On 2/3/2013 10:55 PM, Eric NO3M wrote:
also the phase inverter
At the risk of being a PTA, let me remind all that phase is a
continuously variable quantity that is directly related to frequency and
time. It cannot be "inverted" or "reversed."
There is another parameter called "polarity," and it can be inverted by
reversing the wiring in a circuit (often using a two-winding
transformer), or by running through an inverting amplifier. Polarity
does not vary with frequency.
In the old days, when almost everyone here also used the word "phase" to
describe both things, but they are very different. That's unfortunate,
because it leads to most of us failing to realize that simply reversing
polarity in a transformer makes the cancellation not sensitive to
frequency, while cancellation produced by a half wavelength of
transmission line will be sensitive to frequency. Further, the
half-wave line will act as a resonant circuit, and can, in many
situations, be used to behave in a manner that it compensates for
resonance in an antenna.
The late Dick Heyser, a scientist at JPL and inventor of Time Delay
Sprectrometry, introduced those of us in the pro audio world to this
difference back in the 70s, and it helped us understand some rather
complex acoustic relationships. It is equally important in the RF
world, and it is very important in the design of any directional antenna
system.
73, Jim K9YC.
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Topband Reflector
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