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I have put this page on for any newcomer to HF who may be a bit confused regarding the best way to feed a HF antenna, especially a multiband one. Working with 50 ohm antennas and 50 ohm transmission line is pretty much straight forward and is probably the most common way of setting antennas up on the vhf/uhf frequencies when working FM. Most of the antennas used at these frequencies are pre tuned out the box and only require that you mount them high and feed with 50 ohm coax at whatever length is convienient, and require no matching or impedence transformation. The 2/70 FM bands are what many newcomers will cut there teeth on and many get confused when setting a hf antenna up as they are moving away from matched systems to systems that may or may not be balanced and may need matching sections and in the case of multiband antennas will almost certainly need an atu.
Above is a typical way of feeding a mulitband antenna
Firstly i feel i need to ad this as many newcomers simply don't know and this may help some understand the reasons for the way things are done. Firstly, the radio output ( SO239) is whats called an unbalanced socket. Coax is an unbalanced feedline. Most vertical antennas are unbalanced . So if you have an antenna that is 50 ohms unbalanced and you feed with 50 ohm unbalanced coax to the unbalanced socket on the radio then this is called a matched system. Dipole antennas are called balanced antennas, open wire feeder,ladderline is also balanced and in some antenna systems you may need to transform the balanced side of things to an unbalanced one before you get to the radio, which is unbalanced (see baluns). Ladderline is used in many multiband systems because of its very low loss when compared to coax .The impedence of the ladderline isn't critical in these systems unless they are being used for matching stubs ect.Below are three common feeders. the top example is 450 ohm ladderline with 300 ohm in the middle. The bottom is 50 ohm coax.
An ATU (antenna tuning unit) or AMU (antenna matching unit) as some like to call them DOES NOT tune your antenna. It simply shows your radio a good load to work into(50 ohms).That is all,nothing more.What happens after the ATU on its way to the antenna is still very important. In the above picture you will see a typical set up for a mulitband system. Let's imagine the antenna (not shown) is showing a feedpoint impedence of 200 ohms (vswr 4.1). This is no good for the radio which wants to "see" 50 ohms. The atu will tune out any mismatch and let the radio "see" the 50 ohm it wants. The balun will give the transition from a balanced antenna and feedline to the unbalanced side of the system.The use of ladderline is to reduce any losses due to the mismatches accociated with multiband systems. If using coax in the above set up the losses due to mismatch would be a lot greater than when using ladderline.(the balun would be at the antenna feedpoint if coax fed)
Mono band use
It's possible to use a resonant monoband HF antenna fed with coax and no atu(see dipole). Although its quite possible to feed a dipole with coax it is also possible to feed with ladderline for even lower feedline losses, although i doubt anyone would tell the differance between a resonant dipole fed with coax and that of one fed with ladderline unless the run was a long one. Although a dipole at resonance has an impedence of around 70 ohms,at the hights most people can mount them at it's more likely to be around the 50 ohm mark making it possible to feed with coax, or a halfwave of ladderline (or multiple of) which will simply duplicate the antenna feedpoint impedence regardless of the characteristic impedence of the ladderline (300-450 ohm ect).I have only touched the very tip of this subject but hope it has given the reader especially if new to the hobby a little better understanding.
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