An interesting set of articles about VHF propagation in SE India.
If you have any comments about the following, please address them to:
QSL: Prof. K. Venkata Reddy D.Sc.,
11-2-18 Wilderness Drive,
Visakhapatnam 530 003, INDIA.
Please note, new Email address.
E-mail: vu2bbu*vu2sjv@bkk.win-net.org
Packet: VU2BBU @ VU2SJV.VZG.IND.AS
VHF-DX in SE INDIA - Part 1
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ABSTRACT:
Vhf Dxing had evinced a lot of interest in our Ham's residing
along the east coast of India. So we were prompted to write our
experiences on VHf Dxing from Visakhapatnam (18N 83E) and since
we have the expertise, to enlighten our VHF users by explaining
the probable modes for extended VHF propagation through the
various regions of our terrestrial atmosphere. Radio Climatology
and its role in creating conditions suitable for the same is also
discussed, with special reference to east coast of India.
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VHF DXING AT VISAKHAPATNAM (18N 83E):
It is a known fact that the most enjoyable mode of DXing is
establishing contacts with outstation hams on the VHF. This
obviously refers to the contacts made between stations located
beyond the line of sight.
Hams from Visakhapatnam first tasted the thrill of VHF-DXing,
in April 1988, when they could hear VU2RM and make VHF contacts
with him at Kakinada, a coastal town 150 Km. south of
Viskahapatnam. As a follow up to this, in March 1989, they were
even more thrilled when they could make contacts with hams at
Madras, Tuticorin, Nagapattanam, Baangalore and Pondicherry through
VU2MRR, the Madras Amateur Radio Society's VHF repeater. From
Visakhapatnam (18N 83E) the Madras and Colombo, Sri Lanka repeaters
could be accessed between the months of February and June.
SODAR (Sonic Radar) investigations at the Andhra University in
Visakhapatnam had shown that an Inversion Layer with a thickness
ranging from 100 to 200 m. was formed at a height between 40 to
100 mtrs. above the sea level during pre- monsoon seasons. So the
Marine Duct formed by the conducting sea surface and the lower
level of the inversion layer made it possible for extended VHF
propagation. Using a Yagi it was experimentally confirmed that the
VHF signal coming from the duct was Vertically polarized.
In 1993, the most glittering aspect of VHF DXing was when
our HAM's from Visakhapatnam could establish contacts with those
from Phuket (7.9N 98.8E), which was across the Bay-of-Bengal and
2000 km. away. In addition, they had also the thrill to contact the
Russian Orbiting MIR Station U2MIR, U3MIR, U4MIR, etc. over the
past few years.
This is a pointer to the fact that as years roll by, our
technical knowledge, understanding and the range for VHF DXing is
going to increase.
VHF-Dx in SE INDIA - Part 2
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PROBABLE MODES FOR EXTENDED VHF PROPAGATION:
While exploring for literature to explain this phenomena, the
following references gave us details of various aspects of
extended VHF communications:
1. Atlas of Tropospheric Radio Propagation parameters over
the Indian Subcontinent, Published by the National Physical
Laboratory, New-Delhi, India, 1985.
2. The lecture notes at the workshop on "HF, VHF, and
Microwave Communications" held in Feb. 1987, organized by the
Radio Communications Group at NPL, New-Delhi, India.
Before we take up the main topic, we shall discuss some
aspects of extended VHF propagation (ie. beyond the line of
sight) through the various regions of our terrestrial atmosphere;
namely troposphere, ionosphere and their anomalies.
The frequency range 30 to 300 MHz. is known as the VHF band.
Ionospheric effects dominate the lower part of this frequency
band, while the upper part is influenced by the tropospheric
phenomena. The VHF frequency range (144 to 146 MHz.) allocated for
Amateur Radio Service is considered to be in the upper part of the
VHF band.
Terrestrial atmosphere below 60km. is considered to be
non-ionized for practical purposes of radio communications. The
neutral atmosphere below 10km. is known as the troposphere. The
tropospheric region mainly consists of Nitrogen and Oxygen and
their percentage concentration does not change with the altitude
except for the water vapor content in the medium.
