CENTRE FED MULTIBAND HF DIPOLES - SHORT VERSION
EASY AND SIMPLE HF WIRE ANTENNAS
If you want to read a whole lot of theoretical stuff - try here.
There are plenty of radio websites with piles of rubbish on them including this one. There are detailed analyses of all different lengths and they are simply a waste of time. If you have the appropriate tuner, a paper clip will radiate pretty much the the same power as a tin fence or any other conductor. See quick theory of operation below if you really need to.
Some will maintain an exact resonant length is required to radiate any effective signal. They say anything else is turned into heat whether you have a tuner or not. BOLLOX. I will be the first to admit that an exact resonant length (under the right conditions) can be the most efficient because the radiating resistance is at its lowest BUT that doesn't mean signals with only extremely small losses can't be radiated at any length. Put quite simply, changing the length away from resonance changes the radiating resistance. A tuner modifies the voltage (and therefore current) into that radiating resistance thereby modifying the power radiated. Again - See quick theory of operation below.
GO FOR IT
1) Find some place to put and work out how long you can make the radiators and allow half a metre each end for insulation. Make the wires as long as you can.
2) Make up some sort of centre insulator (ideas below) and connect some balanced feeder to the centre. DO NOT USE COAX BETWEEN A TUNER AND AN ANTENNA. It loses too much at high SWRs.
3) String it up between the shed and the tree (or whatever) using a length of fishing line, "whipper snipper" cord or similar to pull it up so you can let it down again anytime. Put it as high up as you can.
4) Next to the radio, connect to a balanced coupler of some kind (tuner and balun or whatever) and try to get a tune especially on the bands which are an even multiple of the others eg. 40m, 20 and 10m. 10m is the best place to start. If the system tunes on 10m it will probably tune on 20m and 40m as well. If it doesn't, try 15m. If neither 20m nor 15m will tune at all, you have something else wrong. If you can't get a suitable tune on all bands, add or subtract a metre of feeder and try again. The length of feeder is only irrelevant when the transmission line is properly matched to the load and, in this case, it isn't.
ALL DONE, NO FUSSING AROUND, NO CRAZY DESCRIPTIONS.
A FEW EXTRA IDEAS
For the ends, good, thick fishing line works OK. It sheds water quickly. The line used for lawn edge trimmers or "whipper snippers" is also great stuff. A simple knot in the ends will soon take shape and hold under a lot of strain.
Antennas work better than others because someone made them (even my own). I have tried an off-center fed dipole and found it noisy. I have tried end-fed but that has to work against ground and is also VERY noisy. I found a properlly and fully balanced system easiest to errect and has the lowest noise level. Why not try others for yourself and see if you agree?
Some say solid wire is no good and multi-strand is better because it is more flexible. I have had 1mm enamelled copper wire stretched up for ages. It presents the least resistance to the wind for its strength. If it ever breaks I'll join it back together. I have had my current HF antenna up for only six months. Before that I had one on Kangaroo Island, a very windy place, on the top of a hill for 10 years. If you don't like that idea, fine, use power cable if you like. The principles stated above are the same.
Any balanced line for a non-resonant antenna is better than coax. Some say TV ribbon is lousy, some say not. I am probaly better off with ladder (or "window") line but TV ribbon works fine for me. It can't radiate because of the opposing signals. Provided common mode currents are kept to a minimum, even with disgustingly high SWRs, the currents oppose each other and are virtually lossless.
For the spreader in the middle, I used a length of PCB etched in the centre to allow enough of a gap to run max power plus a bit. Fibreglass material is incredibly strong. It also sheds water fairly quickly. The feeder can be folded over and held in place with a wire tie. Here are two examples of using PCB material for the centre tap.
THEORY OF OPERATION
The secret is all in low loss transmission lines (of some sort) and a decent tuner. A properly resonant dipole without a tuner is always preferable. There is an insertion loss associated with extra plugs, sockets and wire in a tuner. There are also ohmic losses in the transmission line and the wire the antenna is made out of. These remain fairly constant but the further the radiating resistance is from the driven resonant resistance, the more significant all these other losses become.
A good tuner consumes very little power. You must use balanced line between a tuner and an antenna. Coax loses too much about 1.5:1 SWR while balanced line doesn't until you hit some high value like 30:1. I know of one fellow who used car speaker wire - no problem. The characteristic impedance was about 50Ω but even that is irrelevant once you use a tuner.
The theory of operation is as follows,
Radiated power = radio output - losses
WHAT DOES A TUNER DO
Any difference from resonant length or any mismatching of the line to the load creates reactance. All a tuner does is balance that reactance out to make the system as a whole resistive (resonant).
SEE THE FOLLOWING LINKS FOR MORE DETAIL ON RESONANCE
Driven and natural resonance.
Series and parallel resonance.
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I think that's pretty fair, don't you?