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I know that in the High Frequency regions of the ham bands, the antennas have to be sub-wavelength because most people just can't put up a 160 meter full-size antenna.

Let's say we're in the 23 centimeter or an even higher band. Can the antenna lengths there be, let's say, twice, four times, etc., as large?

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    $\begingroup$ Well, length, I doubt it. Size, yes, absolutely. A high-gain antenna is much more practical at 23 cm than at 20 meters simply because the physical dimensions are smaller. Stacked dipoles can provide you with vertical gain and horizontal omnidirectionality; this is reasonably practical on two meters, and very practical on 70 cm and up. $\endgroup$ – a CVn Dec 20 '13 at 19:17
  • $\begingroup$ @MichaelKjörling precisely what orientation and stacking do you have in mind for that statement? $\endgroup$ – Phil Frost - W8II Dec 21 '13 at 12:23
  • $\begingroup$ @PhilFrost Something like e.g. this one (a 5.1 meters long vertical antenna giving you a specified 8.5 dBi on 144 MHz and 11.9 dBi on 432 MHz; the web page is in Swedish, but the data should be understandable enough and there is a picture for illustration). I've seen plenty of similar constructions. $\endgroup$ – a CVn Dec 21 '13 at 15:50
  • $\begingroup$ @Optionparty How does "Discone antenna" answer the question "is a multiple-wavelengths antenna possible"? $\endgroup$ – a CVn Mar 24 '14 at 15:15
  • $\begingroup$ @Optionparty I believe you are reading the question wrong. It's not "multiple wavelengths" as in "works equally well on a number of different frequencies", but rather "multiple wavelengths" as in "the physical size of the antenna is several wavelengths at the operating frequency". This interpretation is supported by the accepted answer being what it is. Is that the reason for your original comment? $\endgroup$ – a CVn Mar 25 '14 at 8:32
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People can, and do, put up full size 160m antennas. Usually "full size" means a 1/2 wavelength dipole, or a 1/4 wave vertical. The vertical works because the 1/4 element is "mirrored" by the ground plane, creating an image antenna, which makes it very much like a 1/2 wave dipole.

1/2 wavelength seems like a magic number because it is the minimum length required to attain resonance. Antennas can be made smaller and resonated by introducing additional, non-radiating reactive components like inductors or capacitors. However, shortening the antenna in this way makes the antenna less efficient.

Making the antenna longer than 1/2 wavelength isn't frequently done because in most cases there isn't much point to it. As the antenna first becomes longer than 1/2 wavelength, it stops being resonant. If made longer to 1.5 wavelengths, it becomes resonant again. However, it isn't any more efficient than a 1/2 wavelength antenna. In fact, it's likely less efficient, because the current must now travel over more antenna, which has more resistance.

What making the antenna longer does do is alter the radiation pattern. For example, compare a 1/2 wave dipole (in free space):

1/2 wave dipole radiation pattern

to a 1.5 wave dipole:

1.5 wave dipole radiation pattern

These are from antenna-theory.com.

Is this really useful? Usually no. One possible exception might be for verticals, where making the antenna longer (to a point, usually 5/8 wavelength) lowers the takeoff angle a bit.

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  • $\begingroup$ Not to add another answer to an old question, but I don't see longwire antennas (not beverage, multi-lambda length of wire, terminated in a resistor to ground) mentioned anywhere in this discussion. For the mentioned 23cm band, a "minimum" 4-wave longwire would be just under a meter long, and with 5-10 meters of wire one could get some really impressive gain/directionality without having to "snip and check" to tune for resonance! $\endgroup$ – Robherc KV5ROB Jan 31 '16 at 6:35
  • $\begingroup$ @RobhercKV5ROB Do longwire antennas differ significantly from long dipoles? I think in the end the effect is much the same: you get increasingly more lobes in the pattern but it doesn't "pick up more signal" (and you need an antenna tuner). $\endgroup$ – Phil Frost - W8II Jan 31 '16 at 8:38
  • $\begingroup$ It differs from a long dipole in 3 ways: 1-It's end-fed, more like a 1/4 wave "whip" than a dipole; 2-It's unidirectional, being fed from one end and terminated resistively to ground at the other gives a front-back ratio >1:1 and also makes it non resonant; 3-It's non-resonant. The non-fed end is terminated resistively to ground, so no resonating open end & no requirement for a matching network/antenna tuner (though an impedance-matching balun/unun might be called for if your termination resistor doesn't equal xmitter impedance or you use a balanced transmission line) $\endgroup$ – Robherc KV5ROB Jan 31 '16 at 10:51
  • $\begingroup$ I'm confused. Earlier you said "not beverage", but now what you are describing sounds like a Beverage antenna. $\endgroup$ – Phil Frost - W8II Jan 31 '16 at 11:13
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    $\begingroup$ @ KV5ROB- I did a long wire antenna for receive on UHF TV (ch 57 analog) years ago before I became a ham....directional is putting it politely. So directional that a gust of wind would make it go from full signal to just raster. If Memory serves it was roughly 15/4 wave long. $\endgroup$ – Old_Fossil Jul 3 '17 at 23:12
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Yes, you could create, instead of a 1/2 wave dipole, a full wave dipole. They get very directional after you pass full-wave in length as shown in graphs at http://www.antenna-theory.com/antennas/dipole.php.

However, most folks instead use a collinear dipole array of multiple dipoles stacked, or side-by-side, or both, for gain.

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You may be asking about what is known as a “log-periodic antenna.”

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    $\begingroup$ Could you say more about how a log-periodic antenna is relevant? As the article says, a LPDA consists of a number of half-wave dipole elements, not longer ones. Is there a type of log-periodic antenna that has elements larger than 1/2 wavelength? $\endgroup$ – Kevin Reid AG6YO Feb 2 '16 at 16:23

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