The lower part of the troposphere up to 1.5km. is of interest
for extended VHF propagation. Refraction, scattering and
attenuation are the principle features of the troposphere and they
exercise profound influence on the propagation mechanism. For
extended VHF communication, suitable favorable conditions have to
exist in the lower part of the troposphere (ie. from surface to
1.5km).
The following characteristics are relevant for such communication:
1. Pressure, temperature, water vapor and their variations with altitude.
2. Super refraction and ducting causing anomalous propagation conditions.
3. Depolarization due to non-spherical rain drops.
4. Wild variations of the above parameters in space and time at tropical latitudes.
VHF-Dx in SE INDIA - Part 3
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PROBABLE MODES FOR EXTENDED VHF PROPAGATION:
Sporadic-E (E-s) is a very thin layer of intense ionisation
in the form of a sheet of about 1km. average thickness and
horizontal dimensions of about 100km. formed at an altitude
ranging between 100 to 120km. These sheets appear in a random
manner and are sporadic in character.
It has been reported that in temperate latitude region high
frequency E-s and other irregularities sustain long distance
anomalous VHF propagation, up to 2000km.
In the equatorial zone, E-s is believed to be due to the
presence of a strong eastward electric current during the day,
called electrojet. In the temperate latitudes, its occurrence is
higher during summer months than eqinoxes. E-s propagation in the
VHF region rarely occurs at distances less than 500km. since the
limiting distance for one hop is around 2500km.
During the periods of very high solar activity there will be
blanketing E-s and large Spread-F and these anomalies also
contribute to the Vhf propagation over long distances.
Each day billions of meteors enter the earths atmosphere and
while burning form long columns of ionized particles. During their
brief existence, these ionized columns reflect radio signals.
Digital Meteor burst communication circuits are operated in an
intermittent mode. They transmit only during the brief intervals
when meteor propagation is possible. When a burst occurs, its
presence is indicated by the reception of a control pulse and the
compressed mode message transmission begins. In the intervals
between meteor bursts, the digital system accepts messages,
compresses and stores them for subsequent transmission. These
meteor scatter communications are basically weak signal systems
because the signal loss associated with meteor reflection is
relatively high and hence the transmitting power should be high.
Recent studies have indicated that strong transmissions can
occur, particularly during high sunspot years, over the
North-South paths spanning the Equator. Such Trans-equatorial
propagation in the VHF region had been observed between Japan and
Australia. This propagation could be also due to magnetospheric
ducting (ie. magnetic lines of force starting from Japan ending up
in Australia.)
VHF-Dx in SE INDIA - Part 4
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PROBABLE MODES FOR EXTENDED VHF PROPAGATION:
The bending of radio signal in the troposphere can be
defined in the form of a Radio Refractive Index. It varies with
pressure, temperature, partial water vapor pressure, etc., and it
influences the propagation in many ways. The initial refractivity
gradient is responsible for the bending of the radio waves and
thus extends the Radio Horizon. The refractivity gradient can be
obtained by measuring the refractivity index at ground level, at
heights of 100m., 200m. etc. above the surface depending on the
requirements.
Line of Sight: When the transmitting and receiving antennae
are within the radio horizon, the VHF waves propagate in the line
of sight mode. The line of sight depends on the elevation of the
antennae and the initial refractivity gradient. If the gradient is
more negative, the line of sight can get extended further.
Diffraction: In the diffraction mode, when the wave front is
partially obstructed by an obstacle some energy gets diffracted
into the shadow zone of the obstacle. When an obstacle is visible
from both the terminals of the diffraction path, it produces what
is known as "Obstacle Gain". This gain is the difference between
the attenuation over the radio path having such an obstacle and
the greater attenuation that would have been in the absence of any
such obstacle.
Scattering: The radio fields produced by scatter and
reflection from elevated layers dominate when the diffraction
losses become large. The forward scatter power depends on the
spatial distribution of the irregularities and radio wave length.
Reflection is more frequent when lower frequencies and wide beam
antennas are used.
