|The SRC X65 with its original box, now long gone.|
Wednesday, 20 August 2014
I've been using a 65ft inverted L with a 9:1 unun for some time. It was originally made by the Snowdonia Radio Company (SRC), but I have had to rebox it as the unun box cracked and filled with water.
With a new box and stainless steel bolts and fittings, the antenna works well from 20m – 10m, with coverage of 40m and 80m as well, although performance isn't fantastic on these lower bands.
The antenna is a vertical for the first 8m, supported on a fibreglass fishing pole. It then goes off at quite an acute angle down to the garden fence. The pole is held against a tree trunk with a couple of bungee cords and has a single earth stake. It is fed with RG213 coax with a 10-turn choke at the feedpoint.
What I did notice was that the antenna didn't work very well on medium wave, long wave or with LF aeronautical NDBs, so I thought about trying something.
The unun box now has a DPDT switch attached that switches out the unun and feeds the antenna directly. This has made a big difference to medium wave with signals romping in. The switch is waterproofed so all I have to do is go outside, flick it if I want to use the antenna on medium or long wave, and flick it back to engage the unun, which gives a better match on the upper HF amateur bands.
Without the unun I also get a better match on 80m, which is a bonus. Simon and SRC are no longer trading unfortunately, but you can build your own 9:1 unun quite easily.
While it is no Flag, Kaz or Wellbrook Loop, it does make a simple omnidirectional antenna for SWLs and hams.
|The circuit diagram - you can easily make this on |
a piece of PCB.
This has to be the ultimate in small antennas – one that will receive everything from VLF to HF (even VHF) in a package less than six inches long!
The antenna was conceived after Roelof, PA0RDT, had several attempts to make an active loop work in a city environment.
What he discovered was that the electric field from local noise sources was generally contained within his house. But, the magnetic field of noise sources was not, making weak signal reception at LF virtually impossible.
From that he decided that an antenna mounted outdoors that was receptive to the electric field rather than the magnetic field might be useful.
After extensive tests he said it became clear that at LF an active whip is effectively a “capacitance coupled to the electric field”.
Roelof added that in practice the “whip” can be tiny, such as a small piece of copper clad printed circuit board. Hence his Miniwhip was born. Roelof's practical design is only 100mm long and 40mm in diameter yet will receive from 10 kHz – 30 MHz.
It is an active antenna and that feeds the power to the antenna via the coax and a bias T circuit.
The antenna details and schematic are available on the internet for you to make one, but Roelof also makes them to order (you can email him at email@example.com). At the time of writing the cost was €48 including shipping within Europe.
The antenna and its amplifier circuit are built into a sealed grey plastic pipe leaving you only to connect a suitable length of coax via its BNC socket. You will need to provide a 9-15V supply, such as a small “wall wart” PSU. But it must be a “clean” supply as some of these can be electrically noisy.
Extensive tests with the antenna showed that it is very prone to receiving noise (which I had been warned about). In my shack it picked up all manner of interference, from switch mode power supplies to a low-energy light bulb. Even a TV on standby in a room 20ft away caused problems. To be fair, this was to be expected. Roelof says it is an excellent noise sniffer! He has used it extensively to investigate several local noise sources.
It is best mounted outdoors and as high as you can get it. In fact, I mounted mine on a telescopic fibreglass pole so that I could test it at different heights up to 8m.
|The tiny commercial version, direct from Roelof.|
Extensive use of ferrite chokes might also be useful. I found that my two PCs produced a lot of interference, which was picked up on my Perseus SDR receiver. Using my Icom 756 Pro 3 with the PCs switched off made a big different, especially on the LF/MF bands. Roelof says that grounding the shield of the coax before it enters the house or at the bottom of the mast is also important.
What happens is that local noise is received on the shield of the coax inside the house and travels to the antenna. By grounding the shield, the noise will "flow" to earth. This can make a considerable difference. He has also included a RF isolating transformer in the power bias T box. A jumper is used to select between isolated and connected grounds.
What also soon became apparent was that the higher I mounted it, the better the Miniwhip worked. In fact, at about 5m it almost matched the performance of my existing HF antennas, despite its tiny size.
Roelof says that moving the antenna from 1.2m to 4.8m could increase the received signal strength by up to 8dB on the lower bands and I would agree.
I tested it on everything from non-directional beacons (NDBs at LF), long wave, medium wave and all ham and broadcast frequencies up to 30MHz and it worked very well indeed.
Mounted on a fibreglass pole the antenna merrily received aircraft non-directional beacons down in the 300kHz range – signals that were considerably weaker on my main doublet antenna.
It was a similar story with medium wave stations, with the tiny antenna pulling in stations from all over Europe in August. My usual test is to see how well I can pick up BBC Radio Wales on 882kHz from Washford, Somerset, here in Norfolk. It did this with flying colours.
Moving up to Top Band (160m) and there wasn't a lot of activity, but I did pick up some DL and PA CW signals that were at least as loud as those received on my W5GI antenna over the roof.
It was similar performance on 80m (3.5MHz) where the little antenna worked reasonable well, but was down on a dedicated 80m antenna. Noise levels were a lot lower though. On 40m (7MHz) the antenna also picked up signals, but the signal levels were down quite significantly. Interestingly, in some instances, the overall signal to noise ratio was no different, so in terms of copying the signals there was little difference.
|My own version built (very) ugly style and now |
mounted in a piece of 40mm PVC pipe.
Roelof says that he has his mounted at 4m on a non-conductive mast to get clear of bushes in his garden, but he has also had excellent results in an open field at a height of only two metres.
