10w linear amp from QRP-Labs now available to purchase

Hans G0UPL who operates QRP-Labs has released his 10W HF Linear Power Amplifier.  This kit comfortably produces 10W from a 12V supply.  It is a compact design with huge heatsink included, which will not overheat even on continuous 100% duty-cycle operation.   The amp provides 26dB gain with +/- 1dB gain flatness from 2 to 30MHz.

This 10W HF Linear Power Amplifier kit has no Surface Mount Components (SMD) to solder. There are a number of small transformers to be wound, and assembly requires care and patience.

The push-pull driver stage uses two BS170 transistors in the amplifier design used in the SoftRock transmitter stage. The final uses two IRF510 transistors in push-pull. Yes, this humble low-cost MOSFET really is capable of excellent performance all the way up to 10m band and beyond! Short lead-lengths and PCB layout are extremely important, they are the key to success.


  • 10W output from 2 to 30MHz, using 12V Supply
  • Generously-sized heatsink, will not overheat even on continuous 100% duty-cycle modes
  • 2-stage amplifier provides 26dB of gain
  • Push-pull driver and push-pull finals, for high linearity and low harmonic content
  • +/- 1dB gain flatness from 2 to 30MHz
  • 4dB down at 6m (50MHz) and 8dB down on 4m (70MHz)
  • Standard 50-ohm input and output
  • Through-hole plated PCB, all through-hole components (no Surface Mount Devices)
  • Standard inexpensive components throughout
  • Tested for 1 hour at full-power 10W, 100% continuous duty-cycle with no forced air cooling
  • Tested for 15 minutes at 20W, 100% continuous duty-cycle with no forced air cooling
  • Tested at 20V supply
  • Tested into open load, shorted load and various mismatches without instability (oscillation)

As Hans advises on the BITX20 I/O Groups list:

I am not suggesting that this amplifier will solve all problems and without wishing to be offensive, the “garbage in, garbage out” rule always applies – in particular, if you feed it a signal containing spurs, harmonics, or other problems… then you will get the same thing at the other end, just amplified 26dB. It also still applies that all stages leading up to the final amplifier must have flat gain. If the pre-drivers cannot provide a big enough signal then you won’t get full power output.

But, it could be useful or interesting, in the BITX context, to look at the details of this amplifier kit and see the short lead lengths, symmetric PCB and extensively “stitched” groundplanes.

For further information see the product on Hans website.


Outboard filter unit for uBITx

Tom WB6B  wanted to build a new filter for use with the uBITX (to overcome harmonics on his output). However,  he didn’t want to build a filter that would be strictly tied to the uBITX.

So he built a standalone Low Pass Filter box that works as an add on filter for the uBITX as well as becoming a general purpose piece of Ham equipment he can use over and over for other needs.

It uses an arduino to measure the frequency you’re transmitting on and select any individual or combination of filters you like. The proposed design uses a set of filters and a relay switch board; such as to ones by QRP Labs. But, the Sketch code should be easy enough to modify for whatever you like.

The controller can show the selected filter with LEDs.  Alternatively  it can be configured to use an LCD display, which will display the transmit frequency as well.

Tom’s unit is still under construction, but he has built and tested a prototype with a signal generator.

He is releasing the preliminary design now because it is complete enough for people to try out on their own filter designs. It makes using outboard filters really easy, and he thinks  people will like it.

Tom welcomes feedback on improvements people make while building their own “Auto Filters” with the controller.

The code for the controller is here: https://github.com/mountaintom/TX_Auto_Filter


Distilled wisdom with respect to HF linear amplifier kits

Many constructors will have thought about adding on to their µBITx one of the cheap linear amplifier kits that can be found on eBay.com or Aliexpress.com.

A thread covering these options was started by Arvo KD9HLC.  This article attempts to succinctly cover this ground for those exploring amplifier kits in future.

Arvo asked about the kit illustrated above that costs US$36.   There is another cheaper version available and Lee N9LO says:

“I’ve read some reviews on these and it seem the big difference between the $36 100w and the $18 70w is the 100w is on 16v and the 70w is 13.8v. The heat sink for either is $8. They both need a low pass filter for the band you use it on”

Richie notes:

“The 45 watt and 70 watt amps use the IRF530, and I own both. They do work, but not for long at the advertised power. It’s very easy to push too hard on the drive and blow the FETs. The 45 watt version only takes 10 milliwatts of drive to get full power, so it can be connected directly to the BitX 40 T/R relay with a 3 to 6 dB pad (bypassing the PA). You can get about 25 watts out on 13.8 volts. The 70 watt amp needs about 1 to 3 watts of drive to work, but can produce 40 to 50 watts from 80 to 20 meters. The output falls off sharply after that. Either way, but some extra IRF530s… you will need them!”