Ducting: Radio horizon is defined by the horizontal ray
touching the ground. When the modulus of refractivity becomes
negative then a WAVE GUIDE is formed in the troposphere. Under
this condition the radio horizon cannot be defined. When the
refractivity gradient exceeds -157 N-units/km. the atmospheric
situation comes under a special category called DUCTING, because
the curvature of the radio waves equals or exceeds the curvature
of the earth.
VHF-Dx in SE INDIA - Part 5
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INVERSIONS:
Nocturnal Inversion: The planetary boundary layer receives
most of its heat energy from the earth's surface and for this
reason, prevailing temperature normally decreases with increasing
height above the sea level. The vertical gradient of temperature
in the lower atmosphere is of the order of 6 to 7 degrees/km. This
decrease in temperature continues up to the troposphere which is
about 16km. in height over the equator and about 10km. over the
poles.
At times when the sun is very low in the sky so that the
outgoing radiation exceeds that coming from the sun and sky,
inversions of temperature are liable to occur. Under inversion
conditions the temperature of the air increases with height up to
a certain altitude. The inversion caused by radiative cooling of
the earth's surface is known as Radiation/Nocturnal Inversion and
is basically earth based.
Ideal conditions for development of nocturnal inversion are:
1. Long nights, so that radiation cooling takes place for maximum period.
2. Clear sky condition for rapid loss of terrestrial radiations.
3. Relatively dry air that absorbs little outgoing terrestrial radiation.
4. Calm atmosphere so that very little vertical mixing takes place.
The ground based inversion formed due to radiative cooling of
the earth's surface at nights, start rising as the earth's
surface becomes warm. There is an entrainment of air into the
mixed layer as it erodes away the overlying inversion base. This
process causes the inversion base to rise and finally the layer
gets dissipated at some high altitude leaving the atmosphere in a
convective state.
Advective Inversion: Surface based layers can also be
produced by advective process, in which warm air moves
horizontally over a cold surface, the warm air is cooled from the
bottom from the colder surface producing temperature inversion.
VHF-Dx in SE INDIA - Part 6
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INVERSIONS:
Subsidence Inversion: Subsidence inversion layer is an
elevated layer associated with the developing anticyclone or
high pressure areas. It develops at the base of a subsiding
(sinking) air as a result of the dynamically forced warm air by
the anticyclone velocity. The persistent anticyclone leads to a
persisting elevated inversion, which can come down to as much as
300m. above the surface of the earth. There is generally low
humidity above such inversions.
Frontal Inversion: When two air masses of different
temperatures come together, they do not mix freely. So a
transition zone called a FRONT forms between them. Fronts are
zones of steep horizontal temperature gradients and therefore they
are associated with strong winds and they can cause complex
inversions.
Sea and Land Breeze Inversions: In the coastal regions the
land surfaces respond to changes of the radiation balance and
become warmer by day and colder by night adjoining the sea. The
air over over warm regions/land forms convection and is replaced
by the cool breeze cutting in from the colder areas nearby. Thus
at the coastlines a cool breeze tends to blow from the sea on warm
days forming an elevated layer which is not only different in
temperature but also high in humidity. This layer results in large
changes of the radio refractivity gradient and influences line of
sight links severely. In the nights the air from the cooler land
goes into the sea. Such breezes are best developed in clear
weather favoring maximum changes of solar and terrestrial
radiation at the surface.
The formation of the elevated inversions are attributed to
temperature and humidity stratifications, wind shear, weather
fronts, land and sea breezes and mesoscale phenomena like
anticyclones. Most of the elevated layers are seen during the
night hours. During the day time convective currents do not allow
any elevated layer to develop below 1km. unless mesoscale cyclone
persists.
The important property of temperature inversions is to
produce hydrostatic stability in the atmosphere inhibiting
vertical mixing. Steep humidity lapse rates are therefore
developed under inversion conditions leading to the formation of
super-refraction and ducting particularly over land.
VHF-Dx in SE INDIA - Part 7
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RADIO CLIMATOLOGY ALONG THE EASTERN COAST OF INDIA:
Indian troposphere is dominated by the proximity to sea in
the west, south and eastern parts and by vast land masses, some
desert areas and hills in the north. Kulsherstha et al, Indian
Meteorology Department, had drawn the Homoclines of Radio Climate
for various places in India and found that they fall under four
distinct regimes. Homoclines are plots of refractivity (Ns) for
different seasons in the year.
