There was an error in the feedback resistor value on the schematic near the NE5534. It was shown as 1k5. This resistor sets the gain of the op-amp stage and should be 220k, giving a gain of 146, or 43db.
As shown in the heading pic, the transmitter will interface to the previously published receiver, via the changeover relay, which is VOX operated. The audio from the PC, or laptop, is buffered by the external USB sound-card. Since the computer does all the timing and data generation via the WSPR software, the only thing that needs to be taken care of is that the PC clock must be accurate. There are a number of applications available that will synchronise your PC clock to an atomic standard every time you start your system. Here is a good place to start http://www.worldtimeserver.com/atomic-clock/
This will ensure that your WSPR activities are within the standard 2 minute time slots. I recommend setting your transmission percentage to an odd value. This will tend to randomise your 2 minute listening and transmission slots over a period of time.
There doesn’t appear to be much merit in selecting a variable voltage regulator for this circuit. However, my initial design incorporated a FET PA which needed a bias voltage. Not having had much luck, or any experience, with the FET design, I resorted to a pretty standard bipolar circuit. Just set the output voltage to 8v before connecting the mixer supply Pin 8, to this supply node. The mixer device will surely destroy itself if you exceed 9v supply.
You can of course simply fit a fixed voltage regulator from the LM78xx 100mA range. Anything between and including 5v and 8v should work fine.
Audio Input and Mixer.
The WSPR audio input to the transmitter is controlled by the power slider control in the WSPR software. This control should be set to 75% to start with. I found the signal at the transmitter input, which is also connected to the VOX input, to be near 1V p-p. You may find a different level depending on your sound-card capability.
The preset pot at the mixer input should be set for 200-300mv on pin 1. If you can’t measure this voltage, set the pot to the ground end. Later, during the transmit cycle, adjust the pot until the signal can be seen in Spectran, or heard on your receiver. With no audio there should be no output as there will be no sidebands. Increasing the signal to more than about 300mv will not improve the output power and may well lead to distortion.
The 1nF capacitor is insurance against any RF getting into the audio port of the mixer, along with the audio, and generating unwanted products in the output.
The audio arriving at the transmit mixer, is ‘mixed’ with the oscillator frequency. The oscillator frequency is determined by the crystal, and the other components at Pins 6 and 7. In this case it is 10.140MHz. The NE602 ( SA612 etc. ) data sheet shows the formulae for calculating the values. They are not too critical so I used the nearest preferred values. The addition of TC1 and L1, enables the frequency to be ‘pulled’ slightly, in order to put the transmitter exactly on the 1500Hz marker in Spectran. See Alignment section.
You can of course fit almost any crystal here and by changing the LPF components you will move the transmitter to another band. Replacing the crystal with a VFO or synthesiser will also make it multi-band, again depending on the LPF components. This would be a great opportunity to construct a versatile CW transmitter.
The mixer output at Pin 5 produces the DSB signal that we will use. With 7.5v supplied to the mixer, the output pins carry in excess of 6v DC in my prototype, so a capacitor is used to couple the signal into the gate of the driver FET. There is no need to match the output of the mixer to the gate impedance of the FET, as we are trying to transfer maximum signal voltage not power. The source resistor provides some 2v of gate bias via its voltage drop, and is bypassed at RF in order to maximise the signal gain. You should find over 2v p-p at the drain.
The drain transformer is bifilar wound and is connected as a centre tapped transformer providing a 4:1 impedance transformation whilst feeding RF to the base of the output transistor. I experimented at length with the number of turns and various tapping points. There was little difference in the output, unless I chose ridiculous ratios. However, using a 22uH choke for the drain load and coupling with a capacitor reduced the output considerably. So I stayed with the bifilar solution.
The PA is broadband and runs in class AB, biased by 680R and the 1n4001 diode. Not the best bias configuration, but more than capable at this power level. A ferrite bead on the base leg guards against possible VHF parasitics which destroyed a couple of earlier devices.
