Important  Information

DANGER : If you power up the µBITX without the pull-up resistor the µBITX can randomly go into CW transmit.   Have you installed the 4.7k pullup to 5v on the CW key pin?

SHIPPING:  µBitx orders placed ca. 21 December 2017 currently shipping.


  Every Sunday – 3PM & 7PM Local Time
7277 kHz in North America, 7177 kHz elsewhere.

How to even out uBITx power out

Bill Schmidt K9HZ has come up with a way of evening out the power output from the uBITx.

He says, “I had a couple of hours of spare time today so I started off my measuring the gain of the pre-drivers in the uBITx.. and sure enough, there is a lot of variation from 1.8-54 MHz (where I want mine to work).  Substituting the RD15HVF1 (my choice of RF PA) into the circuit with no other changes gives results similar to what John saw.. but it is because of the pre-driver stages.  I contemplated ways of compensating the drive with frequency but there isn’t a really good solution using reactive components because it causes some other non-desirable behavior (like a peak in drive at 21 MHz that is too high for the PA).

“I did land on a rather bruit force method that does work well… I removed RV1 (drive control) from the circuit board and replaced it with a tiny board with three small relays and four 100 ohm 10 turn pots.  The relays are controlled by the KT1, KT1, and KT3 drivers…  The short story is that now I have gain that is adjustable for essentially each of the bands (at least sets of bands that follow the LP filters).  I’ve adjusted the drive so that the PA puts out the same power (+/- about 2 watts) across the entire frequency spectrum.  I suppose this should have been an obvious answer, but I’m not fond of using relays for stuff like this even if they only draw milliwatts…”


Wind 32AWG onto small toroids


Eric KE6US uses big blunt upholstery needles to wind toroids.  Just twist one end of the wire on. Thread it through the toroid, then just let it drop from gravity. It’ll weight the end down while you sort out kinks, etc., then pick up the end and drop it though again. Sometimes I’ll stand while I’m working if the wire is long. It goes very fast once you get a rhythm going.


Power Amp fix

Erhart DF3FY, as well as providing a fix for the audio pop problem, has upgraded his PA output stage with RD16HHF1s.  Erhart suggests  the IRF 510s are not ideal for portable field operation.  The small output transformer represents a mismatch for power outputs greater than 5 W so he has incorporated a new output transformer.  He is now achieving 18w output on 20m and 12w on 10m, with an average of 15w output on all bands.

For details see his PDF file: fixes PA.

Extron case

There has been a bit of discussion on the list about Extron cases.  These are aluminium cases that can be found with a search on eBay that come with other componentry that may be useful for a uBITx builder.

Vince Vielhaber KA8CSH found one that was big enough for a Bitx and would be a good fit. The BITx40 fits quite comfortably in it (after moving the power supply), but the only way the uBITx will fit is if the raduino is detached.  As you can see in the attached pics, it has plenty of BNCs for other projects and a power supply. Being 12v and 5v, the 7805 on the raduino could potentially be eliminated (provided, of course, the supply puts out over 1 amp).


W0EB/W2CTX Firmware release v2.01R

Jim Sheldon W0EB has announced a further release of Firmware (v2.01R) of the W0EB/W2CTX firmware intended for I2C driven displays (including the newly announced “RadI2Cino” card which is an (almost) drop-in replacement for the Raduino.

This release effectively covers all the enhancements for the non-I2C Raduino released earlier (v 2.00R)

The following changes have been incorporated in the firmware:

“The CW Keyer module has been completely re-written in this version. The keyer is now totally interrupt driven which helps make the transition to CW transmit much faster and it doesn’t tend to clip the first dot as it did occasionally in past versions. This version also moved the DASH paddle input from the analog A7 input. It now shares the analog A3 input with the PTT line as normally PTT isn’t used in CW mode and the Hand Key isn’t used in CW Paddle mode or the Paddles aren’t used in CW Hand Key mode so all three can share the same input easily. This gives A7 back to those wishing to use it for metering applications. Complete, illustrated documentation on everything in this release is contained in the zip file as a PDF. Menu operation has been slightly improved from V2.00R as well. We hope you enjoy using this version. Jim – W0EB and Ron – W2CTX”

You can access the release details file as follows:


Top Band (160m) external LPF

Bill NG1P provides a bit more detail on his Top Band (160m) mod success.

“I’m having a blast on 160m I did add larger heat sinks and a pair of small fans that I can turn on and off as needed to keep the finals cool if I’m long winded or using digital modes.

“Qrp labs makes a great low pass filter kit:
Read previous news iteM

RX-TX Audio Pop Fix

Erhard Haertel DF3FY comments, “There are a lot of proposals for addressing the RX-TX audio pop with the uBITX, but none of them really works”.

Erhard has come up with his own pop-fix that involves muting the TDA amplifier during TX, turning the IC on with the 12V RX line (to prevent SSB noise in headphones during transmit) and additional decoupling for the 12V line.

Erhard says, “This fix cures all the issues”.  Write and tell us if you have tried this fix.

Anderson Powerpole chassis mounts

Dave WI6R showed off his aluminium chassis to the list, but this resulted in questions about how he mounted his Anderson 12v powerpole connectors.

The answer is that you can use either of these mounting systems (the second was used in Dave’s case):


Note, that the mouting clamps do not have to be mounted on the exterior of an enclosure.  They can just as easily be mounted on the inside, making the who affair look more presentable.

