Topband: Take off angles

Dan Robbins
Tue, 24 Apr 2001 07:16:44 +0100

I received some questions on the post about high and low angle. Try to answer
them before I go out of town for a few days (I'll be netless!).

Re: horizontal or vertical

It was strictly vertical on the radar and the backscatter sounder.  The chirp
sounder was horizontal, but it radiated straight up and was not directly used
for take off angle determination.  There was no way to compare horizontal
polarization to vertical.

Re: How is the TO angle figured?

 Determining the angle is complicated, but basically it worked like this:  The
chirp sounder provided ionospheric information in the local area.  The
backscatter sounder provided info for the ionosphere down range.  The computer
cluster contained an ionospheric model which was calculated for solar flux,
the season, the time of day and maybe a few more parameters.  This model was
presented graphically to a human operator who then tweaked the model so that
it matched the  real ionosphere.  Once the model is accurate (and we could
verify this extremely well), then the computer can easily figure out the take
off angle along with several other parameters for a given distance and
frequency.  This was not a statistical answer either, like "50% of the time
the angle is between 10 an 15 degrees".  This was real time and exact.  The
propagation data constantly downloaded at a half T1, or over 14 times the
speed of your 56K modem just to keep the model current.

Re:  Interpolation below 4 mHz

Carl is correct that the Backscatter sounder bottomed out just above 4 MHz and
topped out at 28 MHz.  The chirp sounder went down to 1 MHz.  However, once
the HF model in the computer was tweaked to represent real conditions, the
model was good between 1 MHz to over 50 MHz.  I regularly used the computers
(during my off time!) to find 6 meter openings and there was not one time they
were wrong.  If they said yes, 6 will open, it did open.  If they said no, the
band did not open.  Although I did not have a 160 m antenna or amp, I was able
to work a few stations in the Pacific on Top band by trying on nights when
good 160m conditions were indicated by the computers. While the model was good
over a broad frequency range, it was limited in azimuth and I could not do
accurate modeling through the auroral zone.  Grazing, yes, but not over the
pole.  I believe the gyro freq is less important on the E-W paths.

                                                            Dan KL7Y

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