On 2/20/15 9:24 AM, Grant Saviers wrote:
An NEC4 analysis shows a 5+5 20m stack beats a dipole at the same max
height (125') in absolute gain by 10db (7.6dbi to 17.6dbi). This 5+5 owa
is <1.25:1 swr band edge to edge. The dipole has several -1 to -3db
high angle lobes to pick up local QRM/QRN so the stack has another big
advantage for working DX. There are many bigger stacks out there that
do better.
I would say that the vast majority of single band 3 element
non-short-boom Yagis are in the 6-8 dBi range. The differences are
primarily in mechanical design, and feed-point bandwidth (I doubt the
gain changes all that much across the band.. maybe a few tenths of a
dB). Adding elements (while keeping the length the same) improves the
bandwidth, in general, and the gain (to a lesser degree, maybe 1-2 dB)
your modeled 10dB jump from dipole to 2 stacked Yagis is consistent with
this.. the dipole is 2.15 dBi, the stack should be something like
10-12dBi (assuming you pick up 3dB for running two of them and the 5 el
is 1-2 dB over the 3 el)
Re the other assertions, you may want to check out what designers using
computer analysis have produced for very good multiband interlaced
antenna designs. These interlaced designs tend to be on shorter booms
because it is easier to do, and that is where the market is. Putting 5
bands on one 48' boom has been done, but it is pretty complex.
The one thing that computer analysis lets you do fairly easily is to
assess the design sensitivity to small changes in dimensions or element
position. I think there were a number of "empirical" designs in the
past that were quite good, but very picky, so that if they got slighly
bent, or the wind moved the elements, or it was misassembled, you didn't
get the performance expected. I, like a lot of hams, look at antennas as
I drive around, and I'd say that about 20-30% of the beam antennas I see
have the elements not in the same plane.
Today, it's pretty straightforward for a designer to grind through
several hundred iterations in an hour to really know whether the element
droop will make any difference, etc.
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