Transcript Propagation VHF and Above

Propagation VHF and Above
(an overview only)
Rick Bandla (VE3CVG)
2003-08-30
RAC Forum and Technology Update
1
Radio Waves
Electromagnetic Waves composed of equal
electric and magnetic power levels
Travel in straight lines more or less like light
Change direction if/when encounter atoms,
molecules, ions, electrons.
Wave movement takes place by stimulation of
electrons
Coherent electron activity re-enforces wave
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Slide 2
The Ground – info from OZ1RH
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Low angle of radiation usually desirable
Vertical antennas need perfect ground to give low angle of radiation
Radiation angle of a horizontal beam is a function of its height over
the ground. Quality of ground has minimal effect
Antenna on a hill (land slopes down) will have a lower angle of
radiation than flat land.
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Height on Flat Gnd
Ground Gain
Max Radiation @
1/2w
-1.7dB
13 degrees
1
+2.6dB
11 degrees
2
+4.8dB
7 degrees
5
+5.5dB
3 degrees
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Slide 3
Ionization
Atoms are usually electrically neutral with a +ve
nucleus surrounded by some quantity of electrons
If electrons become separated we have free
electrons and positively charged ions
Ionization occurs as a result of bombardment by
charged particles, UV, X-rays, gamma rays
Radio waves are non-ionizing radiation
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Slide 4
When Radio Waves meet Matter
 Refraction – change in direction of wave
 Refraction due to ionization looks like reflection
 Reflection – some fraction of a wave is turned back
 Scattering
 wave encounters objects smaller than a wavelength.
 Over-dense scattering resembles refraction and reflection.
 Under-dense scattering from objects with order, may result in
useable propagation (meteors, aurora)
 Absorption – energy converted into heat
 Diffraction
 obstacle removes part of a wave front. Causes propagation into the
shadow of the object
 Loss increases with scattering angle and frequency (clear takeoff
more important at higher frequencies)
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Slide 5
Refractive Index or N
 N is the radio wave velocity in free space/velocity in a new
medium
 Calculation: N = 77.6 (pressure in millibars/temp in
kelvins) + 3.73 (105 (water vapour pressure in
millibars/temp2)) which is typically about 324
 N is Independent of frequency
 In normal atmosphere, air temp and dew point decrease
with height above ground therefore N also decreases
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Slide 6
Troposphere
 This is where our weather lives
 Tropopause is upper boundary and is the temperature
inversion layer (about 10 km depending on lat and
conditions)
 No significant ionization
 Physical properties of the air may vary radio refractive
index or N
 N decreases linearly for first 2-3km (particularly after a
rainstorm). Rate of decrease reduces with altitude
 Refraction within the troposphere is what extends the
optical horizon to the radio horizon (4/3 earth)
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Slide 7
Tropo Scatter
Point to point communication on radio horizon is
typically 50km max, S9 signal strength Workable
distance is d1(km)=4.1( h1 +  h2)
Troposcatter Ts is always possible, independent of
frequency 50Mhz – 10GHz
 Range up to 800km is realistic
Troposcatter loss increases with the scatter angle
Note: there is also Back Scatter and Side Scatter
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Slide 8
Tropo Ducting
 Caused by radio waves reflected and refracted between layers
 Waveguide in the sky, with very little loss
 Often one opening and one exit
 May be multiple ducts through the same area
 Often excellent signal strength. Ranges of 2500km or more
 May last a few minutes or several hours
 Typically 144MHz and above
 Surface Ducting
 If temp increases with height, we have a temperature inversion. N
decreases more sharply at the boundary, bending radio waves
downwards and trapping them between earth and boundary
 Usually occur over water, generally between 15m and 400m deep
 Elevated ducting formed by double discontinuity in N.
