Further to that, if you desire, as I might, to have more radiation in one
direction, eg from here to North America, I would have one elevated radial
pointing that way and the radiation pattern tilts that way a little.
One further point not brought out so far: the antenna itself does not have
to be resonant; as long as the impedance is transformed to match the tx, it
will accept power and radiate, minus the transformation and system losses.
Since ocf has been mentioned a few times: I've seen VNA sweeps of ocf
aerials in which the vswr dips nicely in band but the resonance as shown by
X passing thro zero is often nowhere near.
David
G3UNA
IMPT POINT: If you want your antenna actually to have a vertical
radiation pattern (low angle, omni-directional) then the elevated radials
must be symmetric. If there are two, they must be of exactly equal length
and point in exactly opposite directions. If there are four, they must be
equal and point in directions 90 degrees from one another. This symmetry
guarantees that the "radials" do not radiate. The do carry currents, they
do help resonate the antenna, but if they are symmetric radiation from
them cancels and they therefore do not radiate appreciably.
73,
Oliver
W6ODJ
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