ERP on 500kHz (part 2)

Yesterday I did a recalculation of the G3XBM ERP based on the classic formula and re-measurements of my antenna current. My antenna is a 5m long vertical with a spiral top hat of about 7 turns about 1.5m x 1.5m. Not being clear how to model the top hat I assumed effective heights of 5, 4 or 3m. The measured antenna current is 0.12A. My results are as follows:

Assumed effective height 5m     ERP = 1.53mW
Assumed effective height 4m     ERP = 0.98mW
Assumed effective height 3m     ERP = 0.55mW

So, I believe my 1mW ERP previously calculated is not a bad estimate, and it may indeed be lower than this figure.

Rik ON7YD (OR7T) has just sent me this email:

"I simulated the antenna based on your description (with MMANA) and the current through the vertical wire seems to be quite constant. So the tophat works well and the effective height will be more or less the physical height. The simulation gives a radiation resistance of 0.074 Ohm, so with 0.12 A a radiated power just over 1mW EMRP (1.75mW ERP). In the real world it will be some dB's less (as Jim stated recently), probably something like 0,5 to 1mW ERP.  Very impressive that it covers over 1200km."

JT4A mode on 500kHz

The next (operational) project is emerging from the mists..... Several people have started to experiment with JT4A WSJT mode on 500kHz. This allows basic EME/MS style QSO exchanges so should allow me some 2-way contacts on the band with my low ERP. Still to work out how to run the software though: when I tried the other night I could see strong traces but no decodes. Getting this sorted will be half the fun. Then I might try to mode on VHF too. JT4A uses a bandwidth of 17.5Hz with tone spacings of 4.375Hz and can decode down to -23dB S/N. This is somewhat less sensitive than WSPR but would still allow QSOs with quite a large number of the stations who've heard me on WSPR.

QRP Books

International QRP Collection is a new book bringing together articles from magazines around the world on QRP (low power) radio. There are articles on commercial equipment, homebrew ideas, test equipment and operating on bands from 160m to microwaves plus much more. A friend who is not too familiar with QRP thinks it is a good introduction, but I must admit I was disappointed: the collection seems disjointed and, in my view, a better set of articles could have been put together. As an example, the intriguing transceiver on the cover does not appear in the articles inside! Overall, I'd give this one 6 out of 10. The book is available from the RSGB. 

QRP Basics by G.Dobbs G3RJV is a much better book and worth buying.

The Pixie Files

The Pixie and Micro80 are  well known simple transceivers for any HF bands. I've built both in the past and they do work well considering their lack of complexity. The main limitation is the receiver which uses the TX PA as a simple mixer. For those wanting more information on the Pixie and its derivatives check out The Pixie Files which is a collection of articles from SPRAT.

ERP calculation on 500kHz

I've recalculated my ERP based on a better measurement of antenna current. I was using a dust iron core for my current transformer, so swapped this for an FT37 ferrite toroid today and got a lot more current measured now. My antenna current is actually around 0.12A. My antenna is 5m long with a small spiral top so the effective height must be <5m. Even putting in 5m into the classic formula I cannot make my ERP exceed 1mW. I make it more like 600uW. Interestingly this is 20dB more than the original estimate from M0BMU and since then I'd increased the ERP, I believe, by around 20dB which would also make it 600uW. Of course the margins of error must be large.

The Internet in the 19th Century

Yes, the internet was alive in the second half of the 1800s, albeit in a rather more limited form. It was possible to find out information from right across the world in a matter of minutes as long ago as the 1860s by using the very extensive telegraph network that ballooned across the globe within a few decades. There is a fascinating book all about this that I can highly recommend called The Victorian Internet by Tom Standage which is available via Amazon and other booksellers. If you thought it was the coming of wireless that connected the world then think again.

More DX again tonight on 500kHz: 1232kms

Lubos OK2BVG is receiving me well again tonight on 500kHz WSPR. In the last hour two reports received via the database.

500kHz DX with 200uW ERP

Guess what?  Lubos OK2BVG has just spotted G3ZJO's tiny signal on 500kHz. Eddie is running only 200uW ERP. This evening conditions on 500kHz have been truly superb.

Done it at last! 1232 km on 500kHz

This evening, my target of exceeding 1000kms on 500kHz has been exceeded by a great margin. Lubos OK2BVG in JN88ks managed to hear my 1mW ERP signal at a distance of 1232kms. This is by far my best DX and a new country on the band using WSPR. Lubos even moved to his quieter country QTH in order to hear my weak signal. I am SO pleased he has such good ears! This was his message half an hour ago:

Hello Roger, LF!
This moment is just now:
2136  -7  0.2   0.503874  0 G4JNT IO90 23
2142 -29 -0.5   0.503910  1 G3XBM JO02

Congratulation!
Lubos, OK2BVG, JN88KS

A little later the signal was even stronger:
2206 -24 -0.6   0.503903  0 G3XBM JO02 0

K4TWJ - silent key

Dave Ingram, K4TWJ died this morning having lost his battle to overcome a massive heart attack on New Year's Eve. He wrote a regular column in CQ Magazine and was well known in the QRP community. RIP Dave.