Howard Fidel says:

“I built the 70 watt model. I changed the 2 IRF530s to 4 IRF520s. The IRF530s oscillated and failed quickly. They also run very hot. The IRF520s run much cooler (2x as many junctions) and are stable putting out 50 watts on 20 meters. The output on 15 and 10 is less, but the uBitx puts out less on those frequencies. I am working on getting it working on all bands, but for now I just use it on 20.”

David, N8DAH, says:

“Just my 2 cents. After getting two versions of the “cheapbay” amp I can say the best thing I did.  I bought a proper kit (like a WA2EBY) and built it.  The Hardrock 50 was also a great amp but the cost is pretty high. I have not used a HFpacker but know a few QRP guys that swear by them as well.

If you really want 100W look at the HLA-150. I have had much better luck with building a known kit then trying to save a few bones on the cheaper kits with next to no info on them”

Jerry KE7ER says:

“Any amateur amp should be followed by a low pass filter suitable for knocking out the second harmonic and beyond.  That means separate filters for 160m, 80m, 40m, 20m, 10m. You may piggyback some of the other non-harmonic bands into that set of filters, but that makes the filters much more difficult to design and build.  A rig with only a 30mhz LP filter is likely aimed mostly at CB’rs.

“QRP is good for experimenting.  But with a 50W+ eBay amp, you really should be testing for compliance with FCC regs.  It likely fails, and all the nasties will scale up with the output power.”

Two articles to check out

4Z1NT suggests checking out this article on how to make the 70w unit work well.

Andrew Kasurak suggests checking out this article if you want to upgrade a cheap Chinese eBay amp to a working 55w unit.


Some great mods from PH2LB


Lex PH2LB  wrote to uBITx.net to tell us about a page on his website where he describes his uBitx (V3) mods.   This is a very nice build, and he has some good ideas.   Check his page out here


In particular Lex has developed some custom firmware that firmware geeks may be quite interested in …

“Second mod : custom firmware”

Originally based on the v2 software but merged to v4.3 and updated to code to have a lower RAM footprint (usage of F(…) macro and strcpy_P) with about 50%.

Source files can be found here : https://github.com/ph2lb/ubitx4

Over the last few months there have been a range of ideas to boost mic drive output or to add compression.  Here’s a mod designed to work with a dynamic microphone …

“Fourth mod : dynamic microphone amplifier.”

Because I like to work with dynamic microphones, I added a dynamic microphone amplifier based on the microphone preamp designed by Javier Solans Badia, EA3GCY for his ILER transceivers.

There are a whole bunch of ways to add buttons. KD8CEC does this through paralleling up buttons with different series resistor values on the encoder analogue port).   Lex has taken a different approach that will be of interest to some constructors.   He uses a PCF8574 I2C encoder (like the backpacks for a 16×2 or 20×4 LCD display) and uses the existing I2C bus…

“Fifth mod : again adding extra buttons.”

Using a PCF8574AP I2C IO Extender and hooked it up to the all-ready existing I2C bus on the Raduino for more direct menu buttons. Needs the custom firmware to direct switch between bands with a PA bandplan limitation (also has FULL option) and Step size up and down.


Something that a number of constructors have done is to remove the 7805  and supply 5v to the Raduio using a separate 5v supply.  Most are using buck or buck boost modules, but Lex has used a P-MOSFET.   There’s a good description of his approach on his website …

“Sixth mod : removing 7805 from Raduino and reverse power protection.”

Relocating the 7805 is a good idea, but adding a reversed voltage polarity to a uBitx is a must. I used a P-MOSFETs for that (also link to good video about using P-MOSFETS for reverse power protection).

Finally, you may be interested in Lex’s use of the Manhattan style technique for PCB layout.  It can look very professional as per this example:

An improved reverse voltage protection circuit

With four components, and the on:off switch, Bill K9HZ provides the ultimate reverse voltage and over-current protection system for your µBITx.  The fuse protects the circuit from excessive current draw – so you won’t blow your finals when you wind up the bias too far and they try to go into thermal runaway.  The relay must be powered on to power your µBITx (and the switch must be turned on).  With the series diode in place the relay cannot turn on unless the power is wired up correctly.