The output devices may be any of a number of common NPN types. My choice of 2N3904 was determined by the fact that there were at least 100 of them in my spares box, and I got through quite a number of them. I experimented with a couple of other transistors before I realised there was a spurious VHF problem. As previously mentioned a ferrite bead cured that. I tried various resistor values in the emitter circuit, and reached the conclusion that, in this configuration at least, 4R7 is a good compromise between power output and thermal protection. At 2R2 the transistor gets hot. At 10R and higher, the output power naturally reduces.
A small heatsink is recommended. I used a 2 small piece of scrap tin plate formed around the transistor and soldered to the PCB. The whole heatsink assembly is about half the size of a postage stamp. It gets warm during a 2 minute transmit cycle. If your device runs too hot then increase the surface area of the heatsink. Or raise the value of the emitter resistor at the cost of a reduction in output power.
The PA collector load is a trifilar broadband transformer. The 9:1 ratio transforms the collector impedance hopefully a bit closer to 50R in order to match into the following Low Pass Filter. Again I tried a number of different ratios here, and also a bifilar transformer as in the driver stage. The trifilar configuration appears to offer the best performance.
The LPF at the output is essential for removing the unavoidable harmonics. This filter is of a standard design, I have selected the values in order to use off the shelf components. This means that the LPF impedance is not actually 50R, but it is fairly close. At 1uH the -3db corner frequency is 10.7 MHz, the response is 40db down at 21MHz.
If you roll your own, the toroids should be FT37-6 wound with 18 turns. The response now will be very slightly higher due to the fractionally lower inductance of ( theoretically ) 0. 97uH.
I believe there is a 5% manufacturing tolerance with these toroids so minor manipulation of the number of turns may be required. i.e. If the measured output is considerably lower than 12v p-p or 400mW, remove one turn and measure the output again. In the vast majority of cases there should be no problems here.
This is simply an audio amplifier with a gain of 10, followed by a rectifier / doubler circuit. The electrolytic capacitor determines the ‘hang’ time and the resistor fixes the maximum base current to just over 1mA. Almost any 8 pin DIL op-amp will work here. Also just about any small signal NPN transistor that can handle the relay operating current. A shocking 40mA in my case. Pun unintended.
Download and install the free software, Spectran. http://www.sdradio.eu/weaksignals/spectran.html
Or use your favourite Spectrum viewer.
Check that there are no low resistance paths from all supply pins to ground. Ensure the supply polarity is correct. A couple of 100R 1/2 Watt resistors, wired in parallel across the output of the LPF will be a suitable 50R dummy load.
The amplifier may not withstand an open or short circuited load for any length of time, so you should ensure that either a 50R dummy load or a suitable resonant antenna is attached at all times when in transmit mode.
Connect a multimeter in series with the supply, switched to a suitable current range. Expect to see between 40mA and 50mA consumption at power on. The bias voltage at the bases of the output devices should be very close to 0.7v.
The simplest way to align the transmitter is to connect the audio from the PC sound-card, to the PCB, and run the WSPR software. This should have previously been setup with your callsign and locator. Select 50% transmit. As soon as the transmit cycle begins, the VOX relay should switch and the Tx LED will illuminate. Then monitor the transmission in Spectran via a receiver connected to another PC. Alternatively, if you have a tablet, there are apps available for IOS and Android devices that will allow you to drive the transmitter with WSPR data. I borrowed my wife’s laptop for this purpose. ( Thank you Maria )
Adjust the crystal oscillator trimmer, CT1, until the received signal appears at the 1500Hz marker on the Spectran waterfall. Close Spectran and now launch the WSJTx software, on the second PC, and check that you are receiving your own signal.
If you don’t have access to a second PC then you can unbalance the mixer by keying Pin 1 to ground via a 10k resistor. This should be removed after setting up.
