Bob Tests the (tr)uSDX at Low Voltage

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.

DROK DC Buck Module, Adjustable Buck Converter Step Down Voltage Regulator 6V-32V 30V 24V 12V to 1.5-32V 5V 5A

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

3-D printed hamstick AZ-EL adaptor design by AB7E

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

Update from Bob: There has been some interest in the AB7E antenna balcony mount designed for me and he is receiving requests directly as well as comments on the post.

From AB7E:

Well, I’m happy to provide the design files to anyone who wants to get them 3D printed, but it takes roughly 30hours to print them and I have no interest in printing and selling them myself. I would have to charge far more than it is worth. I made that one for my friend Bob because he wanted to mount his Hamstick to point out at an angle like shown in the picture, but in my opinion it would be just as effective (and far cheaper) to simply clamp the metal bracket to the balcony railing with a C-clamp or similar and have the Hamstick or other whip antenna point out horizontally. The bracket can be bought from Amazon: https://amzn.to/4aLVTR4 (QRPer.com affiliate link)

20 thoughts on “Bob Tests the (tr)uSDX at Low Voltage”

  1. Hi Bob,
    I often feed the little wonder with 5v USB, and I have never noticed a degradation of the characteristics in reception, I don’t understand why it should. By the way, you used 6 volts. I’ll do some tests and get back to you here.
    73 IW2EPE Luca

      1. Hi Bob,
        This evening I tried to compare the reception of a signal with 12.6v and with USB power. I didn’t notice any differences. I don’t have the possibility of 6v power, maybe Does it depend on whether the socket expects 12 V minimum? This is my guess. 73

      1. Please hold off on contacting AB7E until he determines how he will handle requests for the adaptor design. When he figures out how to make the design known I will let you know.

        This adaptor can also likely be used for small satellite antennas as well…

        Thanks!
        Bob
        K7ZB

  2. I very much would love to have a (tr)uSDX.

    Or just about any QRP radio.

    Actually, any HF radio would be a great thing.

    Real Hams HF. I am yet a failure.

    🙂

    1. Not if ,but when you take the (tr)usdx plunge, do your homework on resonant end fed half wave antennas. They will not tolerate hi swrs. I do know they are great on cw and also SSB in the field. I know a guy that’s had a lot pota contacts in both modes on one. They have pretty great receive and filtering for a looooow price. 73, Rob

    2. There are so many great affordable HF options now, Bernard. You’re diving in at a great time.
      That said, as much as I love HF (and this is a 26 year love affair) I firmly believe any way you do ham radio is REAL ham radio! 🙂 Doesn’t matter the mode or part of the spectrum. It’s all good!

      Cheers,
      Thomas
      K4SWL

      1. I’m just jealous 🙂 I like radio in all the forms I’ve found it, so far, and I have a few left to try.

      2. By the way, Thomas, your POTA videos are fun for me to watch because I know so many of the places you go “personally”. I don’t watch every second, because I can’t copy CW yet, but I enjoy the place and the setup and the radio components.

        1. I’m so glad you enjoy my videos and I really need to fit in SSB from time to time as well for those who don’t do CW. 🙂

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