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.

New LF Yahoo Group

After some debate, the old RSGB LF-reflector is now migrating to a new Yahoo group at http://uk.groups.yahoo.com/group/rsgb_lf_group/ . If you have an interest in the VLF/LF/MF amateur bands such as 8.97kHz, 136kHz, 472kHz and 500kHz then you may want to join. It is sure to become one of the most useful resources on the web for up-to-date information as well as a valuable archive of schematics, photos and ideas.

137kHz transverter bites the dust

Having re-erected my 80m loop antenna I was about to reload OPERA and give it a try now it appears more stable. I ran up the transverter on WSPR to check all was well only to have the power supply suddenly take around 20A and shut itself down.  I think the MOSFET PA died on me. All being well I will get this fixed next week when my grandson goes back home again and have another try.

Winter project list

In this first post of 2012 I thought I'd list the projects that are "on the to-do list" for the coming few months. As always, this is subject to change without notice as my interest and enthusiasm goes through waves:

1. Lightbeam RX. The aim is to detect the GB3CAM 481THz optical beacon at a distance of 22km from my nearest highspot. So far I have breadboarded several good sensitivity optical detector heads and the next stage is to complete the packaging of this into a screened box and attach it to my optics which also need a bit of engineering to complete. Target is this to be completed next week.
2. Light beam transceiver. Following on from (1), this will either be a transceive head with a transverter or an FM transceiver that I can duplicate so I can talk to others who can borrow the second unit.
3. Rebuilt VLF earth mode beacon transmitter. In a few weeks time I will be able to put out a stable signal on 8.97kHz  (the usual VLF test frequency) rather than 8.76kHz. My intention is to rebuild the whole beacon TX so I can run 10wpm CW, QRSS3, 30 and 60 modes as well as WSPR. I may also add a 100W PA option later. With this kit I hope to improve my personal earth mode DX record of 6km.
4. Improved LF loop and E-field probe antennas. I want to erect a more permanent external antenna for VLF and LF grabber work and mount these away from the house. Up to now I have used my 80m square vertical loop on RX but this is currently down.
5. Case up the 137kHz transverter. This has been a rat's nest on a piece of copper laminate for too long!
6. 28MHz WSPR transceiver based on a 14.060MHz crystal doubled in a DSB direct conversion transceiver circuit.

Now, how many of these projects get done is a matter of conjecture, but I hope at least 50% of them will get finished.

137kHz Transverter RX Desense

When testing my transverter on the bench today I noticed the receiver was being desensitised by the undriven TX strip. I don't switch the TX part off when on RX, I just don't apply 10MHz drive into it. There must be some source of low level noise from these stages when on RX as the desense is around 10-20dB. Despite this I am still able to copy M0PPP at 182km pretty consistently these days on 137.5kHz WSPR. I need to fix this problem before this project is boxed and "finished". When the TX strip is powered down the sensitivity is very good indeed.

Ten WSPR reports now on 137.5kHz

This evening G3WCB (101km) reported my 137.5kHz WSPR signal for the first time bringing my total number of reports on this more difficult band up to 10. My ERP is currently around 500-600uW, although I hope to run the transverter PA from around 20V tomorrow which should increase the ERP by around 3dB.

These are the reporters so far:

G6ALB
G3WCD
M0FMT
G3XIZ (46km)
G3XVL (69km)
M0BMU (69km)
G3WCB (101km)
G3YXM (148km)
M0PPP (182km)
GW0EZY (251km)

137kHz WSPR - big SUCCESS at last!

After yesterday's disappointments today has been a whole lot better on 137.5kHz with my best WSPR DX report ever and a report from my second DXCC country on the band.  First several reports from GW0EZY in mid-Wales (251km) and then one from M0PPP in the north of England (off the side of my TX loop too - 182km) who I heard last night for the first time.  I've copied him several times tonight as well as PA3EGO. The transverter now puts out about 20W, but the ERP is still only around 200uW based on the loop current and enclosed area. I'm tempted to put it in a box at last as I now know this system is capable of decent range on a good night like tonight.

