Thursday, 15 October 2015

Getting more bands out of a 40m OCFD (Windom)

The original design in my book for a flat-top
optimised 40m OCFD (click for larger image).
In my book “Introduction to Antenna Modelling” I talk about how you can get more bands out of a 40m off centre fed dipole (OCFD – sometimes incorrectly called a Windom) by adjusting the feed point position.

The conventional feed point is at the 33%/66% point, but if you do that with a 40m OCFD (roughly 20m long) you find that you get 40m, 20m and 10m, but 15m offers a high SWR.

But the model shows that by adjusting the feed point to 41% you can get a low SWR on 15m too.

But this is just theory – does it work in practice?

I thought I would try it in real life and see how it performs. The only trouble is I wanted to erect the antenna as an inverted V, so would that affect the lengths and feed point in the MMANA-GAL model?

Anyway, after an afternoon of fiddling with MMANA-GAL I ended up with an answer – an antenna with a total length of 20.75m (8.5m and 12.25m for a 41%/59% feed point), fed with a homemade twin core 4:1 balun and mounted at about 8.5 metres, fed with about 25m metres of Mini8 coax.

The model I ended up with suggested low-ish SWRs on 40 (actually, just over 3.1:1), 20, 15 and 10m, with not so good matches on 30m and 17m, as expected.

Now to turn it into a real antenna. If you use PVC-coated wire, you'll end up with an antenna that has to be shorter than calculated due to its velocity factor being less than one. That is, the speed of light is slower in a denser medium.

Every antenna I have ever designed with MMANA-GAL has ended up shorter than calculated.

So applying an estimated velocity factor of 95% I ended up with two legs of 8.075m and 11.637m. I cut the wires a little longer than this and twisted the ends over to allow for a little adjustment – I fully expected to have to shorten the wires once it was up.

Having hauled it all into position at the top of a fibreglass fishing pole I checked the SWR at the end of the coax and was pleasantly surprised. It was pretty much spot on, with an SWR less than 3:1 on 40, 20, 15m and 10, and my internal ATU could also tune 30m and 17m, although obviously its performance is down a little on those bands.

Lengthening it a little might put the lowest SWR points a little more mid-band, but we are only talking about fractions of one SWR point.

The model shows it is a cloud warmer on 40m (good for NVIS contacts), and the multi-lobe pattern on the higher bands is complex and not always ideal for DX, but this is a compromise antenna.

Early tests have been promising, with the antenna performance matching dedicated dipoles on the bands on which it is resonant. So I think it is a success.

Actual SWR figures (at end of 25m of coax)

7.100MHz 1.9:1
10.120MHz 3.3:1
14.175MHz 2.5:1
18.100MHz 3.0:1
21.225MHz 1.2:1
24.940MHz 2.2:1
28.500MHz 1.4:1

Update: Well, I've been using it for a day and have worked Cuba (CO), Moldova (ER), Qatar (A71), Saudi Arabia (HZ) and Ceuta & Melilla (EA9) on 15m where it seems to go great guns. 40m has given strong contacts into Europe. I like it.

Monday, 12 October 2015

RSGB Convention, October 2015

I gave two talks at this year's RSGB convention. The first was on HF Propagation and I'd like to remind readers that I have a video presentation with audio that is available for RSGB-affiliated clubs.

If you would like your club to have a copy get your programme secretary to drop me an email at steve [at] I can also do a Skype-based Q&A after the presentation in most cases.

Otherwise, there is always the free "Understanding LF and HF Propagation" e-book to download.

The other presentation was on "An introduction to antenna modelling with MMANA-GAL". This is available for download. Don't forget that my three books are also available from RSGB ;-) or you can use the links on the right.

Sunday, 27 September 2015

Amateur radio from a submarine - W7SUB

USS Blueback, W7SUB, Portland, Oregon
I'm lucky enough to do some business travelling every autumn (fall) and am currently in Portland, Oregon, USA.

I was even more lucky to do some operating from the submarine USS Blueback at the Oregon Museum of Science and Industry (OMSI) this morning, thanks to Joe KF7UOQ.

