Crossed-wire antenna feeding configurations
Shown below are some of the various possible feeding arrangements for a horizontal cross-dipole antenna.
|Simple dipole (or "doublet").
Un-driven pair interacts minimally with driven pair. Radiation pattern can be switched through 90°. The two dipoles can be driven simultaneously by different transmitters (although reception via one may be a little tricky when transmitting into the other).
|Splayed dipole (Double V).
Radiation pattern is approximately conserved and rotated by 45° relative to simple dipole at low frequencies. Long-wire radiation pattern at high-fequencies is spread-out and modified. Radiation pattern can be switched through 90°. Input impedance is half that of the simple dipole.
|Double T (Vertical Null).
Vertical radiation is cancelled. Forces low angle radiation. Prevents NVIS working.
Electrical length of parasitic section is altered by adjusting load reactance (inductance, capacitance, open circuit, short circuit). Max radiation occurs in the quadrant indicated when parasitic section is electrically longer than the driven section.
Driven Director / Driven Reflector (pseudo Yagi).
Produces radiation patterns similar to that of the Yagi configuration.
Suitable choice of reactances will concentrate radiation in the quadrant adjacent to or opposite from the generator.
Gives ad hoc modification of radiation pattern depending on frequency. Useful on 80m band and above (in freq.). See far field simulations.
Gives ad hoc modification of radiation pattern depending on frequency. Useful on 40m band and above (in freq.).
Additional directional effects can be obtained by using the 2+1 asymmetric feed with a reactance connected from the spare wire to either end of the generator. See far field simulations.
Gives elliptically (i.e., approx. circularly) polarised radiation in vertical direction. Direction of rotation can be changed by swapping delay-line output connections.
A similar effect can be achieved using 2 ganged phasing reactances.
David Knight, 2008