# Discone antenna

### The discone antenna is able to provide a bandwidth of around 10:1 with an omnidirectional radiation characteristic, making it ideal for many uses in the VHF and UHF portion of the spectrum including as a scanner antenna.

Discone antenna includes:
Discone basics     Buy best discone antenna

the discone antenna or discone aerial is often used where an omnidirectional wide band or bandwidth rf antenna design is needed.

## Discone antenna basics

although sometimes called a scanner antenna, the discone antenna receives its name from its shape: it consists of a top disc underneath which is a cone.

the discone antenna 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.

## Physical aspects of the discone

the basic rf discone antenna or scanner 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. it is represented by the distance d on the diagram.

$A=\frac{75000}{f}$

Where:
A is the length of the cone elements in millimetres.
f is the frequency in MHz

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:

$B=\frac{52550}{f}$

Where:
B is the length of the disc elements in millimetres.
f is the frequency in MHz

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. it is represented by the distance c on the diagram.

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.

## Additional vertical element on discone

the downside to the addition of the vertical element is that it can reduce the efficiency at higher frequencies.

## Discone operation

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.

in terms of its operation requirements, the antenna is self contained. unlike various vertical antennas it does not require the use of any ground connection, etc. also the discone is an unbalanced antenna and can be fed directly from a coaxial feeder.

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