We operated SSB and CW on 17m, 20m and 40m and I managed to work a few stations, including as far afield as Wisconsin (17m CW) and a SOTA station Todd W7TAO using 5-10W CW on 40m - conditions were not brilliant.

We organised a sked with Dean KG7MZ in Washington State via 2m (after a fire alarm went off!) and worked him on 40m CW too. I thought the fire alarm was part of the sub's sound effects and stayed put - duh!

I used QRP to stop interference to Joe on 20m. Joe worked Texas and a host of others on 14MHz SSB.

My CW was a little jerky at first until I got used to the sub's straight key - W5NNS must have wondered what he was working!

Antennas are a vertical for 20m and a dipole for 6-50MHz (from memory). W7SUB is obviously at river level and surrounded by buildings so not the best location for HF.

USS Blueback (SS-581) is a Barbel-class submarine formerly in the United States Navy. I was amazed to see it actually has three decks and was nowhere near as claustrophobic as I thought it would be. Having said that, not sure I'd want to be underwater on active service on it!

USS Blueback appeared in the 1990 movie "The Hunt for Red October".

On previous trips to the "left coast" I've been lucky enough to operate on the Queen Mary, USS Midway and at K6KPH at Point Reyes/Bolinas north of California.

The USS Blueback Radio Room
Very nice to be able to include amateur radio on a business trip. Thanks again to the team at W7SUB (what a callsign!) including Joseph Noecker K7FGN who helped organise it.

You can click on the images to see a bigger version.


Friday, 11 September 2015

SSTV images found from Space Shuttle Challenger, 1985

I had a really big surprise this week. My local club - Norfolk Amateur Radio Club - were having a retro technology evening. This involves people bringing in equipment like Sinclair Spectrums, BBC Bs, old calculators etc - even an Oric Atmos showed up this year.

The highlight for me was a 1982 Betamax video recoder and Sony UHF telly, complete with videos of the news and BBC's "Multi-Coloured Swap Shop" with Noel Edmonds, thanks to Robert G4TUK.

Anyway, I took my Sony Walkman Professional, a couple of old Macintosh computers and a 1978 Russian Vega Selena shortwave radio - still working. While I was looking for the Walkman I found an old box of cassettes with one marked "Space Shuttle August 1985".

This turned out to have 2m SSTV signals on it from Tony England W0ORE's STS-51-F mission.

The recording was a bit noisy as I think I used a Slim Jim, and in those days I used a Sinclair Spectrum to decode the images. But what could I do with it in 2015 - 30 years later.

I researched this and he used a Robot 1200C to encode the SSTV images. I was able to use RX-SSTV to decode some of the black and white 8-second images and the Robot 36 ones. They won't win any awards but you can definitely see what they are - one is Tony himself and the others are of the Shuttle's cargo bay, with the telescope it was carrying, and the earth.

The most chilling thing is the Morse ident in between the images which reads "W0ORE/CHALLENGER".

Less than a year later, in January 1986, Challenger was no more after exploding shortly after launch.

Tuesday, 18 August 2015

Current solar conditions added to propagation chart pages

I've just made some changes to my monthly propagation charts.

I've added some more data sources so that you can see the current solar and geomagnetic conditions as well as the predicted HF coverage maps from the UK.

This means that you can get an at a glance look at likely HF propagation conditions – all on one page. The data include the current solar flux index, the Kp index and also solar wind characteristics.

We have been suffering from a number of recurring coronal holes recently (areas on the sun where the magnetic field is weaker, letting plasma out to form the high-speed solar wind).

If the magnetic polarity of this solar wind is “south” (we say that its Bz is pointing south) it is more likely to couple with the earth's magnetic field and the hot plasma can flood in.

The net result is the earth's magnetic field is distorted and we see this reflected in the Kp index, which normally rises - a geomagnetic storm is in progress.

