Many thanks to Bob (K7ZB), who shares the following guest post:

Experimenting with Low Voltage on the (tr)uSDX

by Bob (K7ZB)

I wanted to see how the (tr)uSDX would perform at lower voltages, so I decided to power it with a 6V Eveready lantern battery. With this setup, the radio delivered 1.1W to the antenna through an ATU-10, connected via 20 feet of RG-316 coax to a 20M Hamstick mounted off my condo balcony. I also used a wire counterpoise to complete the system.

Initially, I had planned to use a variable buck converter to step down the voltage from my 12V battery packs incrementally while measuring output power.

Unfortunately, the buck converter couldn’t handle the radio’s load and tripped offline each time I tried to use it. While it wasn’t suitable for this experiment, it’s still a handy and flexible little converter for other purposes.

Testing with the Lantern Battery

Switching to the 6V lantern battery, I used an antenna analyzer to fine-tune the counterpoise length, geometry, and placement. I achieved an acceptable SWR just under 3:1 at the radio end of the coax, which the ATU-10 easily matched down to 1:1.

Transmit performance was promising—I received a 10dB RBN report from AC0C in Kansas City at sunrise, running 1.1W. I also monitored my own signal using an ATS-20 receiver inside the condo.

The Receive Issue at 6V

However, reception was a different story. While the transmit side worked well, the receiver performance was significantly degraded. At 6V, my Malahit (tr)uSDX, which is normally a solid receiver at 12V, struggled to pick up signals. The noise level was noticeably high, and I could barely hear anything. I’m curious if others have experienced similar issues when running this radio at lower voltages.

Success at 12V

To compare, I switched to a Talentcell 12V 3000mAh Lithium-ion battery pack, and the radio instantly performed as expected. With 5W output, I quickly made a QSO with a station in Vermont—a night-and-day difference from the 6V test!

What a delightful little QRP rig this is!

73, de K7ZB
Bob
Gilbert, AZ