VK2ETA variation updated (based on KD8CEC firmware)

John VK2ETA has uploaded two updates of his Raduino firmware and ATU firmware in the folder: Variations on KD8CEC Software (by VK2ETA) + ATU sketch .

This firmware is targeted at portable and /PM operations. John doesn’t plan to add much in the way of further features unless they are of value to portable operations.

If you want to use sections of code in your projects, follow the #ifdef/#endif segments that mostly enclose the sections of code of interest. John is happy to help with extraction of code if requested.

The software is organised in options that can be enabled/disabled (mostly) independently.

The key features added in this version are:
1. Reduction in libraries size by 1K byte (this allows for more features to be added in the limited memory of the Arduino).
2. Added a “Stock Standard” option for unmodified rigs wired as per HF Signals instructions.
3. Added a two adjustable levels supply voltage monitor with visual and audio indication. Level two will disable TX to protect the battery.
4. Added the WSPR beacon option from KD8CEC software version 1.061.
5. Changed logic for CW modes: frequency displayed is carrier frequency, with Rx frequency shifted up or down (CWL / CWU).
6. In split modes, the VFO A/B label and frequency change when going from Rx to Tx.
7. RIT display:  frequency is kept fixed, RIT is now shown as +/- frequency difference.
8. Added ALC levels for MAX level too. Used to be no attenuation.

The second update is part of his approach to continuously look for program size reduction to have as many features as possible concurrently in the Nano’s memory.   Inspired by Ian Lee’s KD8CEC version 1.701, he has included a modified version of his TinyLCD library.  Compared to the complete default LiquidCrystal library this cut down version saves 452 bytes of program flash memory.  He has made this code into an object oriented class so it can be easily retrofitted into other projects as there were only two lines of code to change.


Evening up power output on high bands

Howard WB2VXW received his µBitx, assembled it, and started testing.  He performed a number of modifications to the µBITx in his lab before moving it to its permanent home in the shack.

In particular, he modified the transmitter to improve output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz.  His modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On his board, there was not enough range in VR1 to increase the power to anywhere near this level without modification. The other bands remain unchanged in output, at around 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve this further, but Howard was happy with the improvement for now.  He plans on adding a power amplifier later, something between 50 and 100 watts.

Howard’s fix involves adding 3 passive parts:

  • 33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.
  • Adding a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

The theory is very simple. The closed loop gain of Q911 and Q912 is set by the ratio of R86/R85, or a gain of 10. Adding an inductor into the path in the feedback loop increases the effective impedance at higher frequencies, adding “peaking” to the circuit, thus making the closed loop gain higher.

I think the poor frequency response is in the transformer as well as the transistors Hfe falling to under 25 typically at 30 MHz.  The open loop gain is also increased by bypassing the emitter resistors for higher frequencies.

The transformer is a 2 to 1 step down, (it looks like a trifilar winding) so the theoretical gain of the stage is 5.

Offer of parts

Howard also made a generous offer to  anyone wanting to try the fix and needs the parts.  He has a full reel of each of the parts (2x 270pF capacitor and 33µH inductor), so just Howard a stamped self addressed envelope, and he will send it to you by return mail. The offer is good until he runs out of parts.

Others try it on

Joel N6ALT completed the mod not long after it appeared on the BITX20 IO Groups list with spectacular results. Before the mod he had only 300mw at 13V on 28.500.  After the mod he had 3.8 watts, and after increasing the drive slightly, he now has 4.5 watts. All of the other bands also benefited from the mod.

Tim AB0WR also tried the mod, winding an FT37-43 ferrite to get 33uh. Not as neat as an smd inductor but he didn’t have any in the shack!


Version 4.00R latest W0EB/W2CTX I2C firmware release

Jim W0EB and Ron W2CTX have released Version 4.00R.   This is the I2C release firmware for modified hardware on the display.  The W0EB/W2CTX firmware also requires a minor CW keying control mod.