Keying the mixer input pin produces a carrier at the crystal frequency. A simple crystal controlled CW transmitter results, this can easily be monitored on an oscilloscope for clean waveform and amplitude. Or you can listen for it on an SSB receiver tuned to the carrier frequency, NOT the WSPR frequency, which is 1500Hz higher due to the sideband generation. A receiver and monitoring software such as Spectran, are used as above. You will need to tune 1500Hz lower on the receiver because there is no USB to demodulate, just the carrier. Switch in your attenuator, as the local signal is quite strong. Over 12v p-p should be observed on the scope, or more than 400mW into 50R at 110mA or so.
You will still need an audio source in order to test the action of the VOX circuit. With 1v of audio drive, the output of the op-amp is 8v p-p clipped sine wave, and there will be at least 7v DC after the voltage doubler. The 4k7 resistor in the VOX transistor base circuit allows about 1mA or so of base current. With a nominal gain of 100 there will be more than enough collector current to drive a relay.
Current consumption during the transmit cycle should be in excess of 100mA, or about another 70mA more than standby. Indicating about 850mW power input with a 12v supply.
You should find around 18 to 20v p-p on the ‘scope, at the collectors of the PA. This will include lots of harmonics. After the Low Pass Filter the output power is in excess of 450mW, or 13v p-p, indicating about 55% efficiency.
450mW doesn’t sound like much power but believe me worldwide coverage is certainly possible. CW enthusiast have known this for a long time. On the first evening during a short test period, a couple of USA stations and some Aussie neighbours spotted my little signal from ZL.
If you have previously built the receiver, you can at this point wire the Rx and Tx modules together via the relay, as shown in the VOX schematic. Note that the VOX circuit is always powered, but that the receiver and transmitter are powered up depending on the state of the relay, which itself depends on the WSPR software tx/rx cycle. The relay also transfers the antenna to the correct board.
Connect the USB sound-card to your PC and set up the WSPR timing to your preference.
A suitable antenna cut for 30m is recommended. Such as a half wave dipole or a quarter wave vertical, with radials or groundplane. You don’t want to waste any of that meagre power in antenna losses through a random length of wire, while transmitting, using this exciting and interesting weak signal mode.
Joe Taylor K1JT invented WSPR and other weak signal modes. My thanks go to him for my renewed interest in ham radio.
Ver 1.2 Corrected some spelling mistakes. Added better image of receiver. Updated schematic.
Having developed an interest in WSPR last year, I purchased a very neat transmitter kit from QRP-LABS. The Ultimate3S. Mentioned elsewhere on this site.
( WWW.qrp-labs.com )
The popular kit worked as soon as it was powered up. After a couple of days of successful ‘beaconing’ on 30m WSPR, and despite owning a couple Elecraft transceivers, the urge to construct a suitable WSPR receiver overcame me and I began researching the project in earnest. Most designs seem to be direct conversion architecture mainly based on the ubiquitous LM386 and NE602 et al. I built a couple of these designs and wasn’t happy with the performance. The 602 is not known for its signal handling, and the LM386 is another problem area as it is usually pushed to its limit in an effort to simplify the circuit. I would happily trade simplicity for proportionally better performance.
The 602 is an elegant part even with its limited performance. This got me thinking about a really narrow Band Pass Filter in the antenna circuit, thus limiting the energy reaching the mixer input. I decided that a narrow bandpass crystal filter would be ideal. A superhet design, although more complex, would give single signal reception, avoiding the usual image problems associated with direct conversion, apart from other known issues. I was surprised to find that some Elecraft QRP rigs use the NE602 as a receive mixer, and even the very well respected K2 uses it as a product detector. Suitably encouraged I pressed on.
I already had a number of 10.140MHz crystals and hundreds of others. While checking through my collection of crystals, in order to select a suitable I/F to work with, I found some 10.7 MHz parts. A quick calculation determined that the VFO or local oscillator would need to be 560 kHz. Oscillator stability shouldn’t be a problem at that low frequency. Something about the frequency jogged my memory about remote controls for TV, Hi-Fi and such devices. A quick search of the web revealed some 560kHz ceramic resonators for sale on Ebay. The seller was in Germany. A pack of 5 of the 560kHz parts were ordered that same day. Shipping cost was reasonable, and the parts arrived within a few weeks. In the meantime a number of circuit designs were sketched out.
