Minimum length of a non-resonant multi-band dipole

Considering a centre fed non-resonant (or not necessarily resonant) dipole for multi-band use, the feed point impedance will depend on many things, but mainly length, frequency, and to a lesser extent height above ground.

Feedline losses will be prohibitive using coax of any substantial length, so the practical choice for most hams will be some form of open wire line. The open wire line will have standing waves, so the impedance looking into the line will vary mainly with load impedance, frequency and line length (amongst other things). Feedline loss under mismatch depends on load impedance, frequency, line length and line parameters.

The nominally balanced antenna and open wire line will need adaption to the unbalanced output of most modern transmitters, and impedance transformation will usually be needed to provide the required load impedance for the transmitter. This function can be performed with an balun / ATU combination. ATU losses depend on the load impedance. For voltage baluns, loss depends mainly on differential load impedance, for current baluns, mainly on common mode current.

So, when you bring all this together, it turns out that for practical antenna systems of this type, for dipole lengths more than about 40% of a wavelength, system efficiency (the ratio of power radiated to transmitter output power) is quite good (>70% typically, depending on line type, length, ATU, balun etc).

As you shorten the length of the dipole, losses (principally in the line, ATU (and voltage balun)) increase, fairly rapidly,

Note, these numbers are for a practical scenario, but the numbers for each scenario depend on actual dipole length, height, line length, line type, balun, ATU etc.

What is good or bad, what ‘works’ and what doesn’t? Well, that depends on your own criteria. They all work, they are all good, so long as your criteria are fairly relaxed. But, if you want to be heard, better is the goal, albeit one of diminishing returns.

Unless you model your own scenario in some detail, it is probably a good idea to strive for a dipole length of 35% of a wavelength or longer.

Some examples:

The figures worked up above assume 25m of low loss open wire line, a practical L match and a low loss current balun. Common ladder line is lossier, longer line is lossier, T match ATUs will usually have worse losses, and voltage baluns are likely to have severe losses when employed in the region of voltage maxima with this type of antenna. Note that most commercial ATUs are T matches, and 4:1 voltage baluns are widely represented, especially those integrated in ATUs.

© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.