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.

The 481THz Band

My tests on 137kHz WSPR are now all but completed and I do not intend to take this work any further now I've "got the measure" of what is possible. I shall put the transverter in a box after a tidy up and use it periodically over the winter.

Now I am about to start something new: 481THz band communications! This is 623nm red light for the uninitiated. This evening I've made a start by ordering some suitable components to allow benchwork to start next week. My initial tests will be low powered beaconing with a portable receiver walking down the road. Assuming this is promising, I'll then refine the kit and organise optics to give some "antenna gain" to allow much greater range.

There are a lot of resources on the internet about light beam communication, especially using high power LEDs and there was an excellent series of inspiring articles about this in the March-May RadCom this year.

10m Chirpy Measurements

Last week G6ALB made a copy of my Chirpy 14 component transceiver for 28MHz CW. Andrew had access to some better calibrated test equipment and carried out some RF power and sensitivity measurements both on his version and on my second original unit - the first was just a rat's nest on the bench.

Both on my version and G6ALB's version the measured RF power out was in excess of 200mW, which is around 2-3dB more than I had crudely measured. The RX sensitivity on both was such that below -100dBm (around 2uV) was audible in our earpieces in a quiet room. Backwave carrier on TX was rather too high at around -10dBm. Second harmonic was also only around -6dB, so a low pass filter is really a necessity apart from casual short tests.

It does seem that the simple design is reproducible and its performance not at all bad for something this simple. The only major shortcoming is the chirp.

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

Simple VLF Receiver

SM6LKM's excellent PC based VLF receiver

In testing my Chirpy 10m transceiver, I used the simple PC based VLF receiver by SM6LKM as a bolt-on SDR. This little VLF receiver works very well as a stand-alone VLF receiver that tunes the 0-22kHz frequency range. Simply by sliding the cursor to any frequency it produces an audible signal just as you would get by tuning a conventional receiver.  Bandwidths are adjustable down to around 300Hz. For my video showing this in action listening to VLF signals (via my loop and small preamp) see http://www.youtube.com/watch?v=TDxRhQYg7lQ. With the SM6LKM VLF receiver you can clearly hear the Omega beacons around 11-14kHz, SAQ when transmitting on 17.2kHz, and various MSK military signals around 18-22kHz. It has also proved useful when carrying out earth-mode (through the ground) VLF communication tests.

A good night on 500kHz WSPR

Not bad for 2mW ERP on 500kHz and a smallish wire loop antenna!

Very good activity levels again this evening on 500kHz WSPR with 13 unique stations reporting my signal in 6 countries. Best DX report was from OH1LSQ at 1733km, the first time this season.

I really must make an effort to get the 137kHz WSPR activity stirring. Maybe if I can get my ERP up this next week I'll swap over to that band for a few nights and see what turns up. Mind you, the WSPR activity on 137kHz is very low currently with most people using QRSS beaconing.

The "Chirpy" 14 parts QRP 10m transceiver video

This is a short video showing "Chirpy" (XBM10-2), my 14 component 28MHz CW transceiver. More details can be found at my website http://sites.google.com/site/g3xbmqrp/Home/xbm10_2.  Best DX is still IT9QAU/QRP but I am hoping for further soon. This is a simple project and, as with all very simple radio projects, it is a compromise between cost, complexity and performance. Nonetheless it DOES work remarkably well for something so simple. AM breakthrough is almost non-existent too, which was a bonus. Couple the audio into a PC rather than a crystal earpiece and  as a bonus you have an SDR receiver capable of copying from about 28.040-28.080kHz. Why the name Chirpy will be evident when you hear the keying!

Elecraft KX3 or FT817 successor?

For some time now we've been expecting the successor to the FT817 to appear on the market as the current offering is now over 10 years old, with just a minor update in this period. Sunspots are rising fast now and I'd expect Yaesu-Vertex to release this within 6 months or miss a big window of opportunity.  The potential sales volumes are very large indeed as a large part of the amateur community owns an FT817 and would aspire to upgrade.

One wonders how Yaesu-Vertex is viewing the Elecraft KX3 and whether any changes to their new product will be made as a result of the KX3 pre-release data. The KX3 does look impressive but its form factor may not appeal to many. It also looks very "square" and Elecrafty! Although it can be used handheld, I understand you have to plug in a local microphone for example - a bit clunky for true handheld portable use, although you need an external mic with the FT817.

It is widely assumed that an FT817 successor will have a Li-Ion battery pack (2hr charge), built-in wide range auto ATU, good DSP features and possibly 70MHz coverage. The form factor is unlikely to be very much different from the current model, but with more inside.

Does anyone have any inside knowledge of Yaesu's release plans? If they don't get a move on the KX3 will steal the show. The KX3 is due to be released at the end of 2011 and order placement is starting very soon.