I reworked the circuit topology on paper and decided to use the resonators in the I/F and also in the Carrier Insertion Oscillator driving the product detector. The 10.7 crystal would then be used as the local oscillator. This was beginning to look interesting.As soon as the parts arrived I quickly determined that the NE602 would oscillate well with the resonator,using a small inductor and a trimmer capacitor.
The first part of the circuit that was built is shown here. The product detector and Carrier Insertion Oscillator based on a SA612A. Applying a 560kHz signal from my home-brew synthesised signal generator, to the mixer, produced an audio tone in the high Z earpiece that I used for monitoring the very low level output from pins 4 and 5.
The inductor is 470uH and the capacitor is 15pF fixed, in parallel with a 60pF variable. These are parts that were available at the time and a better balance of values should be selected. The oscillator could easily be ‘pulled’ high or low as witnessed by the changes in audio tone.
I/F Phase splitter.
The novel part of this circuit is the phase splitter feeding the input pins of the product detector. I have not seen this done before and I’m not sure if it is more effective than a transformer. However it appears to work fine. This takes the single ended I/F signal and drives the mixer in push-pull. The source and drain resistors are 1k5 in an effort to match the input impedance of the mixer. I removed the internal capacitor from a 455 kHz I/F transformer ( yellow core ), then measured the inductance and from that calculated the required capacitor value. Despite my efforts I could not peak the signal. I then accidentally discovered that the transformer was self-resonant at 560kHz, so no capacitor was fitted. I then easily found a peak in the signal, which was audible even off tune. The FET gate resistor is 1M, making the input very high impedance and less likely to affect the transformer circuit preceding it. An inductor and capacitor were selected as a 850kHz LPF, and fitted between the transformer and the gate of the FET. This removed a 1MHz approx, spurious signal that I could see on the output pins when monitoring with an oscilloscope.
The first mixer.
This has the 10.7 local oscillator used to convert the antenna signal to the 560kHz I/F. The output of this stage is DC coupled into the base of the emitter follower I/F stage. The emitter resistor is 1k5, in an effort to match the impedance of the mixer. The 560kHz resonator is used as an I/F filter, driven from the emitter circuit, and feeds the low impedance winding of the I/F transformer. I later added another resonator. See the alignment notes. The front end
The antenna input circuit uses a ready wound transformer of 2.6uH, available from ( WWW.GQRP.COM ) as 2u6L. This needed 100pF to resonate at 10.140 MHz. It’s fairly broadband, but the tuned circuit’s main function is to match the low impedance of the antenna (hopefully 50R) into the high impedance of the crystal. It will of course provide some filtering thereby protecting the crystal from out of band high energy signals. The wanted signal filtering will be done by the crystal.
An alternative antenna circuit could be built around a T-37-2 toroid holding 26 turns, 50pF fixed capacitor and 60pF variable in parallel. The primary winding could be 4 turns. The toroid is best mounted clear of the PCB. I moved the voltage regulator to enable short routes to all parts of the circuit.
Finally the audio stage, which is also driven in push pull. The NE5534 is fairly low noise and at this low signal level is unlikely to overload, despite the high gain of the circuit. A preset resistor is fitted at the output in order to facilitate level setting of the audio into the PC or laptop. In many years of experimenting with computers and radio interfaces, I have never damaged a PC. However I suggest you fit an isolating transformer if you have any misgivings.
As an alternative to an audio isolating transformer I have used a USB sound-card with great success. These devices are very cost effective ($3NZD) considering their utility, and certainly cheaper than a transformer. Windows 10 recognised them immediately and automatically loaded the requisite driver. No further software installation is required. You will need to select ‘PNP USB Device’ or something similar in the WSPR-x audio setup. On the PC or laptop you will need to set the USB microphone gain to at least 50, maybe more.
