Connect with me today

  • Call me on 44 (0)7970 190437

All sorts of interesting stuff about Ham Radio and old gear from Steve G3ZPS, near London in the UK

BITX 40 QRP, WWII Paraset and The 'Sphinx Transmitter'

QRP Project Jan 2017 – BITX40

The ready populated BITX40 became available at the end of 2016. Ashhar Farhan's design from 2003 has been copied and adapted many times over the last 13 years - starting out as a simple 20m transceiver to encourage hams around the world to build and experiment – especially those with limited funds.

In 2016 he started another enterprise to get 40m boards assembled by local Indian workers – thereby providing local employment. These complete QRP 5W radio PCBs using surface mount parts come in at a very low the UK it cost me £37.50 delivered to my front door. The first kits came with an onboard varicap VFO that was bit unstable and without a counter hard to tell which frequency you were on. The second batch come with a suitable (if basic) DDS synthesizer on a separate small PCB.

Mine was one of the first batch, so I either used the onboard varicap VFO or my own DDS...which is what I did.

First Test

My first test was just to check if the board was working – all the connectors are provided so only 12v and antenna are needed to check things out. My first impression was that the RX audio had rather too much hiss for my was not the band noise on 40m, and it was not all coming from the LM386 audio appeared to be present at the input of the 386. The LM386 was configured with quite a bit of gain so perhaps this was a feature of the simple design (no AGC on the RF / IF stages means you cant have too much gain in this part of the receiver)

The on board VFO was not really good enough for me. Although it stabilised after a while – it was hard to tell what frequency you where listening to – and a multi-turn pot would not allow quick frequency changes - a DDS would be required


I wanted a DDS that did not require too much programming – this would ensure I could get things working pretty quickly. I chose to go with a DDS control board from the UK Company '6V6'. This board is based around a PIC controller and interfaces directly with one of the popular AD9850 DDS units which are dirt cheap on eBay. The pre-programmed PIC board is easy to assemble with no SMD components and comes with a 16 x 2 LCD display and the rotary encoder.

Instructions are fairly minimal but the build is fairly painless. Once assembled the complete DDS and controller make an oscillator that can go from 0 – 40MHz in steps as small as 1Hz, it also allows storage of an IF 'offset' to allow the display to show the actual RX/TX frequency.

I tested the DDS on my scope and it worked a treat with about .5V out. The BITX needs 4.8 – 5.0 MHz to cover the 7.0 – 7.2MHz 40m band.

The Box

I had picked up a nice aluminium box with wood wraparound top and sides at a ham radio rally – and it looked just right, with a bit of extra room for either a 25W PA or a PSU. There followed the most time consuming part of the build which was designing the layout and cutting the metal. I had a nice meter from a KW204 transmitter which would look good - and that damn 16 x 2 display needs a perfect oblong cutout with accurate screw holes. I designed the front panel in MS 'Draw' so I could play with the layout and produce a drilling template. I tried to ensure almost every hole and cutout was completed before any other components were added.

Final build

The final build went fairly quickly, the supplied BITX connectors really speed things up – a bit like an old Heathkit...kit. To use the DDS it is necessary to remove the existing analogue VFO coil. There is a DDS connector on the board and the only extra component required is a DC blocking cap from the AD9850 output. The receiver came up straight away, and after a bit of programming through the 4 push buttons connected to the controller the display showed the correct 40m frequencies. Although the radio has a 12MHz IF, the actual offset must take account of the LSB carrier shift, this works out at around 11.998.5MHz

The damn hiss was still present, and my simple but very effective solution was to roll off the audio response a bit with 100n and 10k preset across the volume control – very mellow sound now.


AGC and Mic preamp

My next test was to try and transmit - I lashed up some wiring to use a condenser mic capsule..the board provides the bias. It sounded awful and with little drive either - even swapping capsules was no help. I put an isolation capacitor in place and plugged in a comms dynamic mic, much much better but still low drive – I had to shout to get peaks anywhere near 5W. My solution was an additional BC108 mic amp which allowed me to add a mic gain control – The pic below is of a generic single transistor pre amp - I used a smaller 10uF emitter bypass cap to keep the gain a bit lower, and added a 10K preset pot at the output. With this set up I was able to have contacts around the UK and EU.

Whilst listening on another receiver in the workshop I noticed residual carrier on the SSB signal..and try as I might I could not null it out with the preset resistor on the board. I note that there are no small balancing caps as you normally expect to find, so that's a little mod for later.

The lack of any AGC and an S meter was next on the list...searching the net on different QRP rigs indicated that audio derived AGC controlling the audio level to the LM386 was the most simple if not crude option. A simple 4 transistor design which allowed the connection of an S meter was build up on stripboard and to my amazement works a treat, just a little bit of popping on voice peaks, I substituted BC108 transistors in this design  The S meter is in PARALLEL with R12, and ideally should be less than 1mA, you must put a 20K trimmer in series with the meter and adjust so that a strong signal is around 75% of full scale- click the picture to go to the webpage or here to see another version of this circuit in English - but note that for the BITX40 you must have the input and output trimmers to set it up properly. I also changed the input preset to 20K to get the levels right - even then you will find the input level can be set quite low.

