Sunday, 31 May 2009
The Rybakov 806 Multiband Antenna is a little different to other antennas I have made in that it is designed to not be resonant on any of the amateur bands. What makes it special is that because it is a non-resonant antenna it tunes up equally as well (or as badly) on the WARC bands as on 7/14 and 28Mhz with only 7.6m of wire. Some have found that with a longer wire, in the region of 8.6 to 21m that they can use the antenna on 3.5mHz.
I became interested in this antenna when Peter ON3WAB mentioned that he and Lieven ON4CVL were using one, made by Lieven, on a SOTA activation from Hotondberg ON-022 using an Icom 703 and 10w and putting out a fabulous signal. It seemed ideal for those summits where you cannot get a dipole up and a number of constructors have claimed it out performs trapped verticals. Pictures from the ON activation - http://www.flickr.com/photos/9924647@N08/
The only thing I did not have to make one of these was the T-200-2 Toroid which I found on eBay for under £4 from Spratreader (firstname.lastname@example.org) who is actually Les Jackson G4HZJ a member of West Manchester Radio Club.
The US has been equipping its emergency crews with digital radios because they take up half the radio spectrum of the old analogue ones. But fire crews complain that they cannot hear what their colleagues are saying.
Read the rest of the article at New Scientist
All I feel like saying is "We could have told you so!"
Dave is the secretary of Mold & District ARC of which I am chairman and also a member of Flintshire Raynet. I know Dave has been very busy with work and his young son but his new site doesn't seem to have quite as much on it as his previous site, but his change of callsign (some time back) prompted him to revamp his site. I'm sure it will soon be much better than the previous one. He seems to make nice sites. He has a blog on the site that may be worth watching too.
I have never seen anyone with a ham radio tattoo so I was quite surprised to find these...
Sorry guys they may not be as geeky as the watch in my previous posts but you must be poor sad buggers to go through life sporting those.
Friday, 29 May 2009
There are still plenty of experts out there and the Internet gives everyone the chance to draw on that expertise and become less dumb. Amateur Radio is after all about 'self training'.
Even amongst the self styled amateur radio gurus with sites on the web there are those that demand more respect than others. One of those is Martin Steyer DK7ZB who I personally consider to be the Antenna Miester. If you want to build antennas this is where to go...
Interestingly when I read G4HJW's interesting web site I realised the original idea came from my fellow Mold & District ARC member John MW1FGQ, so I have an expert to help me get started right under my nose. Who knows maybe with John's help and advice I might even get QRV in the near future. I had better order those lead underpants.
Wednesday, 27 May 2009
Justin Johnson G0KSC makes public details of the LFA ‘Loop Feed Array’.
For more details visit - http://www.southgatearc.org/news/may2009/g0ksc_loop_fed_array.htm
or download the construction details here - http://www.g0ksc.co.uk/file-download/category/1-antenna-design-files.html
Yours truly on Pendle Hill after a Summits on the Air activation.
My SOTA activation report can be viewed under the title of 'Ramblings and Reports 2009' on the SOTA Reflector http://www.sotawatch.org/reflector.php
or my web site -http://gw7aav.googlepages.com/sotaactivationreports2009part2
…and which includes photographs that are available at http://www.flickr.com/photos/gw7aav/collections/
I have recently updated my website at http://gw7aav.googlepages.com including my activation reports for 2008/2009.
Tuesday, 26 May 2009
Have you got one of those cheap Chinese hand held transceivers that you see sold on eBay? You have! Well in that case have you ever checked that rubber duck antenna on an analyser? If the answer is no maybe you should.
Cheap hand held radios are flooding into the UK through eBay and names such as Wouxun, Jingtong, Puxing, Linton and Quasheng are becoming familiar to radio amateurs around the country. I must admit that at first I was very sceptical about the quality of these radios, but when you check them out they turn out to be in the main of at least acceptable quality. You can of course get a duff one but most people I speak to have been very happy with their purchases. There is no doubt that you tend to get what you pay for but putting a few of these radios through their paces has shown them to be at least as clean and sensitive as their more expensive Japanese rivals.
