I am in the process of designing a tower for my new vacation place, and I
have a steeply sloped foreground in one direction. I plan to install a
C-31XR and a 2 or 3 element 40 meter yagi. Experience at other similar
QTH's suggested to me that I wanted the tribander low and the 40 high - but
'lore' suggests that this type of inverse x-mas tree installation can be
problematic (as the lengthy thread here recently indicates). So I modeled
something similar using NEC4WIN95- a C-3 tribander and a 2 element full size
40 meter yagi on a 22' boom.
I started out by modeling just the C-3 at 60' over average ground. Then I
modeled the 40 meter yagi at 75' over average ground. Then I put the two
together, and modeled them as a stack with 15' separation (about the most
you can reasonably hope to get with a 20' mast that has 3-4' in the tower).
Then I lowered the 40 meter yagi to 65' - 10' separation. Here are the
results:
Freq, main lobe gain @ angle, R + X = SWR
C3 by itself @ 60' over average ground
14.1Mhz, 11.47 dbi @ 16, 50.29 + 9.75 = 1.21
21.2Mhz, 11.82 dbi @ 10, 42.89 - 20.27 = 1.58
28.4Mhz, 12.18 dbi @ 9, 61.08 - 15.75 = 1.41
40m 2 ele @ 75' over average ground
7.1 Mhz, 11.11 dbi @ 25, 57.13 + .002 = 1.14
Stack, 60' + 75', over average ground
7.1Mhz, 11.27 dbi @ 24, 56.71 + 19.95 = 1.48
14.1Mhz, 11.54 dbi @ 16, 50.73 + 21.61 = 1.53
21.2Mhz, 12.08 dbi @ 10, 45.8 - 15.19 = 1.39
28.4Mhz, 12.34 dbi @ 9, 58.63 -21.02 = 1.52
Stack, 60' + 70', over average ground
7.1Mhz, 11.04 @ 26, 61.71 + 24.17 = 1.61
14.1Mhz, 11.27 dbi @ 16, 41.72 + 22.66 = 1.69
21.2Mhz, 11.34 dbi @ 10, 38.26 - 13.91 = 1.51
28.4Mhz, 11.81 dbi @ 9, 68.81 - 20.99 = 1.61
In all cases, the pattern looked perfectly normal.
My conclusion from this exercise is that with 15' of separation, the
interactions between these particular two antennas are completely
acceptable. There is no negative impact on gain, and minimal impact on SWR.
At 10' of separation, things begin to degrade more significantly. Based on
this limited study, I plan to model the two particular antennas that I want
to stack - but with the assumption that I will be able to design a 40 meter
yagi that I can stack over the C-31 with 15' of separation and see no
significant degradation in either antenna.
Hope this sheds some light,
***dan, K6IF
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