VK2ETA automatic L-match using separate arduino nano

John VK2ETA has uploaded an updated folder to the BITX20 IO Group list files section.   This adds new schematics for his Antenna Tuning Unit (ATU).

ATU Design objective

1. Fits in the limited space of my Jameco case (Jaycar case here in Oz), on my second level board.

2. Tunes long wire and EFHW (worst case with the help of a 9:1 balun).

3. Works 80m to 10M.

4. Memory tune to save power and time.

5. Negligible power consumption when not tuning.

6. Must integrate with the extra (planned) features like SWR measurements, Finals’ current limiting, and power supply monitoring, and more


John settled on using a second Arduino and an L-Tuner network despite some limitations when compared to T or Z-match networks that seem to require three adjustable elements for 80 to 10M coverage.

The 2nd Arduino has the following advantages: at around US$3-5 it is much cheaper than extra I2C analogue and digital I/Os, plus it gives another 30K of programming space and 1K of EEPROM for memory tuning. It can be put in low power mode when not tuning,  as John uses the Mini Pro version (no USB port), and communication is via the I2C bus to the Raduino.

John needed an SWR meter.  He chose the Don Cantrell (ND6T) circuit as a perfect match.  He made it on a daughter board that plugs directly into the connector after the LPFs.

Browsing the internet and looking at previous solutions like the SLT+ and the Altoid Long Wire Tuner, I settled on 6 inductance values.

Switching the inductance could be done with relays, but that means 5 bi-stable relays and ten digital outputs. Same issue with the variable capacitors.

I decided to use an RC servo controlling a mylar variable capacitor and another one controlling a rotary switch for the coils.

The first challenge was to have a way of switching the capacitor from the antenna side to the transceiver side to match both high and low impedance antenna loads.  One option was a bi-stable relay.   The solution John settled on was to use a double wafer rotary switch with 12 positions and dedicated 6 of them to the capacitor on the input and 6 on the output.

He needed two digital outputs for the PWM generation for the servos and one for cutting the power off to the servos (using a common positive supply).

His main concern was the possibility of the servos not handling RFI. But in the end they were easy to tame.

The next challenge was to find a servo that could do 360 degree rotation (or at least 345 degrees) to cover all 12 contacts on the rotary switch.  There are quite a few servos that manage 180 degrees, but he was unable to find one that did a full 360 degrees. Please note there are many so-called “360 degree” servos available but they are “continuous rotation” servos and do not move to a position, just rotate at a certain speed, with no position feedback.

The first solution he tried was to use a 2:1 gearing and a 180 degree servo. It worked but was not very reliable due to the additional backlash, even with a larger servo to compensate for the power loss in the gears.

Luckily there are now “Sail Winch Servos” available in 1, 1.5, 2 and more turns, but that retain a position control. John chose the 1 turn version, which worked successfully.  It is a “GWS S125 1Turn 2BB Sail Winch Servo”.

The key challenge he faced was to ensure that the servo would settle pretty much centred on the rotary switch contacts. The angular resolution of the servo is sufficient for this but he needed repeatability. Otherwise he would destroy the contacts through arcing.

Since John controls the supply of RF power to the antenna, he can cut the power off when he changes contacts on the rotary switch.   He used a digital input on the Arduino to measure whether the contacts had been established or not, and thereby form a map of the location of the contacts relative to the angular position of the servo. When the contact is established you should get a short to ground through the coils. A pair of 1 M Ohm resistors to feed the 5V and connect to the Arduino pin,  worked very well.

He  builds the contact map once, at first tune, and uses it thereafter until the rig is powered down. It may be possible to store the map in EEPROM, but stability over time and with temperature changes hasn’t been checked.

When the servo is moved from one contact to the next you can again check at what angle the contact is established or lost to compensate exactly for the backlash. A bit of software does this, and it works quite reliably.

John has shielded the ATU with sides made from PCU board to prevent stray RF.  Apart from the capacitor servo, which occasionally displayed small jitters, the rest did not really need shielding and worked quite well without additional effort.

