Simple UBITX Test Set

Scott McDonald


You're right, I think, a simple cheap standardized UBitx test set would really be a great tool, not just for a Ubitx but for anyone starting out without any RF test equipment.

I started down that road for a club talk last year and built a single 2N3904 oscillator with a color burst crystal that gave signals in or very close to all the HF bands except 60 meters.  

By juggling a few values I was getting reliable output on all those freqs at -20 dBm plus or minus probably 5 dBm, and this was reproduceable with 5 different color burst crystals (tho from the same batch).  Not enough to drive a diode ring mixer but otherwise pretty useful and consistent, I think close enough that you could count on the output being what you expected without measuring it.  

As an option adding a buffer with another 2N3904 would make it more useful, or maybe just use one of Diz's inexpensive new SMT HF amp kits instead of reinventing the wheel.

Added an IF crystal from Kits and Parts (it was for a V3) with a cap and coil to fudge it a bit and got a useful IF signal at the same level. 

Added a couple 50 dB Pi attenuators to give a choice of 50 over 9, S9 and a few tenths of a microvolt for signal level.  Standard resistors used that way yielded decent attenuators plus or minus 3 or 4 dB.

Did it on a piece of perf board using 0.1 inch headers and jumpers to switch crystals and attenuators so there were no switches or other hardware needed.

It was dirt cheap and pretty handy, adding the diode detector would be simple, and the attenuators possible useful with that, same for adding an audio generator.  That's got to be a 10 dollar or less project with bulk parts purchases.

It would make a great first build project too.  

If I had any board skills I would make a board and put in Osh Park... :)  But if anyone with some skills wants to collaborate on it, I'd be happy to get together, as I expect people troubleshooting with a known test kit would make the great advice you, Evan, Allison and others give on here even quicker and more powerful.  

73 Scott ka9p

-----Original Message-----
From: Jerry Gaffke via <jgaffke@...>
Sent: Sun, Oct 25, 2020 10:50 am
Subject: Re: [BITX20] My son's V6

The primary selling point of the uBitx is its simplicity,
would be good to keep it that way.
The circuit improvements moving from V4 to V5 are a major win,
badly needed and no significant added complexity.

Though I would have kept the +12V for the IRF510's separate, much safer to 
debug all but the final amp of the transmitter with those IRF510's disabled.
And allows flexibility in setting the output power by adjusting the IRF510 supply voltage.
A simple way to adjust RF gain would be good.

Moving from the V5 16x2 LCD to the V6 TFT display adds lots of overhead
to the Nano's firmware.  I'd prefer the uBitx had kept the 16x2, at least as an option. 

An SWR meter and antenna tuner with a suitable dummy load for the uBitx 
might be a good accessory.  Audio and RF signal generators, a diode RF probe
(or AD8307), complete instructions to debug the rig, those would be most welcome.   
Perhaps make them all part of a single accessory?  An audio CW filter too?

Jerry, KE7ER

On Sat, Oct 24, 2020 at 05:49 PM, Bob Lunsford wrote:
Using a tuner on an antenna that is designed/dedicated to a specific band or frequency would theoretically funnel the signal to that specific frequency but it would also, theoretically, suppress received signals up and down frequency from the tuner's settings. Since it is also a good approach to getting as much power as possible from a QRP radio to the antenna by allowing the transmitter's output stage to see a much better load and lessen the complications of a mismatch, it would/should be an automatic part of the system unless weight or system complexity are concerns. It is undoubtedly a prime candidate for experimentation to see if a tuner does indeed reduce the influence of a high powered transmitter either nearby in frequency or location.
Thanks, Curt, for your feedback on this. Sometimes a simple solution may be best even if not a complete solution. I agree that AGC may be nice but it is not always the best or necessary. For example, on my G90 if you turn off the AGC it reveals many weak stations and even if some others come booming in, it's a small price to pay to guarantee that all stations on the [net] frequency are heard. Unless it is truly excessive in volume, of course, and this is "in the ear of the hearer."
Bob — KK5R

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