The present invention relates to an antenna for trains.
Currently, antennas are generally provided with devices
that allow to protect them from so-called static charges that can accumulate on
the radiating rods.
Such devices are generally constituted by an inductor,
which is sized according to procedures known per se and is interposed between the
radiating rod and the ground so as to discharge the static charges towards the ground.
Therefore, the presence of the inductor allows to eliminate
the danger that may arise from the conduction of the charges towards devices connected
to the antenna, i.e. downstream of the feeder cable, and on which several personnel
performing various tasks work.
Since static charges are rather weak, the inductor does
not have to withstand strong currents, because as the charges reach the antenna
they are immediately dissipated to the ground before they can accumulate thus become
potentially dangerous to personnel.
For this reason, the inductors currently used to eliminate
static charges are generally constituted by a wire that has a rather small cross-section,
and they are sized so as to generally withstand the flow of a current comprised
between 2 and 100 amperes according to the type and size of antenna being considered.
However, antennas used on trains may be subjected not only
to static charges but also to dynamic voltages or currents, such as those carried
by high-voltage cables suspended from electric power line pylons or by electric
lines that supply power to the trains.
Moreover, it should be noted that this risk is increased
by the fact that the antennas used on trains are generally located in the uppermost
regions and possibly at regions that are clear of obstacles.
From the above description it is evident that currently
an antenna for trains is particularly exposed to the danger of electrical shock,
which is extremely dangerous not so much for the possible destruction of the antenna
but rather because of the consequences that such shocks can cause to the personnel
using the services of the antenna on board the trains.
If an antenna accidentally makes contact, for example,
with a high-voltage line, the current carried by the line, in contact with the antenna,
in fact immediately fuses the grounding inductor and therefore discharges along
the antenna feeder.
It is evident that this phenomenon entails as a consequence,
the carrying of high-voltage to the vicinity of operators, with great danger for
their safety.
In order to solve the drawback noted above, antennas have
been devised in which an attempt has been made to increase so-called robustness.
However, in the application described above these solutions
are difficult to use, essentially because of the fact that in order to ensure indestructibility
of the inductor, since high-voltage lines can carry currents with tens of thousands
of amperes, it would be necessary to size such inductor with wire diameters of approximately
ten centimeters.
It is immediately evident that such an approach is difficult
to provide, first of all because of the highly problematic dimensions of the inductor
and secondly because of the fact that by working frequently with antennas operating
at a frequency around 900 MHz (for example in the GSM band), the dimensions of the
inductor would no longer allow resonance of the antenna, since they would be physically
larger than the wavelengths related to these frequencies.
Accordingly, antennas are commercially available which
are also used as train antennas and are provided with safety devices constituted
by "compromise" inductors, which however cannot ensure safety to the operator.
DE 25 50 915
,
GB 255 987
and
US-A-5 122 921
all disclose an antenna with a protective circuit.
DE 201 06 188
and
US-4 395 713
disclose an enclosure for the radiating element of the antenna.
"
Resettable fuses" Automotive Engineering, society of Automotive Engineers,
Warrendale, US, vol. 104, No. 9, 1 September 1996 (1996-09-01), pages 137-139
, XP000627074 ISSN: 0098-2571 discloses the self-resetting action of polymer-based
PTC devices, such as resettable fuses, that can be advantageous in vehicles.
US-3 968 411
discloses a transmitter-receiver protection device.
An object of the present invention is to provide a train
antenna that ensures operator safety even if it is subjected to intense electrical
shocks, by ensuring optimum receiving capability of the radiating element.
Another object of the present invention is to provide a
train antenna that has a very simple structure at competitive production cost.
These objects that will become better apparent hereinafter
are achieved by a train antenna as defined in claim 1.
Advantageously, an antenna according to the invention is
characterized in that the spark-gap unit comprises a spark gap provided with two
electrodes facing each other.
Conveniently, an antenna according to the present invention
is characterized in that it comprises means for adjusting the distance between said
pair of facing electrodes according to the critical trigger voltage.