This is a FOUR line display version designed to work with a 4 line by 20 character LCD display that has an I2C “backpack” or, one with the I2C interface built in. As long as you know the I2C address for your display and can properly set that in the .ino source file, it should compile and run on an I2C modified Raduino card as well as on our RadI2Cino card which can be purchased by contacting W0EB (email address on QRZ). Jim Sheldon, W0EB

You can access this file at the URL:

David  N8DAH who is using the well regarded SOTAbeams LaserBeam Dual variable kit. He didn’t like the limited mounting options or the point-to-point wiring of the filter kit so he designed a board for his own projects and let the list loose on the remainder (which all went quickly).


KD8CEC v1.072

Ian KD8CEC has released version 1.072 firmware.  This includes support for both 20×4 and the standard 16×2 LCD screens (using i2c).  It enables constructors to include or remove bits of code, and it allows the integration of an US$8 RTL-SDR to work on all HF bands to give full DSP, waterfall display, etc.

For more details see his website at www.hamskey.com

Download the manual (ug1072_087) for the KD8CEC v1.072, which has been updated by Rod KM6SN and his peer reviewers.

Separating your display from the main board

Gordon W2TTT asked on the BITX20 Group list, “Does anyone have some links to the display/ Raduino extension cable parts?  I want to remote the display from the main board.”

If you want to separate your display from the main board, you should separate the display from the rest of the raduino and plug the “arduino nano” bit into the connector on the main board.  The reason for this is that you don’t want long leads spraying RF around from the oscillator outputs from the si5351a.  This is a sure recipe for birdies in your RX.

Use Dupont connectors (Male to Female) to do this.  Most will find either 10cm or 20cm connectors are suitable.

Jack W8TEE  comments “Jumper wires (aka Dupont wires) are great for breadboarding and experimenting, but my experience is that some of them don’t make a solid connection after recycling (connect/disconnect) a few times. We were getting a lot of noise on a TFT display line and finally tracked it to a faulty Dupont. The Chinese imports seem especially susceptible. If you experience noise or hashing on a display or in the audio, this would be one of the first places I’d look.”


Transverters for 2m and 70cm

Gerry W1VE has 2m and 70cm transverters from Ukraine, which should be a good fit for the µBITx:
These take from 1 – 50mW of drive, so he asks where to find the right tap off point before the final.
The boards cost just US$21.   They put out about 8 watts in theory (less in practice) and they are sufficiently small to fit in the µbitx nicely.  Imagine, a HF/V/U station for around $200.  “Awesome” says Gerry.

Jose CO2JA says “Put an L attenuator on the driver output and turn off the finals. Use a saturated NPN switch driven from the +TX to key the transverter”.

Allison KB1GMX says “The ubitx sans driver and finals will put out roughly the right power for most modern transverters without the problem of too much power. The receiver is a good match as well.”


New AGC system

Don ND6T has been tinkering with AGC mods, and has come up with a new solution that gives much high levels of RF compression.

The µBITX presents some challenges with RF AGC system design: There is no RF preamplifier to use as a voltage-controlled attenuator, it is broadband and includes no tuned circuits in the receive path, and the intermediate frequency amplifiers are not configured for variable gain.

If we try to use a PIN diode in the RF, the high insertion loss raises the receiver over-all noise figure appreciably and the driver circuitry begins to become complex. The diode array also necessitates a fair additional current drain.

Original solution

Don’s original solution, designed for the BITX40, was to use a 2N7002 MOSFET as a shunt from RF to ground. The advantages were the simple circuit, the low current drain, and no insertion loss. The disadvantage was the limited dynamic control range (about 20 dB ). This was primarily due to the finite series resistance of the MOSFET when it was driven to full conduction, about 5 ohms. You can see this project by clicking here.