 .5 – 2 km above ground or sea & shallow compared to surface ducts
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Slide 9
Atmosphere
Chart
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Slide 10
Ionosphere
 MUF or Maximum Usable Frequency is highest freq returned to earth
 D Region at 60-90km
 Absorbs HF but almost transparent to VHF and above
 Contributes to VHF and above forward scatter during intense solar
activity
 E Region at 90 to 120km
 Ions predominate neutral atoms
 Sporadic E, Aurora, Meteors capable of reflecting VHF and above
 F region
 F1 – some refraction at 50MHz
 F2 at about 350km elevation, between 100km and 200km deep
World wide communication at 50MHz but insufficient
ionization to effect 144MHz and up
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Slide 11
Sporadic E (Es)
 Range 500-8000km on 50MHz (multi hop)
 Range 800 – 4000km on 144MHz (double hop)
 Occasional openings to 222MHz
 Openings last a few minutes - few hours
 Higher frequency = shorter openings
 Geographically selective
 Work several stations all in one or 2 grid squares
 Signals are often very strong
 Types
 Temperate Zone Es – solar driven, summertime, 50-200MHz
 Auroral Es – see Aurora slide
 Equatorial Zone Es – 50-100MHz, around the magnetic equator
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Slide 12
Sporadic E (Es)
 Some Temperate Zone predictability/probability:
 50MHz Apr-Sept, between 6am and 10pm
 144MHz May-Aug, between 11am and 8 pm
 mid-morning peak & larger peak at abt 6pm
 Tends to occur repeatedly between same spots
 Es layers contain high concentrations of ionized metal, which
gradually fall to earth
 Constantly being replaced by meteor debris
 No definite relationship between solar cycle and E layer
 Possible relationship with thunderstorms
 Theory: Wind shear causing ion concentration
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Slide 13
Aurora (Ar)
 Complex! Charged particles emanating from sun in the solar wind
become trapped in earth’s magneto tail which streams behind earth away
from sun. Increase in solar flux stretches magneto tail until it snaps and
reconnects into a more stable configuration. Some snapped particles eject
into space, retained particles collapse back to earth. These collapsing
particles somehow pick up enough energy to penetrate the E region.
 Auroral effects are in E layer
 Daily rotation of magnetic pole about geographic pole & solar wind make
the auroral zone approach and recede twice daily
 Usable from 50-902MHz but mainly 50MHz and 144Mhz
 Visible aurora (400km high) & radio aurora (100km high) not consistent
 Both stations point antenna at aurora
 High power helps
 Range is about 2000km (usually less because of lack of stations)
 Region where auroral contact is possible is referred to as the “boundary
fence”. Oval 2000km to your east and your west and 1000km to your
north (forward scatter) and south (back scatter).
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Slide 14
Auroral Es
Follows time and location of aurora
50-200MHz
Higher latitudes, night or day, other times besides
summer
Ionization originates from auroral particles
Usually formed from ionization remaining after
auroral storm
Ion concentration probably due to wind shear
Usually northern latitudes but can spread south to
mid latitudes
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Slide 15
Aurora Map
See http://www.sec.noaa.gov/SWN/
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Slide 16
Meteor Scatter (MS)
 Propagation is actually from trails of ionization as high velocity
fragments (grains) burn up at about 100km.
 Trail (tail) 20 – 65km long 1m in diam
 Meteors are random, any time works, but midnight to dawn best for MS
 Meteor “showers” are very predictable.
 Quadrantids 3-4 Jan
 Arietids 8 June
 Nu Geminids 12 July
 Perseids 12-13 Aug
 Geminids 13-14 Dec
 Pings are abrupt and rapidly fades (semi-coherent scatter)
 Bursts are much longer and may come from over or under dense trails
 Bursts from over dense trails increase in strength & oscillate then fade
 MS significantly easier on 50MHz than on 432MHz
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Slide 17
Meteor Scatter – What Works?
