[TowerTalk] Re: Can takeoff angle be too low?
n4kg@juno.com
n4kg@juno.com
Mon, 18 Dec 2000 19:11:27 -0600
Bill,
Are you quoting from N6BV's statistics / model where
he ASSUMED 60 ft towers at BOTH ends of the path?
(which exhibit a NULL from 13 to 20 degrees on 10M).
That same study showed NO propagation on 20M
from ~20 to 28? degrees, assuming 100 ft high towers,
but an opening in the 30 degree range. He didn't
seem to realize that his Propagation Null was coincident
with the PATTERN NULL of 100 ft high antennas.
I don't put much weight in his propagation "statistics"
based on ioncap and fixed (high) antenna heights.
To determine the best reception angle for the LOW
transmitting antennas that most DX stations are using,
it would seem best to model a Low Transmitting antenna
and look for the angle that produces the highest received
signal strength which is NOT the approach taken by N6BV.
During previous sunspot maximums I found my 35 to 40 ft
antenna best to Europe and especially Africa most of the
time once the band was open. Several others in the area
have made similar observations when comparing 70 to 90 ft
high antennas with 35 to 40 ft (1 to 1.25 WL peaking at
12 to 14 degrees +/- 6 degrees).
When the MUF approaches 50 MHz, the ionosphere will
support up to 18-20 degrees on 28 MHz. For the majority
of Europeans / Africans with antennas at 30 to 40 ft, it
would follow that THEIR signals would peak at higher
angles. My "guess", based on pattern peaks of low
antennas and pattern NULLS of high antennas is that
the peak angles from Europe and Africa on 10M are
in the range of 10 to 15 degress during midday.
de Tom N4KG
On Mon, 18 Dec 2000 Bill Tippett <btippett@alum.mit.edu> writes:
> 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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