:: : : : For a half-wave antenna you would need 1.5 metres long antenna for 100Mhz, but half-wave dipole is not the only type there is out there . . .-- G 08 : 25, 10 September 2007 ( UTC)
42.
The most common form of log-periodic antenna is the "'log-periodic dipole array "'or "'LPDA "', The LPDA consists of a number of half-wave dipole driven elements of gradually increasing length, each consisting of a pair of metal rods.
43.
Mills said it " . . . consists of two rows of 250 half-wave dipole elements backed by a plane wire mesh reflector; the individual dipoles are aligned in an E-W direction . " The cross operated at a frequency of 85.5 MHz ( 3.5m wavelength ), giving a 49 arcminute beam.
44.
Many types of coaxial cable have a characteristic impedance of 75 ?, which would therefore be a good match for a half-wave dipole, however coax is an unbalanced transmission line ( with one terminal at ground potential ) whereas a dipole antenna presents a balanced input ( both terminals have an equal but opposite voltage with respect to ground ).
45.
The antenna itself consisted of a series of 36 half wave dipoles backed with reflectors, arranged in three bays, each bay with twelve dipoles arranged in a three-high four-wide stack . ( Later production versions of the SCR-270 used 32 dipoles and reflectors, either eight wide by four high ( fixed ) or four wide by eight high ( mobile ) ).