High ERP an advantage, horizontal polarization at
both stations
“Point a not too directional beam along the great
circle path at the other station and hope for
meteors”
With meteors at 88-100km high max range is 2100
– 2250km
Longer contacts probably single hops assisted by
other modes
WSJT software by K1JT makes MS much easier
2003-08-30
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Slide 18
Summary of Propagation Modes
Medium
Mode
Abbrev
Troposphr
Refraction
Tropo
Ducting
Tropo
Scatter
Ts
70cm
Above
X
X
X
X
X
X
X
X
X
X
Es
X
X
Auroral E
Ar-Es
X
X
Aurora
Ar
X
X
X
X
F2
F2
X
Meteor Scatter
MS
X
X
?
?
Iono Scatter
X
?
Diffraction
X
X
X
X
?
X
X
X
X
X
X
X
Ionosphere Sporadic E
Obstacles
Moonbounce
Reflection
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EME
6M
2M
X
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Slide 19
Polarization
Refraction and reflection of a wave is more
probable when polarization of the wave and the
surface are in the same plane. (less loss)
Horizon, Ionosphere, clouds, various layers are
horizontal with respect to the surface of the earth.
Horizontal polarization more successful for DX
propagation at VHF and above frequencies.
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Slide 20
So What Mode Was That?
 Sometimes it’s obvious from frequency or characteristics
of the signals
 Sometimes more than one mode contributes to a path
 Short Es and long MS sound the same
 Very short tropo ducts sound same as Es (same time of day,
same season)
 Important to know whether a burst of signal is a prelude to
an Es opening
 Tools include weather maps, books, beacons, broadcast
signals, fm repeater behavior, DX spots, web sites, sun
stats, aurora stats, celestial patterns, discussion and info
sharing
 Listen a lot! Experience!
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Slide 21
What You Need to Succeed on VHF/UHF & Up
 A transceiver that covers the desired bands & has reasonable
performance
 Extra power useful but not mandatory
 Good Antennas are mandatory!!!
Horizontally polarized
Rotator
The more gain the better
 Low loss feed lines very desirable
 Clear uncluttered horizon (there are other options)
 Flat or elevated site very desirable
 Antenna height very helpful
 Computer and some software in the shack helpful but not
mandatory
 Internet access in the shack can be useful
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Slide 22
When’s A Good Time?
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Slide 23
Methods and Modes
Point-to-point
Meteor Scatter
Moon Bounce
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Voice on SSB
CW
JT44 & JT6M
FSK441
PSK31
Other
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Slide 24
WSJT by K1JT Joe Taylor
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Windows PC software (freeware) which uses a computer sound card
connected to a transceiver for:
 Tropo propagation using JT44 or JT6M
 Meteor Scatter using FSK441
 EME (moon bounce) using modified JT44
Excellent way to learn about propagation
See http://pulsar.princeton.edu/~joe/K1JT/ to download a copy of the
software
See http://www.chris.org/cgi-bin/jt44talk for co-ordinating JT44
contacts.
See http://www.pingjockey.net/cgi-bin/pingtalk for co-ordinating
FSK441 contacts.
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Slide 25
References
The VHF/UHF DX Book Vol#1 RSGB –
Editor Ian White G3SEK
See the list of web sites
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Slide 26
Web Sites
 Bill Hepburn’s VHF UHF Tropo Ducting as well as links to MUF, Aurora,
Meteor Scatter, F2 Skip http://www.iprimus.ca/~hepburnw/tropo.html
 Space Weather Now (solar, aurora, etc) http://www.sec.noaa.gov/SWN/
 Gary Kronks Meteor Observing Calendar
http://comets.amsmeteors.org/meteors/calendar.html
 Weather Related Interference
http://www.bbc.co.uk/reception/factsheets/docs/reception_weather.pdf
 VHF UHF and up Spots http://dxworld.com/
 IW9CER Spots and info http://www.iw9cer.com/
 Paul Kelley N1BUG - VHF and Above http://www.n1bug.net/
 Palle Preben-Hansen (Denmark) OZ1RH http://www.qsl.net/oz1rh/
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Slide 27