The Miniwhip makes an excellent SWL antenna, as long as you spend some time calming all the various noise sources in your shack.
I even mounted it in the loft, where it worked, but again noise levels were higher.
In the interests of experimentation I also decided to build one myself from the plans I found on the internet.
The antenna is made on a piece of single-sided PCB, with the top half acting as the antenna and the lower half holding the circuit. I made my circuit board by carefully cutting away the copper with a Dremel-type tool. I then built the circuitry “ugly-style” (very ugly actually!) onto the board, using Superglue to mount some of the components securely with their wire leads being used for the interconnections. The end result was a little Heath Robinson, but did it work? The answer was a resounding yes.
As a test I also tried it on VHF and found it was a reasonably effective little antenna for airband and 2m signals.
Monday, 19 May 2014
The book looks at the free MMANA-GAL antenna modelling program that lets you design and optimise a whole host of antennas – all on your PC.
It shows you step-by-step how to input antenna designs into MMANA-GAL, how to adapt designs you are given and how to optimise your designs for the best performance.
By the time you have finished you should be able to model a whole host of antennas including dipoles, the G5RV, the W3DZZ trapped dipole, verticals, off-centre fed dipoles (OCFD), magnetic loop antennas and many more.
The book also includes a CD-ROM that not only contains the MMANA-GAL software so you can get started immediately, but sample antenna files too. It also includes other antenna modelling software including EZNEC, MININEC Pro and 4nec2. Plus there are also more than 30 other amateur radio programmes included.
The RSGB has also managed to keep the price below £10.
See the RSGB shop.
Saturday, 22 March 2014
|Predicted 10m propagation at 1600UTC March|
Despite the VOACAP prediction suggesting that there would not be much propagation into the USA from the UK on 10m, there were plenty to be copied.
I think 10m propagation to the States will start to disappear as we move towards summer.
I even heard two from California.
I like beacons. They are usually low powered, use single element antennas and the beacon owners appreciate a quick e-mail to show that their efforts are worthwhile.
You can find G3USF's list of 28 MHz beacons at http://www.keele.ac.uk/depts/por/28.htm
Anyway, here is what I heard, all with a dipole:
|N2PD/B||New York, 5W||28.285|
Tuesday, 18 March 2014
Keith M0DZB operating at GB0CMS
It is also a good opportunity for you to gain a very nice certificate. All you have to do is work the requisite number of award stations and send in a log extract - you don't need QSL cards.
I gave a talk to a couple of clubs about the best way to win the award and you can download the presentation in PDF format (14Mb). It is aimed at UK operators.
I shall be helping to run GB0CMS again this year at Caister Lifeboat. There is short video that looks at the equipment the club used to make 165 contacts in 24 countries on Saturday 30 April 2011. And another for the 2012 event when we made more than 500 contacts.
See http://www.youtube.com/watch?v=JUXzJcaHThs and http://www.youtube.com/watch?v=C8ZRW0q6SyI
You can also view my propagation predictions online.
You can find out more at: http://www.gb4imd.org.uk/
Friday, 24 January 2014
I have now updated my HF propagation prediction charts for the first quarter of 2014 to take into account the latest predicted smoothed sunspot numbers. It is generally thought that we may now be at or past the second hump of this sunspot maximum, but the sun continues to throw some surprises.
For example, the observed sunspot numbers for the last six months of 2013 were 57.0, 66.0, 36.9, 85.6, 77.6 and 90.3, which shows the great variation.
The current predicted smoothed sunspot number (which is that recommended for VOACAP-based prediction programs) is about 61-62 for the next three months.
This equates to a 10.7cm solar flux index of about 110 (on 24th January it was 136 with an actual sunspot number of 121).
A flux over 100 suggests that 10m will continue to open on a fairly regular basis and the next few months should throw up some good opportunities for DX on the higher bands. However, the longer-term trend may be downwards.
On the downward side of the cycle we may also experience more unsettled conditions due to flares and coronal mass ejections. These may cause short-term D-layer absorption, poor or noisy conditions (especially on routes over the poles) or depletion of the F layer (with corresponding poor conditions on the upper HF bands). Looking on the bright side it may also give raise to aurorae.
The short-path HF predictions from the UK can be found via a link on the top right or by clicking here.
If you want to produce more detailed point-to-point calculations (and don't have your own program) I recommend VOACAP online at http://www.voacap.com/prediction.html
You can keep track of the current state of the sun at www.solarham.com
Chairman, RSGB Propagation Studies Committee
Sunday, 29 December 2013
|The SAQ 17.2kHz transmitter at Grimeton, Sweden|
Last tranmission I forgot to turn my Wellbrook ALA1530S loop to point North East, so it was in the null and I received nothing - doh! I used a Perseus SDR and the loop, which worked well and all was copied.
The SDR broke the recording into two files and knocked off the "C" in "Merry Christmas", but I have put it back in using Audacity sound editing software.
If you don't read CW the message was:
VVV DE SAQ SAQ SAQ.
100KW ALTERNATOR ON 17.2KHZ.
WE AGAIN WISH YOU ALL A MERRY CHRISTMAS AND A HAPPY NEW YEAR. SIGNED: THE WORLD HERITAGE AT GRIMETON AND THE ALEXANDER-GRIMETON VETERANRADIOS VAENNER ASSOCIATION.
FOR QSL INFO PLEASE READ OUR WEBSITE: WWW.ALEXANDER.N.SE.
DE SAQ SAQ SAQ SK.
You can listen to the broadcast yourself using the controls below.