[TowerTalk] Re: Can takeoff angle be too low?
alsopb
alsopb@gloryroad.net
Tue, 19 Dec 2000 11:58:42 +0000
Guys,
Perhaps what is being reported here is true.
However, it seems to require inventing new propogation modes.
I am skeptical that all the experts over the years haven't found
them. Keep in mind codes like VOACAP are part theoretical and partly
experimental based. They do reflect the huge number observations made
over the years.
So either the modelers choose to leave out such modes of propogation
because they were rare, they never observed such modes, or there is
something inconsistent with the ancedotal reports.
Having spent most of my life "benchmarking" analytical models against
nature (admittedly not in the propogation area). I know the great
pains the modelers go through to produce such codes as VOACAP. Yes,
the codes are not 100 % accurate. However, the betting man would be
better off using their predictions than some "tip" from ancedotal
reports. But it's your money. Feel free to spend it as you wish.
Keep in mind the data collected and used in benchmarking codes like
VOACAP had the ability to know what angle the arriving signals were
coming in from. I doubt that the observers here have any idea what
elevation angles the "stronger" signals reported arrived from.
My 2 cents.
73 de Brian/K3KO
n4kg@juno.com wrote:
>
> 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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> >
>
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