The NE602 maximum rated working voltage is 8v. On no account exceed this. A low noise linear power supply, rather than switch mode, is highly recommended. I used a SLA 12v battery.
You will need an accurate signal source of 10.140MHz. I recommend using the filter crystal as a temporary test oscillator. Fit a 4p7 capacitor in its place in the receiver circuit. Replace it with the crystal once alignment is done. If you have a transmitter, such as the QRP Labs Ultimate3S already running WSPR things will be a lot easier
First, it always pays to check that there are no low resistance readings from supply pins to ground. Ensure your supply polarity is correct. ( ask me how I learned this ! ) Check that the CIO variable capacitor is set to half mesh. Connect a pair of headphones. Apply 12v DC. If you can power it up with a milliammeter in circuit, the reading should be less than 30mA. My measurement was 23mA.
I have used this oscillator circuit many times with no problems. Just place the oscillator close enough to the receiver so that it can be heard in your headphones and can therefore be detected by the monitoring software later. Set the audio output pot to about 3/4. Tweak the antenna coil for loudest signal. Use the correct tool or you WILL break the core. Similarly tweak the I/F transformer. Connect the audio output from the receiver to your sound-card. I monitored the audio in Spectran.
My transmitter was sending the WSPR messages every 2 minutes. By adjusting the variable capacitor in the product detector oscillator circuit, it was possible to line up the waterfall signal in Spectran exactly on the 1500Hz marker. The waterfall in WSPR-x is unsuitable for this. Switching from Spectran to WSPR-x and waiting a few minutes confirmed reception of my own signal. At this point I strongly suggest you put the receiver in a screened enclosure and surround it with some polystyrene foam in an effort to stabilise the temperature. The device that I found to be most temperature sensitive is the 470uH inductor in the CIO. I might replace this with a different type of coil. Maybe a toroid. It may be necessary to re-tune the CIO after boxing up and allowing the temperature to stabilise.
Later testing using a signal generator and headphones revealed that the opposite sideband was not very well suppressed. So I added a further resonator, in series with the first, and a 180pF capacitor to ground at their junction. This greatly improved things. Best dx heard on the first day using a 35ft vertical antenna, was EA, at over 19,000 km from New Zealand.
Travelling around in a motorhome I don’t have the luxury of a workshop. I am sure that those with access to more/better test equipment could come up with something with even better performance, or at least optimise some components that I have only guessed at. Have fun.
Since retiring to NZ in 2013 we have been travelling the country in our motorhome. Its an ex-rental 6 berth Kia Dreamtime Deluxe. Plenty of space for Maria and me. We both have our hobbies and the van is sufficiently spacious to allow us to occupy different parts of the living space without getting in each others way. Maria has the ‘lounge’ area with its picture window, and I have the 4 seater breakfast table. This doubles as my operating desk and construction bench. In the last 3 years this has worked very well.
When we first went mobile I was determined that I would have a radio station that was compact and easy to use. I had an Elecraft K2-100 and a QRP rig, the KX3. I have since acquired the latest Icom rig, the 7300, which is mentioned elsewhere on this site. I spent considerable time ( months ) scouring Ebay and Trademe looking for the LDG remote ATU, the RT-11. I had owned one of these back in the UK and never had any issues with it. I disposed of it when I purchased a rig that had an auto ATU built in. That was a mistake ! Not many integral tuners can handle such a wide mismatch as an external tuner can. So when we arrived in NZ I had no auto ATU. My patience was rewarded when I eventually found what I was looking for in USA. Rather than ‘peck’ at the bidding, I immediately bid what I was willing to pay. I think that was equivalent to £80GBP. The bidding didn’t get that far and I got what I considered to be a bargain. Exactly what I wanted for less than I was willing to pay. The total cost including shipping amounted to £78GBP if I recall correctly. The plan was to install the ATU inside a rear locker of the van and control it remotely using a homebrew button box. In the meantime ……..
