[TowerTalk] Stacking Problem

[Pete Smith]

At 04:54 PM 2/15/04 +0000, David Robbins K1TTT wrote:
>I am not surprised.  The added gain when running both is not the big
>reason for stacking on hf. though you do get some small gain with both,
>the big reason is to fill in gaps in the takeoff angle pattern.  On 40m
>at 150' you may notice this if you happen to find a station who is much
>weaker on one than the other, but when running both he is almost as loud
>as on the single antenna that was the strongest before... that is the
>real test that stacking is working, but it takes a lot of comparisons to
>find a perfect example because of all the variables.

The only things I would add are --

It's unlikely that the RCS-4 is any part of the "problem."

It's not at all surprising that you can't hear any particular difference 
between the top antenna and the stack.  I modeled the stack versus the top 
antenna over flat terrain with HFTA -- from 2 to 8 or 10 degrees takeoff 
angle, the traces are virtually identical.  At peak gain, the stack 
registers 14.4 dBi at 15-16 degrees takeoff angle, while the top antenna 
shows 12.8 dBi at 13-14 degrees.  Both peaks are a little high for best 
coverage of the statistical distribution of signals from Europe, but the 
real point is that the difference in signal strength is probably too small 
to hear on most all signals.

The dramatic difference in the modeled patterns is in the vertical breadth 
of the first lobe.  The single antenna has 11 dBi gain or better only from 
8-18 degrees takeoff, while the stack extends that out to 8-25 degrees. 
Statistically, to Europe again, this should be trivial, because the 
percentage of signals that arrive at angles above 18 degrees is very 
small.  However, the real ionosphere often supports multiple propagation 
modes, some perhaps at higher angles than modeling would predict, and the 
mix can change rapidly.   Adding of these various modes at the receiving 
array could account for some of the benefit from stacks, particularly in 
resistance to fading.

In my experience on the east coast, I almost never hear a difference 
between my top antenna and my full tribander stack, though fading does 
usually seem to be less with the stack.  On the other hand, there are times 
when the bottom antenna is distinctly better, particularly in the middle of 
a European opening.

73, Pete N4ZR
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