Struggling on 137kHz - time to move on?

Today I did some changes to my 137kHz transverter and put it on the air this evening for about 4 hours sending and receiving 137.5kHz WSPR. Power from the PA is now around 20W and the ERP must be around 200uW.  I was really quite hopeful that, with enough stations active on WSPR tonight, I might get a few reports.  The band was indeed quite busy with a few stations like G0KTN and G3WCB who have regularly copied my 500kHz WSPR on the band and looking.  What a disappointment! Not only did I get not a single report, but I was unable to copy any of the active TX stations like F5WK or M0PPP. The latter was visible as a trace but not strong enough to decode.

Without increasing the power another 6dB at least to around 80W and improving the antenna considerably I am now of the opinion that, QRSS30 apart, 137kHz is not going to be a success. My feeling is I need at least 20-30dB better system performance (TX power and antenna efficiency that is) to even start to approach the level of reports I manage on 500kHz. So, do I try just that bit more or do I throw in the QRP towel on this band? I don't want to run lots of power and I don't want to erect a monster antenna: I was hoping my unobtrusive loop would have done me as well as it did on 500kHz, but clearly not.

I am beginning to think that this really IS a band where you need lots of power to make a success of things.

UPDATE: Just as I finished writing this I manage to decode M0PPP twice at 182km.
2252 -27 -0.5   0.137562  1 M0PPP IO93 17

500kHz WSPR

Yesterday I fired up the 500kHz WSPR kit (2mW ERP from the loop antenna) for the first time in several months and was pleased with the reports and level of activity. Several people joined in the fun and some new stations are now monitoring and reporting on the band. I want to get my 137kHz WSPR kit sorted this week with around 25W RF and maybe 0.5mW ERP if I am lucky.

Success on 137kHz QRPp

 My 137kHz 0.2mW ERP signal by M0BMU at 69km

Today has seen some very encouraging results achieved on 137kHz with the QRSS30 0.2mW ERP beacon. Firstly copy from G4AYT in Whitstable Kent at 118km (in the null of my TX loop antenna) and excellent reception by M0BMU using his ferrite rod antenna at 69km - see screen shot. Several more stations are looking out for the signal which will remain on overnight.

LF kit improvements

24W 137kHz QRSS30 beacon TX (ERP 0.2mW)

137.5kHz 24W transverter (for WSPR) 0.2mW ERP

Today I completed the first stage of my ERP improvement programme on 137kHz completing the rebuild of the QRSS30 beacon and the transverter used for WSPR on the band. Photos attached show both of these. My ERP has now increased to around 0.2mW with both rigs. The output from the PA stage in both rigs is around 23-24W. More power will be easily possible by raising the PA supply voltage later.

Higher power on 137kHz

Today I did some work on increasing the output power of my 137kHz transverter used for WSPR, with mixed results. When driving the IRF540 PA with a decent square wave I can get around 24W out (was around 5-8W) from a 13.8V supply with decent efficiency. When using the drive from the mixed-down transverted signal I am having less success. The issue is driving the MOSFET gate from a really low impedance and ensuring it is as square as possible with the right mark-space ratio. Most 137kHz transmitters do this by driving at 274kHz with ICs designed to drive a pair of MOSFETs whereas I want to go in at 137kHz using discrete parts. This is a solvable challenge and I'll have this up and running in a few weeks. 25-30W would give me 6-8dB ERP improvement and more should be possible from a higher PA supply voltage. This would take my ERP up to around 0.5mW  ....heck that's high power for me!

More QRPp testing on 137.7682kHz

My QRSS30 137.7682kHz beacon is on until 0800z Tuesday morning sending XBM in QRSS30 with the full call at 10wpm periodically for full ID.  Reports (including no copy) would be much appreciated.  I'm hoping to improve on the recent best DX a few days ago - a report from G3WCB at 101km who got the signal pretty well at times despite the 140uW ERP.

