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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