WSPRing again on 137.5kHz

Today I built a breadboard version of my transverter for 137kHz and now I have it running WSPR-2 for the evening.

RF power from the IRF640 transverter PA is around 30-40W (depending on the PA voltage) and the ERP should be around 25-30uW using the earth-electrode antenna.  At present, the transverter is only on 20% duty cycle TX and I have still to add the RX preamp and switching.

My recent 137.766kHz QRSS3 tests using a measured just 6.6uW ERP from the earth-electrode antenna have now concluded.  Best reports were G3WCB (101km), G3XDV (61km) and G4FEV (56.7km).

G3XBM just detectable on G3WCB's trace at 101km

Stronger signal with G3XDV at 61km

Similar signal at G4FEV at 56.7km

Judging by the QRSS3 results with 6dB lower ERP than my WSPR signal, I think reports out to >100km should certainly be possible as long as people are looking.  WSPR2 is able to detect at about the same level as QRSS10, so overall I have around 11dB more "system gain" (5dB QRSS3 to WSPR, 6dB ERP improvement).

Oscar 6 and 7 satellites

Way back in the 1970s I took my first steps into amateur space communications by listening to the SSB and CW signals coming down from the new (then!) Oscar 6 and Oscar 7 satellites as they passed over. Equipment was an HB9CV antenna in the loft, a 2m-80m homebrew converter feeding a Codar T28 receiver. It was crude, but it worked fine. The same set-up worked well for monitoring 2m CW and SSB terrestrial activity as well.

Using my "osculator" - a map with the satellite paths and an overlay showing when the satellite would be in range on the various paths I could predict when signals would first appear from the south and disappear in the north.

Best passes were ones over the mid Atlantic when , for around 10-15 minutes, sometimes less, US and Canadian stations would appear at the top of the 2m band. It was very exciting at the time. If I recall correctly HB9HB was a beacon near the top of the band and that could be copied too via the satellites.

Although I had one satellite QSO years later with a 10m-15m transponder on one of the later Oscars, I never did get into it on TX.  In the Southgate News today I read that Oscar 7 is still good for DX QSOs even though it is now 37 years old. Whatever happened to the Phase III satellites in geosynchronous orbit like Oscar 40? Did that fail to work?

You can see where the satellite is now at various websites allowing tracking. For example http://www.n2yo.com/?s=7530 .

137.766kHz ERP - now measured at 6.6uW

So, today I repeated my field measurements to establish the ERP of my 8W 137.766kHz beacon feeding my earth-electrode antenna. This time I did the measurement correctly. Here's how.

1. Travel 2km from the home QTH with E-field probe, FT817 and PC running Spectran.
2. Choose a location in the best direction for the "loop in the ground" so the measured signal strength is close to the maximum possible i.e. not off the sides of the loop.
3. Set up FT817 so the RF gain is adjustable and turned well back and the AGC is inactive.
4. Tune in my LF beacon and adjust the RF gain to the lowest possible detection level.
5. Measure the S/N of my signal with Spectran and note reading.
6. Retune to DCF39 (138.830kHz) and, without adjusting the RF gain at all, measure the S/N with Spectran.
7. Repeat these measurements 5 times.
8. Calculate the difference in dB between my signal and DCF39 (in my case 41.3dB)
9. Using the assumption that DCF39 has a field strength of 1mV/m, work out my own field strength (in my case 9uV/m)
10. Using the formula ERP = (E^2 * d^2)/49 work out the ERP.

Answer? My measured ERP on 137.766kHz is 6.6uW

Caveats:

1. If DCF39 is not approx 1mV/m then the ERP needs to be adjusted up or down proportionally.
2. Measurement error is +/- 2dB.
3. The ERP is the figure measured in close to the best direction. Off the sides of the loop the ERP will be much lower.

Conclusions:

1. Stations able to detect the QRSS3 signal at any distance are doing well as the signal is very weak indeed.
2. Using my proposed WSPR transverter at 32W, for example as a reasonable target output, would give me 6dB more ERP at around 25uW.
3. Based on results with QRSS3, I should be able to be copied using WSPR-2 at up to 100km on ground wave. At night with sky wave, considerably further is possible. 
4. Using WSPR-15 and 32W my ground wave range should be up to around 200km, possibly a little more. 

The QRSS3 beacon is still running over the weekend and more reports would be welcomed

SMA version of the G3XBM 472kHz transverter

In an email,  John G4BAO tells me he has built an SMA version of my 472kHz transverter. At present, he is getting 5W RF out using standard SMA parts and a different FET.  If you wish to enquire about this version please contact John and NOT me!

G4BAO's SMA version of the G3XBM 472kHz transverter

Amateur activity on VLF in the last few days

Several amateurs in Europe have reported strong signals on 8970.022Hz VLF, a frequency used in the past by DJ8WX near Hamburg. It is some months since there has been any amateur "dreamers band" activity, so this is very welcome. This is the signal as seem in the Czech Republic on the OK2BVG grabber. Note the timescale on the grabber picture. It is necessary to watch the signal for very many hours to see the signal appear. Hence, very stable transmissions are needed, and a receiving system that is able to hold to a few uHz for hours and hours on end. Just listening on VLF and you will have ZERO chance of detecting such signals.
Amateur VLF reception is usually a "whole new ballgame" compared with listening for the strong MSF signals sending to submarines which are very strong.

