Evening up power output across bands

There has been discussion recently about how to even up the power output of the uBITx across bands.   Maximum output (in excess of 10w PEP) is achieved on 80m, while output on 10m can be as low as 1w PEP.

Method 1:  Preset replaced by Pot on Front/Back Panel

John  G0UCP says “Drive to the finals is controlled by the preset RV1. This could be replaced by a variable pot, perhaps located on the back panel.  However, I think it was Farhan who pointed out a long time ago that the only problem with this is the temptation to keep turning it up! “
[NB:   as RF is carried on RV1, so if you are locating a replacement potentiometer some distance away from it’s location on the main board, it would be advisable to use coax to/from the board to the potentiometer.]

Method 2:  Mike Preamp with Mic Gain Control (works on SSB only)

Jerry Gaffke says “Alternately, build a new mike preamp with audio gain control, though that solution will not affect CW power level.”

Method 3:  Relays to different driver pots by band group

Bill Schmidt K9HZ has relays driven by the TX-A, TX-B and TX-C control lines that connect with 4 different multi-turn pots to set drive level by band grouping (80m/60m, 40m/30m, 20m/15m, 12m/10m.   Details of this straight forward solution can be found here.

Method 4: Vary the Power Supply Voltage to the finals
Marco – KG5PRT suggests,  “It’s kind of crude, but you could power the entire radio with 24v. Use a 12v regulator for the receiver and a LM350 for a variable regulator to the PA. You could set it up to be variable for only a few volts change to vary the output. You alternately could use a 12 v source and the LM350 to vary the voltage to the PA. Again, you could design something that would vary by a few watts.  Running from 24v lets you get the higher output but allows you to turn things down, as needed “
A number of alternative means of lowering voltage were suggested by list members.
Method 5:   Attenuator method with PIN diodes

Karl Heinz – K5KHK introduced another suggestion, “There is an HP application note about PIN diode attenuators.   The device used in this particular configuration reached it’s end of life, and may be hard to get soon. There are alternatives:


The data sheet has a chart that shows insertion loss based on frequency and control voltage.

Jerry KE7ER refers constructors to old threads that are well worth reading that touch on pin diode attenuators, mostly in regard to AGC for the receiver.    https://groups.io/g/BITX20/topic/5945215

Here’s another old thread:

Method 6:   Use a digital attenuator

Carl, K0MWC  ran across this web page  where  a 6-bit, 50 ohm, 1-4000 MHz digital RF attenuator chip from Peregrine Semiconductor is used to vary RF attenuation from 0dB to 31.5dB in 0.5dB steps via an SPI serial interface.   The newer PE4312 chip (the replacement for the obsolete PE4306 used in the design above) goes for less than $5 in small quantities (less than 100 units).

Carl observes, “The PE4312 datasheet has a maximum allowed input RF power that is much lower for HF frequencies, going from roughly 11.5db @ 1MHz to about 23.5dB @ 50MHz (see Figure 4).  The PE4312 also allows control via a parallel interface for those that would rather control it that way, perhaps using an I2C digital I/O expander chip to control the attenuation rather than using SPI to save pins on the Raduino.”


An alternative to using the PE4312 could be to use the HMC470 module (pre-built) ex China for around US$13.    Mike ZL1AXG suggests that when inserted between the pre-driver stage and the bandpass filter the attenuator could be activated on both TX and RX under the control of the Raduino.  It would require 5 i/o lines. 

On TX: the digital attenuator would control drive level to the pre-driver stage, allowing RF output for both Phone and CW on all bands to be near uniform.  Phone and CW output could be controlled for on each band to within 1dB.

On RX it would be able to be used as:

an RF gain control (selectable in the menu and using the encoder to control 32 steps from 0dB to 31dB of attenuation)  AND

as part of an AGC circuit to reduce gain for strong signals.

An S-meter sensor  taken off the input to the volume control could be captured on the A7 analogue line and software could be used to display S level and control the attenuator to reduce gain on stronger signals. The setting of the RF gain control would adjust the “floor” of the S-meter.    Some work would need to be done in calibrating an S meter for typical uBITx sensor readings with the floor varying between s1 (no attenuation) and s6 (maximum attenuation) according to the (virtual) RF gain control setting.  If you want a larger AGC range, you could use two attenuators in series, to give 62dB of AGC (at a cost of 10 digital lines).

Using an i2c display would release 6 digital I/O ports, or an i2c port expander could be used to get the additional I/O lines required.

There are many options to choose from, but Mike likes the ultra-inexpensive CD74H4067 16 channel analogue/digital multiplexer module (US$0.50 or thereabouts on AliExpress). This takes 4 digital i/o ports and a single analogue port, but switching between these multiplexed ports can be done very quickly by turning on/off the four digital ports used to select 1 of 16 ports. Just read or write via the analogue port.

Even after grabbing an analogue port and 5 digital lines for the attenuator from the multiplexer, you are left with 9 digital or analogue I/O ports for future use in the µBTIX.   Analogue out is another bonus feature of this module.

Because the RV1 drive level control (in the uBITx circuit) is in the RF line, it is not recommended to use a DAC variable potentiometer, as this is likely to get RF back into the raduino.    PIN diode attenuators should work, and could be used with a DAC I2C chip or similar.   However, a digital attentuator is an easier arrangement.


New uBITx: This is what you should not do!

Berenstain Bear tells the new uBITx owner:

  • Check and recheck every connection.   Wrong connections can result in major damage to your uBITx.
  • Check that you have your power supply wired around the right way.  There is little protection for reverse voltage mistakes on the uBITx withouth modification.
  • Check that you have not inserted the digital connector in the audio connector (or vice-versa).  This WILL damage your uBITx!   Mark these connectors now.
  • Do not power up without a 4.7K pull up resistor to 5v on the CW key pin.  Without this pull up resistor, the uBITx will arbitrarily go into TX.
  • Connect a 50 ohm dummy load, or a well matched antenna to the uBITx before attempting to transmit.  Failure to do will result in finals (IRF510s) that will blow up.
  • Do not touch any settings on the board. Boards are adjusted at the factory for correct operation. Do not adjust driver or bias pots, without having carefully read the instructions.  Over-driving the finals or setting abnormal bias levels can be catastrophic to your IRF510s.  Generally speaking, these controls should not be adjusted and do not require adjustment, even if you have read comments to the contrary on the email list.
  • Ask for help on the IO groups BITX20 list.  Fellow amateur operators are only too happy to provide assistance.

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.


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.

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.”