to push them around helps.

Tighten the screws until they're both just barely snug, then use them to adjust folding tension as desired. The screws need to be turned with the short end of an allen wrench a little bit at a time. When they're all the way snug you should see the screw just barely poking out of the nut as shown below.

Step 4: Terminate the Cable

Now's a good time to put a piece of shrink tube over the antenna cable to cover up the solder in later steps!

The braided insulation needs to go over the knurled part of the connector, but if you're using a connector like mine the tiny antenna shielding just won't expand that far. Expand the exposed shielding braid with a tweezers and cut a slit in it with a pair of wire cutters.

Pull back the braid and slide the insulated inner conductor into the connector. Wrap the braid around the outside of the knurled shaft on the connector. Solder the inner conductor to the post inside the RP-SMA connector, and solder the braid to the outside of the knurled shaft.

If it seems like there's too much shielding braid to solder on cleanly you can cut a little bit off. Similarly, if some of the braid comes apart when you pull it back don't worry.

Cover up your shameful soldering with some shrink tube. Pound in the plastic and brass disks that came with the RP-SMA connector to cover up the inner conductor post.

Step 5: Installation

Screw on the RP-SMA connector so that it's facing straight back, then bolt on the transmitter bracket using the M3x14 screws and go fly!

RF Performance

I have not done thorough range testing, but I compared the performance of this antenna vs. the stock antenna using an ImmersionRC RF Power meter. Without the plastic case the Happymodel antenna soldered to the RP-SMA connector measures close to or maybe ~0.5 dBm under the stock antenna. Once enclosed in the Galaxy Black PETG Prusament printed case the antenna measures ~1dbm lower than the stock antenna at a distance of ~2" with 250mW transmit power. 

This is a very crude measurement method where small changes in the position of the meter or antenna can affect the readings by a few tenths of dBm very easily, so take these results with a grain of salt. The readings fluctuate quite a bit as well, so pictures don't tell the whole story, however it does seem like there is some measurable decrease in performance vs. the stock antenna.

Settings used on the transmitter and the RF Power Meter:

Testing pictures and results vs. the stock antenna:

Antenna measurements without the case:

UPDATE 10/28/23: A commenter pointed out that aluminum in the glitter of the Prusament Galaxy Black PETG could have a negative impact on RF performance so I also printed an antenna from Prusament Chalky Blue PETG. I was not able to measure a difference between the two with this setup, so glitter filaments seem to not affect RF performance substantially.