More on LF transformers

Following on from the earlier blog entry about using a 3C90 core for a VLF and LF transformer, I got this reply from Jim M0BMU last night on the RSGB LF Yahoo group. I post it here as it contains some useful additional information. See also the mini-Ring Core Calculator from DL5SWB at http://dl5swb.de/ .

"Dear Roger, Andy, LF Group,

>> Four turns minimum for 137kHz 25 Watts. 60 or so for 9kHz
> Yes these values look quite practical ones.

...But now the inductance of the winding and AL value of the core do become important. (BTW, the value of 2000 is the relative permeability of the 3C90 material. The "inductance factor" AL, the "inductance per turn-squared", is a different number which depends on the shape and size of the core as well as the permeability.) AL for this core is given as 2690nH nominally. With a four turn winding, the resulting L is about 43uH, with a reactance of only 37ohms at 137k. In a 50 ohm circuit, this will cetainly mess things up a bit. As a general rule, you would probably like the reactance of the 50ohm winding to be at least 250ohms at the operating frequency. This requires an inductance of more than 290uH, so a winding of 11 turns minimum will be needed for a 50ohm impedance level.

This is a typical result when using a core that is much larger than what is  required by power handling considerations - the number of turns needed to keep the flux down to an acceptable level becomes so small that the inductance becomes the deciding factor. It also obviously makes it tricky to match to low impedances, which is often what you are trying to do in a PA or
loop-matching transformer - you may well find that you end up with windings of less than 1 turn! In these cases the inductance or the required turns ratio becomes the determining factors. In the more normal situation where you are trying to design a transformer with an economically-sized core for a given power level, the inductance is usually large enough not to be an issue, as Andy stated.

At 9kHz however, the 60turn winding is quite reasonable from the inductance point of view, giving 9.7mH and about 550ohm reactance. Also, the core losses would be lower at 9kHz, so you could allow a higher flux density and reduce the number of turns (or increase the power level, which might be better!)

Cheers, Jim Moritz
73 de M0BMU"

Azores Islands

This evening I switched on the FT817 not expecting to work anyone on 10m when I heard CU7AA, Faial Island in the Azores archipelago, calling CQ on 28.520MHz. A quick single call and he came back to me with a 57 report on SSB. Although I've worked the Azores several times, including on 6m QRP, this is the first time I've worked Faial Island I think

On-line LF toroid transformer design tool?

I have some 42mm diameter 3C90 toroids and want to use these in output transformers in 3 applications:

(1) in the output of a 137kHz (up to) 25W transmitter
(2) in the output of an 8.97kHz (up to) 25W transmitter
(3) as an impedance transformer for a TX loop antenna at 8.97, 137 and 500kHz.

I was looking for an on-line calculator to help me work out secondary turns needed, but could not find one.  Andy G4JNT helped with this input:

"The magic equation is Vrms = 4.44.F.N.A.B    all in SI units.   

rearranged  Nmin = V / (4.44 . F . A . B)

Al is irrelevant for transformers.

Use a Bmax of 0.1 Tesla for Ferrites, allowing a decent safety margin.

Your A  (of 25 mm^2)   = 25*10^-6    ,   F = 137000, 

25W in 50 ohms is 35V"

To aid calculations in future I have produced a small spreadsheet to work out the secondary turns from the input data (freq, cross sectional area and RF power out).

As an aside, I use http://www.66pacific.com/calculators/toroid_calc.aspx very often to work out the turns needed for the common HF toroids such as T37-x and T50-x.

A return to earth-mode VLF experiments

This afternoon I installed a more permanent earth-mode ground system to use in forthcoming tests at VLF through to 500kHz. Instead of bringing the 2 earth connections into my upstairs shack, as I had done previously, I have now installed a couple of grounds and wires that come into my "designing" shack downstairs. This means I can now run a lot more tests using the test equipment at my disposal. It also means I do not tie up equipment in my "operating" shack upstairs when doing earth mode beaconing.

The diagram shows the current arrangement of the grounds and wire. At its highest point the wire is 1.5m above ground, running along the back garden fence. It is invisible.

Tomorrow I hope to get the ULF/VLF earth-mode beacon TX on-air initially on 8.97kHz and 1.147kHz in QRSS3 and QRSS30 and carry out my usual reception test at a point 1.6km from home where the signal is usually strong. Subject to satisfactory results with the new TX "antenna" I then intend to do a series of RX tests using new equipment out to around 10km from home.

The OXO QRP transmitter

OXO schematic on the G3PTO website

One of the most simple and popular HF transmitters is the OXO, originally design by GM3OXX. The circuit appeared in the GQRP club's SPRAT magazine about 30 years ago. It is essentially a 2 transistor QRP transmitter (plus another for keying) capable of working as a fundamental crystal controlled or VXO controlled transmitter on an HF band. I used this design as the TX part of my Pipit 800mW transceiver for 15m and later the Tenner transceiver for 10m. On the higher bands there is more chance of a little chirp, but perfectly usable. On the lower HF bands the OXO is capable of over 1W. It is a very easy transmitter to build, is almost guaranteed to work first time, and is great fun to use.

