Re: [TowerTalk] Funniest thing I've seen in weeks

[Jim Lux <jimlux@earthlink.net>]

At 06:12 PM 6/30/2004 -0400, Tom Rauch wrote:
> > This would be true for ALL arrays.. the performance is
> proportional to the
> > aperture.  Yagi-Udas just happen to be end fire arrays
> where the aperture
> > is in the same direction as the preferred propagation
> direction.  There are
> > some subtleties in superdirective arrays, but, as you say,
> directivity/gain
>
> The electrical aperture or performance is NOT defined by the
> physical area an antenna occupies.

Sort of true.

Performance IS proportional to effective aperture, and for simple, 
moderately efficient antennas, the effective aperture IS proportional to 
the physical extent.  Performance was (deliberately) left undefined, as well.

Physical extent determines the resolution of the antenna (that is, the 
smallest beam that can be formed).

Effective aperture determines the maximum "gain" of the antenna (that is, 
the amount of energy it can extract from the wavefront)

The Rohmbic and longwire are relatively inefficient antennas, so the 
physical extent is much larger than the effective aperture (but, they can 
have very high resolution or narrow beams.... lots of directivity, not much 
gain)

The real challenge of course (and one that is the subject of much 
theoretical work) is going the other direction.  how do you get a physical 
extent SMALLER than the effective aperture.  Superdirective arrays are one 
way, but the large reactive power stored in the system makes the Q high and 
raises practicality problems.  (Thiele published an interesting paper on 
this last year (2003), which I'm just now reading)



> The electrical aperture is defined only by gain and
> frequency, that's why it is more correctly called the
> *effective* aperture.

Where gain = directivity - loss



> For example, a Rhombic occupying an area of several
> wavelengths with 10dBd gain has the same effective aperture
> as a 10dBd Yagi occupying less than a wavelength on the same
> frequency.
>
> A longwire 20 wavelengths long can have less effective
> aperture than a dipole.  When you throw a resistor in a
> given antenna, the electrical aperture decreases.

But both of those can have directivity substantially better than that 12dBi 
Yagi.


> I've noticed over the years many people think physical area
> makes an antenna more effective at receiving. The verbiage
> is something to the effect of  "it's a darned good receiving
> antenna because it has a large capture area". That would
> only be true for size if you are talking about the antenna
> catching the wind, not radio signals. The capture area or
> effective aperture not only NOT determined by physical size,
> gain or effective aperture does not determine receiving S/N
> at HF.


Only because in HF, the received sky and atmospheric noise is the limit.  A 
bigger effective aperture captures more noise, as well as more signal.  On 
UHF and higher, where receiver and scene noise dominate, bigger is 
certainly better.




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