Simple 28MHz Polyakov Mixer WSPR RX

As the first part of my WSPR transceiver project for 28MHz, I breadboarded a Polyakov harmonic mixer based, crystal controlled, direct conversion receiver this afternoon. It was a great success!

28MHz Polyakov mixer based WSPR RX

The circuit shows the idea implemented. A low cost GQRP club 14.060MHz crystal is pulled to 14.0623MHz in the oscillator/buffer stage. This is then injected into the back to back diode "Polyakov" harmonic mixer which switches at 2x injection frequency i.e. 28.1246MHz, the WSPR USB dial frequency. The 28MHz input from the antenna is filtered and directly applied (no RF amp) to the diode harmonic mixer with the audio output amplified and filtered in the 2 stages of audio gain. The output then goes to the PC sound card. I may improve the audio filtering and add a 600ohm line transformer between the audio output and the PC mic or line input.

Minimum discernable signal (MDS) is around -124dBm, possibly a shade better. AM rejection was tested by injecting a 100% AM modulated carrier at 28MHz. An input of -53dBm was needed before the AM signal was detected, so 70dB AM rejection, which I think is very good.

More bench work to do, then I'll try looking for 10m WSPR signals on-air with the RX, but for about an hour of work, a very fine result. Once the RX is air-test proven, the TX part will be added to make a complete WSPR transceiver.

Autumn project list

My wife often calls me a "fireside fusilier" as I am always gunner (going to) do this or gunner do that.

Well here is my current projects ideas list, some of which I hope to address in the coming weeks and months. How many will be replaced with other ideas first?

1. Finish the 472kHz transverter and write this up.
2. Simple 28MHz QRP WSPR transceiver to use with a netbook.
3. Stand alone 137kHz receiver (for use with PC).
4. Semi-permanent E-field probe antenna and on-line 137kHz/472kHz grabbers
5. More non line-of-sight light beam experiments (get better distances).
6. Semi-permanent 481THz QRSS beacon for NLOS light beam tests.
7. Alternative HF antennas - maybe re-erect my Par Electronics 40/20/10 end-fed.
8. Further earth-mode (ground conduction) experiments - tests at around 30 and 73kHz as well as more tests at VLF aiming to improve best though ground DX (currently 6km with 5W).
9. Experiment with horizontal loop antenna on TX at VLF (maximising coupling into ground).
10. Build some better LF/MF test gear such as resistive SWR bridge, 50W dummy load and a noise bridge.

I'll give you all a progress update in about 6 weeks time!

PC based LF/MF receivers

There are some very neat ideas for simple LF receivers around that make use of programs like Spectran or Argo effectively as the second IF and detector stages.

DK7FC's simple 137kHz receiver

DK7FC recently posted his schematic for his 137kHz receiver which used a 4MHz crystal divided down to 125kHz using a 4060 IC with the IF at 12kHz (corresponding to 137kHz) being fed straight into the PC software. With programs like Argo the scale can be offset so that the correct frequency is displayed. Of course, there needs to be some front end filtering to remove the image, in this case at 113kHz. Using this approach one can make a very simple "stand alone" LF receiver for 137kHz weak signal reception. Such an approach is ideal for grabbers which are permanently connected to a PC anyway, uploading the received signals to the internet.

Baofeng UV-3R very inexpensive VHF/UHF handheld

You may or may not have seen the adverts on eBay for the Baofeng UV-3R handheld transceiver. These are available in several colour versions (including the yellow version shown here) for prices between £25-40 including shipping. For example Solidradio of Hong Kong have these available for just £29.90. Reports suggest these are not bad rigs at all.

What I've just noticed is that, the same supplier shows there is a 3 year UK SquareTrade (no matter what) warranty available for £8.49. So, for less than £40 you can buy a dual band VHF/UHF handheld with a decent warranty for 3 full years.  I've heard of some deals, but this takes some beating.

