Terry Settergren wrote:
> Hi, I can't find many references to this- could an HF antenna, say a
> vertical, use a shroud of rf-absorbent material to achieve receive
> (and transmit?) directionality? What materials, and how close to the
> element could they be? I have seen tubular rf absorbent materials
> that appear to be intended to cover a cable, and I keep imagining a
> rotatable vertical surrounded by a "slotted radome" (pipe), filled
> with rf absorbent material, with a vertical directional slot exposed.
> Durable, stealthy, receive directionality, maybe? But I'm ignorant of
> the effects of the absorbent material near an antenna element.
>
> Regards, KE5FGX
Unfortunately not (unless the tube is really, really big, as in many
wavelengths)
As to exactly why this doesn't work... I'm sort of struggling for a
simple example/explanation, but it has to do with the fact that the
energy stored in the fields around the antenna is as important in
determining the radiation pattern as the actual antenna.
The other problem is that Babinet's principle says that a slot in a
infinite conductive plane is the same as a conductive thing the same
shape, in an infinite insulator. Ultimately, the slot is a good an
antenna as a wire (why shielding boxes is tough.. the gap between cover
and box is a slot). In fact, slot radiators are widely used in microwaves.
It's true that if your absorber were carefully made of "space cloth"
(something that has characteristic impedance of 377 ohms/square, but is
lossy), you wouldn't have the slot problem, but you'd then have the
problem that the radiation escaping from the slot is the same as having
an antenna the same physical size as the slot (Huygen's principle), so
it wouldn't necessarily be any more directive than the antenna inside.
But you've raised an obvious and interesting question to which I can't
think of a simple answer. I'll have to send it to some of the undergrad
EM professors I know and let them churn on it.
Jim, W6RMK
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