The initial effects can be a short improvement in HF conditions, but these can be short lived. We then see an overall drop in maximum useable frequencies, the bands can get noisier and signals drop away, often with lots of heavy fading (QSB). It can take 24-48 hours for the ionosphere to recover, if it is not hit again.

The net effect is a lowering of overall critical and maximum useable frequencies as the plasma hits and excessive absorption, especially on polar paths.

We've seen a lot of this recently with poor conditions on HF.

So for good HF conditions look for settled geomagnetic conditions with a low K index for a day or so, a low solar wind speed (less than 450 km/s) with a Bz that is neutral or pointing north and a high solar flux index.

If you have a high Kp index, a high solar wind speed and a Bz pointing south don't be surprised if the bands aren't so good! This would not normally be seen in the monthly charts, which are an "average" for the month.

The charts are also now being carried on the RSGB web site too at which is why I have kept them long and thin to fit into the frame on

Tuesday, 14 July 2015

UK astronaut Tim Peake announces shortlist for amateur radio contact with ISS

UK ESA astronaut Tim Peake.
Image: Steve Nichols
I wanted to be one of the first to break this news from Liverpool, especially as Norwich School is local to me. Congratulations to the school for making the shortlist for a two-way contact with Tim Peake when he is on his six-month mission to the International Space Station, currently targeted for December 2015 or later.

Here is the full announcement:

The shortlist of UK schools that will have the opportunity to contact British ESA astronaut Tim Peake via amateur radio during his mission to the International Space Station (ISS) has been revealed today at the UK Space Conference.

Tim will launch to the ISS in December of this year and will spend six months working and living in space. The Amateur Radio competition is a collaboration between the UK Space Agency, the Radio Society of Great Britain (RSGB) and the European Space Agency (ESA).

Selected schools will host a direct link-up with the ISS during a two-day, space-related STEM workshop which will be the culmination of a large range of learning activities using space as a context for teaching throughout the curriculum.

ARISS UK (Amateur Radio on the International Space Station) will provide and set up all necessary radio equipment such as low earth orbit satellite tracking antennas and radios, to establishing a fully functional, direct radio link with the ISS from the schools’ very own premises. In a ten-minute window when the ISS will be over the UK, an amateur radio contact will be established with Tim, and students will be able to ask him questions about his life and work on board the ISS.

Owing to the nature of scheduling the links, which is dependent on geography, the  exact orbit of the ISS and the crew schedules, the exact dates and times for possible links will not be known until 2 weeks before the link up is scheduled.  The shortlisted schools will all be prepared for such scheduling challenges and, by having a number of schools, we can ensure that all links are used. 

Jeremy Curtis, Head of Education at the UK Space Agency, said: “We’re delighted with the amount of interest in this exciting project and look forward to working with the selected schools as they make a call into space.

“Both Tim’s space mission and amateur radio have the power to inspire young people and encourage them into STEM subjects. By bringing them together we can boost their reach and give young people around the UK the chance to be involved in a space mission and a hands-on project that will teach them new skills.”

The following schools have been shortlisted for a possible ARISS call with Tim whilst he is in orbit on the ISS:
  • Ashfield Primary School, Otley, West Yorkshire
  • The Derby High School, Derby                   
  • The Kings School, Ottery St Mary               
  • Norwich School, Norwich
  • Oasis Academy Brightstowe, Bristol           
  • Powys Secondary Schools Joint, Powys  
  • Royal Masonic School for Girls, Rickmansworth          
  • Sandringham School, St Albans                
  • St Richard’s Catholic College, Bexhill-on-Sea 
  • Wellesley House School, Broadstairs        
John Gould, G3WKL, President of the RSGB, said: “The Radio Society of Great Britain will be delighted to support shortlisted schools by teaching their pupils about amateur radio and helping them through their licence exams where appropriate. Members of our Youth Committee are based across the UK and will be keen to visit the chosen schools in their area and chat to the pupils.”

The ARISS UK Operations team will now work with the shortlisted schools to prepare them for this exceptional opportunity during the mission of the first British ESA Astronaut.