One useful item that I brought with me from UK was a 8m fishing pole. I had planned to repurpose this as my mobile vertical antenna. After some trial and error I eventually mounted some 40mm waste pipe to the rear ladder using heavy duty cable tie wraps. I removed the bottom section of the pole as it was too large to fit inside the waste pipe. I also removed the top section as it was too thin to support the antenna wire without bending. The wire is silver plated ( I think ) and white Teflon coated. Courtesy of Westlake if I remember. The overall length is 35ft. No particular ‘magic’ length, just the longest piece of wire that fitted the installation. A few turns of insulating tape near the top and bottom of the outer section of the pole ensured a snug fit inside the wastepipe. The pipe has a stainless steel cross bolt near the bottom, to stop the pole falling straight thro, and its overall length is about 50mm shorter than the fishing pole. This is so that when the pole is collapsed inside the pipe, at least a small part of it is accessible so that it can be easily extended. The wire is loosly spiralled around the pole, down to the top of the white pipe, where it is secured with yet more insulating tape. The loose end then drapes down the back of the van and is routed into the rear locker. Here I had a balun mounted inside a plastic sandwich box. The coax ran alongside the van, and was routed thro a window to the radio. Erecting the antenna, deploying the counterpoise and running the coax to the radio took a few minutes, but was a real pain in wet weather. The auto ATU inside the rig would match this antenna setup on 40 and 15 with quite a low VSWR. However some of the other bands of interest were not so good. So about 14 months later ……..
I read some really complimentary revues of the Icom 7300, and subsequently ordered one. Despite being told by Icom NZ that the radio was in stock, I had to wait a month before it arrived. I assume they meant it was in stock in Japan, they simply neglected to mention that detail. Nevertheless when it arrived I was very impressed with the whole thing. My only minor criticism is the lack of a bandswitch. You have to use the touchscreen.
I soon dicovered that the 7300 had an auto ATU control port and to my delight, that it was compatible with the LDG tuner, which was still languishing in a storage box. Other than initially testing it I had never used it. I ordered a suitable Molex connector from a local on-line trader that would fit the ATU port on the 7300. The next thing to do was install a run of coax and some 4 way screened cable from inside the van to the back locker. This turned out to be considerably more difficult than I had imagined. I purchased the cables at a local outlet and was disappointed to find that the screening of the coax was nothing like as substantial as previous examples of RG-58 that I have used, and still have some short lengths of. Satisfied that the distance from the radio to the ATU was quite short I decided to use what I had rather than spend more time search for something better. Not a lot of choices in Northland NZ. Trying to purchase some flexible conduit was another problem. It was available from a number of places, but at a price that I considered too expensive. I eventually bought a couple of 3m lengths of 20mm water pipe. Being in a campsite with a hardstanding ( Waitangi ) and in dry weather, the day arrived when everything was ready for the installation. I squirmed under the van on my back and checked out the cable routing only to discover that I would have to cross a chassis member to get to the rear compartment, mainly due to having to avoid the double rear wheels. After much indecision and not a small amount of bad language, I fitted the water pipe along the chassis member as best I could, whilst avoiding the shock absorber, the wheels and various other bits of automotive hardware. This took far longer than I had planned on spending on the whole project. A few heavy duty cable tie wraps eventually held it in place. The cables would be exposed for about a meter at either end but at least the majority of the run was protected.
