Message-ID: <003e01c0df11$39e01a00$1fb51bca@xtr743187>
From: "Vernall" <vernall@xtra.co.nz>
To: rsgb_lf_group@blacksheep.org
References: <65AECDF1F89AD411900400508BFC869F9C04AB@pdw-mail-1.dera.gov.uk>
Subject: LF: Re: RE: LF Prop.
Date: Fri, 18 May 2001 08:09:01 +1200
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Hi all,

Andy's information is also my recollection of the meaning of LUF:

> Seem to remember reading that LUF is a calculation based on D layer
> absorbtion.   Propagation predictions software giving LUF uses sunspot
> number / solar flux and time plus lat / long, but unlike MUF, also needs
> transmitter and antenna properties in order to calculate signal strength
>
> LUF is the lowest frequency that will propagate over the skywave path(s)
> that is subject to the D layer absorbtion, whereas MUF is a geometrical
> consideration only, so doesn't need power and antenna details.

For F layer one hop geometry, a ray is subject to two doses of absorption
from the D layer.  Absorption increases with decreasing frequency, which can
be considered as a path loss matter, and is not a sharp cut-off mechanism.
Path loss can to some extent be compensated for by increasing transmitter
power and antenna gain.  Even for really high radiated power, there is a
point where only a one hop path is useable (there is twice the loss in dB
for two hop geometry), and even a one hop path can have too much loss for a
low powered system.

The practical impact is that D layer absorption greatly attenuates skywaves
on lower frequency bands during hours of daylight (the sun being the
stimulus of D layer ionisation).  Best DX is at night-time.

73, Bob