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!

Lightning across Europe

Lightning map of Europe http://www.blitzortung.org/Webpages/index.php?lang=en

Sometimes it is useful to know where there are lightning strikes taking place across Europe: for example, there are some theories linking sporadic-E propagation to a secondary effect of lightning, although I forgot where I read this. Also, it is useful to know how noisy the VLF and LF bands are likely to be as a result of sferics from such strikes.

Well, http://www.blitzortung.org/Webpages/index.php?lang=en gives a map showing this data.

Homebrew RF Circuit Design Ideas website

HA5KHC's useful website

Wandering around looking for 50W PA designs for 137kHz this afternoon,  I chanced on a website that I don't remember seeing before by HA5KHC. It consists of a HUGE list of very useful links to other websites arranged by design topic (transceivers, receivers,  etc).   It is a massive list of useful links that I could spend days trawling through for ideas and inspiration.  It was quite flattering to see a couple of my designs linked.

Unfortunately there is nowhere on the page with a contact address for the website owner, although I expect HA5KHC's details will be on QRZ.com.

UPDATE:  Adrian YO5PBG said:

"I believe it is a mirror of the original site... of Iiulian - YO3DAC/VA3IUL - http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Homebrew_RF_Circuit_Design_Ideas.htm" 

New frequency for LF QRSS3 beacon : 137.766kHz

Using the strong German commercial station DCF39 on 138.830kHz (it is S9+ with me) as an accurate frequency reference, I have checked the frequency of my QRSS3 QRP beacon and it is running higher up the band than I thought due to the way the crystal is loaded. 

Frequency to look is 137.766kHz   +/- . It should be within 1Hz of this frequency. 

Reports and screenshots (looking now in the right place) would be very much appreciated. I shall leave the beacon running all today at QRSS3 speed.

137kHz E-field probe mag-mount

Tiny E-field probe on the car roof on mag-mount

This is the schematic of my simple mag-mount based E-field probe for 137kHz reception in the car. I use a low cost MPF102 FET and a low cost 2N3904 transistor. Performance seems to be totally adequate, detecting my very low ERP QRSS3 beacon as I drive around the local area up to around 10km (yet to test at greater range) from home. The whole unit is attached at the base of a 15cm whip on a mag-mount stuck on the centre of the car roof. The coax from the EFP connects into an FT817 with the IPO button ON - this makes the sensitivity better on 137kHz, a tip worth knowing.

QRSS3 tests with QRP on 137.685kHz.

This afternoon I started my tests on the 137kHz band again. With my 8W (RF out from the unit - ERP will be uW level) crystal controlled beacon sending QRSS3 feeding the 20m spaced earth electrodes, currently under 8cm of snow, I ventured forth in the car complete with an E-field probe built into the bottom of a mag-mount antenna on the car roof. The E-field probe whip is about 12-15cm long only. It looks like a small UHF quarter wave, rather than an antenna for 2200m! The TX frequency is currently around 137.685kHz and this QRSS3 beacon will remain on overnight. I am hoping that stations out to around 100km will take a look for it.

QRP earth-electrode antenna signal at 8.2km on 137kHz band

Well, results have been spectacular with some excellent signals around 8.2km away in the line of the earth-electrodes. Orthogonally, the signal was not copyable at all at a similar range. This is classic loop directionality again.

/M RX signal at 6km orthogonal to TX antenna.

The receiver consists of the 10-15cm long E-field probe feeding the FT817 with IPO button pressed (improves sensitivity when ON at 137kHz) feeding the PC running Spectran at 0.34Hz resolution.

I do find it unbelievable that such a TX "antenna" buried under the snow with NOTHING in the air at all and with earth rods just 20m apart can put out such a decent signal on the 2200m band.

New ERP calculations on 472kHz with earth-electrode antenna in 8cm snow

With several repeatable ground wave reports on 472kHz WSPR using the earth-electrode antenna, it is time for a recalculation of my ERP based on a formula (for 500kHz) supplied some years ago by M0BMU.  The assumption (from M0BMU, that I am not at all sure is right) is that the noise floor is around 3uV/m in a 2500Hz (WSPR) bandwidth. So, based on the WSPR S/N ratio the field strength when using the earth-electrodes can be calculated.

• G3ZJO at 79km gets me at around -19dB giving my field strength around 355nV/m. 
• G3XDV at 61km he gets me around -12dB S/N giving my field strength around 810nV/m.  
• M0BMU at 69km he gets me around -30dB S/N giving a field strength around 100nV/m

ERP Calculations

• ERP using G3ZJO's results = (E*d)^2 / 49; with E = 355nV, d = 79km   ERP = 16uW
• ERP using G3XDV's results = (E*d)^2 / 49; with E = 810nV, d = 61km  ERP  = 49uW
• ERP using M0BMU's results =  (E*d)^2 / 49; with E =100nV, d = 69km  ERP = 1uW

Now, to me these figures seem far too low, so perhaps the noise level is much higher than 3uV/m. Certainly that is the case here where I have an S4- S8 noise floor due to SMPSUs and other noise sources. I think these numbers can be doubled based on ground wave losses but, even so, the figures appear to be extremely low.  My Marconi antenna is some 8dB (average) better than the earth-electrode antenna and I am sure the ERP is well above 1mW using this antenna.

Conclusions

I am totally puzzled: measuring antenna current on the Marconi I get ERPs that are an order of magnitude higher yet measured results with the earth-electrode antenna average just 8dB down on the Marconi. This suggests ERPs for the earth-electrode antenna in the low mW region, which I find much more believable.

Any thoughts please?

Is this Chinese HF radio a wind up one?

As the person who started the FT818 hare running a year or so back (my thoughts and ideas for an FT817 replacement - thanks to a little Photoshop editing!) , I'm wondering if the attached is real or also a spoof?

It purports to be a new Chinese HF radio retailing for £299. It looks very much like an IC718 with some stickers on. It was brought to my attention by Steve G1KQH who found it at
http://s3.zetaboards.com/copythat/topic/7467118/1/ but the date on this is 2011. Surely if true the radio would be on sale now? Somehow I am of the opinion this is (also) a bit of wishful thinking, but I would be delighted to be proved wrong.

A real Chinese HF radio at this sort of price is not totally out of the question: labour costs are low and there are already some very nice Chinese VHF/UHF handhelds on the market that under-cut Yaesu and Icom by around 100%. It is surely just a matter of time before an FT817 or an IC718 look-alike radio is available from one of the Chinese makers.

UPDATE 1755z: maybe this IS real. See http://www.tradekey.com/product-view/1538868-518398/Hf-Transceiver-ft-808-.html

Back on the earth-electrode antenna on 472kHz

The red dots show where the wire to my far earth rod goes - under the snow!

This afternoon I've installed a coax cable from my operating shack down to the back of the garage where I do my radio building and where I can connect to the earth-electrode antenna running down the back garden. Now I have my 472kHz transverter in the garage too so it can connect directly into the earth-electrode antenna without any matching. Even with 8cm of snow on the ground and nothing of the antenna visible above the snow line (!) this antenna is still radiating a decent signal with reports already this afternoon from G3ZJO (-22dB S/N) and G3XDV (-12dB S/N) at good range. I find it truly amazing this system works at all.

I am signing G3XBM/1 on the WSPR database to distinguish my signal from that transmitted using a Marconi where I use G3XBM.  I hope no-one minds me using this suffix for a short period, but it is very much in the interests of good research. The signal level with G3XDV at 61km is about 8dB weaker on the earth-electrode antenna than on the vertical Marconi.