The main components are:

  • Arduino mini pro or nano
  • Variable capacitor and micro servo’
  • Rotary switch 12 positions with two wafers (it could be one wafer and less positions and a bi-stable relay)
  • A “one turn sail servo”
  • a P-Channel Mosfet for servo supply
  • a 5V regulator dedicated to the servos’ power,
  • the components for the ND6T SWR and Power bridge.
John uses the following I/Os on the 2nd Arduino:
  • 4 digital I/Os of which 2 are PWM
  • two analogue inputs
  • the I2C (A4/A5) lines.

John found he had enough I/Os left for the other functions that he wanted to implement on the second arduino.

Performance: With a 21m (69′) long wire and a 10m (33′) counterpoise on the ground John found he coul  tune all bands ( 60M wasn’t tested, as VK still doesn’t have access to this band), with an SWR of under 2 at all times.

A full tune sequence takes 32 seconds if the matching coil is in position 12, and a memory tune is around 3 seconds. At first tune after power-up, there is an additional delay of 15 seconds for the rotary switch contact mapping process to complete.

Total parts cost is around AU$130 (US$100 approx.), but a lot cheaper in the USA and other countries I am sure and quite a few items could already be in the junk box.


1. A complete view of the unit with the shields in place. Also the Android hands free headset (with modified software for push-on/push-off PTT).

2. Second Board (double sided fibreglass as a ground plane, plus sections of vero board) with the MAX9814 AGC, the SSM2167 mic compressor, the ATU circuit and Arduino.

3. The back of the unit with the ATU toroids, variable capacitor (the angling is to align it with the servo’s angular range), the SWR/Power bridge. Note that three toroids are used to minimise losses and prevent high voltages since the unused turns are not shorted out as in some designs.

4. A top view of the coupling of the micro servo and mylar variable capacitor.

5. A top view of the rotary switch, toroids and the contact detection circuit.

6. The SWR bridge daughter board’s back with it’s female header to provide solid ground connection and mechanical rigidity.

7. Tuning completed . P = forward power, R = SWR …. front panel labels to come!

A critique  

John provides his own crritique of the design, now that is is completed:

  • the winch servo adds 50g plus coupling of around 30g, which is a fair amount of weight, but the total build is still under 2Kg at 1.25Kg or 2.8 Pounds.
  • a single bi-stable relay instead of the second wafer for switching the capacitor over may be a simpler solution, and leave more steps for the inductance.


John has now also provided the two parts of the schematic. Part 1, the L-Network:

And the Control part:

The software can be found separately.

Reference #1
Reference #2

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.”


Extensive VK2ETA mods to KD8CEC firmware

John, VK2ETA, has implemented a range of changes in Ian KD8CEC’s software targeted at portable operations (the software can be downloaded here in the files section of the BITX20 IO Group).

VK2ETA Software modifications to KD8CEC firmware

The scope of these modifications is described below:

Options for various features – These can be turned on or off. Key objective is to be able to customise the rig based on your needs and unfortunately on the restricted memory size of the Nano. So not all features can be selected at once. Choices, choices…

ATU control – A servo-based L-Network ATU. The communication between the Raduino and the ATU Arduino is via I2C. There is a separate sketch for the ATU Arduino (Nano or Pro-mini).   ATU operating mode can be set to OFF, Manual as in on-demand, or auto-RX meaning that it pre-tunes based on historical data on a change of band and after first change of dial frequency (for a quick scan of the bands). It uses the EEPROM data of the closest stored frequency for pre-tune or tune on-demand to accelerate the tuning process.

Handsfree microphone/headphone – Using an Android style 3 rings (TTRS) handsfree earpieces/mic combination, with 1 or 3 buttons (Play/Pause, +, -), the PTT is controlled by Play/Pause as toggle, and I use long presses on + and – as respectively pre-tune and smart-tune of the ATU. Short + or – presses could be used for frequency up and down. Requires a very simple hardware mod to free-up A6 (see below).

S-meter measure and display – using analogue input A7 from an 2N7002 based AGC or a MAX9814 circuit or any other for that matter.

Software based AGC range extender – to augment (as in double or triple) the dynamic range of an audio AGC. This uses the slope of the 1st If filter at 45Mhz to attenuate the Rx signal when the audio AGC reaches its limit. Adds over 50dB of dynamic range.

Forward power and SWR measure and display – Currently assumes that the ATU is providing that info over I2C. Otherwise could be adapted with a pair of analogue inputs for measure. See the excellent NT6D design on the wiki.

