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Thread: Multyband vertical

  1. #1

    Default Multyband vertical

    Hi I've decided to go down the vertical antenna route.
    I have a 7 meater pole for my antenna. It will be a multyband antenna for all ham bands as I will be sitting my foundation license soon.
    I have 135ft of wire witch will be twisted around the pole. At the bottom of the pole I will have a 9:1 balun. Now my problem is do i connect the balun straight to a ground rod or can i use a old tape measure as a ground plane witch will be cut i. To 4 radials if so how long would the radials have to be. Bearing in mind this will be 38 ft above the ground and about 4ft above the roof ridge. I have a mfj tuner to tune the antenna. Can anyone help
    Thanks in advance

  2. #2

    Default

    I'm at a loss to understand what it is you are trying to build? A multi-band antenna needs to resonate at frequencies within each band. You need to do some reading on antenna theory - there's a very good ARRL publication and of course some RSGB books too, but usually there needs to be some way to trick the transmitter into thinking a decent antenna is connected, when it won't be.

    This info isn't 100% correct, because there are some electronic effects that have an impact, but the basics will match what you're learning for your exam. If you have a quarter wave antenna, with a ground plane, it's a match for the transmitter with a 50Ohm impedance. The maths is simple too to give you the cutting length. The radials are there to give the antenna something to work against. The best ground plane would be the sea, under the keel of a boat. A huge conductive ground plane. If you use a number of wire rods, then clearly it's not as big as the sea, so not as good - but they're good enough to bring the VSWR down. Doesn't make the antenna suddenly good though. Some radials might well be a wavelength long. If you have a quarter wave going up, and you bend one of the quarter wave radials down, you've made a half wave dipole - that radiates pretty well. Bend them back to horizontal, and the match and performance stay about the same - ish.

    Now to your antenna. If you are making an antenna for the 40m band, the quarter wave going up will be 10m high - far too tall, but if you can arrange it, with a decent ground plane, you've made a pretty effective antenna. It's too long, so you shorten it, the resonant frequency goes up, and the match on your 40m gets rotten. If you wind some of the bottom section into a helical coil, this adds inductance and tricks the antenna into thinking it's longer again. Great - but of course in practice, the shorter length means its' not as good as the wave. If you want to make it work on 20m, it's too long to match, so what you can do is use a tuned circuit in the antenna itself. Something that acts a bit like an isolator at certain frequencies. It blocks some, passes others. This way you can make antennas that resonate on lots of bands.

    It gets complicated though. An antenna that is a wave for 2m, is a wave for 70cm, and a wave is also a 50 ohm match - so mathematically related frequencies can also work as multi-band antenna basics. So, if you follow this through, you have multiples of frequencies helping plus traps (the tuned circuit blocking components).

    If you take your idea - 135ft of wire twisted around a non-conductive pole, then I've no idea what frequency it will resonate on. 12-15m maybe? Pretty certain that you will need a decent antenna tuner to add L and C at the bottom to make it electrically match. The 9:1 balun. Balanced to unbalanced? Your antenna is unbalanced, so is the feeder, unless you use ribbon cable to the tuner, so it's probably not the right component for your design. You've sort of got a long wire vertical. How well will it work? No idea, but it will need excellent radials - probably lots of them, buried just beneath the surface all soldered together, and even then, the soil or whatever will be important. Banging in a ground stake is a good start, but my own experience suggests a simple cheap multi-band dipole and a tuner could work much, much better. For this type of antenna, the radials HAVE to try to be 'the sea'. They are not supposed to be part of the radiation system at all, they're meant to be the 'ground' as in the infinite reflector an antenna can work against. There's some good information about the old failure in the design of the US over the horizon radar built at Orford Ness in Suffolk, where they buried miles of copper and the attempt to make the antenna work. The old CB thing from the 70s where people attached shortened quarter waves with helical windings to biscuit tins, got good VSWR and couldn't get more than half a mile down the road come to mind. Increase the ground plane to car roof size and the same aerial got better, but stick it on the front of a fishing boat on the sea and the range zoomed up as the antenna worked so much better.

    Loads of comments here - you'll have to consider how they apply to what you're hoping will work. You've clearly got the basics, but then confused them in practice. However - ham radio is all about experimenting. So get your licence, then start experimenting with crazy ideas - but a decent tuner pays dividends.

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