|To:||"Al Williams" <firstname.lastname@example.org>, <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 11:03:48 -0700|
At 08:41 AM 6/30/2004 -0700, Al Williams wrote:|
>----- Original Message ----- >From: "W0UN -- John Brosnahan" <email@example.com>
I'm going to wing it here...
Probably it derives from earlier antenna design techniques relying on empiricism and intuition. Consider the classic curtain array with a reflecting screen behind it.
Then, you'd go to two element arrays with a passive reflector behind the driven element (just because, conceptually, it doesn't look like there would be any "blockage" from having a shorter element in front). I'm pretty sure that a dipole in front of a single reflector was developed empirically, with the optimum length for the reflector determined by trial and error as opposed to analysis.
Yagi and Uda did their work long before all the basic analytical techniques we use now were developed. (the Yagi paper is 1928, Uda's in 1927) I'm sure that their design was an outgrowth of experimental methods, and with the term reflector already established, the addition of the term director was a natural extension.
A fascinating paper by G. Sato, in IEEE Ant & Prop Magazine, June 1991, talks about Yagi antennas and their history.
Carter published his paper on circuit relations in 1932 (which started the idea of considering multielement antennas as coupled radiators in a circuit)
The Hallen paper (upon which most of the modern analytical techniques for mutual impedance are based) was in 1938.
Hansen and Woodyard did their paper on superdirective arrays in 1938 (Most all amateur beams are a case of a superdirective array), but they were concerned more with "all driven" arrays (such as used in broadcast or radar)
Schellkunoff published his paper on arrays in 1943.
Walkinshaw did one of the first analytical papers on Yagis in 1946 (for special cases 4 directors), Green published a paper in 1966 giving experimental designs for some cases; and even as late as 1969, Thiele published his paper on Analysis of Yagi-Uda-Type Antennas, which probably was the first real "computer optimization" of these sorts of antennas (on a 7094, which had enough memory for 27 elements!)
You're right, though, they all reradiate because of mutual coupling, and there's no particular reason why you couldn't for instance, make an antenna with 3 reflectors and a driven element. Of course, you have to consider other aspects of optimization: minimizing mass and size (and cost of aluminum, and structural support, etc.).. this tends to drive you to one element behind and many in front.
There may be advantages to multidirector designs in multiband antennas too.
You've also got to consider marketing. People expect to see one reflector and many directors, and if you were to design and market something different, you'd have to do a lot of explaining to sell it. Consider, for instance, a design like the Force 12 C3SS, where the driven element is somewhat tricky, with the 10 and 15 meter driven elements excited by the 20 meter element, rather than a direct wired connection.
I'll comment that designing (and optimizing) an antenna design like the C3SS would probably be impractical without a computer to do the analysis, which is why you don't see similar designs from the 40s and 50s or even 60s. In that era, you'd be doing your analysis with analytical means, and you'd naturally gravitate to a trapped design for multiband use, because the analysis is at least tractable. Even an "interleaved" design where you've essentially got multiple monoband arrays on the same boom would be a nightmare to analyze without a computer.
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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