Discone antenna
| Discone antenna | |
|---|---|
| Type | Wideband omnidirectional antenna |
| Invented | Armig G. Kandoian, mid-1940s |
| Construction | A disc above a cone, usually made of rods |
| Typical use | VHF and UHF receiving, scanners and SDRs |
| Performance | |
| Polarization | Vertical |
| Pattern | Omnidirectional in the horizontal plane |
| Gain | About that of a dipole, or less at the top of its range |
| Lowest frequency | Set by the cone, whose sides are about a quarter-wave long there |
A discone antenna is a wideband, vertically polarized antenna made of a flat disc mounted just above the point of a cone. It covers a frequency range of many times its lowest frequency with an acceptable SWR and no adjustment, which makes it the standard antenna for scanners, software-defined radios and anyone who wants to listen across the VHF and UHF spectrum with a single antenna. It can also transmit, though it does no job as well as an antenna built for one band. The design was developed in the mid-1940s by Armig G. Kandoian.
Construction
In a discone the center conductor of the coax connects to the disc and the shield to the top of the cone, with a small gap between them. Solid metal works, but most practical discones replace the disc and the cone with sets of rods, often eight or more of each, which cut weight and wind load while behaving almost the same electrically. The disc is somewhat smaller across than the base of the cone.
The size of the cone sets the lowest usable frequency: its sides are roughly a quarter-wavelength long there. Below that frequency the SWR rises steeply, so a discone acts like a high-pass filter. Above it, the SWR stays moderate over a very wide range. Commercial discones sold to hams and scanner listeners are commonly advertised to receive from about 25 MHz to well over 1,000 MHz, often with a vertical whip on top to extend the low end, and specify transmitting only on certain amateur bands.
Performance
A discone radiates equally in all horizontal directions, with a pattern like that of a vertical dipole at the low end of its range. Its gain is about that of a dipole, or a little less. At higher frequencies the main lobe tends to tilt upward, so less energy goes toward the horizon, where most terrestrial signals are.
That is the price of bandwidth. On a single band, a dedicated antenna does better: a 2-meter collinear vertical has several decibels more gain toward the horizon, and a Yagi much more in one direction. Because the discone is vertically polarized, it is also a poor match for weak-signal SSB and CW work on VHF and UHF, which uses horizontal polarization.
Uses
- Receiving. The discone's main strength is covering aircraft, public-service, amateur and other services on one antenna. It is a natural partner for an SDR, whose screen can show a wide slice of spectrum at once.
- Transmitting. For FM simplex and repeater work on 2 meters and 70 centimeters at modest power, a discone is usable, and some hams use one as a general-purpose station antenna. Stay within the manufacturer's power rating and listed transmit bands.
- Monitoring. A discone makes a good second antenna for listening to one band while transmitting on another antenna, or for checking which frequencies are busy.
Installing
Mount a discone as high and as clear of surrounding objects as practical, since VHF and UHF signals travel mostly by line of sight. Use low-loss coax, because cable loss rises with frequency and a long run of thin coax can waste most of the signal at the top of the discone's range.
A wideband antenna also delivers strong signals from outside the bands of interest, such as FM broadcast stations, pagers and cellular sites, and they can overload a sensitive receiver. A band-pass filter or a broadcast-band notch filter at the receiver often helps, and adding a preamplifier can make overload worse rather than better.