[Aavso-sid-list] Too close, too far?

[ChrisAtUpw at aol.com]

In a message dated 21/02/2006, pws at rechtsmedizin.uni-kiel.de writes:

Dusty  wrote:
> Q.  How close is too close to avoid the VLF ground  wave?
A.: if you cannot see the typical differences between night  
and day time propagation.
> Q.  How far is  too far not to receive the reflected VLF signal?
A.: if you are not able to  detect the signal strengths or
phases reliably around  the clock.
Hi Peter,
 
    Sure, but this rather 'begs the question' and does  not give even a rough 
estimate.

It's  nearly impossible to specify miles or km for both cases. It depends on  
transmitting power, transmitting antennas, both latitudes, propagation path,  
your antenna, environment and intrinsic noise figures, etc
    This may be much too pessimistic, particularly  since we are considering 
high power transmitters. Note the possible ground / sky  wave interference 
between 600 and 1100 km with DCF77. See 
_http://www.ptb.de/en/org/4/44/442/dcf77_weite_e.htm_ (http://www.ptb.de/en/org/4/44/442/dcf77_weite_e.htm)    and also 
_http://tf.nist.gov/stations/wwvbcoverage.htm_ 
(http://tf.nist.gov/stations/wwvbcoverage.htm)    I can confirm DCF77 day and night coverage to 2,000 km in 
southern Spain. For a  more comprehensive account of 137 kHz 1 watt signals 
see _http://www.qsl.net/dk2fi/lw/lfprop1.pdf_ 
(http://www.qsl.net/dk2fi/lw/lfprop1.pdf)   

If  ground waves seem too strong for the detection of sky wave fluctuations 
you  could try nulling out the ground wave propagation by means of loop / 
ferrite  antennas. This would seperate it from sky waves which are coming in at 
higher  angles. It was a well-established practice by researchers some 40 years 
ago  here in Europe.
    The transmitting antennas are designed for horizontal  propagation. The 
reflectance and absorption of the ionised layers depends on the  angle of 
incidence and on the frequency. Consequently the sky wave is very  weak close to 
the transmitter and you are unlikely to be able to null  the ground wave within 
about 2x the height of the D layer, ~120 km.
 
    Regards,
 
    Chris Chapman
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