|To:||Tom Rauch <email@example.com>|
|Subject:||Re: [TowerTalk] Funniest thing I've seen in weeks|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Wed, 30 Jun 2004 16:29:34 -0700|
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
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.
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.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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