VLF test by OE3GHB

OE3GHB's VLF loading coil and transmitter

OE3GHB has reported on his 8.97kHz VLF tests this weekend. He was copied by several stations in W.Europe. Gerhard's signal was clearly detected by OK2BVG, DF6NM, DK7FC and by Paul Nicholson in Todmorden. There was a trace seen by G3ZJO. At the moment my VLF system is out of action so I was unable to take a look. The picture shows Gerhard's enormous VLF loading coil. Antenna current was around 600mA and his antenna was a vertical on an 18m fibre glass pole with a 100m horizontal top wire with 600pF capacitance.

Mixed results on 160m

The small loft antenna for 160m has produced some impressive results tonight with spots from all over NW Europe of my QRP signal. Best was GM4SLV in Shetland at 5W and OZ7IT at 2W.

But the real reason for erecting the antenna was to allow QSOs on QRP AM over a 3km path to G6ALB so we could use topband for a talkback link when doing other tests. Now, at 9.30pm we went onto 1.973MHz AM to see how easy it would be. It wasn't easy at all! In fact it was very hard work copying even a few watts of AM. The killer is the noise floor that is around S9 both at Andrew's home and mine. On WSPR, with its much narrower bandwidth this is not such an issue, but on AM it ruins the band for what we'd intended. There is no way that 0.25W of AM would make it, certainly not easily with simple rigs as we'd planned. 10W would be OK, but that defeats the purpose. On 70cm AM (yes AM) we had armchair copy between us with about 1W. On 160m FM it was better copy until we dropped the power down and the signal was drowned by the QRN/QRM.

So, what will we do? There is now no point in continuing with the 160m AM transceivers as this isn't going to work at our noisy QTHs with QRP over 3km, sadly. The most probable choice will be 6m AM using my SixBox rig at my end.

The lower HF bands like 160m are radically different from 30 odd years ago when the noise floor was some 20-30dB lower.

160m loft antenna erected

160m loft antenna

Today I erected a small loft antenna for 160m use. The prime purpose is to allow me to test my QRP AM transceiver (that is under construction) with G6ALB in the next village. The idea came from an old RSGB book called, "Practical Antennas For Novices" by John Hayes G3BDQ. It consists of a long coil wound on a PVC pipe about 1.5m long with a couple of wires along the top of the loft as a top capacity hat. In John's design he used 21mm pipe but I chose 32mm instead and my inductance measured 590uH. Just a little more inductance in the shack was enough to make the antenna resonant when tuned against ground (my copper hot water tank and pipes in the house). Now it is time to find out how well (or not) it works. Incidentally without the additional loading in the shack it was a good match on 40m and 15m too.

UPDATE: My first 2 WSPR reports on the new (indoor) antenna were from OZ7IT at 853kms! The antenna must work, HI. Shortly after PA0A spotted me.

DMR products

My old company, Sepura, entered the Digital Mobile Radio market last year (as an addition to the TETRA products they already design and make) and are, as we speak, designing DMR products for sale in the near future. In recent days they've had a wake-up call from their major competitor Motorola in the form of two new DMR radios that set the benchmark. See http://www.motorola.com/Business/XU-EN/Product+Lines/MOTOTRBO/SL4000+Series/SL4000_SL4010. These look very attractive products and I am glad I no longer have the task of trying to beat them! Instead I can tinker with lightbeams and VLF.

Getting Going on "Bottom Band"

Well, guess what, I am a year younger (amateur radio wise) than I thought. Having looked up some old RSGB Bulletins this evening I see that I actually joined the RSGB in 1963 and not 1962, so I've only been a member for the last 49 years. Suddenly I feel a year younger!

I was looking for an article about lightbeam communications that was in the very first RSGB Bulletin (later Radcom) I ever received and this was in April 1963 and not 1962. It was called "Getting Going on Bottom Band". The article was an April Fool's one but I didn't realise this and got the reference books from the library. But, the fascination with communicating a long way over a beam of light was kindled in that very article and only now am I giving the subject proper attention, some 49 (and not 50) years later. I feel as if I have a whole year extra in which to enjoy the hobby, HI.

