Wednesday, 30 November 2016

Quarter wave verticals for HF

The radiation pattern for a quarter wave vertical.
I had an email from a radio amateur who was struggling to work DX after putting up a very off centre fed dipole cut for 40m and fed with open wire feeder.

This is probably not the best way to go about it as the off centre feed can cause an imbalance and create RFI problems.

I suggested a better DX antenna might be a quarter wave vertical cut for the band in question and fed against a decent ground plane. But what is a decent ground plane?

Rudy Severns N6LF has done extensive research on this, but his conclusion was that you really need as many radials on the ground as possible. Up to 120 is optimal, but you will notice an improvement as you add more and more with perhaps 16-32 being the minimum for good performance.

Don't be misled by your SWR meter as a single earth stake may give you a low SWR, but what you are seeing may be the effect of ground losses.

A quarter wave vertical should have a theoretical impedance of about 35-36 Ohms, so if you have a 1:1 match you are seeing 35 Ohms, plus 15 Ohms of ground losses.

As you add more and more radials the SWR may INCREASE. This shows it is starting to get closer to the optimum 35 Ohms.

The goal is to keep on adding ground radials until the SWR stops changing. Then the vertical is working about as good as it can.

Rudy found that once you get above 32 ground radials the improvements start to get more subtle and increasingly minimal.

But how long should the radials be be? A quarter wave radial laying on the ground is detuned so a true quarter wave is no longer a resonant radial, although it is a good overall compromise.

So the golden rule is that for a given amount of wire more shorter radials are better than fewer longer ones. This helps to collect the ground currents around the base of the antenna and improves the antenna's efficiency.

If it is a multiband vertical then the compromise is to make them as long as the antenna is high. If it is a monoband antenna then perhaps a quarter wave is best, although eight "eighth wave" radials might work better than four quarter waves (if on the ground).

In tests though you will find that two resonant elevated radials fitted so that they are at 180 degrees to each other may work as well as eight or so random radials on the ground. Rudy suggests that more resonant quarter wave elevated radials may be better still, but its starts to get a bit unwieldy.

A few years ago my club used a quarter wave vertical cut for 40m and fed against two elevated quarter wave radials and it worked very well. For contacts out to Germany from the UK there was little in it compared with a horizontal half-wave dipole at about 40 feet. Closer-in contacts were louder on the half wave horizontal dipole due to the different radiation pattern, but for DX the vertical was better.

You can see this with the MMANA-GAL antenna modelling software.

We also used it on 21MHz where it was a three quarter wave vertical and ended up working India (VU).

As you can buy 10m fibreglass fishing poles for about £30 you can make an effective quarter wave vertical for very little money.

The length of the radiator will then be 300/7.1MHz = 42.25m/four = 10.56m.

If using PVC-coated wire the adjusted length will be about 10.56m x 95% = 10.03m, although start a little longer and fold or cut to get the SWR minimum.

For a 30m quarter wave vertical the sums are:

300/10.1MHz = 29.7m/four = 7.42m or about 7.054m if using PVC-coated wire.

Once optimised expect to see an SWR of about 50 Ohms/36 Ohms (the impedance of a quarter wave vertical) = 1.4:1 or 1.5:1 NOT 1:1.

Although putting a quarter wave vertical (or Hustler/Butternut) on a single earth stake will work, you are throwing away its efficiency.

Why not try building one and let me know how you get on?

Thursday, 24 November 2016

Christmas present ideas!

As Christmas is coming, I thought I would remind people that there are some great radio-related goodies for sale at CafePress.co.uk and .com.

You can choose from a number of items, including:

  • Three different types of ship's radio room clock, with silent period sectors marked
  • "Remember QRT SP" Merchant Navy Radio Officer merchandise
  • "Keep Calm and Work Some DX" merchandise
  • "Keep Calm and Work Some CW" items
  • Nikola Tesla merchandise, featuring him sitting in his Colorado Springs laboratory in 1899, surrounded by electrical arcs.

You can have the last three slogans added to T-shirts, sweatshirts, mouse mats, calendar, mugs and much more.

Just go to the Radio Room!

Or there are a number of my radio-related books that make good presents , including "Radio Propagation Explained", "Antenna Modelling", "Stealth Antennas" and "Getting Started in Amateur Radio". Use the image links on the right for more information.

