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Originally Posted by Mike. Watterson
A folded dipole is efficient if the size 1/2 wave overall. Such an aerial curved to be a "halo" type is very inefficient.
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Almost any metal antenna of sufficient size has efficiency approaching 100%. The halo has low gain because it is approximately omnidirectional but it is quite efficient. Maybe you use the term "efficient" to mean something other than power efficiency?
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A Yagi, dish or some Log periodics can have apparent high efficiency because the beamwidth is narrower.
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Narrow beamwidth means high gain, not high efficiency. The efficiency is probably high anyway, as I said above.
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The halo is evil for VHF radio as it has negative gain and for omni, a vertical whip is best.
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The halo was commonly used in the past for 2m amateur mobile operation because in the days before channelisation most fixed stations were horizontally polarised.
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A whip needs a ground plane, so on a mast it needs earthed radials.
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It needs radials or a ground.
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As the aerial gets shorter than 1/4 the aperture is less, thus a 1/40th wave (10m long on MW) will pick up very much less radio signal.
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Aperture relates to gain, which in turn relates to directivity. The gain of a halfwave dipole is 2.15dBi. The gain of an infinitesimal dipole is 1.5dBi. Something in between will have a gain and therefore an aperture somewhere in between. The problem with a short antenna is matching, not gain/aperture.
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It will not pickup much less interference that is very local, E near field).
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Local E field pickup will probably vary something like antenna self-capacity, which reduces as it gets shorter.
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Putting more than 1/10th wave of coax or dual feeder between the aerial and an ATU at the radio can't match the aerial properly.
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That would depend on the matching range of the matcher, and the impedance presented by the antenna. There is no fundamental problem with putting some feeder in the way.