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.

Successful 481THz NLOS test (8.63km)

8.63km non line-of-sight reception of 481THz optical beacon

Bernie G4HJW set his high power optical beacon running a 1.082kHz tone this evening pointing in the Burwell direction on a non line-of-sight path from his home QTH at Little Wilberham. He then came over to Burwell to compare his RX and my own from a test site on the edge of the village clear of the village lights. Distance was 8.63km.

Both on Bernie's RX kit and my own we got solid copy by ear of the beacon at a distance of around 8.63km NLOS using 100mm optics. Once found, the signal was solid.  I then set up the optics on the tripod and fed the signal into Spectran where the signal was 30dB over noise in a 0.17Hz bandwidth. Listen here for a recording (best played back using Spectran with 0.17Hz bandwidth settings)

G4HJW's beacon TX

There was hardly a cloud in the sky, so this was largely by scatter from the mist/dust in the air. Bernie says signals have been stronger on other nights. Best reception was with the RX aimed just above the horizon, maybe 5-10 degrees at most above.

This was very gratifying as it is the furtherest NLOS signal I have copied and it verified my RX kit is now working at good sensitivity.

When I got back home I did try to copy the beacon by ear out of the bedroom window but the tree cover and lights made copy not possible. However I will try again later in the week with Spectran running and narrow bandwidth.

Failed cloudbounce 481THz optical test (7.6km)

Possible weak 481THz reception at 7.6km NLOS

This evening I tried a 7.6km non line-of-sight (NLOS) path between Burwell and Newmarket Heath with the 0.7W input red LED beacon running QRSS3 on an 820Hz subcarrier (at home) and modified KA7OEI receiver at the RX end, both with 100mm optics. My PC was running Spectran.

The best I can say is reception was very doubtful:  the screen capture above shows a signal at the correct subcarrier frequency in the right direction, but too weak to identify CW characters. The recent 3.6km NLOS test produced good 10dB S/N signals in 0.67Hz bandwidth - much easier.

With a largely clear sky, few clouds and a slight haze, not ideal conditions for cloudbounce testing. This test will be repeated when cloud cover is better and visibility is clearer. QRSS30 would give me another 10dB so this may be worth a go if QRSS3 doesn't work on a second try. There are several variables that make weak signal forward scatter detection difficult: (a) what vertical angle to use at both ends, especially with few clouds (b) amount and height of cloud cover, (c) horizontal direction of aim.  With QRSS3/30 modes it is a case of small aiming increments and wait to see if a signal appears on the screen. It is quite hard work.

The result is disappointing, but there will be plenty of other occasions to repeat this test and do others. All good  fun.

Some 160m AM QRP rig links

Looking around for some ideas for a local natter box, I came across these links for simple AM transmitters and receivers so far:

http://www.gameangler.eu/delboy/m0dad/construcion/poppet_top_band_am_transmitter.htm
http://www.angelfire.com/de/RadioAnarchy/
http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/1.65MHz_AM_QRP_TX.gif
http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/AM_RX_Superheterodine_NE612.gif

If you are aware of other, simple, ideas for 160m QRP AM rigs please let me know.  I've still to go through my old SPRATs to see what is there and there have also been some ideas in Practical Wireless.

A 160m AM local natter rig

Further to my post a few days ago about possible bands to use for a very local "natter link" over a 3km path between G6ALB and here, my thoughts are turning to 160m, partly for nostalgic reasons. This is where I first started my ham radio operating with a small 400mW AM transmitter from Practical Wireless (Dec 67 I think).

Although 160m is VERY noisy now, especially at night, I figure that 100mW AM will be enough to kill the QRM/QRN over such a short distance. So, I am gathering ideas for something incredibly simple to build.

For the TX the most sensible rig would be a crystal oscillator (or ceramic oscillator) followed by a buffer and series modulated PA using the modulation circuit from the 2m AM Fredbox. On receive, either a regen receiver (operating just out of oscillation) or even an MK484 (like the ZN414) single chip receiver are worth a try.

This would be no DX rig, just a very basic transceiver to get me into the next village and not much further. I'll keep you posted. Incidentally I am unable to do much building right now as my house is in the middle of some redecoration. I can't find anything!