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G8MNY  > TECHNI   10.03.24 17:04l 71 Lines 3069 Bytes #999 (0) @ WW
BID : 2955_GB7CIP
Read: GAST
Subj: Understanding a Multipath
Path: DB0FFL<OE2XZR<OE6XPE<DB0ERF<DK0WUE<PD0LPM<OK2PEN<GB7CIP
Sent: 240310/0932Z @:GB7CIP.#32.GBR.EURO #:2955 [Caterham Surrey GBR] $:2955_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                          (New Feb 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

If you Rx a RF signal by more than 1 path this is what you get..

Reflection or                     Tx
tropo refraction.                 /'
The angle BC is                /  '
proportional to    . .    B /    '
frequency with  .'     ' /      '
refraction or  .      /  .     '  Path A is
defraction.    .    ³    .    '   a direct
                .   ³   .    '    or another
                 ` .³. '    '     tropo path.
path length = B+C   ³      'A
                    ³     '
                   C³    '
                    ³   '                        / 1st nul 150/(B+C-A) MHz \
                    ³  '      Nulls due to path |  further nulls repeat at  |
                    ³ '       length difference  \ 300/(B+C-A) MHz spacing /
                    ³'            ³       ³
                    Rx        _       _       _       _  ___ up to +6dB
                            /' `\   /' `\   /' `\   /' `\    enhancement
The affect of reflection,  |     | |     | |     | |     |   over 1 path
refraction or defraction          º       º       º       º
due to the anomaly is the  ÄÂÄÄÄÄÄÅÄÄÄÄÄÂÄÁÄÄÄÂÄÄÄÁÄÂÄÄÄÄÄÅÄ Null > -20dB
make the tropo beam        300   400   500   600   700   800 Frequency
heading appear to change
with frequency! Of cause          _       _       _       _
as the anomaly moves       `\   /' `\   /' `\   /' `\   /' ` Path difference
causing the path lengths     | |     | |     | |     | |     moved by « a
to change all the null        º       º       º       º      wavelength
spot frequencies slide up  ÄÂÄÁÄÄÄÂÄÄÄÁÄÂÄÄÄÄÄÅÄÄÄÄÄÂÄÁÄÄÄÂÄ
or down as well, changes   300   400   500   600   700   800 Frequency
in dual path amplitude
will cause the null depth        _       _       _       _
to change, from not        \   /' `\   /' `\   /' `\   /' `\ Unequal signals
detectable to extreme       `Ä'     `Ä'     `Ä'     `Ä'      no deep nulls
nulling.                   ÄÂÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÂÄ
                           300   400   500   600   700   800 Frequency


LENS EFFECTS
My experience with a group of well scattered users of 23cm GB3HV ATV repeater
at High Wycombe, may put more light on the subject. During a Wx front it became
apparent that large lens of heavy damp air was moving through many viewers Rx
paths....
                             GB3HV
                             _..._
                          /''     ``\
                         ( DENSE AIR )
  BASINGSTOKE             \.._   _../
      0dB                     """
                   READING                CROYDON
                    -20dB                  -3dB

                            ALTON
                            +20dB

Signal were lost in some palaces & much enhanced in others by getting on for
ñ 20dB as the Wx front moved west to east. Most sites saw the effect over an
hour or so. 


Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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