Discone antenna
- overview, summary, tutorial about the basics of what is a discone antenna
for wide band or bandwidth omnidirectional applications.
The discone antenna is widely used where an omnidirectional
wide band or bandwidth RF antenna design is needed. It finds many uses,
particularly for all type of radio scanning and monitoring applications from the
commercial or military monitoring services to the home scanner enthusiast for
frequencies above 30 MHz.
Discone overview
The discone antenna receives its name from its distinctive
shape. The RF antenna design consists of a top "disc" formulated from a number
of elements arranged in a disc at the top, and further elements pointing
downwards in the shape of a cone. Although the RF antenna could be made as a
full disc and a cone, this would considerably increases its weight and wind
loading, which would not be advisable from mechanical considerations.
This type of RF antenna design can operate over frequency
ranges of up to 10:1 dependent upon the particular design, and it also offers a
relatively low angle of radiation (and reception). This makes it ideal for VHF /
UHF applications as its greatest sensitivity is parallel or almost parallel to
the Earth. However towards the top of its frequency range it is found that the
angle of radiation increases slightly.
Although it is widely used for receiving applications, the
discone antenna is less commonly used for transmitting. There are several
reasons for this. Although it offers a wide bandwidth, it is not optimised for a
particular band of frequencies and is less efficient than many other types of RF
antenna design. Additionally the wideband with of the RF antenna means that
spurious signals can be radiated more easily and the level of reflected power
will vary over the operating range and may rise above acceptable limits in some
areas.
Physical aspects of the discone
The basic RF antenna design consists of three main
components: the insulator, the cone elements and the disc elements.
Of the RF antenna components the insulator size governs a
number of factors of the performance of the antenna. It is made from insulating
material and acts to hold the disc and cone elements in place, keeping them a
fixed distance apart. In fact this distance is one of the factors that
determines the overall frequency range of the particular RF antenna design.
Secondly, the cone elements should be a quarter wavelength at
the minimum operating frequency. This can be calculated from the formula A =
75000 / frequency (MHz) millimetres where A is the length of the cone elements.
Thirdly the disc elements should be made to have an overall
length of 0.7 of a quarter wavelength. This can be calculated from the formula B
= 52550 / frequency (MHz) millimetres. The diameter of the top of the cone is
mainly dependent upon the diameter of the coaxial cable being used. This
determines the upper frequency limit of the antenna. The smaller the diameter
the higher the frequency. For many designs operating in the VHF / UHF region of
the radio spectrum it is around 15 millimetres. The spacing between the cone and
the disc should be about a quarter of the inner diameter of the cone, i.e.
around three of four millimetres.
Operation
The way in which the discone operates is relatively
complicated, but it can be envisaged in a simplified manner. The disc and cone
elements sufficiently simulate an electrically complete disc and cone from which
the energy is radiated. As a result the greater the number of elements, the
better the simulation, although in reality there is a balance between
performance, cost and wind resistance. Often around six elements are used, but
the number is not critical.
In operation energy from the feeder meets the RF antenna and
spreads over the surface of the cone from the apex towards the base until the
vertical distance between the point on the cone and the disc is a quarter
wavelength. In this way it is possible for the energy to be radiated or received
efficiently.
The RF antenna radiates and receives energy that is
vertically polarised, and the radiation pattern is omnidirectional in the
horizontal plane. The antenna radiates most of the energy at a low angle which
it maintains over the most of the operating range. Typically there is little
change over a range of 5:1 and above this a slight increase in the angle.
With the feed point at the top of the RF antenna the current
maximum point is also at the top. It is also found that below the minimum
frequency the antenna presents a very bad mismatch to the feeder. However once
the frequency rises above this point then a reasonable match to 50 ohm coax is
maintained over virtually the whole of the band.
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