G8MNY > TECH 09.07.18 15:03l 71 Lines 3069 Bytes #999 (0) @ WW
BID : 28253_GB7CIP
Subj: Understanding a Multipath
Sent: 180709/0834Z @:GB7CIP.#32.GBR.EURO #:28253 [Caterham Surrey GBR]
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 ' / 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
300 400 500 600 700 800 Frequency
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
( DENSE AIR )
BASINGSTOKE \.._ _../
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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