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

[Jim Lux <jimlux@earthlink.net>]

At 05:44 PM 7/1/2004 -0400, doc wrote:
> > I should think that if you optimized the antenna, there wouldn't be a 
> > significant difference between feeding at the back or the front.
> > Feeding in the middle is always going to be better.  In fact, doing a 
> > quick optimization, I get F/B ratios within 0.5 dB and gains within 0.5dB 
> > for feeding either the front or the back element
> > > 73, ... Joe, K4IK
>
> If I recall from a friend who was optimizing 6M beams he said that
> other than the initial gain/pattern value of the first reflector the
> second best value of the reflector was keeping signals "off the back"
> to a minimum.  I think he added a second to optimize that effect and
> then added a couple of directors to end up with a very narrow focal
> point in his pattern.
>
> I am guessing that no reflector and all directors would make for a
> gain antenna with poor FB ratio.
>
> I'd guess that all reflectors and no directors would have a broad
> pattern off the front and severe restriction of signals off the back.
>
> Are your software runs not showing that to be so?


Indeed they are not.. the pattern looks pretty much the same.. The 
differences are more attributable to the different magnitudes of the 
current (aperture illumination taper in antenna speak).

All directors with the driven element at the back should be able to produce 
almost as good a F/B as all reflectors with the driven element at the 
front.  Both have the problem of feeding the power the length of the array.

There's no particular reason why feeding at one end or the other (i.e. all 
directors or all reflectors) would cause the pattern to be broader or 
sharper in that direction, other than the "weighting" of the currents in 
the elements.  For instance, a log periodic (which has all elements driven) 
doesn't really care which end it's driven at (aside from some matching issues).

There is probably some effect on REAL antennas because of the matching and 
practicality issues. In a real antenna design you're optimizing not only 
pattern, but SWR bandwidth and cost, both of which favor feeding an element 
in the middle.  Cost and wind load kinds of things might favor directors 
over reflectors, all other things being equal.

I'm going to set up a series of models and see if I can come up with a more 
consistent explanation.


Excellent suggestions though... This is turning out to be quite interesting. 

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