VLF amateur signal, believed to be DJ8WX yesterday seen on Lubos, OK2BVG's grabber

My ERP on 137.766kHz is.......

Thanks to G4FEV, I've just realised that I made a fundamental mistake measuring my field strength, forgetting to turn off the AGC when taking the measurements of my signal relative to DCF39. Whereas my initial measurements indicated the ERP was in the low mW region, in reality my signal is much, much weaker. For example, G4FEV reckons my signal is some 70-80dB weaker than DCF39 where he is, if not more! So, tomorrow I shall have to try the test again and see if I can get a sensible figure with AGC turned OFF.

G3XDV's screenshot of my QRSS3 LF signal this afternoon.

Mike G3XDV (61km) has just sent me a very comprehensive report with 178 screenshots of my QRPp signal over the last 30 hours. Copy was remarkably good considering everything. Again, I am amazed how well this works.

G4FEV reports my QRSS3 LF signal at 56.7km

Path between G4FEV and G3XBM

This afternoon I was out and about with my E-field probe checking signal levels of my little 137.766kHz QRPp QRSS3 beacon at various locations around the Cambridge area. The furthermost point I tried was near Comberton, 21.5km WSW of the home QTH in Burwell. I have taken a lot of measurements, including the level of DCF39 at the same locations so that later I can work out my field strength and thence ERP.

G4FEV's screen shot of my QRPp QRSS3 137.766kHz beacon at 56.7km

When I arrived home I had an email from Dave G4FEV 56.7km due west of me to say he had copied my signal and had sent a good clear screenshot. Signals with Dave looked quite good, suggesting the signal due west may be stronger than signals in the Cambridge direction (SW - WSW). The earth-electrode antenna has directivity but, aside from a zero copy at one location orthogonal to the "loop" I have still to work out exactly which directions have the strongest signals. Based on the above I think it is close to W-E.

Disappointment on 137.766kHz

Despite running my QRP LF beacon for over 2 days continuously I have received no successful reports. A couple of people looked without success, but at least they tried. I am leaving it running QRSS3 today as I want to do some further reception tests this afternoon and measure my ERP by taking actual field strength measurements out to about 15km from home. Let's hope I get a few positive reports as the signal should be detectable out to at least 60km.

Checking my 137.766kHz ERP by measuring field strength

Well, today has been a bit of a disappointment: my QRP QRSS3 beacon has been running for over a day on 137.766kHz and, apart from my own highly successful reception tests out to around 8km (as far as I went in the best direction) not a single station has reported seeing it  ....yet.

Tomorrow, I am going to do one more experiment using the QRSS beacon: knowing the field strength of DCF39 on 138.830kHz, a commercial station in Germany, is around 1mV/m here in the south of the UK in daytime, I want to measure the S/N of this station about 5km from home using the mag-mounted E-field probe. Then, at the very same spot, I will measure the S/N of my beacon with the same bandwidth settings and RX kit.

Based on these two measurements, I should be able to work out the ERP of my beacon to an order of magnitude and probably to within +/-3dB. The field strength of my own signal can be worked out just by comparing the S/N with that of DCF39.  Knowing the field strength and the distance from the home QTH, I can work out the ERP needed to produce this.

Any guesses what ERP I am using? I'll tell you the answer tomorrow!

New 137kHz Transverter thoughts

IRF640 FET

This evening, I've ordered a reasonable sized heatsink to use with the new 137kHz transverter. I am aiming for a power from the transverter of at least 30W and want to be sure that with mismatch the PA is able to survive quite hard abuse. The IRF640 looks a good choice for the PA, maybe a pair, as it is rated at a high voltage (200V) and high current (18A) so I can run the PA from a 30V (or even 50V) supply to get more out. Even with 30W RF, the ERP will still be in very low milliwatts.

As I want to ensure the 10MHz LO signal is very stable so I can use it with WSPR-15 and QRSS30 modes, I may put this in a separate insulated enclosure and pipe the injection signal in via coax.

What I am most concerned about is the lack of activity on the 137kHz band: the last time I gave it a go with WSPR the biggest issue was the very few people actually monitoring WSPR on the band. At the moment, the new 472kHz band is very much the focus of attention with up to 50 WSPR users monitoring and/or TXing on any evening. On 137kHz however the number of WSPR users is often just 3 in the world with no-one on in Europe at all.

Still, I am convinced that with WSPR-15 I should be able to reach near Europe even with my 20m earth-electrode antenna if I can get the power up to around 30W from the transverter. It may even be possible, just, with WSPR-2.This is worth a go, before getting back onto other projects ...like finishing off the Tenbox transceiver!