From the 3 way volume control connector on the board take the red wire to the AGC input trimpot, now take a wire from the AGC output trim pot and wire it to the top connector of the volume control (where the red wire was before the mod). Keep the wires short and use screened cables for the all the connections

The final little addition was relative power indication. A few turns on a small toroid slipped over the antenna connection with a diode and one cap was ample to move the 1mA meter upscale on voice peaks. Now it looks and works like a nice little radio

AGC and Mic Preamp












DDS and Controller










Relative power toroid and diode

October 2016 - A real oddity - The British 'Sphinx' HF Transmitter

Well this is a real odd one - the 'Sphinx' HF Transmitter made for a brief period in the 1960s by SSB Products of Derby England. This is another bit of kit that came to me through a friend and it sure is a strange piece of ham radio gear. SSB Products seemed to have an obsession with naming things after ancient Egyptian culture, they sold a 'Pyramid' HF linear as well

The Sphinx must have been an attempt to cash in on the UK market for ham radio kit in the mid 60's. KW were going strong and maybe the founder (G3EKX) thought there was room for a competitor...but this was made down to a price (but sold at a high price!) and even at the time it looked decidedly basic - in fact its almost homemade, I think it was available as either a kit or ready built. Corners were cut in both the design and construction of this transmitter.

It uses a very basic mixing technique with readily available 455 KHz crystals on only one sideband on 3 bands. 160 and 80 come out as LSB and 20m as USB..there is no 40m band as that would have come out as USB without either adding another SSB filter or a 2nd carrier oscillator...too complex so leave it out!. 15m and 10m would have needed more crystals and switching - so leave that out as well (or buy an extra HF adaptor!).

The SSB filter uses only 2 crystals - so not the best suppression

Contacts to either mute a receiver, switch the antenna or have PTT are all missing...that needed another box called the 'Delta control unit'

It has 2 separate band switches

Only one HV winding on the transformer, - so how to get from 500V for the PA to 230V DC for the rest of the TX.. Answer : use a very big dropper resistor.

No winding for a -100 V bias supply either so how to do that??..Answer : use another little transformer to step UP from 6.3V to 100V

How to bring all the PSU and control sections out to the back???..Answer : put highly dangerous voltages on ' chocolate block' connectors that you can easily put your finger on..and maybe die!

VFO calibration ??? forget it. Make your own chart (or use the one supplied) to match frequencies to the vernier drive

Having said all that - I ran it up from a bench PSU with low volts on the PA and got about 15W out on manually switching the antenna I was able to have a few contacts -and no one complained about poor audio, a bit of carrier, or drift...I should have told them it was a IC7300 !...nice

The Sphinx must have been an attempt to cash in on the UK market for ham radio kit in the mid 60's. KW were going strong and maybe the founder (G3EKX) thought there was room for a competitor...but this was made down to a price (but sold at a high price!) and even at the time it looked decidedly basic - in fact its almost homemade, I think it was available as either a kit or ready built. Corners were cut in both the design and construction of this transmitter.

October 2016 Paraset WWII Spy Radio

This next project is all thanks to Colin G3VTT - on a visit to his home I was impressed by the lovely little QRP valve rigs he handbuilt - each one was a work of art. He persuaded me that I should build one and even gifted me the octal valves to get started. He reccommended I look at the 3 Tube 'Paraset' design which uses a 1 valve CW transmiiter and a 2 valve 'Regen' receiver...this design has copied by hams across the world - some have built amazing replicas and other have used some modern twists in the design - after all the world of electronics has moved on a bit since 74 years ago !.

The original MI6 (Secret Intelligence Service) wartime name for these units was the MK VII - designed in 1942 and initially made at Whaddon Hall. After the formation of the wartime SOE (Special Operations Executive), it was discovered that this simple little radio was ideal for parachuting into occupied countries for use by local resitance groups and spies - it operated between about 3 and 8MHz. The simple design was not without its drawbacks though - one of the worst was that the simple regen receiver radiated a little tiny bit of RF - this meant that if a DF unit got close to the location, the receiver alone could give its position away. More info here


I looked at the designs across the web and starting raiding my parts store for the my surprise I had most of the parts to get started...but first the metalwork!!. In most ham radio projects (especially tube ones) the metal work is the longest and hardest part of the build - it sets the course of the whole job. I wanted  a reasonably professional job..and found a large die cast box that was little bigger than most of the paraset replica's..this was a good thing because it gave me some extra room to play with, especially options for PA antenna matching

I put quite a few days into reasearching the schematic, layout, parts and typical builds by other hams....research always pays off.  I was not preapared to pay a fortune on ebay for rare parts - that would defeat the whole object. My biggest substitutions were on the PA Tank capacitors - the Jackson 100pf ones are not readily available or demand a very high price. I had 2 broadcast twin gang capacitors which fiited nicely - although as they are nearer 250pf, tuning will be more critical. The authentic copies use a slow motion 'friction' drive to turn the main RX tuning dial..I didnt have the parts for this so opted to use a very low value variable cap as a bandspread arrangement (control at bottom keft on front)..even then I found it hard to use both 40m and 80m on the one receive I may have to revert to a bandswitch for the receiver.

I also had to scour the internet to buy some older style TX crystals for 40 and 80m...and ended up with 3.530 MHz, 3.562 MHz, 7.010MHz

The original unit was built to load into a random wire so uses a link coupling on the simple PA - I decided to include an option to use a more standard 'Pi Tank' arrangement that would allow the radio to be connected to a matched 50 ohm antenna - pics below shows my copy of the original link coupled coil design,