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

David McKenzie k1fsy at vhfwiki.com
Mon Dec 20 16:52:27 EST 2010

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
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
> > year. He found that when single Yagis for different bands were  stacked
> > one mast, they could be practically on top of one another (as  little as
> > inches apart, I think) and had virtually no effect on one  another. When
> > 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
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


The stack lays out like this:

-----------------------  (222-5wl w/23' boom)
 two feet
------------------------------- (17b2 w/31' boom)
 two feet
------------------------------- (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
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
of a mast that is long enough to conform to "perfect" stacking distances for
these antennas.

Ev, W2EV
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