At 08:28 AM 7/19/2005, Richard (Rick) Karlquist (N6RK) wrote:
> > With close elements you can design an antenna to have increased bandwidth.
> >
> > John KK9A
>
>That's news to me. I always thought
>wide spaced Yagi's have greater bandwidth.
>
>Rick N6RK
>
Always a tradeoff.. Size, Bandwidth, Gain, Efficiency. I suspect that a
physically large antenna (widely spaced) might let you have a bit more
design freedom (more solutions that achieve a given gain, bandwidth, or
efficiency). Also, widely spaced elements tend to have less mutual
coupling, which in turn makes the design less sensitive to small changes.
Interestingly, it's also well known that superdirective antennas relying on
passive coupling among the elements(which most ham beams are) have a
certain maximum size. Get much bigger and the coupling drops off, so you
can't get the excitation current you need in the elements. So if you plot
gain vs length, you get a hump at the bottom end, but in the limit, it
converges back to a straight line that tapers off to flat eventually.
Allow "non-Foster" connections to the element feedpoints (i.e. elements
like Gyrators with negative inductance) and the relationship's not quite so
fixed, and practical implementations (often called Negative Impedance
Converters or NICs) of these consume power, so the overall efficiency of
the "system" is poor.
A recent paper by R.C. Hansen:
Wideband dipole arrays using non-Foster coupling
Microwave and Optical Technology Letters, Volume 38, Issue 6
A planar array of dipoles, with collinear dipoles connected by lumped
negative inductances, is simulated by the piecewise sinusoidal method of
moments. Over a multi-octave bandwidth, the gain is close to the area gain,
which is a remarkable result. The non-Foster couplings smooth out and
increase the current at low frequencies. Recent advances in transistor
circuits and topology allow this new array to be considered.
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