New book: Radio Propagation Explained

Blog readers might be interested to know that I have a new book out. “Radio Propagation Explained” is based on Ian Poole's excellent “Radio Propagation Principles and Practice”, (published in 2004) but has been updated throughout.

It is bang up to date with the current amateur radio allocations in the UK and has new chapters on propagation prediction software, web resources and propagation on the LF and MF bands.

While I was at it I added a lot more information about Sporadic E, tropospheric propagation and the Sun and its impact on the ionosphere and HF.

As such the Radio Society of Great Britain felt it made more sense to give it a new title to avoid confusion with people who already owned the original book.

Giles Read at RSGB reviewed it and wrote : ”It's not an expensive book, yet it's worth its weight in gold – highly recommended.”

The other good news is that the price has been kept low – the RSGB member price is £11.04 and non-members pay £12.99.

It can be bought from the RSGB if you are in UK/Europe, or as a Kindle version on Amazon.com and Amazon.co.uk

Friday, 7 October 2016

GB2RS Propagation Presentation

On Sunday 9th October I'm giving a talk at the RSGB Convention on how we prepare the weekly RSGB GB2RS Propagation report.

This looks at HF, VHF, moonbounce, aurora, rain scatter and much more.

To save people hurriedly scribbling down all the URLs in the presentation I have turned it into a PDF for download with all the URLs hyperlinked.

I'm not sure if this is of too much use if you haven't seen the presentation, but it does show all the resources that are used each week.

You can download the PDF at:
http://www.infotechcomms.co.uk/downloads/GB2RS_Propagation.pdf

Wednesday, 28 September 2016

Coronal holes causing chaos on HF bands

Coronal holes appear as dark patches when
viewed in extreme UV light via the SDO spacecraft.
The past few months have seen very poor HF conditions with the higher bands often closed and maximum usable frequencies below 14MHz during daylight hours. This has been coupled with high K/A indices and aurora.

But what has been the cause?

HF conditions are generally worse in the summer months, with lower ionisation levels overall during daylight hours in the northern hemisphere.

This is believed to be due to a change in the chemical composition of the F layers of the ionosphere, with a predominance of molecular rather than atomic oxygen and nitrogen [1]. That is, atoms that are paired up have stronger bonds, which means it is harder for ionising radiation to liberate electrons. The actual process is quite complex and depends upon the ratios of [O]/[O2] and [O]/[N2]. The reaction is also temperature sensitive.

But over the past few months we had had an added problem – coronal holes (CHs) and coronal mass ejections (CMEs) from the sun.

A CH is an area on the sun where the corona is darker, colder, and has lower-density plasma than average. The magnetic field around a CH is also different – instead of returning to the surface, the magnetic field lines remain open and stretch out into space.

This can allow the charged solar wind to escape at high speeds, often up to 600-700 kilometres a second.

When a CH is positioned near the centre of the Earth-facing solar disk, especially when the “frozen in” magnetic field of the escaping plasma (Bz) points south, it can more easily couple with the earth's magnetic field.

The hot gasses can then flow to Earth as a solar wind high-speed stream (HSS), causing geomagnetic disturbances, including enhanced auroral activity and absorption at high latitudes. We can detect this geomagnetic storm as the Kp index rises to four or more.

Other effects can include a lowering of the critical frequency as the F layers are depleted causing the upper HF bands to close down. Higher noise levels may also be evident.

We can “see” CHs on the sun thanks to spacecraft. When viewed in the extreme ultraviolet light or X-ray spectrum they appear black due to their lower energy levels. The Solar Dynamics Observatory (SDO) has an X-ray imager called the Atmospheric Imaging Assembly (AIA) and its imagery can be found at www.solarham.net/latest_imagery/ – look for “AIA 211b W/ Coronal Holes”.

CHs can also be long lasting, their effects often being observed on several solar rotations (27 days) [2].

But the CH effects have also been augmented by CMEs. These are often linked to a solar flare and occur when the magnetic field within a sunspot breaks down allowing millions of tonnes of plasma to be ejected. This has similar effects to a CH as the plasma hits the earth, especially if the CME's magnetic field (Bz) is pointing south.

But why the sudden increase in CHs and CMEs recently?

