Wideband Discone Aerial For Vhf Receivers

48 WIDEBAND DISCONE AERIAL FOR VHF Most scanning receivers are capable of receiving signals over a relatively wide frequency range: 80 to 470 MHz with some gaps is not uncommon. Unfortu- nately, it is practically impossible to use a single aerial for the full frequency range covered by such a receiver. Most omnidi- rectional aerials for use in the VHF band have a bandwidth of about 10 MHz only. The gain of the discone aerial at any fre- quency within its pass-band is lower than that of the dipole reference dimensioned for that frequency. The disadvantage of the lower gain is, however, made good by the much greater bandwidth, which is a must for scanning receivers. Marconi aerial and ground-plane The discone aerial is derived from the so- called Marconi-aerial, which is essentially a vertical rod or wire with a length corre- sponding to a quarter of the wavelength of the radiated signal. The Marconi aerial does not work unless it is placed above a relatively large conductive surfac...