Digi-POTA with a Battery Powered QDX and an iPhone

The QRP-Labs QDX is a five band, five watt, digital mode HF transceiver. It’s highly efficient, easy to operate, and configurable for bands from 2200 meter through 4 meter. As usual, Hans has created a very clever bit of kit for the homebrew-minded ham.

For my QDX build I added a rechargeable LiPo battery pack, charge circuit, and a USB power supply. I packed the build into an inexpensive extruded aluminum enclosure that I sloppily machined for access to the various connectors. The result is a very lightweight and nearly self contained digital rig that can be operated with nothing more than my iPhone and a simple wire antenna.

For details on the kit check out QRP-Labs.com.

Pros

• Lightweight & portable.
• Very efficient 5W amplifier.
• Generates pure signals, no side band image or carrier leakage.
• Highly configurable firmware.
• Supports any band from 2200m LF to 4m VHF.
• Audio input decoupled from RF chain, preventing overdrive and distortion products.

Cons

• Closed source hardware & software.
• Hardware lock out prevents running any firmware not signed by QRP-Labs.
• Densely populated circuit board can be a bit difficult to work with; desoldering SMD components accidentally is very easy (ask me how I know).

Building the QDX

Safety Matters!

Soldering irons are hot. Everything they touch gets hot.

Have a fire extinguisher nearby!

Solder splatters. Your eyes are not easily replaceable.

Wear your PPE!

Fumes from heating the enamel coating on magnet wire can be dangerous.

Work in a well ventilated area!

Tools and Materials

1. Soldering Station
2. Multimeter
3. Helping Hands or PCB Vise
4. Lead-Free Solder
5. Flux Paste
6. Brass Tip Cleaner
7. Solder Sucker or Solder Wick
8. Mini Pliers & Cutters
9. Jewelers Loupe

Enclosure & Battery Parts

1. Extruded Aluminum Enclosure
2. Battery Cells (3)
3. Battery Management System
4. USB Power Supply
5. Mini SPST Toggle Switch
6. USB Battery Charger (to charge the internal battery from any USB power source)

The QDX Kit

I won’t bore you with the details of building this board. The QRP-Labs team publishes fantastic instructions. The QDX comes in kits for the 80, 60, 40, 30, and 20 meter bands or the 20, 17, 15, 12, and 10 meter bands or you can provide your own filters to support other bands.

Whatever you choose wind the toroids to create the filters and transformers that you need. The official instructions include the values for the bands available in kit form. Take a look at my tutorial on the Twine Lock method for any easy way to create neat toroidal inductors.

When assembling be careful not to desolder nearby SMD components, which are often very close to the through hole pins. Take your time, be careful, and you’ll have no problems with this build.

Installing in the Enclosure

Now for the fun bits. I wanted to reduce the number of external components, cables, etc that I need to bring to operate from this rig. Ideally I wanted to operate with nothing except the rig, an antenna, and my phone. That means I need a built in battery and to supply USB power out from the rig. The reason for the USB power output is to feed power back to the phone. iPhone specifically does not appreciate USB peripherals drawing any amount of power. The QDX draws a bit of current on the USB connection, that’s probably not intentional, but it prevents operation from an iPhone unless that iPhone is charging.

All of that is relatively straight forward. Put the QDX in an enclosure, add a battery, a battery management circuit, a switch, a DC to USB power supply, and wire it all together.

That’s all this is but let me go over the details.

• On the front panel I drilled a small hole for the toggle switch and tightened it down. Just for fun I added a 3D printed switch cover to prevent the radio from turning on and draining the battery in my pack.
• I soldered three 3000mAh LiPo cells to the battery management circuit. A lot of people might skip this step but the BMS is an important component – it takes the battery out of the circuit if anything bad is happening (overcurrent, overcharging, overdischarging, etc).
• The enclosure I used is intentionally a bit wider than the QDX but that means I needed a method to hold the QDX in place. For this I just 3D printed a small raft that filled the gap next to the QDX. The raft has a groove on one side for the QDX to sit on and a tongue on the other to slide into the groove in the enclosure.
• On top of that raft I hot-glued the USB power supply. There weren’t any mounting holes on the USB buck converter I purchased for this purpose. While I was at it I secured other things with glue – the toroids, battery, etc. It may not look great but it holds up to being bounced around in my backpack.
• Finally I wired all the power connections. From the QDX’s power input connector I ran power to the USB supply and one side of the toggle switch. From the other side of the switch I connected to the BMS. Grounds are just all tied together.

The only truly challenging part of this build is the front panel. The QDX has a USB (B) port, 3.5mm TRS jack, 5.5mm barrel jack for power, and a BNC connector for the antenna. My added USB power supply has to be accessible too. I machined the blank aluminum panel that was supplied with the enclosure but I realize that’s not accessible to most people.

A 3D printed panel would be almost as good, not quite as strong and not a great RF shield, but overall a good choice. I actually 3D printed panels just to double check my measurements before machining the aluminum panel.

Operating from an iPhone

Operation from the iPhone is limiting but completely acceptable in my opinion. I purchased the iFTx app for a small fee (no subscription nonsense) and everything just works. I’m not affiliate with the developer and don’t receive a commission if you buy the app. There are other options but iFTx seemed to be well made and it’s worked fine for me so far.

USB serial for CAT control doesn’t seem possible on iOS, it likely is on Android but CAT isn’t mission critical for me. I just turned on VOX and tuned the thresholds at home and the QDX remembers my configuration. Most of the time I just leave the QDX configured for 20M FT8 but I sometimes configure another band and mode combination before setting out.

The USB audio device presented by the QDX is sometimes not detected by the iPhone, I’m not sure why but disconnecting and reconnecting the USB cable or just power cycling the radio usually resolves the issue.

Starting with a charged iPhone is best because the radio will export power until the phone is full and then keep it topped up. Powering from an external supply is possible too (link to USB battery charger).

The radio weighs in at about 450g (1.1lb) with a battery large enough to operate for days. The full kit including cabling, a simple wire antenna and tuner, and my arborist kit for getting the antenna up a tree comes in at 1550g (3.4lb). The whole thing fits in a bag that’s barely big enough for my lunch.

Conclusion

Keeping the rig as small and lightweight as possible were my primary goals. I’ve got a few extra pounds on me and I’m not in peak physical condition. A lightweight radio means I can get out more often and go an more interesting adventures. Digital modes are perfect for getting a successful activation even in poor conditions.

This QDX has performed very well, it’s never failed to successfully activate a park or peak with me. The battery pack lasts several hours, I estimate five to seven hours of operation are possible on a single charge even while charging my phone too.

There are a couple negatives. Operating from an iPhone is somewhat limiting. I have no CAT control and no ability to debug or configure the radio since Apple locks down iOS so much. An Android phone would be more capable but something like a Raspberry Pi would be best. My biggest pet peeve though is how Hans has locked down the hardware so that it’s not capable of running any firmware besides his.

In the end the pros beat the cons by a mile. I plan to build a couple more rigs based on the QDX for various projects in the near future. Stick around to see how far we can push this little transceiver.

73, AI6YM