...Discs & Cones

- Broadband performance for airband and surveillance

Used by the military in the second world war and patented by Armig Kanddoian of New York in 1945, the discone antenna provided a practical solution for air and surveillance communications. The new system not only achieved a wide bandwidth, but also a aerodynamic design that was inherently balanced for wind loading, keeping torque to a minimum...

Essentially, the discone is a modification of the biconal antenna, where the second upper cone is replaced by a disc. The cone itself can be compared with one half of a dipole. The function of the disc is similar to that of a mirror, reflecting the radiation from the cone below to make up the full half wave. The shape of the cone provides broadband characteristics with stable impedance across a wide range of several octaves in frequency, uniform gain in horizontal directions and omnidirectional radiation. These characteristics have made it a popular solution for use in commercial and military radio scanning and monitoring and also with the home scanner enthusiast, as well as for the 225-400 MHz MHz air band both on aircraft and as fixed stations.

The size of the discone is governed by frequency and increases as the frequency is lowered. It is frequently used at VHF and UHF frequencies, where it is sometimes possible to cover both bands, depending on required VSWR. The sizes required at HF frequencies are considerable, usually making it impractical. A single discone covering 3 to 29 MHz, for example, would require a 23 metre (75ft) vertical structure with a base diameter of 20 metres (65ft).

While this type of antenna can be used for transmitting, it is more usually found in receiving applications. This is because its wide bandwidth makes it subject to more interference and levels of reflected power can vary over the operating range. Also, antennas that are designed over a smaller frequency range, can usually provide better performance and greater range. Discones which cover extremely wide bandwidth in the one receiver with one feed line for example from 25-1,000 or 2,000 MHz are not likely to provide good performance over the whole range. When the 30-50 MHz and 350-470 MHz scanner bands are to be covered, it is usually better to use a second discone with separate feed line above 400 MHz as well.

Discones can operate over a range of ten times the base frequency, so that, depending on the individual design, an antenna with a base frequency of 140 MHz can work up to 1.4 GHz with a VSWR of 2:1 up to the second harmonic and 3:1 above this. Within the resonant range, the antenna provides an excellent match to 50 ohms. Below the minimum frequency, however, the VSWR rises rapidly.

VSWR Plot Moonraker BDA500: 450-550 MHz

These antennas are very stable electrically and are ground independent with essentially consistent unity gain across the frequency range.. Lack of gain can be improved by low loss feeder cable and amplification of the received signal. While they are not affected by precipitation static or radial ice build-up, the far field pattern can be obstructed by objects in the Fresnel zone.

The design consists of a top conducting disc with a vertical cone beneath, separated by an insulator. The insulator supports the disc and fixes the spacing between disc and the cone, thus determining the overall frequency range of the antenna. The disc and the cone are balanced for wind loading to minimise torque caused by the wind. When a vertical whip is placed above the disc to extend the low frequency limit, antenna characteristics may be changed to resemble a ground plane antenna or coaxial dipole having a detrimental affect on the higher frequency response.

The coaxial feed outer conductor is connected all around to the top of the cone and the centre conductor is connected to the disc. The diameter of the disc, height and base diameter of the cone and degree of spacing between the two are all chosen to give the best possible impedance match over the desired bandwidth. Dimensions are determined by the lowest frequency of use.

If the cone elements are electrically long, the antenna will resemble a long wire antenna and the low elevation angle response will be severely distorted. The diameter of the top of the cone, which is mainly governed by the coaxial cable diameter, determines the upper frequency limit with the smaller the diameter the higher the frequency. Actual impedance depends on the cone angle, frequency and disc to cone spacing.

At the higher frequencies and especially in aircraft types, the disc and cone are often made of solid sheet metal, providing considerably increased weight and wind loading. At lower frequencies, closely spaced radials are often preferred, and indeed the only option at HF frequencies where sizes are considerable. Using radials simplifies construction but reduces wind loading. The greater the number of elements used, the better the radials will simulate the solid disc and cone. If as few as 8 are used, there will be loss of around 1 to 2 dB. Using less than 8 will result in considerably reduced efficiency.

Due to the fact that maximum current area is near the top of the antenna away from ground interference, radiation characteristics are like an ideal dipole antenna in free space. For this reason,. discone antennas are ideally suited to mast mounting or on top of buildings. However, unlike the dipole, impedance varies little with changes in frequency.

