Nick Coleman wrote:
I'm in the process of fine-tuning the behaviour of the ADF
(src/Instrumentation/adf.cxx), maintained by David M.
One aspect of calculating the transmission range is the difference in
elevation between the aircraft and the transmitter (aircraft at 10,000'
get better reception than aircraft at 2,000').
Not necessarily true for the ADF. The normal Nav and Comm radios
work in the (roughly) 108 to 136 MHz range, having a wavelength a bit
under 3 meters. A typical ADF like the KR-86 covers 200 to 1799 kHz,
with wavelengths from 167 m to 1500 m. Propagation conditions are much
different in these 2 cases.
Currently, if the transmitter is higher than the aircraft, the elevation
is capped at 200ft. Does anyone know why? (It has the effect of
precluding transmitters on a hilltop from having a longer range than
one on the flat.)
Actually a hill or mountain is usually a poor choice for a medium
or low frequency (MF or LF) transmitter. At the longer wavelengths, it
is commonly impractical or uneconomic to build an antenna on the order
of 1/4 to 1/2 wavelength high. For a shorter antenna, the quality of the
surrounding ground becomes more important to efficiency. A hill or
mountain often exists because of underlying rock, and rock is generally
a poor conductor, hence poor efficiency. A good site, particularly for
the lower frequencies, is a salt marsh, which offers naturally better
conductivity.
Also, I'd like to model the interference from a mountain range between
the transmitter and the aircraft. My plan is to find if there is
terrain higher than aircraft altitude in a line drawn from current
position to transmitter position. I know nothing about OpenGL so any
clues on how to do this are gratefully accepted. (I imagine that this
effect could eventually be ported across to the VOR code too.)
To interfere with the signal propagation, an object generally
needs to be large relative to the wavelength. This lets small hills and
large buildings block VHF signals, hence the "line-of-sight" behavior.
At the longer wavelengths of MF, it takes a pretty decent hill to have
much effect, while in the LF range where many NDBs are placed, a pretty
good sized mountain range is needed to have much effect.
Finally, I plan to model night-time increased range, which is easy
enough.
Day to night propagation changes can increase range, but can also
decrease it where the ground wave and sky wave interact to cause severe
fading and a generally unreliable signal.
Any comments welcome, even "You're wasting your time, no-one uses the
ADF anymore." ;)
Go for it, but be cautioned that the problem is more complex than
it initially seems. The FAA recently announced plans to remove a long
list of NDB approaches where GPS or other facilities make the NDBs
redundant. Not mentioned is the likelihood that such unused NDBs will
disappear. Other countries may still have need of ADF equipment for
quite a while.
Nick
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