I check into the HamAntennas Yahoo Group fairly often and it occurs to me that collectively, there seems to be a lot of smoke and mirrors surrounding the design of low-band antennas and everyone seems to forget the basic principles, that low-to-the-ground antennas will form bubbles of RF above you. We call them NVIS or Cloud Burner antennas. The graphic opposite is a plot of my home-brew 40m loop in the garden from MMANA. It’s 25 feet above the ground (which is exactly the same far-field plot as an 80m loop at 50 feet). Clearly it’s going to work well at what it’s designed for. Up to about 1,500 miles. In practice, it conforms precisely to my software plot.
Most people though seem to think that their home-brew contraption is particularly good because they checked into a 75/80m net 1,000 miles away. Yes, you can do that with NVIS, so don’t worry! Your radio works. Big deal. All that you are doing is forming a healthy bubble of RF above your antenna. That’s the easy bit and why all the folks are out there on 75/80m bands. Heck, I like NVIS. It’s fun and it means that you can talk to you friend up the road. Great!
I read a lot of literature on antenna theory (and practice) and as many of you know, I have built hundred (I exaggerate, but not my much!) of antennas and most have been put to the 500 QSO test. Frankly, there is so much bull about them that I can hardly contain myself sometimes. There are a number of camps, like those that like to design antennas and circuits that relate to a near 50 ohm impedance. I can buy that. It’s difficult to get an antenna to just the right SWR curve based on 50 ohm coax and I appreciate the time and attention to detail that the coax freaks enjoy. I am one of them. This is probably the biggest topic out there and it’s good for amateur radio.
Then there are those that don’t care about 50 ohms, although they still feed with coax and dial it out with a tuner. They heat up coax and probably cause TVI. Not a good practice and we need to help these guys.
Then we have the guys that throw the book away and feed everything and anything with 450 ohm, or open wire feeder and sort it all out with their favorite tuner, well good luck to them. They brag about loading up trucks, guttering, fence posts and gates. Yeah, you can do that. Get over it. Of course, that’s not to say that people who use 450 ohm feedlines throw the books away, but for my convenience, I’ll forget you quiet guys. You say little because you know how it all works and I respect you for that :)
The only way you can get either a bigger or flatter bubble above you (or a lower angle of radiation which is the same thing really) is to become either extremely efficient, high or very clever and there’s only a few clever AND low designs that I know about. Most of these revolve around vertically polarising the signal to keep the heights manageable by using quarter wave vertical prinicples. They tend to use similar concepts to Yagi; they squirt the appropriate RF in mostly one direction. The Half Square, the Bobtail Curtain are very good at this, as are 4-squares and other phased arrays.
The rule is that if it’s horizontally polarised and under about a third of a wavelength in height, it will successfully shove a bubble of RF over your head, the higher - the flatter it will be. You will be happy regardless of length or how you feed it, I’m sure. If it’s vertical it will probably be pretty inefficient but it will no doubt secure some DX for you, however it will only work best between about 500 to 1,500 miles. You will miss your friends locally. See the plot, green is a quarterwave vertical compared to a red plot, my 40m loop - it’s pretty easy to grasp, I’m talking to licensed amateurs here.
All camps will be happy though, because they’ll work anything they can hear. Be aware, that working only those stations you can hear (and working up to 1,500 miles on 80m) can be done with a set of step-ladders so it’s not a good argument about how good your antenna is - be carefull about what you are bragging about.
To conclude, by all means, let’s all tinker away with every conceivable gadget and idea, just remember that regardless of how we feed these things, or what they look like - if it’s low down to the ground, the best you’ll do is get a bubble of RF into the sky, but that’s fine. They work. The good news is that regardless how you do this, most of the bubbles will be the same size, full stop. It’s not about whether your antenna goes around a tree up a gutter, round the house or inside the attic or drivel like this. For the low bands, it’s completely irrelevant, just get some wire out there and find something that will match it to your radio. Did you hear about the guy that worked 100 countries one year on 160m with an antenna that was only 10m (30′) above the ground? That’s the same as having a 40m antenna, only 2.5 meters (7′6″) in the air! Look at all those people who got fed up with antennas - they went and bought an SG230 and strung up a couple of pieces of wire over the roof to an apple tree. It worked great and all their problems went away. The matching was taken off their hands, all they had to do was come up with an idea where to shove their wire. Let’s face it, that’s certainly not the difficult bit, eh?
Cheers for now.
Callum.
January 29th, 2009
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Not much to report this month and I’m only typing because I’ve currently got man-flu and feel like sh*t warmed up.