SpectrumView for iPod/iPhone

SpectrumView 0-24kHz displays of waterfall and spectrum

Looking around for useful (free) software to help me with VLF and lightbeam tests I came across SpectrumView yesterday. This is a fabulous application for the iPhone, iPod Touch and iPad which provides a 0-24kHz spectrum analyser and a waterfall display.  The app is produced by Oxford Wave Research.

To make best use of this I need to connect an external audio input into my iPod Touch 4g.  A 4-pin jack plug when inserted, rather than a 3-pin one, allows an external audio input (or a mic) to be connected so I can feed my VLF receivers, used for both 8.7-9.1kHz earth-mode comms and for lightbeam baseband/sub-carrier detection, directly into the iPod.  Resolution will be down to around 2Hz at best, so this will be of somewhat limited use, but certainly easier than a laptop for quick field tests or quantitative measurements.

The mic input (on a 4-pin jack) is on pin 4 (nearest the plug cover) and the ground is the next one down the connector (pin 3). The tip (pin 1) and pin next to the tip (pin 2) are the two audio outputs for a headset.


UPDATE 16.3.12: to tell the iPod Touch 4g that an external mic is connected you seem to need to have a 4k7 resistor to ground across the terminals. See later post for details.

RSGB member since 1962

Next month is an historic occasion for me as it will be exactly 50 years since I joined the RSGB. In those days applications had to be supported by 2 existing RSGB members and living in the wilds of Devon I had to look hard to find two!  One of the two who supported my application was Roger Thorn G3CHN, now SK, who later overhauled my ex-military DST100 radio and thereby opened up the world of shortwave listening to me. Roger worked at the local Decca Navigator station and I remember being thrilled when he showed me around the station.

Many people are critical of the national society, but behind the scenes the RSGB does a LOT for the UK amateur population. A society is only as good as its members and the contributions we make towards its success. A lot of hard work is being done currently to get the society back onto a firm footing after a turbulent period. It deserves our support.

472-479kHz available Jan 1st 2013 in UK

G3XBM's 500kHz transverter. Ideal for 472-479kHz.

OFCOM are now going to renew NoVs for the 501-504kHz band for existing permit holders and this will be possible via an on-line webform on the RSGB website.

"Following the agreements concluded at World Radiocommunication Conference 2012, Ofcom has started discussions with the RSGB and others about the timescales for implementation of the new secondary allocation to the amateur service between 472 and 479 kHz which, it was agreed at the Conference, would be effective from 1 January 2013.
Ofcom has therefore determined that, to allow these discussions to be completed, the current NoVs to allow experimentation between 501 and 504 kHz should have their validity extended to 31 December 2012. This notice confirms this fact."

See http://www.rsgb.org/operating/novapp/500.php. So plenty of time to get an antenna up and a TX or transverter ready. You might like to start by looking at http://sites.google.com/site/g3xbmqrp/Home/500k where I describe my 5W QRP transverter which will work on 472-479kHz. I've plans to make a Mk2 version of this during the summer with higher power and other improvements. It would be nice to make a PCB this time so that others could build it.

A very tiny Chirpy from Eldon WA0UWH

WA0UWH's version of Chirpy

Eldon WA0UWH has built an SMA version of my 10m Chirpy transceiver on a VERY tiny PCB. He is currently debugging it. See Eldon's blog at http://wa0uwh.blogspot.com/2012/02/my-xbm-10-2-chirpy.html for more details.

Optical RX schematic

Schematic of the G3XBM optical RX

Several people have asked for the schematic of the optical receiver used in the recent "over the horizon" red LED 481THz tests. Here it is. The BPW34 photodiode does not have any bias and is connected directly (in the air) to the gate of the FET. I have tried reverse biasing but it made little noticable difference to sensitivity. The cascode "head" is from KA7OEI and this is followed by an op-amp buffer and gain stage which feeds the PC. There is an additional stage of gain to drive a small crystal earpiece. I am sure the circuit can be further simplified and optimised but it has allowed me to copy a baseband scatter signal at 8.63km by ear and is as sensitive as any receiver I have yet tried.