[TowerTalk] Stacking 204BA's

[Jim Lux]

At 08:06 PM 3/7/2007, Bob Selbrede, K6ZZ wrote:
>I'm not sure if my version of YO (YO5) will do the job as
>described below.  I've done the HFTA analysis and the
>elevation diversity attainable with stacked 20M beams at
>100' and 50' is pretty substantial regardless of the type of
>antenna used (204BA, 205CA, KLM 20M6, etc).  For all
>practical purposes my local terrain is flat (+/- 100') for
>3-4 miles in every direction.
>
>Therefore, I'm down to how to optimize the 204BA's for this
>application.  YO5 allows the user to model yagis
>individually over real ground or in free space.  Both the
>W6QHS dimensions and the N6BV version (BV204CA) look pretty
>good individually.  However, when you model a stack of two
>yagis with YO5 it only does it in free space.  When doing so
>the results (if believable) do not look so hot for the N6BV
>variant.  The F/B goes from better than 20 dB across the
>band to around 12 dB at the lower part of the band.  The
>W6QHS and original Hy-Gain dimensions do not appear to be as
>bad.

This is where plopping the model into another program can be useful 
to see if you've got some weird failure mode in the modeling code.

Are you looking at the F/B of the pair, or the F/B of one with the 
other one disconnected?
So you've basically got a couple antennas with decent F/B that are 50 
feet apart (almost a wavelength).  At that kind of distance there 
shouldn't be a heck of a lot of interaction. I would check things in 
your model like how the feed system is modeled (i.e transmission 
lines, termination impedance, if any, etc.)

20dB F/B means that the power in the Back direction is 1% of that in 
the Forward direction.  12 dB means the backwards power is about 6% 
of the forward. High directivity systems rely on the currents in the 
elements all "cancelling" in the undesired directions, so a drop in 
F/B would imply that a current is wrong by enough to degrade the 
cancelling by that much. Radiated power goes roughly as square of 
current in the element (neglecting IR and mutual coupling), so, if 
the effect is "real" (and not just a model artifact), look for 
something carrying significantly different current.

Jim, W6RMK