Options for displaying the S-Meter, SWR and forward power –  in either easy to see “fat” bars with no number, or “skinny” bars with more text and numbers.

Enable a “Memory mode” – selectable by menu, which cycles through all the populated memories (channels). Dial lock also locks the change of channels.

Made some rarely used or once-off functions as options  – to recover program memory after initial tuning and allow for more options to be selected.

Fixed some issues with the IF-shift option – Ian has resolved these in his new V1.06 and later releases. Two issues were present: IF-shift in USB would change the receive frequency and it was applied to TX as well. Now applies to Rx only.

Hardware modifications required to use VK2ETA software mod

The only required hardware mod is to connect the CW key input to the PTT. Since in Ian’s software we select the mode by menu, there is no need to have a separate analogue input tied-up for the CW key. This frees-up analogue input 6 for use by other functions like the handsfree option above.

Still to come

John plans to apply Ian’s improvements in v1.06, especially the CW transmit frequency option and if possible the WSPR beacon mode (as a further add-in option).

How to use VK2ETA software

Download the zip files, and unzip these in your Arduino sketches folder.  Edit the ubitx_20 options sections, using #define for enabled and #undef for disabled.

Perform a CTRL-R to compile and check how much memory is used. If you go over the limit, a warning is issued.  Providing you have enough memory to run the software, upload the sketch to the Arduino.

John has uploaded both the Raduino as well as the Arduino sketch for the ATU and SWR measurement. They can be found in the folder “Variations on Ian Lee’s Software (by VK2ETA) + ATU sketch”. 

All software is released under GPL V3.

Antenna Tuners for the uBITx

Introduction to tuners and tuning

Arv  K7HKL  suggests the type of antenna tuner depends on the type of antenna:

  • L-network for end-fed or high impedance antennas.
  • T-network for medium to low impedance antenna.
  • Pi-network for dipoles.

He suggests it is unfortunate that specifications for ATU’s usually do not include the adjustable impedance range for each band that they cover.

Bill Schmidt K9HZ suggests there is a false supposition here that you must tune under full power.   It is considered a good design to tune with just the amount of power needed in order to tune… not full power.  This can very easily be implemented on the uBITx with a relay that substitutes in a “Tune RV1” set for a much loser tune power.

List members suggested options for simple external antenna tuners for the µBITx transceiver.  The list of potential tuners below (organised by type) is not intended to be exhaustive, but illustrative of the choices available.


LDG Z100+

The LDG Z-100Plus tunes with only 100mW of power. It holds eight AA batteries internally, making it ideal for portable QRP operation. Small, light weight and self-powered.

However, Rahul VU3WJM found that the match at low power levels was inconsistent. He had to reduce the resistors in the ADC sample line for QRP operation.


From the marketing description, this unit handles up to 125 watts SSB or CW but requires only 0.1 watts to tune, making it ideal for QRP operation.

Elecraft T1

Manual Tuners

SOTA Beams Mountain End-Fed antenna tuner

Sotabeams Mountain End-Fed Tuner


Mike WA1MAD has built the Sota beam Mountain tuner.  He says, “It is easy to build, but very manual and only covers 40-17.”

Emtec ZM-2


Dave K8WPE likes the Emtech ZM-2 better as it has air variable capacitors whereas the 4SQRP uses poly variables.

4SQRP tuner

GQRP tuner

Dave K8WPE prefers a small Z match like the Emtec, 4SQRP tuner, GQRP tuner, etc.  At the home QTH, Dave uses an MFJ 300 watt roller inductor tuner.

He says that the reason he likes manual tuners is he can look at the numbers on the dials and if they are different from what he usually sees he knows he has antenna troubles, i.e. an antenna down, ice coated, wrong antenna, a short somewhere. With the automatic tuner it just tunes and you don’t know if its the antenna or the tuner that is making your transceiver happy. And even at 5 watts or less we can fry an IRF510.

Hendricks SLT

Allison KB1GMX suggests the ZM-2, 4Sqrp, SLT, and L-tuner all work.  The 4sqrp, L-tuner, SLT, and Elecraft T1 are in use in Allison’s QRP rigs for power up to 10W. For higher power a home-built L-tuner easily takes 100W. Those get used for the inverted L and the 160M Inverted V  as all the other antennas are 50 ohm resonant removing losses though a tuner and coax.

Allison says, “By far the best antenna is a matched one. Second best is any needing a tuner.”