Further characteristics and advantages of the invention
will become better apparent from the description of preferred but not exclusive
embodiments of an antenna according to the invention, illustrated by way of non-limiting
example in the accompanying drawings, wherein:
- Figure 1 is an electrical diagram of a train antenna according to the present
invention;
- Figure 2 is a partially sectional side elevation view of an example of embodiment
of a train antenna according to the invention;and
- Figure 3 is a rear elevation view of the train antenna according to the invention
shown in Figure 2.
In the examples of embodiment that follow, individual characteristics,
given in relation to specific examples, may actually be interchanged with other
different characteristics that exist in other examples of embodiment.
With reference to the figures, an antenna according to
the invention, generally designated by the reference numeral 1, comprises a radiating
element 2, constituted for example by a rod, which is connected to a feeder cable
3 that is meant to connect the antenna 1 to devices that use said antenna 1.
The radiating element 2 is connected to a grounding inductor
4.
Furthermore, the antenna 1 is provided with an unbreakable
protective enclosure 10 for the radiating element 2.
Moreover, according to the present invention, an antenna
1 has a fuse element 6 that is interposed between the radiating element 2 and the
feeder cable 3.
In practice, according to a preferred embodiment, the fuse
element 6 has a first connecting end 6a that is connected to a first end portion
2a of the radiating element 2 and a second connecting end 6b that is connected to
the feeder cable 3.
Conveniently, as clearly shown in the electrical diagram
illustrated in Figure 1, connection to the inductor 4 is provided at a connecting
region that is located substantially proximate to the second connecting end.
According to a further important aspect of the invention,
the first end portion 2a of the radiating element 2 is connected to at least one
grounding spark-gap unit 7.
According to a preferred embodiment, the spark-gap unit
7 can be constituted by a spark gap that is provided with two electrodes 8a and
8b facing each other.
Conveniently, the antenna 1 is provided with means 9 for
adjusting the distance between the pair of facing electrodes 8a and 8b.
For example, as shown in Figures 2 and 3, one of the two
electrodes, for example electrode 8a, can be supported by a threaded element that
is coupled to a bracket 11: screwing or unscrewing the threaded element moves electrode
8a closer to electrode 8b.
Advantageously, according to a preferred embodiment the
fuse element 6 can be constituted by a microfuse supported by a printed circuit.
In particular, it has been found that the antennas 1 described
above may operate in single-band mode at a frequency comprised between 440 and 470
MHz or at a frequency comprised between 870 and 960 MHz. However, an antenna 1 may
also be of the multiband type and more precisely may operate in dual-band or tri-band
mode.
Operation of an antenna according to the present invention
is as follows.
Practically simultaneously with the flow of current, the
fuse element 6 fuses and vaporizes.
If the antenna 1, or rather its radiating element 2 (or
the unbreakable protective enclosure 10) collides with a high-voltage line, the
current flows through the antenna 1 following the path constituted by the radiating
element 2, the fuse element 6, and the inductor 4.
Due to vaporization of the fuse element 6, an electric
arc is established which in any case limits the current that will flow through the
inductor 4.
The current that flows through the inductor is further
reduced by the action of the spark gap.
The distance between the electrodes 8a and 8b is in fact
advantageously shorter than the length of the electric arc generated at the vaporized
fuse element 6, and this entails the triggering of an additional electric arc between
the tips of the electrodes 8a and 8b.
This subsequent triggering divides the current into two arcs, with a consequent
further limitation of the current that flows through the inductor 4, preventing
its fusing.
All the characteristics of the invention that are described
above as being advantageous, convenient or the like, may also be omitted or replaced
by equivalents.
Thus, for example, the fuse element 6 can be replaced by
a low-voltage capacitor, which when subjected to a high voltage is destroyed, simulating
the fusing of the fuse element 6.
The invention thus conceived is susceptible of numerous
modifications and variations, all of which are within the scope of the appended
claims.
In practice it has been found that in all the embodiments
the invention has achieved the intended aim and objects.
In particular, it has been found experimentally that an
antenna according to the invention can withstand the flow of intense currents for
times on the order of one tenth of a second without voltages that are dangerous
for users appearing on the feeder cable.
This result appears to be highly satisfactory also in relation
to the fact that the safety devices (such as disconnectors) that operate on high-voltage
lines have reaction times on the order of hundredths of a second.
