Posted 05 March 2013 - 12:29 PM
Phased arrays and Yagi-Udas have been around a very long time.
The typical phased array TV antenna has its dipoles a half wave length long in total length for each one. Reflectors are usually 10 % longer. This also applies to the Yagi-Uda Vertically the dipoles are connected with a quarter wavelength long harness, with the dipole/reflector a half wavelength spacing to the one above. At the junction of the quarter wavelength harness, the signal is available. Putting the dipoles in parallel reduces the impedance. The folded dipole acts as if it is a pair of dipoles. The quarter wavelength connection increases the impedance. Now the result is a 300 Ω output. In the latest Hills catalogue they are including a balun to go from 300 Ω balanced to 75 Ω.
The old Hills CA16 4 bay phased array was designed for channels 6 - 11 ie 174 - 222 MHz. The current use is from channels 6 - 12 which is 174 - 230 MHz.
The centre frequency for the old requirements is 198 MHz giving a half wavelength of 755 mm. The new requirements 202 MHz, 740 mm.