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More Stack Modeling

Subject: More Stack Modeling
From: bhorn@netcom.com (Bruce Horn) (Bruce Horn)
Here's more data resulting from modeling stacked yagis -- in this case
5-el 20m monobanders rather than tribanders.

Modeling Software: NEC-2
Ground Model: Sommerfeld-Norton
Ground Dielectric Const: 13
Ground Conductivity: 5 mS/m
Taper: W6QHS Method
Segs/Halfwave: 50

Antenna: Hygain 205CA, Phone Setting
Mast Position: Center of boom
Frequency: 14.2 MHz


Stacking Heights: 72/105 feet

Lower Ant   Upper Ant
 Azimuth     Azimuth    Gain
-------------------------------------------------------------------------
  0 deg       0 deg     16.02 dBi @ 10 deg elev   Both in phase
  0 deg       0 deg     13.21 dBi @ 14 deg elev   Lower only
  0 deg       0 deg     13.45 dBi @  9 deg elev   Upper only
 None         0 deg     14.14 dBi @  9 deg elev   One 205CA @ 105 ft
  0 deg       None      13.87 dBi @ 13 deg elev   One 205CA @  72 ft
180 deg       0 deg     10.99 dBi @  9 deg (up) 10.66 dBi @ 13 deg (low)


Stacking Heights: 52/105 feet

Lower Ant   Upper Ant
 Azimuth     Azimuth    Gain
-------------------------------------------------------------------------
  0 deg       0 deg     16.17 dBi @ 11 deg elev   Both in phase
  0 deg       0 deg     13.50 dBi @ 18 deg elev   Lower only
  0 deg       0 deg     14.07 dBi @  9 deg elev   Upper only
 None         0 deg     14.14 dBi @  9 deg elev   One 205CA @ 105 ft
  0 deg       None      13.52 dBi @ 18 deg elev   One 205CA @  52 ft
180 deg       0 deg     12.31 dBi @  9.5 deg (up) 11.34 dBi @ 14.5 deg (low)


These results show that there is less interaction between the monobanders
than between the tribanders when both antennas are pointed in the same
direction, particularly for the larger stacking separation. However, the
gain degradation for the monobanders is actually greater than the tribanders
for the smaller stacking separation when the antennas are pointed in 
opposite directions:

Gain Degradation
----------------------------------------------------------------------------
                        72/105 Stack               52/105 Stack
----------------------------------------------------------------------------
Force 12 C3 Upper        -1.92 dB                   -2.66 dB
Force 12 C3 Lower        -1.93 dB                   -2.88 dB
HG205CA Upper            -2.46 dB                   -1.76 dB
HG205CA Lower            -2.55 dB                   -2.16 dB

Where gain degradation is the difference in maximum gains of the same
antenna at the same height as a result of pointing in opposite directions.

It appears that you pay a performance price when you point stacked yagis
in opposite directions. For practical purposes this will only
be of importance for signals that are marginally above the noise. On the
other hand, contesters spend a lot of money eeking another one or two dB out
of their antenna systems.



73 de Bruce, WA7BNM   (bhorn@netcom.com)

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