[TowerTalk] 80m yagi

[Jim Lux]

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.