Yesterday, I set up a separate antenna for the uBitX on my front porch, to test battery operations, potentially to use the rig in the field. The antenna I set up was not ideal, and my Elecraft T1 ATU would not tune it. But now I'm not so sure. I was powering the uBitX from a Stanley automotive jump and power box, the kind that has a lighter-type socket on the front, and automotive battery clamps on the back for jumping a car. The voltage is 13.7v, not a lot higher than what I was putting into it on the bench, which was 12.5v, but the available amperage was of course much higher -- this is a BIG jump box that I purchased for jump-starting my wheelchair van in an emergency.
I haven't measured the available amperage on the lighter socket, but I suspect it's much higher than the 1.5A I limited my bench power supply to giving. The bench supply has been set to 12.5v @ 1.5A, and with those settings I get almost exactly 5W out on both 40 and 20m into a big dummy load (the MFJ paint can model). But when I power it from the battery box, I get well over 5w, around 20 watts from whistling into the mike. The key gives a very high output power as well. It may be that it was too much output power for the tuner to handle, and it wouldn't such a strong signal.
My bench supply takes 19vdc in from a switching supply that gives 19v @ 3.5A, and allows the user to set a buck-boosted output between 0 and 18v, with commensurate amperage. The DC in socket I installed is the same size as the one on the uBitX, and most of my other projects, as I thought ahead to powering them. I just tested connecting the jump box to the bench supply, and the bench supply to the uBitX, and can keep the higher power from the battery from overpowering the radio. I'm going to order an extra bench supply module -- cheap on Ebay -- and install it permanently on the back of the uBitX to give me controllable flexibility in the output power. Running the uBitX at 12.5v @ 1A limit brings the output power into the dummy down to 2w.
Is there a point below which I might damage the radio from running it too low? Has anyone else had similar experience with too much amperage?
I'm going to rethink the speaker I have mounted in the case, as it doesn't have enough oomph to make it worthwhile. Or, I may add a small audio amp for the speaker. I have several in my junque box. Listening with a pair of headphones sounds great. I'm having no trouble with the audio on phones, just on speaker, and the incoming signals sound good. If nothing else, I have a 10w musical instrument amp from Monoprice on its way. I was going to use it for amplifying my harmonica, but I can use it for this without difficulty. It's a battery-powered model, so I'll be able to take it into the field as well.
I have some antenna kits on the way, including the multi-band antenna on the QRPGuys site and the parts for a "flimsy" W3EDP antenna on the way (I didn't have a 4:1 unun on hand; that's on order as well), and will report my results with them. I'm starting with a Sotabeams 40m end-fed with their "Pico Tuner", which I will be building today.
Gwen NG3P
Gwen Patton