The Mega-Loop is working superbly at the Scout hut, although I’m trying to find something better for 160m. The impedance is already slightly low on top band and with a 4:1 balun in line at the feedpoint, it seems throws the SWR out by quite a bit. The ACOM 2000 seems to handle such a mis-match, in fact, it only trips out with a return power equating to around 575 watts. Technically, does this means that I can fire 1,000 watts up the coax and have nearly 600 watts come back to me? I don’t know - and certainly not above 1,850 anyway (UK band plans & license conditions etc). I’ll need to check.
Here’s some fun. The picture shown is the design for a Hyper-Mega-Loop, this is twice the size of our current Mega-Loop at 340 meters in length sitting at 18 meters height with a design sag of 5 meters in the middle of each leg. Using MMANA, it seems to still develop amazing potential for 40m band. How about 8dBi at 10 degrees take off for the US? I wish I hadn’t thought this up, I’m tempted to build it. Yes, the RF will fire just North of West at around 285 degrees.
Last Monday night, Tim and James helped me to demonstrate amateur radio to the Beavers. We did a standard JOTA style 90 minutes with little ones starting with Morse code, continuing to QSL designs and passing greeting messages on 80m. Thanks to Chris, G0MLY who was was an excellent victim in managing the little ones pass their greetings message. Also thanks to the many others who helped including our friends, Barry (G0DGQ) and Chris (G0EYO).
At the Exec Committee for Dorridge Scout Group last night, I was asked to step in as an Interim Group Scout Leader for 6 months whilst we find a parent to fill these boots. I accepted and I’m very keen to get stuck in. I immediately announced the 2009 mega-clean-up. If you do live anywhere near Dorridge, we’re going to be after all the white paint (gloss, undercoat and matt emulsion), brushes and sandpaper that we can get our hands on. Every Beaver, Cub, Scout and Explorer will contribute in painting the hut from top to bottom, inside and out. A clean floor and a new kitchen is also planned. If you are reading this and you can help, please let me know.
This also means that we can get the technology room built and set up for remote control ham radio too. We already have three Dell PCs and matching LCD monitors together with a Dell 2400 server to donate to this project. Internet will be sent from my house to the hut via a point-to-point 2.4Ghz link. It’s only 900 meters, so a very interesting project. We’re on the look out for anything that we can either use or sell for funds for our TS2000 appeal or use. We’ll also need PSUs, coax, connectors etc, etc. Can you help?
Callum.
January 28th, 2009
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callum |
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This weekend, we took the boat out for the first time as a family since the summer. Each time we have previously tried to take the boat out, we seem to have hit various mechanical snags and had to return home. This time it was the weather that nearly pushed us home. However my long lost genetics of the Outer Hebrides encouraged me to continue.
On Friday afternoon, we motored Wherethehell-Rwe out the marina via half-inch thick ice and up the three locks outside the marina. Luckily the ice finished at the canal. We stopped discretely opposite the tow path between Calcutts and Wigrams turn conveniently between two trees.
I have recently done some fault-finding with our marine TV antenna. This is the flying-saucer style one from Shakespeare Marine. I have raised it up 14 inches from the deck with their smallest custom extension mast and it’s made a fantastic improvement. I’ve also got a 4 foot extension mast too. It only takes a few minutes to add the large one when we’re moored up for even better signal quality. I’m extremely pleased. It means that in the main, each TV can now receive the big digital channels as well as Sky. Last summer, we only had Sky which was pretty annoying as we all had to watch the one Sky channel across all the TVs. Experiencing proper TV now on each telly has made a terrific improvement for all on board - and it even possible to receive TV whilst on the go! We can’t have Sky whilst moving though. I did look at it but it’s pointless for narrowboats because of the bridges, cuttings and trees etc. It would be forever losing signal.
I digress. By 6:00pm, we had the wood-burner flat out, the central heating on and a 500 watt electric heater in the back bedroom. We could only just keep the temperature up as the ice formed around the boat and dumped the overnight temperature to nearly minus 10 degrees centigrade. Not good for live-aboards I fancy. I have a cool electronic weather station at home that I have never put up, I might use it on the boat instead since it’s got a computer interface etc. It could be an interesting addition. I’ll let you know.
Nearly an inch of ice had formed on the canal surface during the night which surprised me. I didn’t think it could form that fast. It certainly made a bloody racket though. Everytime you moved, it would scrape and groan on the hull.
We had a laugh as I zoomed blocks of ice up the canal for hundreds of yards, listening to the tinkling of the blocks skittering across the surface. We waited and waited for someone to be bold enough to come down our bit of the Grand Union canal but nobody did. Finally I asked a dog walker what the conditions were up at the Oxford junction (600 yards ahead) and he confirmed that boats had already broken the ice ahead. Wendy encouraged me to make a run for it, so at midday, we gingerly set off, my first ice-breaking adventure.
