ARLA/CLUSTER: Como trabalhar estações distantes em VHF e superiores.
João Costa > CT1FBF
Segunda-Feira, 26 de Dezembro de 2016 - 18:13:52 WET
How to work distant stations on Ham Radio
Radio Hams around the world have the great privilege to use part of
the Very High Frequency radio spectrum to enhance their communication
hobby, but how do you work long distance stations on V.H.F.? This part
of the radio spectrum is by nature a line of sight band under normal
weather conditions, line of sight plus about a third of the distance
is the normal range, however there are conditions that do exist
sporadically that offer the keen operator the fun of working many
hundreds of miles.
Keep an eye on the weather forecast, sporadic E and Tropospheric ducts
occur from time to time and offer good long propagation conditions.
Areas of high pressure over land often contribute to this condition.
Meteor shower trails left behind meteor showers help propagate what is
normally a line of sight frequency over several hundred miles.
Temperature inversion over the North Sea has given me the opportunity
to contact stations in Scandinavian, this type of propagation occurs
during differences in temperature between the sea and the surrounding
atmosphere. Coastal ducts often happen for those who are lucky enough
to live by the sea, this phenomenon sometimes happen during times when
the coast is bathed in thick fog.
There is a tremendous amount of loss of signal strength on V.H.F.
between the transmitter and the receiver, to counteract this physical
fact many operators use a directional aerial comprising a boom where a
number of elements have been attached to amplify the power from not
only the transmitter, a directional beam amplifies the incoming signal
too. The Yagi aerial is an example of this design. Commercial radio
masts erected for utility operators can easily overcome this problem
by having multiple hilltop repeaters that receive the transmissions;
they are then sent to other hilltops via microwave radio links or
connected together via fibre cable and the internet.
Working longer distance on V.H.F. is possible if you engage in this
activity during certain organised events. Contests and other planned
events including field days offer the chance for radio Hams to cover
larger distances with their equipment. Many people set up their
stations well away from urban areas where masts can be erected and
larger beam aerials can be utilised. Linear amplifiers are used to
boost the outgoing signal. Head for higher ground during these events,
you will be amazed how many contacts can be made from a hilltop
I live in an area that is only 120 feet above ground level, not the
best location for V.H.F.; however the Pennines, a mountain chain
running through the middle of the United Kingdom is within easy
driving distance from my home. Operating from the Pennines offer the
V.H.F. Ham radio operator the benefit of height, I have operated in a
particular spot on the Pennines that is over 1300 feet above sea
level. Using a transportable transceiver and a homemade Yagi aerial, I
have communicated with other stations well over 200 miles away under
flat radio conditions. Proving that height above sea level gives the
operator a great advantage.
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