A WSPR receiver for 30m

Derwyn Williams. ZL4SAE.

Published in QST Feb 2017.          

 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.

Crystals.

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.








Product detector.

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.

phase splitter and product detector
 
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.

completed receiver

Audio.

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. 

Sound card.

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. 

external USB sound-cards

Alignment.

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.

Test Oscillator. 

xtal oscillator courtesy G4CLF

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.

( WWW.sdradio.eu/weaksignals/spectran.html )

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. 

Best 73 Derwyn.

ZL4SAE ex GW4SAE

Advertisements

Motorhome Mobile operation on HF

A homebrew vertical for HF WSPR ‘Mobileers’


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 ……..

Home is where you park it.

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.

LDG RT-11 inside the storage locker.

 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.

4:1 balun

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. 

Milliwatt meter for WSPR

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 !