Putting another FET in series

Numerous experiments revealed that adding another 2N7002 in series with the receive RF path made it possible to add another 20 dB (or so) dynamic range across the HF spectrum. This was controlled with the same control bias as the original but the configuration requires driving the source connection of the transistor with the control and biasing the gate at 2.5 volts so that, at idle, the series transistor is driven to full conduction. This control topography requires that the new series transistor to be DC blocked from the RF line.

Pre-requisite Mod

This project assumes that you have already installed the manual RF Gain Control modification since it requires that the trace between relay K1 and relay K3 be cut. I designed mine to simply be glued on the back of the control potentiometer and cut the single-sided un-etched printed circuit board stock to be about 20 mm by 13 mm. The thickness of the completed circuit is only 3 mm and so fits easily.


Detected audio from the receive audio pre-amplifier is sampled by bringing a twisted pair of wires from the volume control. Just attach the pair across the outside terminals of the control, attaching the wire from the “hot” terminal (which also attaches to “audio1” plug on the main board, pin 4) to the vacant side of C1 on the new board. The other wire attached to the “cold” terminal (which also attaches to pin 3 of the “audio1 plug) connects the the “GND” area of the new board.

This audio sample is then amplified by Q1 and fed through C2 to be rectified by diodes D1 and D2 and filtered by C4 to become the AGC control bias. Signals below 30 millivolts RMS leaves the circuit idle, with Q2 (the series element) biased to full conduction and only adds about 0.6 dB of loss. Q3 (the shunt element) is biased to cutoff and has no effect on the receive signal. Louder signals (S9 and above) create higher bias voltages until maximum is reached (about 1 volt RMS audio) at about 3 volts DC control bias. At this point attenuation is about 50 dB at 3.5 MHz, 34 dB at 30 MHz.

The time constant of C4 and R6 sets the “AGC release” rate, several seconds from full to idle. Charging time is fast, milliseconds, so that sudden strong signals are handled without discomfort. This ratio prevents oscillation and diminishes “pumping” on strong signals. For faster recovery rates, decrease the resistance value of R6. Values down to 100 K work satisfactorily but 1 megaohm worked the best for me.


A hobby knife was used to isolate the connection pads as shown in the sketches. The remaining copper is then lightly tinned. Resistance checks should then be made to insure that there are no copper scraps or solder bridges between the lands.

I often find it convenient to simply begin in the top right corner of a board and work my way down and left so that I avoid working over previously completed circuit areas. Just place a component, hold it down, and touch the soldering iron to the board near one of the component leads. The thin solder will make a weak attachment to the component (called “tacking”). The builder can then move to another connection on that part and properly solder it using a hand to hold the solder. Then the rest of the component is soldered properly and the value checked with metering to assure a good connection and that the part wasn’t damaged in the process.







A spot of fast-setting glue holds the board to the back of the manual RF gain control. Remove the coax center conductor from the potentiometer wiper and connect it to the junction of Q3 drain and C5. Run a jumper from the wiper lug of the gain control and connect to the vacant pad of C3. Leaving the manual RF gain control in place will give you extra control should you wish it. Very short jumpers keep things solid and have purer control. Power can be supplied from any nearby 5 volt source (like pin 3 up on the Raduino, usually green wire). It just needs to be from 4 to 5 volts and only draws less than 2 milliamps.

If you want to add a switch to shut off the AGC, do so at the audio input. I would suggest a single-pole double-throw switch with the connection to C1 at the center, ground to one side, and the other to the hot side of the volume control. If power is removed from this circuit Q2 will not be biased “on” and will reduce signal levels significantly.

A good, solid ground is most important. I recommend a ground lug on the potentiometer shaft or on any other nearby ground point. A poor RF ground will yield poor attenuation. 40 dB is a ratio of 1 to 10,000! Hundredths of an ohm count.

