Dear Jim, LF Group,
The main variables involved are:
TX ERP
TX directional pattern (probably only if a TX loop is used)
Distance - leading to "geometric" propagation losses
Other propagation losses - for DX signals, basically due to ionospheric
effects
Band noise level at the receiver
Directional patttern of RX antenna
Received SNR required for mode in use
It is difficult to make a direct calculation because some of these are not
known to any great level of confidence - in particular, the "other"
ionospheric propagation losses, and the band noise level at a particular
receiver. Noise levels can be determined by field strength measurements,
although few people have done this - my best estimate at M0BMU is about
3uV/m in 2.5kHz bandwidth under quiet conditions, although obviously varying
with time of day, band conditions, direction etc. There does not seem to be
much info available for ionospheric losses at LF/MF - this is of course part
of the motivation for using modes such as WSPR to probe propagation. But you
can be sure that simply extrapolating results with closer stations to long
distances won't give a reliable answer - there is no simple relation between
loss and distance.
But on a practical level, you can use the WSPR database to get a fairly good
idea of how much TX ERP you need to be received by particular stations, e.g.
WD2XSH/17 has spotted M0BMU 40 times during the past week, with SNR
between -31dB and -16dB, but quite frequently around -25dB . G7NKS is a
similar distance and path, so the same ERP (estimated at 2W for M0BMU)
should give similar results. If you would be content with rather fewer spots
at around -30dB, close to the minimum for WSPR, you could reduce the ERP by
about 5dB, or perhaps even 10dB reduction (200mW) if you would be happy
with just the odd 1 or 2 spots at peaks in the propagation - this is the
kind of level G4JNT is running, and has indeed been spotted a couple of
times at WD2XSH/17. You could do a similar kind of estimate with other
stations, of course.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "James Cowburn" <[email protected]>
To: <[email protected]>
Sent: Sunday, January 10, 2010 6:33 PM
Subject: LF: Link budget calculation or estimation of dist for given power
on WSPR
Dear LF,
Has anyone any ideas or suggestions on a resource for calculation of a
link
budget for WSPR over a set distance?
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