----- Original Message -----
From: "Guy Olinger, K2AV" <olinger@bellsouth.net>
To: "Jim Lux" <jimlux@earthlink.net>; "W3YY" <w3yy@cox.net>; "TowerTalk
List" <towertalk@contesting.com>
Sent: Saturday, October 16, 2004 9:04 AM
Subject: Re: [TowerTalk] Tower height increments
>
> ----- Original Message -----
> From: "Jim Lux" <jimlux@earthlink.net>
>
>
> > and the peak elevation angle for propagation on 40 is only 3
> > degrees above the horizon
>
> While I agree with the admonition to consider the effects of tower
> height on the lower takeoff angles...
>
> It is neither proven nor universally accepted that on 40m the "best"
> or "peak" takeoff angle is three degrees for all amateur radio needs.
>
> That three degree figure probably comes from VOACAP or VOACAP derived
> sources. Those programs have an entirely different agenda that is only
> a portion of typical ham radio uses and needs. VOACAP has broadcast
> derived blind spots that have to be worked around carefully.
> Particularly, VOACAP heavily weights takeoff angles that are
> continuously successful over long times and calendar intervals. A one
> hour strong opening somewhere, that would be a ham's delight, is
> largely dunned by VOACAP.
>
> VOACAP assumes that a broadcaster can actually construct high, large
> and efficient enough antennas, and run high enough power levels to
> make use of such openings.
I agree with Guy that one needs to be aware of the actual modeling, etc.
However, the propagation model in VOACAP is independent of the application
(yes, it can generate link probabilities, etc, of great interest to
broadcasters, but it also generates other useful data, like takeoff angles)
However, these statistics are the ones from N6BV, and yes, the propagation
model IS VOACAP, but, I believe, independent of the antenna type. That is,
they looked at the angle of propagation supported in some sort of steps of
time, season, and sunspots. So, to a certain extent, they probably get times
when the angle is some number, but at that time and condition, nobody would
be on the other end. Until someone actually figures out a good model for
that you kind of are shooting in the dark. It's pretty easy to get ham
density maps for the US, but the rest of the world is a challenge, and, of
course, license density doesn't say anything about whether a given ham has a
capability for a given frequency.
>
> The experience at large contest stations with switchable antennas
> confirms that higher angles, even in the 20's, are significant on 40m
> over the course of an evening, depending on conditions, path to DX and
> distance.
There are lots of things that contest stations (and casual users, for that
matter) experience can provide with respect to information about
propagation modes and optimum antenna systems. (like the odds that someone
is listening for your call on the other end?).
However, it's exceedingly unlikely that any contest station actually
measures the takeoff angles being used. You're inferring that 20 degree
propagation angles are useful based on an understanding of what the antenna
pattern might look like in the vertical plane, and, also, the implications
as far as what the other end is doing. I seriously doubt that many hams
actually KNOW what their vertical radiation pattern looks like. Heck, most
have no clue what the actual ground dielectric properties are within a few
km of their station, and that has a HUGE effect on the vertical pattern.
Most hams don't even know what the dielectric properties of the soil in
their back yard is, and just use "average soil Epsilon=13, conductivity =
0.006"
It's true that a lot of people (including myself) have studied the
sensitivity of the patterns to the changes, and might be able to formulate
some fairly accurate guesses as to what might be going on, but they are just
that, guesses.
There is remarkably little published information (in any form) on the actual
angle of arrival of signals, which is kind of a shame, since modern
technology makes it pretty easy to determine. Certainly, the equipment cost
would be fairly small, in comparison to the investment in a competititve DX
contest station with multiple antennas at multiple heights. For a couple
thousand dollars (at most.. probably less than a kilobuck) one could make
some meaningful measurements. A couple or three PCR1000 receivers and a PC
would do it quite nicely.
>
> It was discovered, only last year, that VOACAP systematically
> discarded any data regarding a fourth incoming angle, that combined
> with a low starting angle (such as one or three degrees), made it
> incapable of reporting the existence of significant high incoming
> angles. It is a restriction that can be removed by completely recoding
> the application and its spin-offs in something other than its
> sixties-based Fortran.
This is interesting. Is there any more description of this available?
There are other ionospheric propagation modeling codes available, some of
which are much better suited in "real time" kinds of applications (VOACAP is
validated against statistics over relatively long periods). The
Australians, in particular, seem to have some really good stuff going on
here (probably because they use a lot of HF links on a day to day basis in
the outback).
As far as the gratuitous comment about needing to be recoded in something
other than 60's-based FORTRAN:
There's nothing wrong with FORTRAN, it's still a dominant language for
computationally intensive applications. It is, for instance, what NEC (and
the vast majority of it's successors) is coded in. The changes necessary to
deal with what you describe certainly would not require recoding the whole
program. However, computer languages are a religious issue, and not
suitable for further discussion on this list.
Jim, W6RMK
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