[TowerTalk] Stacking question

[Ed Sawyer]

I could be wrong, but I believe that HFTA adds 3dB for stacked antennas and
uses the center point of the stack to do its calculation.  I base that on
the fact that you can stack 2 yagis at the same point (ie zero ft
separation) and it will show a 3db gain vs the one yagi alone.  We all know
that doesn't play out in real life.

 

You can replace a stack of 2 yagis with a single larger yagi at the midpoint
and see that they are the equivalent if the single larger yagi is 3dB larger
gain than the 2 yagis being stacked.

 

HFTA tells you where to put antennas based on terrain and desired angle.  It
doesn't tell you how to achieve the assumed gain of the selected antenna.

 

You may find that ideal stacking distance puts the combined lobe at the
wrong height.  The 1dB or so of stacking variance for a normal range of
stacking distances me be quickly overtaken by the right height of the
combined effective takeoff angle.

 

What I have found useful with HFTA is to put dipoles every 5 ft up the tower
and see where the lobes max out in the desired angle.  WHATEVER you put
there (or combines to look like its coming from there) is the best antenna
height choice.  Large individual yagi or stack center point.  The stack
choices are obviously the real world best system but the max combined stack
gain is not necessarily the right spacing for your location and desires.

 

Ed  N1UR