[TowerTalk] [Bulk] Mosley (Antenna Gain)

[Jim Lux]

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.