[TowerTalk] Re: Can takeoff angle be too low?

[Bill Tippett]

        From my QTH (actually for GA not NC), EU arrival angles (per N6BV 
stats based on VOACAP modeling) are as follows for 10 meters:

Degrees         Percentage
3                   8
4                  15
5                  19
6                   8
7                  19
8                   8
9                  11
10                 11

        Over flat terrain, my 6/6/6 stack has the following takeoff angles:

Height            TOA        Vertical Beamwidth
35'                13                 13
70'                 7                  6
105'                5                  4
35/70               8                  7
70/105              5                  5
35/70/105           6                  6

        Thus for approximately 30% of the time, when signals are >= 8 degrees,
lower antennas will be the better performers.  For my specific terrain, there
is NO case in which the 35' antenna will ever beat the 35/70 stack, because the
35' antenna does not have more gain for signals <= 12 degrees which, according
to the statistics, is 99% of the time.  Of course this may not apply to 
unusual propagation such as sporadic-E which I suspect is at very high angles.
It is quite common for the lower 2 antennas to beat the full stack by >10 dB
during the latter half of the opening.  The YT model for my QTH confirms this
by showing the full stack is -10 dB versus the 35/70 combination at a TOA of 12
degrees.

        I also think the N6BV/VOACAP statistics are only an approximation and
angles may be too low for this specific part of the sunspot cycle.  To add to
Yuri's comment about non-reciprocality, VOACAP confirms this for paths >10K
kilometers...i.e. the takeoff at TX and RX may indeed be different...and I've
seen this happen many times during antenna tests.  The only way to really know
what is going on is to model your antennas and your specific terrain using a
program like YT.

                                                        73,  Bill  W4ZV
 




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