What is the difference
between Backscatter and Batscatter ?
Some Experiments with Microwave
Scattering (August 2005):
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- during the ERAU Estonian
Microwave Field Day in mid-August, I went portable with 1.3, 2.4, 5.7
and 10 GHz
- the microwave part was split with 1.3 GHz on Saturday morning and
higher bands on Sunday morning
- note the family sized fire extinguisher and the two thermos
flasks: one for the coffee and the other for milk ;-)
- "outside" view of the 23
cm station (dipole mat, with 50 W HB linear behind it,
TS-790R transceiver and SMPS PSU)
- the weather on Saturday was beutiful for the larger region with
light clouds most of the day (view
from behind the 23 cm dipole mat)
- most 23 cm QSO's were via normal tropospheric
scatter and the closer ones via refraction (see Wikipedia for
Snell's law etc.)
- Sunday brought heavy rain between my KP21 location and the Gulf of
Finland with many simultaneous rain fronts
- needed to put a very hurried 13 cm feed together the
previous Sunday for my Microwave Associates OB 120 cm prime focus dish
together late Friday...
- here is photo of
the heavily modified ex-1.8 GHz microwave radio now used for
2304/2320/2400/2424 MHz as installed on the 120 cm dish
- and a view of the portable setup for 13 cm (1 W RF output, NF=0.7
dB) -
made some fine QSO's, one with obvious rain scatter
- the transverter
is behind the dish with a short length of flexible semi-rigid to the
modified 5.7 GHz waveguide feed in front of the dish
- the rain scatter was most spectacular on 5.7 GHz and with OH2AXH we
had a long clean FM QSO, but boy did the S meter wiggle about !
- and finally got to 10 GHz
too: my location was such, that I simply could not copy any of the OH3TR microwave OH3SHF
beacons, but then things started to happen...
- got the dish round to ES2U for a pleasurable CW and SSB QSO and after
this, had a go at finding the OH3SHF 10 GHz beacon via rain backscatter
- another large video file - listen to the Doppler-spread signal pitch (WARNING: file
is about 13 Mb !!!) as I turn the dish around from 90 through 270
degrees
- note also the slight dish elevation required and the fact that the
beacon is not copiable directly (dish pointing to the left).
- you can also see that the wind vector is approximately orthogonal to
the image plane, blowing towards you as you watch the video :-)
- at this time the wind direction was approximately south-eastern, as
can be testified from the Doppler shift
- the beacon - rain front - observer vector relationship (file missing
still) needs to taken into account when working out the wind vector
and velocity
- also made some very nice CW and SSB QSO's on 10 GHz via forward rain scatter
- all this stuff got me thinking for a change, hoping to pick up the
OH3SHF beacon from my home QTH in KP21MI, so I put up my 120 cm dish in garden
- side view of the dish pointing
at the top of a local mobile phone site mast giving the best reflection
(dish elevation is about 5 degrees)
- pointing to a local mobile phone site
mast
gave me a dB or two of C/N from the beacon using backscatter via the
passive mast refletion
- on rainy days I see enhancements from the S2 noise floor up all the
way to tens of dB's over S9
- sometimes both tropo enhancement and rain scatter may be observed
simultaneously with a clean carrier & doppler spread spectrum
several hundred Hz higher or lower :)
- here you may find some
spectrum plots, spectrgrams & sound recordings of Rain Scatter and
Tropospheric Enhancement
- on most rainy days, there is absolutely no place in the garden where
the beacon cannot be heard - it appears to be copiable from every
direction, including downwards...
- these field tests were with my 28 cm dish, converter and FRG9600 - so
I took out my homebrew
converter with dish feedhorn
only - still copiable at several dB C/N in the rain !
- so now I have taken out my G3JVL diode mixer
transverter (my version III) from the 1980's and guess what...
- after thinking about all this for a few days, I then dug out from my
junk box a 10 mW Gunn oscillator,
attached a 15 dB horn &...
- not a very good recording , but here is a sample sound file, (469 kb)
- there is
some "static", but you can clearly hear the Doppler shifted individual
raindrop echoes
- using an outside lamp radome
to prevent the horn from filling up with rain to quickly ;-) The
"static" you hear in the recording above is caused by the radome.
- trying to figure a way other fellow experimenters may have a go at
building a Doppler Rain Radar with a zero budget, I modified a
microwave intrusion detector
- the intrusion detector has a small feed horn with a few milliwatts of
RF output and a reasonable mixer noise figure - you can also hear the
swallows and bats :-) :-) :-)
- and of course a very favourable feature is that this older Racal
device is pulsed and real power-miser at approx. 25 mA
- as an afterthought: rain scatter is a greatly underestimated mode of
propagation - it is definitely THE mode for 10 GHz if you live between
mountains or hills !
- most typical 10 GHz rain (and snow) scatter ranges are 200 - 500 km,
but as recent news from our American microwave colleagues shows:
To: Microwave Reflector
Subject: [Mw] Rohde Island & Florida to North Carolina - 10 GHz
Contact
21 Aug 2005 @ 2012 AF1T in FN41ee, Rhode Island to W4DEX
EM95tg, North Carolina, 1010 km via rain scatter
22 Aug 2005 @ 1836 WA8TTM in EL98cl, Florida to W4DEX EM95tg,
North Carolina, 763 km via rain scatter
25 Aug 2005 @ 2301 UTC VE4MA EN19lu, Manitoba worked KM0T,
EN13vc Iowa , 753 km via 10 GHz rain scatter
(Signals were S9 both ways on CW, SSB and FM. The QSO lasted for
approximately 30 minutes with little change in signal strength.)
Updated 13.01.2019