QRP 10m WSPR

As I cannot access my "lab" currently (kitchen refit materials are everywhere!) I am restricted to operating from my shack, so this morning I've been trying out 28MHz WSPR with 500mW out to the halo in anticipation of my QRP WSPR transceiver design coming soon. This will be DSB, so I will need 1W RF out to get the equivalent of 500mW on the WSPR frequencies.

500mW 10m WSPR reports today

Not unexpectedly, there are loads of reports on the screen when I last looked from all over Europe. The best report so far today is +8dB S/N which suggests just 500uW would have been sufficient to be spotted. I know that others have achieved similar, indeed very much better, success when the band is open. What is also intriguing are that I have been spotted within the UK at over 300km, presumably by Es or backscatter, although maybe by tropo?

I have still to finish off the 472/500kHz transverter and write this up before starting on the 10m WSPR rig, but should have the latter done quite soon. It will be fun to have a stand-alone 10m WSPR transceiver running with a little netbook.

Lack of recent posts

My apologies for lack of recent content, but we've been away from the home QTH whilst our kitchen is being refitted. The local builder, a good and reliable person, advised us to go away and avoid the chaos, so we have been exploring Kent and East Sussex from our son's home near Canterbury. No ham gear with me, just the iPod Touch 4g.

Update Aug11th: we have now returned home (PW awaiting me) to find the kitchen almost finished - 2 more days work - and looking good. Problem is access to my "lab" is impossible because of boxes and builders tools. With luck all should be clear by Wednesday when ham radio building can resume.

CW from Mars

The latest NASA rover "Curiosity" apparently imprints JPL in CW on the Martian surface as a result of its tyre pattern. See http://www.tyrepress.com/News/OFFBEAT/25967.html . If I recall correctly my callsign is on a microchip on "Curiosity" (see post last autumn).

MePADS and MeSQUARES

MeSQUARES

MePADs

As a fan of sticking small bits of cut-up PCB material to a solid piece of copper laminate ground plane in most of my projects I get through a LOT of bits of copper laminate. Getting lazy in my old age, I've just ordered some MePADS and MeSQUARES from www.qrpme.com in the USA. These are arrays of pads or IC footprints for dead bug or Manhattan style construction that can be snapped out and glued to the main ground plane making a neat layout possible without a real PCB.

Finally RIP Heathkit

It was a long time finally dying after the last Heathkit amateur radio product was made by this once fine company, but I read today on the Southgate Amateur Radio News page that what was left of the company has now filed for bankruptcy.  Latterly it had been involved in educational systems but funding was drying up.

VE6CN's fine line-up of Heathkit products

Founded in 1926, Heathkit was famous for its kits in the 1950s, 60s and 70s. They were the Elecraft of their day with sound designs and clear step-by-step building instructions.  My first introduction into electronics and radio was with a kit for Christmas 1961. I never looked back thanks to that wonderful product.

I hope VE6CN wont mind me linking to his picture showing his fine line-up of Heathkit products. For a lot more pictures of Heathkit amateur radio products see http://www.ultrawebb.com/OHP/gallery.htm .

Sub-harmonic mixer based 10m WSPR transceiver

My favourite HF band is 28MHz and I often put my FT817 on and run WSPR in the background whilst getting on with other things in the shack. You may be surprised how often good DX reports are received. This ties up the FT817 so I have been rethinking how to make a very simple WSPR transceiver for 28.1246MHz.

One neat idea is to use a 14.060MHz crystal (cheap from GQRP) pulled up to 14.0623MHz (it is possible) and use this to drive a Polyakov sub-harmonic mixer, which needs half frequency drive, in a simple direct conversion receiver feeding a PC sound card via an isolating transformer. The same oscillator, doubled would be fed into a single balanced DSB modulator and a small QRP PA stage. The resulting WSPR signal would appear as a DSB signal , not SSB, so half the power would be wasted but who cares with milliwatts?

The resulting full WSPR transceiver would be extremely small and simple, need no VFO or VXO and could be left running almost continuously when the PC is switched on. This would be ideal with a small netbook PC.

The idea is not new: see for example http://www.qsl.net/d/dl1gsj/html/qrssrx30.html .

Sounds like this is my next project......