In reading the comments on this subject there would appear to be a
controversy, which is absolutely false, as I will show. In particular, phase
and
polarity are NOT the same thing, and I see no mention of the root of the
controversy.
That lies in the concept of the electrical magnitude of the signal in
question. The definition of polarity is really related to this concept, as the
following simple case shows.
Imagine a pulse generator with a 1% duty cycle and a fixed frequency say
100Hz. This signal appears as a voltage of 1 volt between a pair of terminals
(the
generator output) and we arbitrarily ground one of them. By the "signal" we
refer to the voltage between the hot output terminal and ground.
It is reasonable and customary to ask: "what is the polarity of this
signal?", because there are two possible interpretations of "1%". Do we mean
does the
pulse output rest at 0 and rise to 1 volt 1% of the time, or does the resting
voltage stay at 1 volt and drop to 0volts 1% of the time? Every engineer I
know would say that for the first case the polarity of the signal is +, and
the polarity of the second is -. Changing this waveform by shifting it 180
degrees in time will NOT change this, and the polarity will remain unchanged.
Hence
the polarity is a separate issue. It relates to how direction of the signal
changes, where by direction we mean + and - from 0 ( which is, surprisingly,
the most basic definition I know of 'polarity' - the polarity of currents or
electric charges.)
Suppose that this signal is 'biased' by adding a DC component of -12V (this
is commonly done and good signal generators frequently have provision for this
"DC shifting" of the output). Now the signal will ALWAYS be negative,
regradless of the state of the inverter which does the polarity shifting. What
do we
call the polarity then? There is no perfect answer to that question, because
the term 'polarity' relates to the idea of the actual signal amplitude. For AC
signals, it becomes necessary to establish more detailed definitions (i.e.
standards) to avoid confusion. For example, in the last case the prevailing
standard would focus on the actual polarity of the AC component of the signal,
which would be invariant under bias shifts. If the AC content had (-) polarity,
meaning it spends 99% of its time at the higher level, then it would be called
negative, etc.
Okay, that's more than we all need to know about polarity. For the case that
was a subject of reality here, the antenna signals are pure, symmetrical
sinewaves. If they are passed thru an inverter, or simply inverted by switching
the
antenna leads, the result will be indistinguishable from a 180degree phase
shift, because of the perfect symmetry between the two half cycles. Hence, in
this case it is true that a polarity switch (i.e. an inversion) is equivalent
to
180d. shift. For general AC waveforms, that is not true, although it will be
possible to find special sub-classes (e.g. all periodic, symmetrical
waveforms.)
73
Eric von Valtier K8LV
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