[TowerTalk] stacking distances

[Tom Rauch]

> You have to be careful with YT, using stacks with close spacing.
> If you plot a stack of two antennas even one foot apart,
> YT will still show a 3 dB gain over a single antenna.

Very interesting. It is amazing what is in those manuals
that I, like most people, never bother reading. 

Anyway, that's why I like the new Eznec with it's average gain! You 
can look to see if average gain exceeds 0dB as an automatic red 
flag.

A similar effect is common when modeling receiving antennas 
(which actually are modelled as transmitting antennas by the 
program, of course). If you model two close-spaced Beverages they 
show 3dB gain even though there really is no pattern gain. That 
misleads people into thinking they are making things better when 
nothing is changing!

One common-sense test for modelling is to remember that a pair of 
dipoles stacked in freespace only has about 3dB gain when 
spacing is 5/8 wl or so. The tighter the pattern of each antenna 
alone, the further apart they have to be stacked to get the same 3 
dB gain increase.

If you see 3dB gain stacking gain with 1/2 wl spacing using large 
yagis, you better look carefully at the model for errors!

This of course is only talking about maximum gain, and not null 
filling.

  
73, Tom W8JI
W8JI@contesting.com 

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