After very carefully measuring the floor area below the seating storage compartment inside the van, I drilled 2 holes of 8mm diam thro to the underside of the van. Similarly 2 holes were drilled into the sidewall of the rear storage locker. Feeding the cables down thro the floor then thro the length of pipe and into the locker was easier than I expected and was completed in a very short time. I tied both cable ends together inside the van end and pulled all the rest thro to the back of the van, out of the locker and then back inside the van thro a window. This allowed me to use my soldering iron to fit the PL259 to the coax and the DB9 to the multicore. I then pulled the excess cable back into the van, at the radio end, until there was about 150mm showing at the antenna end. This would be sufficient to reach the RT-11. Inside the van I dressed the cables along the inside of the seating compartment and then thro a gap in the seating hardware to a narrow shelf on the sidewall underneath the breakfast table. More cable ties ! On the shelf I mounted a small ( 100mm x 50mm ) plastic box and the cables ended there. On the lid of the box there are BNC, DB9, Red & Black 30A power terminals and 3A aux power spring terminals. The power terminals are connected directly to the house batteries. 2 x 85Ah deep cycle lead acid types. The spring terminals are in parallel.
The RT-11 is mounted inside the locker ( which is on the right hand side just forward of the light cluster ) and the antenna is connected directly to a terminal post on the ATU. Before installing the ATU, I constructed a heavy duty 4:1 balun and fitted it inside the ATU case along with some terminal posts for antenna and ground connections. See this easy design at http://qrznow.com/make-41-balun-cheap-easy/ I used a T-130-2 toroid.
The ground terminal connected to A, is also bonded to the metalwork of the chassis by a very short braided cable. The antenna and counterpoise wires are attached to the balanced terminals. I assume the antenna appears to be an ‘L’ shaped dipole as far as the balun is concerned. The ground counterpoise consists of 2 wires measuring 35ft and 16ft. These are 1/4 wavelengths on 40m and 20m. When travelling, the antenna is collapsed, of course, and the antenna wire is hung in loops around the top of the white pipe. The waste pipe makes the antenna almost ‘invisible’ when on the move. The counterpoise wires are stored in the back locker.
Does it all work ? Oh yes. VSWR was checked with the antenna analyser built into the Icom 7300. On all bands from 40m to 6m the highest SWR was no worse than 1.5:1. The antenna is not suitable for 80m and no attemp has been made to make it so. My best DX on 30m WSPR, is Portugal, using 1W ( the lowest power setting on the 7300 ). From my location in NZ that is just 60km short of 20,000km. Can’t get much further than that on the planet.
Having the need to measure milliwatt powers from my WSPR transceiver, I decided to build a simple measurement device that would indicate power up to 5W. The QRP milestone. I have a couple of SWR / Power meters, but nothing sensitive enough for such low levels of power. A search around the web produced many projects, almost all based on the same circuit.
This is my take on a fairly standard circuit. Note the diode has a forward voltage of only .4v. Max voltage is 30. So the device should handle up to 15W. You will need to beef up the load resistors if you build it for this higher power level. Diodes such as the ubiquitous 1N4148 can have voltage drops up to 1v, according to the datasheet. This will reduce sensitivity and accuracy at really low power levels.
Calibration can be done by applying a known 5W from a transceiver, Elecraft KX3 as I used for example, and adjusting the pot for the correct reading on the meter. My meter was salvaged from an old SWR bridge, so the scale was conveniently marked.
Alternatively apply 15.82 volts to the point marked on the circuit and adjust the pot as above. The more accurate the calibrating voltage is, the more accurate your meter will be. The power reading is equivalent to (v*v)/R. Where R is usually 50R.
I scored a few pads on a piece of copper clad PCB with a sharp blade, and soldered the parts onto that, keeping all leads as short as possible. The electrolytic 4.7uF across the meter, helps to damp the movement may not be needed and is not shown on the schematic.
This version has 2 pots selectable by a switch ( not shown ) enabling 1W or 5W FSD as required.
Of course it should be installed in a shielded enclosure of some sort. This 50c tea caddy was an ideal size. I didn’t realise how battered it looked until I reviewed this pic !
Listened to some contest stations over the last weekend. Best dx was a GM station. Lots of Europeans and Americans heard as well. All on 20 mtr with my 1/2 wave vertical, connected to my homebrew transceiver that I have dubbed Evolution.