If you don’t use an RF gain control

If you don’t have room for a manual RF gain control or a switch there is no reason to worry. This circuit can be left active and placed near the RF path connector without a problem. You could easily build this as a module that simply plugs into the connector. The most critical junction would again be a nearby ground but a third pin, this time through the board to the ground plane, would help. There should be little need to operate without it. If you need to run any tests that need it disabled, just remove the plug to the RF receive path and replace it with a shorting plug across the two RF pins.

If you adverse to surface mount construction, there is little problem in building it with leaded components. Use 2N7000 or BS170 MOSFETs and a 2N3904 or 2N2222 type NPN transistor. Try to keep wiring short and compact in the RF attenuator portion but the rest is very non-critical. If you use an electrolytic or tantalum capacitor for C4, observe polarity.

Parts List

  • 1: NPN switching transistor (1B, 2N3904, 2N2222) [Q1]
  • 2: N-channel enhancement mode MOSFET (2N7002, 2N7000, BS170) [Q2,3]
  • 2: Silicon switching diodes (1N4148, 1N914) [D1,2]
  • 2: 0.01 uF, 6 volt or higher ceramic capacitor [C3,5]
  • 2: 0.1 uF, 5 volt or higher ceramic capacitor [C1,2]
  • 1: 10 uF, 6 volt or higher ceramic (preferred) electrolytic, or tantalum capacitor [C4]
  • 1: 1 Kohm resistor [R2]
  • 4: 100 Kohm resistor [R1,3,4,5 ]
  • 1: 1 Mohm resistor [R6} (or other value, depending upon desired AGC recovery rate)


Signals below 50 microvolts (S9) are unaffected. Above that level, the AGC begins to engage. Q2, the pass transistor, begins to turn off a bit while Q3, the shunt transistor, begins to turn on. Signals are attenuated to where the audio output is held to reasonable levels and the rest of the receiver does not experience distortion. If you are considering adding an S meter this circuit may be to advantage due to the amplifier stage. The control bias is available at the junction of D2,C4, Q2,R5 and R6. This is the spot that you would use for a metering source.


AGC makes for a much more pleasurable experience. Don seldom needs to reach for the RF gain control or the volume when an especially strong signal suddenly appears. During nets I can now wander about the shack and still hear all of the check ins. Round tables don’t require a hand on the controls. Things are getting easier.

Recommended by others

Tim AB0WR says, “I am using this new AGC circuit. It works well. You still get a slight pop when an extremely strong signal first comes on but it is very short and, to me at least, not annoying at all. You may want to lower the 1 M Ohm resistor to something smaller to decrease the pop i.e. decrease the attack time. The next time I open the case I’m going to try 500K.”

Jim W0EB says “Thanks ND6T for a very workable AGC circuit!”.

It is clear that this has become the new ubitx.net recommended AGC mod.


Pesky volume pot

Geoff, G8BMI is cross-fertilising ideas across hobbies…

He has been assembling a uBITX, and found that the volume control has a 5mm shaft rather than the usual 1/4 inch. This means that none of his spare knobs fit.   We’ve all been there.  Most of us threw the supplied pot into the junk box and moved on with another standard potentiometer.

However, a waking inspiration suggested a very short length of 1/4 inch OD copper pipe, as used in model steam engines, could be used to ‘sleeve down’ a 1/4 inch bore knob. And it does!.

Geoff turned down the copper tube in the lathe, using a file just to ease the fit [It could be mounted in a power drill chuck to get the same result].  It only needs a skim. His tubing was 0.210 inches bore, which is just over 5 mm (5.18)


Connecting your rig to a computer

Connecting your µBITx can be done in a number of ways.   Some people will want to guarantee that their computer is fully isolated from their rig and will make or buy interfaces (Signalink, EasyDigi, etc.).  Others are quite happy to wire up a cable for USB control and another for the audio in/out.    In reality, transformers and other forms of isolation don’t necessarily always work.  RF can get into cables or the computer directly or indirectly.

Gordon KX4Z provides some useful links for reading up on rig/computer connections:




Cheaper alternatives to signalink


With relay:


Dealing with RFI