As it is some time since I've built one, I might just knock one up this afternoon and see how I get on, perhaps on 80m or 40m CW.

QRO optical beacon for over-the-horizon tests

12W red LEDs for optical beaconing

This afternoon I ordered 3 off 12W red LEDs, called a Phlatlight PT54, from the USA. These were originally designed for use in projectors and are VERY bright indeed (425 lumens at 8.1A!).  My plan is to use one of these in my new 481THz optical CW/QRSS beacon for over the horizon non line-of-sight (NLOS) tests this winter.  If you are interested, look up item 170884888890 on eBay. The datasheet for the Phlatlight LED is available at http://www.mouser.com/catalog/specsheets/pt54phlatlight.pdf .

My original beacon circuit is shown below and was a good signal over the horizon 3.6km from home when using Spectran to show the received signal, which was not even visible to the naked eye or audible in a headset. I am hoping that a very similar circuit will work for the Phlatlight LED beacon.

The QRO beacon should be much stronger, so I am expecting to be able to achieve much greater NLOS ranges, especially as my receiver is capable of considerably greater sensitivity with more work and using better PIN photodiodes with a more optimal 3dB sensitivity angle. The main issue is a thermal one: although the device is mounted on quite a good heatsink, I need to find out how hot this gets when sending CW or QRSS and derate it accordingly. I am hoping that around 4-5A should be possible which should allow some 20dB more output.

Autumn Projects Update

About 6 weeks ago I listed a number of projects that I wanted to do this autumn. Here is a progress report.

1. Finish the 472kHz transverter and write this up. DONE
2. Simple 28MHz QRP WSPR transceiver to use with a netbook. DONE (but the TX only beacon is boxed)
3. Stand alone 137kHz receiver (for use with PC).
4. Semi-permanent E-field probe antenna and on-line 137kHz/472kHz grabbers
5. More non line-of-sight light beam experiments (get better distances). 12W Phlatlight LEDs and more sensitive SRF213 detectors ordered
6. Semi-permanent 481THz QRSS beacon for NLOS light beam tests.
7. Alternative HF antennas - maybe re-erect my Par Electronics 40/20/10 end-fed.
8. Further earth-mode (ground conduction) experiments - tests at around 30 and 73kHz as well as more tests at VLF aiming to improve best though ground DX (currently 6km with 5W). More permanent earth-mode "antenna" erected.
9. Experiment with horizontal loop antenna on TX at VLF (maximising coupling into ground).
10. Build some better LF/MF test gear such as resistive SWR bridge, 50W dummy load and a noise bridge.

So, the next project is probably going to be (3) a stand-alone 137kHz receiver combined with some E-field probe development. What I am hoping to do is make a 137kHz RX that can be run from home uploading to an on-line grabber and that can also be used in the car with a roof mounted E-field probe antenna. This will allow me to do some fairly accurate field strength measurements with various TX antennas. With a bit of care I should be able to make this work on various bands from 8.97kHz through to 500kHz.  What I am hoping to be able to do is have a car-mounted receiver with PC that can be used for "drive around" tests although this will depend a lot on the LF noise in the car when the engine is running. I may have to limit tests to static ones with the car electrics not running.

Operating 10m QRP SSB

Today I took a break from designing and building innovative new stuff to enjoy 10m SSB whilst it is good. For some days now the band has been opening up to the USA and I've been getting good WSPR reports with WISPY on a regular basis with 200mW.

So, today it was out with the FT817 and 5W pep QRP to see what I could manage. QSOs in the log inside an hour included 5B4ES, TB5ZND, CN2LO/P and KG2KJ. No great DX but solid QSOs on sideband with just the halo antenna. I must admit, the speech processor really does help enormously.

The current G3XBM shack layout

The current G3XBM operating station

This is the current G3XBM operating position. The new additions are the 472/500kHz 10W transverter and the little WISPY 10m WSPR TX beacon, which has been boxed today. These are the two boxes to the left of the FT817. I've also increased WISPY's output power to about 250mW and have been getting loads and loads of WSPR USA spots this afternoon, including some from the west coast (W7).  I do my design and building elsewhere in the house. This is not ideal, but it does make for an uncluttered operating environment.

The vast majority of my equipment is home built and much of it home designed. Not shown in the picture are the Sixbox (6m AM), the Fredbox (2m AM), Chirpy (10m CW) plus any of the VLF kit. The last time I bought a new transceiver was about 10 years ago.

500kHz WSPR over 5 days

The attached table shows the different stations that have reported my 500kHz WSPR in the last 5 days. In summary, 19 different stations in 8 different countries with best DX OH1LSQ (1736km) and last night LB9YE (1489km). My most recent calculation of my ERP is around 80-120mW. Who says 500kHz QRP into a very small antenna with a useless (?) indoor ground system doesn't work then? I can't wait for 472-479kHz to be released in the UK.