Thursday, 9 July 2015

New "Spitfire" 20m Rockmite ][ QRP CW transceiver is born

Click to view larger image
I was walking around a local shop recently when I spotted a range of mints in tins from a company called Stewarts.

They are very attractive and feature paintings of a Supermarine Spitfire, a steam locomotive and a Triumph motorcycle.

At just £2.99 each the tins are cheap and cheerful, and I knew just what I wanted to do with one!

Hence, the plans for a 20m Rockmite ][ transceiver were born.

I have built a 40m Rockmite v1 before, which is a crystal-controlled ham radio transceiver offering about 0.5W output on two very close frequencies – you select each one by pressing a button. The Rockmite also has a built-in CW keyer with a speed control and is a very simple, but well-respected little radio.

The joy is they will fit in a mint tin, such as the famous Altoids. But the Stewart tins are much more attractive.

So, I ordered a 20m Rockmite ][ from Kanga kits ( in the UK (they are actually made by Rex Harper of in the US) and it arrived the next day.

The box is easy to work on as the material is very thin. I carefully took the lid off while I was working on it and wrapped the rest in masking tape to protect it and allow me to drill the holes. Quick tip – run a pencil around the lid when it is closed so that you can see on the tape where the lid will be when it is assembled. You can then measure and drill the holes for the antenna jack, power, key, headphones and frequency switch knowing that they won't bind on the lid.

Click to view larger image
I used a sharp, new 2.5mm drill for all the holes after marking them with a centre punch first. I then used a Dremel tool and needle files to open up the holes to the size needed. With hindsight I would have moved the PCB a little closer to the antenna side of the box to give more room, but I have now managed to put it all together.

The PCB build was simple, but I did end up with some spare components, which worried me! I didn't miss anything, so I thought they must be spares.

Dennis G6YBC of Kanga subsequently emailed and said: "Apologies if the spare parts (bits left over) confused you. I enclose all parts to complete the Rockmite ][ in either of ways that Rex explains i.e. with volume and speed control etc.

"This includes extra resistors for the various mods that he and Chuck have published. This includes the 3866 and 2N2222 transistors. All the above parts are part of the BOM as per the build instructions for Ver 3, but you also have to read the other sheets Rex publishes on his website."

I also had to modify the layout by cutting a track and adding a resistor, but that was easy.

I had the choice of a 2N2222A (TO-18) transistor or a 2N3866 (TO-5). I chose the latter as I presumed it would give a higher output – wrong, as we'll see later!

I also swapped the BNC for a phono socket to match the Mountain Topper Radio (MTR) I also have, which I have written about before.

By soldering on all the connectors I was able to test if on the bench first before boxing to make sure everything worked, which it did (that's unusual for me).

Powering up the the Rockmite gives a “73” in CW through the headphones, which is a nice touch. You can switch to a straight key by holding down one of the paddles or the key on power up. You can also change the speed by holding the button down for more than half a second and then using either paddle to speed it up or slow it down – the speed in WPM is enunciated in CW.

The first tests were a little disappointing as summer afternoons are not the best for 20m. But the evening and next morning showed the little receiver is very lively, picking up stations in the US, Switzerland, Germany, Czech Republic and others with a dipole at about 25ft.

The rig is actually on 14.0604MHZ and 14.0598MHz as far as I can see.

So does it transmit? Yes, but only about 200-250mW with a 12.4V supply. I think the output can be tweaked a little higher, say 500-700mW by reducing the value of the R18 resistor, but I need to check on that.

Needless to say, I haven't managed to work anyone on it yet – it is a little too QRPP for me! I was picked via the reverse beacon network in Germany and Ireland though.

All in all though, it is a pretty little radio with a very nice receiver around the QRP frequency of 14.060MHz. Next step is a 40m FOXX-3 transceiver in the steam train mint tin I think.

Update 10th July 2015
I put a quick CQ out last night and was picked up on the VE2WU skimmer in Quebec. That's pretty impressive! 

Update 30th November 2015
I replaced R18 with a 3 Ohm resistor and replaced the output transistor with a 2N5109 and am now getting 500mW. Happy with that