Experimenting with Mitsubishi RD16HHF1s

John VK2ETA has done a strait replacement of the IRF510s with RD16HHF1s in his uBITx.  You can see from the photo above that he has them installed cross-legged.   John says, “To replace the finals I simply cut the legs of the IRF510s about 3mm above the board and correspondingly cut and crossed over the drain and source pins of the RD16s to match, then soldered in place”.

What follows are the before and after values of output power and PA current that John measured.   All tests were done with the uBitx VR1 drive level in the same position of approx 60% of range.

1. IRF510s and main board at 12.1V. PA idle current checked at 0.20A total (factory setting) so assume 100mA in each final.

(For info, Rx currents: 164mA no volume, about 209mA “normal” volume).
– At 7.1Mhz: 10W, total current: 1.79A, of which PA current: 1.31A, therefore main board current 0.48A
– At 14.2Mhz: 5.5W, total current: 1.39A, of which PA current: 1.0A, therefore main board current 0.39A
– At 21.2Mhz: 2.2W, total current: 0.95A, of which PA current: 0.53A, therefore main board current 0.42A
– At 28.1Mhz: 1.3W, total current: 0.95A, of which PA current: 0.53A, therefore main board current 0.42A

2. IRF510s with 16.5V, 13.8V for main board. PA total idle current checked at 0.21A.
(For info, Rx currents: 188mA no volume, about 230mA “normal” volume).
– At 7.1Mhz: 19W, total current: 2.65A, of which PA current: 2.09A, therefore main board current 0.56A
– At 14.2Mhz: 11W, total current: 2.20A, of which PA current: 1.80A, therefore main board current 0.40A
– At 21.2Mhz: 5.5W, total current: 1.40A, of which PA current: 1.00A, therefore main board current 0.40A
– At 28.1Mhz: 2.2W, total current: 1.02A, of which PA current: 0.60A, therefore main board current 0.42A

John hasn’t managed to find a definitive reference for the safe and optimum values of the RD16HHF1s idle bias current but it seems to range from 200 to 500mA. He would not recommend long term usage of the 500mA bias used for these measurements and will reset his idle current to the 400-450mA range.

3. RD16HHF1s and main board at 12.1VDC, 250mA idle bias each (Total 0.5A PA idle current).
– At 7.1Mhz: 10W, PA current: 1.20A
– At 14.2Mhz: 9W, PA current: 1.21A
– At 21.2Mhz: 4.5W, PA current: 0.65A
– At 28.1Mhz: 5.5W, PA current: 0.95A

4. RD16HHF1s and main board at 12.1VDC, 500mA idle bias each (Total 1A PA idle current).
– At 7.1Mhz: 10W, PA current: 1.18A
– At 14.2Mhz: 9W, PA current: 1.26A
– At 21.2Mhz: 5W, PA current: 0.71A
– At 28.1Mhz: 6W, PA current: 1.11A

5. RD16HHF1s and main board at 13.8VDC, 500mA idle bias each (Total 1A PA idle current).
– At 7.1Mhz: 13.5W, PA current: 1.95A
– At 14.2Mhz: 13.5W, PA current: 1.93A
– At 21.2Mhz: 6W, PA current: 1.38A
– At 28.1Mhz: 9.5W, PA current: 1.79A

John made the following observations:

A. The RD16HHF1 produces a much flatter power curve over frequency (in his device), although it shows a dip somewhere near the 15m band.

B. The IRF510 can produce some nice power in the lower frequencies when increasing the PA supply voltage, but it comes at the price of a steep power drop at higher frequencies.

C. The bias does not seem to influence the efficiency of the finals at full power with RD16HHF1, since biasing at 250 and 500mA produces essentially the same output for the same DC power input. Assuming distortion reduces with higher bias, can we assume a higher bias (within limits) is preferable? Any risk of thermal runaway?

D. The board main current (which includes the current in the driving stages of the power amplifier) does not seem to change with frequency from 20m onwards. Is this because the gain is pretty constant? If so, most of the drop in power with increasing frequency seems to be in the IRF510s, supporting the results obtained with the RD16HHF1s.

E. With the current uBitx PA circuit the RD16HHF1 seems limited in output, but without the appropriate test instruments he can’t say where the limitation occurs.

F. When he increased the drive through VR1, he noticed that at around 40% for the lower frequencies and at around 60% for the top frequencies he gets a compression effect.   The output does not increase much more from increasing the drive level.   John left the drive gain at around 60% and got positive feedback on the voice quality on his first QSO on 40m.  He assumes that any compression/clipping is not significant at that level (but he hasn’t measured the sprectral purity).

So since his target was around 10W on 10m and 10 to 15W on 40m minimum, he is pleased to  have reached his goal just by changing out the finals to RD16HHF1s and supplying the board with 13.8VDC.   This is below the 15.2/15V stated in the respective datasheets of the RD16HHF1.

DK5LV experience with RD16HHF1s

Henning Weddig DK5LV thanked John for his intensive research on the PA stage and commented that in his experience, “the RD16HFF1 really needs a very high quiescent current of about 500 mA each, which is not good for a QRP design”.

He  goes on to say, “The output transformer plays an important role in the design. Normally a 1 to 4 impedance transformation (12.5 ohms to 50 ohms) is sufficient. Each transistor “sees” half of that impedance i.e. 6.25 ohm. The windings of the transformer must be capacitively compensated and the leakage inductance mimimised on the windings.”

“Another big issue is the choke for the supply voltage: the commonly used centre-tapped transformer without the choke is not recommended. Ashhar Farhan uses two isolated chokes, and in my experience a bifilar wound choke is the better choice.”