Improved QRSS30 beacon on 137.769kHz

This afternoon I did some experimenting with some new 42mm 3C90 toroids and my 1kV WIMA polypropylene capacitors. I've managed to increase the RF output of my QRSS3/30 beacon by around 5-6dB so I've put this on beaconing over the evening and overnight period in the hope that someone will see the signal and send me some screen shots. ERP with this new beacon is around 150uW.  Previous tests with the beacon ERP at around 50uW showed that the signal could be detected in The Netherlands. Each dB gained should improve my chances of being seen ever further. The screenshot shows my signal as received 101km away by G3WCB - quite a good signal at the peak of fades.

What is my ERP currently on 137kHz WSPR?

Based on some measurements and calculations using the improved ATU (see photo) today, this is my calculation:

Measured antenna current in the TX loop = 2.13A
Area enclosed by the loop = 80 sq m
P = (I^2 x A^2)/(205 x 10^6) at 137kHz   (see p101 of "LF Today" book available from RSGB)
where P = ERP,  I = current in amps and A = loop area in sq m

P = (2.13 x 2.13 x 80 x 80)/(205 x 10^6) = 141uW

This is not a lot (but more than ever before) and this is before I start on the "ERP improvement" programme.

137/500kHz ERP improvements

Today I made a start on improving my ERP on 137kHz and 500kHz.

Firstly, I've bought a pile of Wima 1000V polypropylene capacitors from Farnell. With these I have now made a new ATU to match my TX loop (breadboard only ATU so far) which will allow me to run some 6-10dB more power into the loop without the ATU components overheating and failing. My old switched capacitor array was an old school capacitance decade box and this was marginal with tuning changing as caps heated up!

Secondly, I am going to change the MOSFET PA design to increase the 137kHz power from around 7W RF to about 30W. This should be worth around 6-8dB. I plan to do something similar on 500kHz too.

Finally, I've got some more wire to allow me to double the thickness of my TX loop. This will be done later and will be worth 2-3dB as long as I alter the 3C90 ATU (step down) matching transformer.

In all, these changes will give me up to 10-13dB more ERP and, with some luck will allow me to get more reports on QRSS and WSPR. It will take some weeks to implement these, but by late October my range should improve on both bands.

137.5kHz WSPR

This evening F8BOJ was testing on 137.5kHz WSPR so I had a go at copying him, although he is around 700km south of me in the null of my fixed loop. G4WGT was copied but so far no sign of F8BOJ. I've also been TXing with 100uW ERP but no reports this evening although GW3UCJ thinks he may be seeing signs of my bursts but too weak to decode.

137.5kHz WSPR

Several stations on 137.5kHz WSPR tonight. G4WGT is testing a new rig, M0PPP is getting decent reports and me getting reports from just G3XIZ so far. Will leave WSPR running for a few hours and see what happens.

QRSS30 QRPp beaconing on 137kHz band

PA3FNY's reception of the QRSS30 beacon XBM (me)

My QRPp QRSS30 beacon sending "XBM" is active on 137.7679kHz (approx) from now until around 2300z. As before, any reports and screen shots would be much appreciated. ERP is still around 50uW. The last time it was on it was copied in Holland and again today, this time by Victor PA3FNY (see screen shot above) at a distance of 328km.

325km with 50uW ERP on 137kHz

Well, Henny PA3CPM has just sent me a new screen grab showing my signal at 1041z today on 137.676kHz in QRSS30. This time you can make out some of the CW (visible just below the continuous Loran line) and the periodicity between callsign blocks is visible confirming this is indeed my signal.  Distance is 325km (202 miles) to JO22mb square. This is the first time my 137kHz signal has got into mainland Europe. I am amazed that a really simple beacon on a tiny board into a simple wire loop in the garden can span such distances. Even more amazed that Henny could find it!