Topband: Phase vs. polarity

[K8LV1 at aol.com]

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