Fwd: [VHF] Close Vertical stacking distances [was: Horizontal stacking distance question]

[David McKenzie]

Interesting reading re: stacking for those of you not on the VHF list

---------- Forwarded message ----------
From: Ev Tupis <w2ev at yahoo.com>
Date: Sun, Dec 19, 2010 at 7:02 AM
Subject: [VHF] Close Vertical stacking distances [was: Horizontal stacking
distance question]
To: Stanford VHF email Remailer <VHF at w6yx.stanford.edu>


I've taken the liberty of re-presenting Jay's informative plots.  Thanks to
Jay,
we can study data rather than shooting from the hip.

I can clearly see that the text attributed to Kent's observation held true:

------ Original Message ----

> > Kent WA5VJB did some experiments that were presented at Central States
 this
> > year. He found that when single Yagis for different bands were  stacked
on
> > one mast, they could be practically on top of one another (as  little as
5
> > inches apart, I think) and had virtually no effect on one  another. When
two
> > Yagis for one band were used on the same mast, this  did not hold true.
----------------------------

The "purist" will say "I told you that there would be interaction!"  And, of
course there is.  The "practicalist" will say, "so what? it is so
insignificant
I don't care."

See what I mean, below...

I've restated Jay's post a little differently below...

Note: "ascii figures" are NOT to scale and are left-justified. The
assumption is
that all antennas are centered on the mast instead.  I'm including this as a
visual aid only.

Antenna Scenario #1

http://s509.photobucket.com/albums/s339/K0GU/K0GU%20MISC/?action=view&current=222-5wl.jpg


The stack lays out like this:

-----------------------  (222-5wl w/23' boom)
 two feet
 spacing
------------------------------- (17b2 w/31' boom)
 two feet
 spacing
------------------------------- (6m7jhv w/31' boom)

Jay's modeling shows a slight "upward" skew to the 222 beam's main elevation
pattern.  However this is only when compared to the same pattern in "free
space"
w/o any ground interaction at all.  Even so, the difference is only
fractions of
a dB between the 222 antenna in free space vs within 24" of a 17b2 "ground".

Overall 222 MHz gain in elevation is -0.18 dB vs. free space
Overall 222 MHz gain in azimuth is   +0.04 dB vs. free space

Mast needed above the tower: 4'

Of significant note:  The wind survivability of a 4' mast is greater than
that
of a mast that is long enough to conform to "perfect" stacking distances for
these antennas.

Cheers,
Ev, W2EV
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