Discussions down at the local radio club and on air has lead me to realise that a lot of amateurs are using these radios straight from the box and are wondering why they are such a poor signal into the local repeater. The problem lies not in the radio but in the antenna and the wide band nature of these sets. The popular four metre Wouxun 689E for example actually comes two antennas and the set covers 66-88MHz receive and transmit. One antenna is for 66-77MHz and the other is for 77-88MHz. For use on the 70MHz amateur band the first antenna is used but try it on an antenna analyser and you will see what would be normal considered a somewhat poor match. Looking at some of the other Chinese hand portables we find the VHF ones cover 136-174MHz which is nearly 40MHz band width on the one antenna, have these guys discovered something we do not know? Unfortunately not! The UHF sets are even worse with some covering 320-470MHz on a single antenna. No wonder they seen to be poor performers. The fact that theses sets work at all straight from the box is probably testament to the bulletproof output circuits on the rigs alone.
The answer to this problem is simple if you have access to an antenna analyser you can simply remove the little plastic end cap on the antenna and trim the antenna to resonance and the glue the cap back on afterwards. The result will not be Earth shattering but you will at least be putting out most of the rigs meagre power instead of a few milliwatts. While a rubber duck is no alternative for a proper antenna it may just be enough to make that ‘scratchy’ signal into your local repeater fully quieting.
If you don’t have access to an antenna analyser you might think about joining your local radio society/club or asking around on the air. Some radio amateurs have been known to actually be quite helpful. The alternative is to buy a rubber duck aerial that is meant for amateur use but that could cost you more than the radio and there is no guarantee that what you buy has been optimised for the band. For example, a lot of two metre antennas are sold cut for the American market and might be tuned for anywhere between 144 and 148MHz were as we might want to optimise ours for 145MHz. If you trim your own you can at least cut it for wherever you want to operate, which I feel is a better solution. Even better and more fun is to make your own but that is another story.
Although I would only have FM on 70mHz I thought that making another horizontally polarised Delta Loop made sense for a number of reasons, first it was simpler to mount and secondly it would give me chance to see if nesting the loops caused any problems.
As I mentioned in my page on the 6m version, a Delta loop has an impedance of about 100 ohms and to match it we use a quarter wave of 75 ohm coax in series to with the 50 ohm feeder. What might be confusing to those new to antenna making is that we need to compensate for the velocity factor of the cable we are using. If you know what cable you are using there are sources on the Internet (such as the manufacturers web site) that will give you this specification. I used a high quality air core satellite TV coax with a good braid and foil shield. The velocity factor for this cable was 66 therefore… a quarter wave on 4m is 1m and 66% of 1m is 66cm.
This matching section is critical and you need to have 66 cm of shielded section. The trick is to cut 72 cm of 75 ohm coax and then strip back 3 cm either end to make the tails which are tinned with solder and screwed into the block connectors.
Well this one appears to be working, and to my surprise the SWR was 1.1 to 1. Right first time again so I am very pleased. Nesting the two Delta loops seems to work okay with a slight but very acceptable rise in SWR on the 6m loop but no change on four metres.
By moving the feed point to a top corner and taping the apex of the triangle to the mast I have tested the antenna vertically polarised, which for FM works a lot better.
My findings on 4m lead me to think that although this good antenna for portable work and it works well from high locations to be truely effective it needs to be mounted much higher at my home QTH than I have it at present. The difference with the six metre version is due to the type of contact I have been looking for. When the six metre band is open I have managed to work some medium distance DX, that is most of Europe including European Russia. On four metres I am looking for fairly local inter-UK. Both six and four metres tend to exhibit both HF and VHF characteristics and so the low six metre antenna can be effective because of the HF like properties but I need to take advantage of the VHF like properties on the four metre band and at VHF hight is everything.
Delta loops have an impedance of around 100 ohms therefore a quarter wave of 75 ohm coax in the feeder will bring the impedance to near enough the required 50 ohms that is expected at the transmitter. This matching section can be rough wound and taped but I choose to neaten things up by using a piece of white PVC pipe as a former.
The matching section is critical and you need to have 99 cm of shielded section. The trick is to cut 105 cm of 75 ohm coax and then strip back 3 cm either end to make the tails which are tinned with solder and screwed into the block connectors.
This was a worked first time design and the SWR is 1 - 1 from 50 – 51.5mHz and reaches a reasonable 1.7 – 1 at 52mHz It is horizontal polarised and was cut for the SSB section.Do not forget to waterproof the matching section if it is to be used outdoors.
I had no chance with just 25watts into a Watson vertical antenna and was kicking myself that I hadn't yet got my four element J-beam erected, but it gave me hope that in the future I will grab some real DX on the so called 'magic band'.
Please visit my web site - http://gw7aav.googlepages.com/
...and the Four Metre Website -http://www.70mhz.org/