Add capacitor to unload antenna?

Question

So add inductor coil(loading coil) is to cancel out capacitive reactance to make the antenna electrically longer. But what if a capacitor is placed on the antenna, to make the antenna electrically shorter, would that increase the efficiency antenna due to its increase "radiating length" ?

For example, what it will like if you add a variable capacitor to a 20m whip, shorten it to match 10m? or even 2m?

The opposite. (Small/medium) Capacitive hats make an antenna act longer (lowers the frequency of SWR match for a given length) — hotpaw2, Jun 17 '19 at 10:58
But importantly, the capacitive hat is placed at the opposite end of where an inductor would be. — Phil Frost - W8II, Jun 17 '19 at 13:58

score 2 · Answer 1 · answered Jun 17 '19 at 11:35

Verticals can use a capacitive hat and/or a loading coil. Small loops use a tuning capacitor.

"length" is not really correct, that's an oversimplification. It's more like phase of the complex reactance.

You aren't really making it shorter or longer, you're trying to get the complex portion of the reactance to be zero, and either an inductor or a capacitor can do that, but one or the other might be able to do it with a smaller value at a particular location on the antenna. Some antennas use both in different places.

score 2 · Answer 2 · answered Jun 17 '19 at 13:55

Fascinating question! I used NEC 2 to assess the gain change of a dipole in free space with capacitive center loading.

The quasi-sinusoidal current distribution on a resonant 20 meter dipole produces the expected 2.0dBi maximum gain:

Adding 10pF capacitors immediately on each side of the feedpoint raises the resonant frequency to 23.6MHz, raises feedpoint impedance to 734$\Omega$ and results in a current profile that produces a 1dB maximum gain increase to 3.0dBi:

The impact this has on efficiency will depend on the loss characteristics of the rest of the antenna system.

Great study. To answer the original question, what happens if you now extend the dipole to make it resonant again, which might be about 50% longer than its original length? If you end up at more than a full wavelength, you should probably increase the capacitor value a bit. I hope it will work, perhaps with an impedance higher than 73... — tomnexus, Jun 19 '19 at 16:42

score 2 · Accepted Answer · answered Jun 19 '19 at 14:59

Changing the length of a dipole changes the radiation pattern. As the dipole gets much longer than 0.5 wavelength, the radiation pattern grows multiple lobes. Here's an animation from 0.1 to 5 wavelengths:

But the efficiency of the antenna isn't just a question of length. Consider conservation of energy: if energy isn't going into some sort of loss (usually being converted to heat) then it must be radiated. A dipole that is far from a resonant length (too long or to short) has a very reactive feedpoint impedance, which must necessarily be accompanied by some sort of matching network. This matching network will have loss, to the detriment of the overall efficiency of the antenna system.

Add capacitor to unload antenna?

3 Answers3