The south-eastern coastline comes under Group I (360 to 390),
which has the highest Ns, while the north-eastern coastline is
shown in Group II (340 to 370) that has a lower Annal Mean Ns
than Group I
They had also concluded that:
1. Ns is usually higher in coastal areas than in the interior parts.
2. For areas adjoining the Bay-of-Bengal Ns is generally at its highest during the south west monsoon and it can also be of the same order during the summer months.
3. Ns is is at its maximum over Madras and Port Blair during post monsoon months.
4. In the Bay-of-Bengal Ns decreases from the west to east except in winter months when it increases from west to east.
From an exhaustive analysis of the Meteorological data, maps
of the radio climatology over India and neighborhood had been
prepared by Sarkar et al, NLP, New-Delhi, for the structure of the
vertical gradient of the radio refractivity in the lower
surface-based layers, effective earth radius factor,
sub-refraction and ducting occurrence probability and occurrence
characteristics of super refraction at higher levels.
VHF-Dx in SE INDIA - Part 8
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RADIO CLIMATOLOGY ALONG THE EASTERN COAST OF INDIA:
From these maps is has been observed that:
1. In terms of seasonal variations, the highest gradient
values are observed during pre monsoon months, while it is
low in the monsoon months throughout the country.
2. The annual mean initial refractivity gradient (Surface to
250m.) over the south east coast is around 80 N/km.
3. In general in the evening hours the gradient is less
intense than during the morning hours.
4. The surface duct occurrence probability in pre monsoon is
30% over the coastal regions, minimum during monsoon and
is higher by a factor of 2 to 3 in the morning than in the
the evening.
5. The elevated layer occurrence probability is of the order
of 50% over the south east coast.
During the monsoon months the air over land is warmer than at
the sea surface for most of the time. Also the winds in the
surface to 250m. layers are anticyclonic. These factors also
contribute to the formation of ducting conditions which dominate
during the pre monsoon. With the advent of the monsoon,
anticyclonic circulation disappears, wind at lower tropospheric
levels become westerly and increase in velocity. Sea breeze effect
is suppressed. High humidity extends up to the higher levels. The
air temperature and dew point curves come close to each other.
Hence the probability of ducting is reduced during the monsoon
months.
During the post monsoon months a diffuse low pressure area is
established over the Bay-of-Bengal and the atmosphere gets well
mixed up and the probability of occurrence of large refractivity
gradients responsible for ducting gets reduced to 18%.
Based on refractivity values at higher altitudes, 950, 900
and 850mb., the probability of occurrence of refractivity beyond
-75 units/km. was found to be more across 0.5 to 1.0km. than
1.5km. levels during all monsoons over the Indian subcontinent.
The probability of occurrence over Madras was found to be 40%
across 0.5 to 1km. level in Feb., while during Dec. it is 18%.
The maximum occurrence frequency across both these levels was
found to be during the pre monsoon months, while the frequency is
low during monsoon months.
VHF-Dx in SE INDIA - Part 9
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AUTHORS NOTE
From the the studies radio climatology over the Indian sub
continent it is clearly seen that the south eastern coast of India
(ie. from Visakhapatnam to the southern tip of India) has the
highest radio refractivity gradient and duct occurrence
probability.
During the past decade the VHF users of the Amateur Radio
Society of Visakhapatnam had shown keen interest for VHF DXing.
Since we have come to know that we are strategically located, we
have designed a very efficient 2m. Yagi Antenna (12 feet boom, 9
elements, SWR 1:1) ) and fine tuned it. It is being used by every
VHF DXer at Visakhapatnam and is now our standard.
Just out of curiosity in 1989, an extensive survey was
carried out for VHF (144 to 146MHz.) signals along the 10km.
coastline/sea front of Visakhapatnam city and to our surprise we
tumbled on an unusually spot ideally suited for VHF DXing. It is
200m. away from the sea front and is 25m. above sea level located
in the Andhra University campus. Using a hand held transceiver
with 3 watts, from the Visakhapatnam beach, we can access Madras
(700km.) repeater and Colombo (1200km.) repeater in Srilanka.
