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Scott, N0EDV wrote:
"O Now if I could only remember the math formula to figure the direction of steering---." All the simple options are bidirectional except the omni which results from 90-degree phasing between the two dipoles. You know that used separately, maximum radiation is broadside to the energized dipole. Fed in-phase or out-of-phase, the crossed dipoles have lobes at 45-degrees and 225-degrees, or at 135-degrees and 315-degrees. The figure-8 pattern is the same as from a single dipole but shifted plus or minus 45-degrees, depending on in-phase or out-of-phase feed of the two dipoles. Patterns of the crossed dipoles are similar to the oscilloscope display of the same signal fed to both sets of defllection plates but with a variable or selected phase angle between the plates. Just by selecting one dipole or the other you could have a north-south or east-west pattern. By quadrature feed of the two dipoles you get a near circullar pattern from the crossed dipoles. From two dipoles and a 90-degree delay for a non-directional pattern, and with some switching you get 5 radiation patterns. That`s pretty versatile. To get the 90-degree phase shift, a T-network with equal reactances in all branches is often used. For a 90-degree lag, coils X1 and X2 are in series with the load. Capacitor X3 connects between junction of the coils and the other side of the line. For a 90-degree lead, replace the coils with capacitors, and replace the capacitor with a coil in the T-network. It`s easy to remember the reactance values. X1=X2=X3=Zo= sq rt (ZinZL) The reactances may well be 50-ohms if we have a match to the usual load impedance. To adjust the phase lag of the T-network by as much as plus or minus 15-degrees without significantly affecting the magnitude of the shifter`s output, X1 and X2 are often ganged variable inductors. An imperfection in the phase shifter may result from uncertainty about its input and output impedances. Nevertheless, many T-network phase shifters are in use. Best regards, Richard Harrison, KB5WZI |
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