Also, because they are not tuned to a centre frequency, they act like a high-pass band filter, giving efficient performance from the base frequency to the uppermost frequency of the individual design. This factor can also help eliminate spurious signals arising from cross and inter modulation, high background noise, false signals and other interference. However, being wideband, they can radiate unwanted emissions from faulty or improperly filtered transmitters.

Discones are vertically polarised with low angle omnidirectional radiation over most of the operating range, changing little over a range of 3:1 frequency range. Currents in the skirt wires flow mainly in the same direction.

Radiation results as energy from the feeder spreads over the surface of the cone from the apex towards the base until the vertical distance between the point of the cone and the disc is a quarter wavelength.

The relatively low angles or radiation and reception, somewhat like a quarter wave monopole, make them ideal for line of site ground to ground and ground to air VHF and UHF communications.

Above this, the useful lobes decrease as two new lobes grow. As a result the angle of radiation increases somewhat in the pattern of a biconical horn radiator, with higher angle lobes which are less than optimum for ground wave and direct signal reception. Radiation from the disc on top is minimal as currents in the disc elements oppose each other and cancel out.

A discone antenna operating at ten times the base frequency acts like a long wire antenna. As a result the pattern is very distorted, especially at low elevation angles.

Due to the fact that maximum current area is near the top of the antenna away from ground interference, radiation characteristics are like an ideal dipole antenna in free space. For this reason,. discone antennas are ideally suited to mast mounting or on top of buildings. However, unlike the dipole, impedance varies little with changes in frequency.

While discones are normally mounted as high as possible, at the lower end of the VHF band it is also possible to install at ground level, though there will be some loss in efficiency. The low elevation will also have some affect on propagation.

Although the disc of the discone is normally uppermost, it is also possible for the cone to be on top. In this case, the cone is supported by low loss dielectric pillars or a thin walled dielectric cylinder, taking care not to impede the critical feed region. Sometimes a solid disc is combined with cone radials.

Biconical forms are also used, replacing the disc by a second cone. The two cones which face in opposite directions have a common axis and vertex, and are driven by potential, charge or an alternating magnetic field at the vertex. Radiation is omnidirectional in the azimuth plane with elevation patterns varying with frequency.

BCA200-1300 200 MHz

BCA200-1300 1160 MHz

While biconicals are vertically polarised, it is also possible to achieve 45 degree slant polarisation. With slant polarisation wider bandwidths are achievable up to 12:1.

At HF frequencies, it is possible to use a number of types, although there may be constraints due to size restrictions. Inverted cones systems, which are both vertically polarised and omnidirectional, can provide extremely wide bandwidths for marine and airband communications. Here again, elevation patterns change with frequency.

The Inverted Cone

Where sufficient space is not available for an inverted cone, a conical monopole type, which offers similar performance, may be a better prospect.

The Conical Monopole		In this type two cones are connected base to base and supported by a single metallic grounded mast or tower. Usually the radiating elements are connected to the top and bottom discs, but may also be connected to a disc at the point where the two cones meet to form an inductive loop. This acts as a built in impedance matching circuit and provides low VSWR despite the deficiency in antenna height. A conical monopole operating from 2-14 MHz would have a height of around 25 metres and a radius of 20 metres. If either the Inverted Cone or the Discage are used, an earth system of around 60 x quarter wavelength radials will normally be required.

A further variant at these frequencies is the Discage antenna, which features an upper disc above two truncated wire rope cones , connected at the base and supported by a central mast. The lower section operates as a cage monopole from 4 to 12 MHz, the upper part acts as a discone radiator from 10 to 30 MHz. A matching network is used to limit VSWR to <3:1.

The Discage Antenna

The proliferation of dedicated antennas associated with single purpose communications systems has created a demand for antennas with wider and wider bandwidths, especially in the surveillance field. Frequently the bandwidths of traditional types are limited by high VSWR. Disc and cone antennas can be particularly useful due to their consistent VSWR over wide bandwidths and omnidirectionality.

When looking at broadband antennas like the discone, it is important to remember that quoted frequency ranges will relate to the intended use. An antenna with a large frequency range and a VSWR of <5 may be quite acceptable when used for surveillance monitoring but may be totally unsuitable if it is to be used for transmission purposes. The main limitation, however, is likely to be the radiation characteristics over the required bandwidth, rather than the VSWR.

To determine antenna suitability, therefore, it is wise to consult radiation patterns. Elevation angles will be important where both high and very low angles are required for ground to air and ground to ground communications – especially where antennas are elevated.

Discone Systems