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If I were to design a dipole antenna that's significantly shorter than usual for the frequency, how can I determine the best length to use? In other words, given a frequency and a maximum length (potentially much less than a quarter wavelength), how should I calculate the best length? Is the answer to simply make it as long as the maximum length allows?

I'm aware that the antenna might not perform well. I'm simply curious about how to find the best solution given those constraints.

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2 Answers 2

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Is the answer to simply make it as long as the maximum length allows?

In essence, yes.

Two reasons:

  1. $A_{eff}$ still depends on physical size; so, you're losing area from which you could be collecting field energy if you make your antenna smaller.
  2. The typical example of a short dipole uses loading with a capacitive or inductive element (depending on at what point you place it) to achieve an electrically larger length. That needs to be larger the more missing length you need to compensate. A simple consideration from the same model you use to calculate the size of the necessary load shows that the larger the load, the smaller the bandwidth becomes. You usually prefer antennas that behave well over a larger range to these that are frequency selective in the band of interest.
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  • $\begingroup$ @webmarc thanks! Yep! It's weird, but these always mix in my head $\endgroup$
    – sina bala
    Feb 2 at 22:06
  • $\begingroup$ Check into End or Capacitive Loading. Significant advantages over inductive. $\endgroup$
    – Dereck
    Feb 15 at 22:54
  • $\begingroup$ @Dereck that depends! On a lot of mechanical aspects especially in the lower frequency range, as well as on quality factor/losses in the higher frequency range. As usual in radio, there's no such thing as a free lunch :) $\endgroup$
    – sina bala
    Feb 16 at 0:41
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@Mrog, I presume you want your dipole to be resonant, otherwise you could simply use a BalUn and an antenna tuner to match to any dipole antenna configuration, that is shorter than 1/4 wave length per leg. So to do this, use this calculator:

Coil Shortened Antenna Calculator

Now while the description on the website says it calculates a coil-loaded (1/4 wave) vertical, the calculation is the same for a dipole, except that you built a mirror image of whatever you calculate for the other side; and then you have your dipole; just keep in mind these facts:

  1. Coils narrow the bandwidth of any antenna.
  2. The closer you place a coil to the antenna feed-point, the less inductance is needed to make the antenna resonant.
  3. The further you place the coil from the feed-point of a 1/2 wave dipole (or 1/4 wave ground-plane antenna for that matter), the greater the current flow along the high current section of the antenna. (which is desirable)

So as you may be able to see, there are some compromises you will have to entertain when designing your antenna.

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