Observation has shown that the sun is more unsettled after a peak in the sunspot cycle, with more frequent CMEs around solar maximum and CHs on the declining phase. The past few months have seen the decline of Cycle 24 – the least active in our lifetimes

In 2013 NASA spotted a CH that was at least 400,000 miles across – more than 50 Earths side by side [2]. And in March this year scientists saw one of the largest polar CH's they had observed in decades. It covered an estimated six- to eight-percent of the total solar surface [3].

CHs were first seen in images taken by astronauts on board NASA's Skylab space station in 1973 and 1974 [4]. So our data only go back two or three sunspot cycles, and we haven't got a lot to go on.

We know CHs can be long-lasting and we think they move closer and closer to the sun's poles near solar maximum.

But the best guess is that we will be stuck with CHs, CMEs and their effects for at least a few more years yet as we head towards sunspot minimum around 2020.


References:

1. “The Solar-terrestrial Environment”, J.K.Hargreaves.
2. www.nasa.gov/mission_pages/sunearth/news/gallery/20130618-coronalhole.html
3. www.nasa.gov/content/goddard/sdo-sees-two-coronal-holes
4. “The High-Latitude Ionosphere and it effects on Radio Propagation”, R.D.Hunsucker and J.K.Hargreaves.

Originally published in RSGB's RadCom magazine


Tuesday, 2 August 2016

More great tins for QRP radio projects

Click to enlarge
Regular readers will know I've built a couple of QRP HF radios into mint tins. The first was a 20m Rockmite ][ built into a Stewarts' tin with a Spitfire on the front. 

The second was a 40m Foxx-3 built into a Stewarts' train tin.

This all came out of a desire to build something in an Altoids tin or similar having been inspired by the fantastic radios built by Colin M1BUU.

He showed me a Steve Weber-designed ATS (Appalachian Trail Special) in an Altoids tin at a Rishworth QRP convention and the workmanship was fantastic.

Anyway, both my radios work, but at around 1W or less QSOs can be quite hard work – must dig them out again soon and have another play.

But this has set me on the trail for more and better mint tins and I found a couple of crackers recently.

The first depicts the White Star Line and was bought for £3.50 at the Bressingham Steam Museum in Norfolk. They had other designs and it appears to be made by a company called Half Moon Bay in Bath. They are a wholesaler, but do have links to an online stockist – Kitsch-a-go-go – which has lots of different tins.

The second tin was found at the British Motor Heritage Museum at Gaydon and depicts an MGA sports car. It is also a little bigger than an Altoids tin and cost £4.

This was made by a company called Red Hot Lemon. It has a minimum order of £100 unfortunately, but you could always club together with someone else to buy some.

Now, all I need is another kit to build. What I'd really like is a 3W 40m and/or 20m radio transceiver, preferably synthesised so that you move around in frequency that would fit in a standard Altoids tin. So basically, a Steve Weber ATS or MTR – shame they are not being made in kit form. I have a three-band MTR v2 which is my pride and joy.

If you know of any other suitable kits please let me know by commenting below.

Thursday, 28 July 2016

Repairing a cross-needle SWR meter

The SWR meter - working 100% again.
I've owned an AEA ET-1 antenna tuning unit for the last 20+ years. In that time I've had to replace the inductor switch as it burned out and also replace the meter.

Luckily, it uses exactly the same meter as MFJ ATUs so I was able to buy a replacement from them. I also resprayed the case at some point.

But recently the meter started playing up – it would only indicate 20W maximum forward power instead of 100W.

It was more of a problem on the less than 30W setting as I often use QRP radios and need to know if I am putting out 2W or 5W.

You could ask “why not just bin it?”, but that's not in my DNA!

Anyway, after asking for advice on the CDXC forum and looking at the schematic I decided that it could only really be down to a failed capacitor, resistor or diode. Given that semiconductors would be the first suspect I ordered two new 1N270 diodes from a UK supplier on Ebay for £3.98.

When they arrived I unsoldered the old ones, and checked them – one measured 1K Ohm resistance in both directions – not good and definitely not diode-like behaviour.

I soldered in the new diodes and bingo the meter is back to normal again. I've since ordered some 10 more 1N270s from China for just £1.12 inc. postage. These will do as spares.

I'm writing this as it might be useful to owners of cross-needle SWR meters in future. If you start to get low readings it may be the diodes. If you get no reading at all it is probably the meter.