[TowerTalk] Re: Can takeoff angle be too low?
n4kg@juno.com
n4kg@juno.com
Tue, 19 Dec 2000 08:11:47 -0600
As I see it, here is what is 'WRONG' with the previous "studies".
N6BV's FIRST study ASSUMED antennas at CHOSEN heights
at both ends for his analysis. These included 4L Yagi's at 60 ft
on 10, 12, and 15 Meters, 3L Yagi's at 100 ft for 17, 20 Meters,
and dipoles at 100 ft for 30?, 40, and 80M.
Notice how the maximum observed angle stayed almost
constant from 10 through 20 meters under these assumptions.
Notice that 15M supports a higher angle than 17M due to the
different heights used in the model. Observe the "hole" between
18 and 28 degrees created by the pattern NULL of 100 ft high
antennas on 20M.
The hundreds of hours of analysis demonstrated that the
angles "propagated" corresponded strongly to the
VERTICAL Radiation PATTERN of THOSE antennas.
NO propagation was seen at (higher) angles where the
antenna patterns exhibited NULLS since the ionosphere
was NOT excited at those angles!
N6BV's SECOND study placed ISOTROPIC Antennas
at both ends of the path. These excited the ionosphere
uniformly which tended to show that LOW ANGLES
had the least path loss. To realize this low loss,
REQUIRES that antennas that MATCH those angles
be in place at BOTH ends (typically very HIGH antennas).
Higher angle modes with (slightly greater path loss) were
IGNORED as if they did not exist, which distorts the
perception by omission. These higher angles would
receive more radiated power from LOW antennas
(more commonly used by DX stations) than the "preferred"
lower angles. This correlates with observations that LOW
antennas often produce the strongest received signals from
DX when the ionosphere supports those higher angles.
In NEITHER study did N6BV account for greater power
radiated at HIGHER angles by LOW antennas.
To accurately reflect the influence of the ionosphere,
the path loss should be reported for ALL modes (angles)
that are supported. THEN examine the resulting signal
accounting for different ANTENNA HEIGHTS.
Another way to get to the "bottom line" would be to
model propagation from antennas at various heights
at one end (say the DX QTH) and use an isotropic
model at the receiving end. I would suggest 3 or
4 heights, say 40, 70, and 120 ft ( 75% steps) or
35, 52, 80, and 120 ft (50% steps) using 2L Yagi's
(~6 dBi) for the transmit antennas which should be
close to the gain of most tribanders. I would expect
that the "optimum" angles would be found to differ
according to antenna heights.
One other consideration. IMHO, IONCAP tends to
under estimate MUF. While the times for peak
MUF tend to be accurate, I have often found the
next band above predicted MUF's plotted in the
QST MUF charts to be open.
de Tom N4KG
On Tue, 19 Dec 2000 alsopb <alsopb@gloryroad.net> writes:
> 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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