Showing posts with label 137.5khz. Show all posts
Showing posts with label 137.5khz. Show all posts

5 Feb 2013

137kHz 30-40W TX transverter schematic

As promised, here is the schematic of the current 30-40W TX transverter for 137kHz. I have used a 10MHz crystal but a lower local oscillator frequency would be slightly better from a stability viewpoint when using narrowband modes like QRSS30 or WSP15. One choice is to use a low cost 1.843kHz crystal mixing with the driver transmitter's output at 1.979kHz. When a stable external LO source is possible e.g. a GPS disciplined oscillator, remove C7 and feed the LO into the double balanced mixer. Although an SBL-1 mixer was used an alternative, available from eBay is the ADE-1. As balance is not super critical, a home made double balanced mixer would also be possible.

For PA heatsink I just used a couple of small TO220 heatsinks in the prototype. With optimal PA matching into 50 ohms there is little heat from the PA device. A larger heatsink could prevent overheating when the antenna is mismatched. My output network was optimised for my slightly higher than 50 ohms output load presented by my earth electrode antenna.

ERP is around 30uW with my earth-electrode antenna.

Some may prefer to use capacitive coupling into the FET gate. If TR3 failed open circuit, the gate voltage would rise leading to the IRF640 failing. Hasn't been a problem so far though.

4 Feb 2013

Website working again

The main www.g3xbm.co.uk website is now working properly. I contacted my domain providers last night who confirmed they'd updated some systems that screwed up web forwarding. If you spot any more problems, typos or link errors please let me know.

Currently I am drawing out the schematic of my 30W 137kHz TX transverter. This should be on the website tomorrow. Still deciding whether to box it, and add the RX preamp section, or stop at this stage.

3 Feb 2013

First WSPR15 transmission decoded successfully

After a LOT of effort, G8HUH managed to decode my WSPR15 transmission on 137.607kHz at 1815 this evening. Even with a very low (1.7MHz) LO, the drift was still 4Hz in the 15 minute TX period, mainly due to the FT817 I think. At only -32dB S/N decode this was actually not as good a result as with WSPR2 over the 250km path. Short of force cooling the FT817 or using a rig with a TCXO reference, there is nothing more I can do.

My WSPR15 transmissions were also copied and decoded by G4FFC and G4FEV.

CONCLUSION: WSPR15 is a much more tricky mode to use successfully unless the transmission is very stable. This is the screenshot Tom G8HUH sent me.
30uW ERP 137.607kHz WSPR15 signal received by G8HUH 

137.5kHz WSPR15 tests

Using a very stable external signal generator as my 10.0000MHz LO 5dBm injection, I'm now in a position to try WSPR15 with a stable signal. After a quick frequency check with WSPR2 I propose to switch to WSPR15 with its greatly improved weak signal capability for this evening and overnight. WSPR15 activity takes place in the 25Hz slot just above the WSPR2 activity.

Please let me have any reports.

The first WSPR15 transmission will occur at 1600gmt.

Update: Tom G8HUH reported an upward drift of 5Hz during the 15 minute TX burst, so I've moved the LO to 1.7000MHz and the FT817 to 1.836kHz. Drift (all due to the FT817) is now 1Hz. Awaiting decodes. 

137.5kHz ...it's getting repetitive

Well, I've had the 137.5kHz WSPR2 beacon running for about 18 hours now, almost continuously on a 25% TX cycle. I've tried to drum up reporters by posting on the WSPRnet chat page, RSGB LF Yahoo Group and the LF Blacksheep reflector, plus a few direct emails to stations likely to copy my signal.
Consistent 137.5kHz WSPR spots from G8HUH at 250km, but no others!
All I've managed are the very consistent reports from good old G8HUH at 250km. The signal is obviously spanning this distance without any issues so why no others reporting?

Later I am going to replace the 10MHz LO in my transverter with the stable source from my synthesised signal generator and have a go tonight at WSPR15 (15 minutes TX periods) and see if that gets me a few more reports. It should be several dB more sensitive. If that fails to get anywhere, then I am calling it a day on 137kHz. In terms of returns for effort, this is not good value.

30 Jan 2013

Lack of WSPR2 activity on 137kHz

Having run WSPR2 now for a couple of days on 137.5kHz, I have still only received reports from 2 different stations, although I have had LOTS of their reports, suggesting my signal is consistently reaching 250km. My signal is clearly visible to at least this distance, but without people looking I cannot be sure how much further can be expected with the earth-electrode antenna and 30uW ERP.

Although I will box up the new 137kHz transverter and tick the "finished" box I am disappointed that there is not more activity. Most LF/MF people are exploring the new 472kHz band, so interest in Europe for 137kHz is not high apart from DK7FC who is putting out a massive signal that is being copied in both North and South America.

So, what next?  My mind is beginning to think of my new QTH in the village, which we expect to move to in the summer.  What antennas can I put up without upsetting the neighbours? What will the noise floor be like? How much better will VHF/UHF coverage be from the top of our little "hill"?

29 Jan 2013

My new WSPR DX record on 137.5kHz

Having run the 137.5kHz transverter with WSPR-15 (15 minute on periods) overnight - around 30uW from the earth-electrode antenna - I wasn't too hopeful about my chances with WSPR-2 this morning. WSPR-15 failed because of the thermal drift in the 15 minute on period I think.

Anyway I put the TX on WSPR-2 on 137.5khz this morning and let it run. After a couple of hours with no success I was VERY pleased to see I had been spotted several times by G8HUH at 250km. My previous record a year or so ago was just 148km (G3YXM) so this is considerably further and entirely in line with my predictions based on the QRSS3 beacon results.

28 Jan 2013

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).

4 Dec 2011

LF tests with the earth electrode "antenna"

Following the great success on 160m with the 20m spaced earth electrodes yesterday, I fired up the WSPR transverter into the same "antenna" today on 137.5kHz. Although I managed to decode G8IMR at 188km several times, no reports of my signal were received. This ties up with tests last year which suggested the earth electrode system was around 8dB down on my 80sq m vertical wire loop. With results on 137.5kHz hard to get with 500uW ERP it was perhaps not surprising that no-one managed to decode me on the earth electrodes.

I now have a choice on 137kHz: either re-erect the wire loop, but double the wire thickness when I do so, or change to a Marconi vertical with a large loading coil. The thicker loop will increase ERP by up to 6dB but I suspect more is to be had with the vertical. It would be daft of me not to give the Marconi a try both on 137.5kHz and on 500kHz. Even with my limited space I can still manage around 8m of vertical with around 20m of top section.

17 Nov 2011

137kHz transverter picture

137kHz transverter with FT817 and SignaLink USB in the background
Thought people might like to see the current version of the 137kHz transverter used for WSPR transceive. The heatsinks are total overkill as they barely get warm when producing over 20W RF into the loop antenna. I am regularly getting reports from a couple of stations 250km away. M0BMU (69km) and G0WCB (101km) are reporting the WSPR signals almost all the time. It works but needs boxing, when I get time.

16 Nov 2011

137.5kHz transverter desense - fixed

This afternoon I fixed the problem with RX desense by adding a relay to switch the RF input and to turn off the PA supply when not on TX. It has fixed the RX sensitivity issue and added about 10-15dB to RX sensitivity using the transverter. PA0A was about 30dB S/N on QRSS and DK7FC about 20dB S/N on QRSS. M0PPP is now a better signal with me too on WSPR. The TX is still running at 500uW ERP and GW0EZY is copying me in mid-Wales OK (251km) this evening on 137.5kHz WSPR.