Getting out of the overnight berth was a problem and it took a bit of diesel to get the momentum and turning correct to actually leave the overnight berth and get the boat into the middle of the channel. I certainly left half a pint of paint on some of the ice plates. Wendy and the kids were quickly up in the bow watching splinters of ice flying out in all directions as we crunched along, virgin ice-breakers. Finally, we reached Wigrams turn and we could followed the tracks of previous boats.
It’s three or four lock-free hours to Braunston from Wigrams - probably less. In the main, it was an uneventful journey other than the tight bridge turns where the stern would catch on thick plate-ice and restrict my turning ability accompanied with a horrible paint-stripping sound as the stern dug deeply into the plates, shuddering against the boat.
By 2:30pm we were at Braunston and after turning around by pushing our nose into Braunston Marina, we stole the visitors mooring directly outside the Millhouse where we received a great Satellite signal but no Digital TV (the pub is right in the way). Once moored up and the fire re-fuelled, we popped into the pub for a coffee (I mean Carling!) and watched the kids mucking about in the play area. I squared it with the barman for us to moor up but it turned out that they had a kitchen problem and advised us of a 45 minute wait for a plate of chips so we bailed out and strolled up to the village to check out the pubs and get a spot of shopping. We found the Old Plough in the High Street and promised to check it out later that night. It opens at 6:00pm.
That afternoon, Wendy and I crashed out - completely knackered because the previous night’s strange noises that had kept us awake as the ice froze all around the hull, however we soon had a second wind and took in the delights of the Old Plough. This is a pleasant pub but remind me to book next time, they’d had a rush on the place and we had to start our meal in the public bar where a skittles match was about to take place. Luckily, a table by the fire became free in the restaurant and they invited us to move back.
Once settled, Wendy started scribbling on Emily’s notebook. After a minute or two, she showed me what she had written. It went along these lines: “The woman right behind you rolled her eyes at her friend when she saw you come in to the bar. I caught her eye and she knew that I had seen her. She went scarlet red with embarrasment“! I immediately turned around to see for myself but the lady kept her head down very low all night in fear of what I might say. Ah-ha! The fun of having a leopard-skin print Mohican hair-cut!
Sunday morning was again very cold. This time without sun and it remained below freezing all day. It took me longer than usual to get some heat back into the boat. I switched on the 500 watt electric fire, sucking the batteries dry for an hour. Eventually at 10:30am, the mains tripped off as the Victron inverter shut down due to lack of DC available. Our four 110Ah batteries were almost out of real juice. This was my signal to start the engine and get the 5kW generator running. It’s also a reminder to get four additional batteries installed!
Luckily for me, a boat called Xenia came past as as I was preparing to cast off. They shouted over that the Oxford had already been ice-broken by someone else. Great! We needed to get back as the kids started back at school in the morning.
Wrapped up with two pairs of socks and a couple of heat jells on standby for heating up my hands periodially, we left the mooring at about 11:00am with a freezing wind on my cheeks. Wendy came to join me on the stern as we turned at the Oxford, west-bound for Calcutts. Nothing to report on the return journey other than the continuing amazing sights of the large sheets of ice up to an inch and a half think that would move out the way -or get snapped as we motored past. I kept the throttle a hundred RPM less than normal at around 900 so we made marginally slower time. I stopped for lunch at about 2:00pm which helped as the cold was starting to drill into me, however it turned out that we were only fifteen minutes from Wigrams Turn and the three-lock flight back to our marina.
As a coincidence, we shared the first lock with our marina neighbours, Muchgiggling who happened to be about 5 minutes behind us. They had been out since Boxing day and had had 12 boaters on board for New Years eve. Bet you they got plastered! They stopped for a pump out but warned me of a possible icy time in the marina. You bet.
Little prepared me for what I found in the marina. The outer marina was navigable like normal and just under the bridge to the new marina was also navigable. But clearly we were the last boat to have motored on this since Friday. I could just see our tracks in the ice from our departure. Nothing had moved since then. The ice was between an inch and maybe two inches in places and half-way across the new marina, it stopped us in our tracks. I reversed and applied full power and made efforts to keep the boat pointing approximately in the right direction. At full power and already going maybe four knots, the ice brought us to a stop - even with our 50hp Beta at nearly 2,000 revs. A single sheet of ice, thousands of tons in weight had us by the short and curlies. As a concerned skipper, I could only continue repeated attempts at the ice breaking to get us closer and closer to the pontoon, 2 meters at a time - and had it not been for a chap called Steve from a boat moored near our friends Graham and Jo, I don’t know how I would have managed to get moored up. Tony missed the fun, they arrived 15 minutes later not realising the seriousness of my predicament.
Anyway, a successful and enjoyable trip. I look forward to some more sunshine next time along with an early blacking!
Pictures here: http://www.m0mcx.co.uk/gallery/index.php?/category/165
Callum.
January 4th, 2009
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