While on the other hand from the special spot mentioned above low
power (ie. 100 to 150 milli watts) is more than adequate for VHF
DXing and the received signal at this place is 59+.
It looks as if this spot is situated at the centre of a
marine duct, formed by a 40 to 50m. high inversion layer and the
conducting sea surface. There is a tremendous focusing of the
radio signal at this place and there is practically no attenuation
of the VHF signals going into or coming from the duct, formed
along the south eastern coast. Sometimes extended communication
is also possible well into the interior land mass in the
southern part of India.
We are planning to conduct VHF surveys for identifying such
unusual locations along the Andhra Pradesh coastline as they are
potential sites for setting up of repeaters. Andhra Pradesh
coastline is susceptible to severe cyclones/typhoons.
I hope that I had given you sufficient info to enlighten you
on some important aspects of extended VHF communication. I would
appreciate VHF users to come forward and share their knowledge
and experience with specific reference to their location.
Please feel free to let me know your comments.
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Amateur Radio Communications during Cyclones and Disasters in India.
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During failure of normal telecommunication facilities Radio
Amateurs (HAM'S) are permitted to handle third party messages
pertaining to natural calamities such as earth quakes, floods,
cyclones etc. The licensee shall inform by a letter addressed to the
licensing authority regarding the use of his amateur station for
such purposes on each occasion.
Under such provision Ham radio operators can render service as
a second line of radio communications and come to the rescue of
Government agencies when all other forms of communications are
disrupted due to disasters. They can give on the spot assistance to
victims and also provide information to all those in the country and
abroad, who need to know about their kith and kin involved in the
tragedy, without and pecuniary reward.
In fact from ones own normal QTH (residence location of the
amateur wireless station), one can render considerable service.
There are several AIR NETS that operate daily at fixed hours on
fixed frequencies to check the propagation conditions between
various parts of the country, handle emergency traffic and to
provide liaison between two HAM's. In the morning we have the
Charminar Net at 0700 IST on 7.070MHz., INDIA AIR NET at 1930 IST on
14.500MHz., GEM NET at 2200 IST on 7.070MHz.
Any Ham operator can check into the NET and ask for emergency
assistance, ask for contact with another Ham at any specific
location (area, city or town) or pass on information to his
counterpart any where in the world.
In the case of GEM (General Emergency and Medical) NET, a few
medical doctors from Madras and other parts of South India, who are
Ham operators, provide Expert Medical Consultation to interested
parties through the NET on the air free of cost.
During cyclones and floods the transceivers used by the
amateurs are very compact and they could be operated by lead acid
12 volts storage batteries. They could be easily transported or
airlifted to any remote site and erected for operation at short
notice. However extreme precautions have to be taken so that these
transceivers are not subjected to shock treatment and extreme
temperature and humidity conditions. Charged batteries are provided
by the authorities at regular intervals. Personnel operating in
those remote locations at disaster sites have to be cautious about
the food they eat and water they consume.
The Naval Oceanographic Command Center (NOCC) in California,
prepares wide area weather maps and sends them to their Naval bases
for their ships and air craft carriers operating in different parts
of the world. The Joint Typhoon Warning Center at Guam transmits
this weather information as GFAX weather facsimile on HF for
dissemination. They give forecasts up to 120 hours, tropical cyclone
warnings, the track and movements of the depressions etc. This
information is collected using the Amateur Radio Digital facility
and passed on to the planners and Government agencies, when ever a
well marked low pressure area is formed in the Bay-of-Bengal.
The moto of HAM'S (Help All Mankind) is reflected in Amateur
Radio Service rendered during natural calamities, disasters and
emergencies.
QSL: Prof. K. Venkata Reddy D.Sc.,
11-2-18 Wilderness Drive,
Visakhapatnam 530 003, INDIA.
Please note, new Email address
E-mail: vu2bbu*vu2sjv@bkk.win-net.org
73's de VU2BBU @ VU2SJV.VZG.IND.AS
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