Topband: Polarity and Phase

[Jim Brown]

>Actually there is a "polarity" reversal of the electric field
>when it reflects off the ground  - hence the null on the
>horizon for horizontally polarized antennas :):)

Yes.  
>
>
>> There is a trig identity that shows that there is a beat note produced
>> by this combination. I wouldn't call that a phase difference. Phase is
>> the angular difference between two vectors (phasors) that are rotating
>> at the same frequency. When they aren't at the same frequency, you've
>> got some complex relationship between them, but it isn't phase.  In the
>> above example, the carrier produces one beat, and each modulation
>> component produces another. The result is akin to modulation.
>
>I think we are arguing semantics here. My point is that
>when the beat note is sub-Hz, you can actually observe
>the baseband phasor rotate around the unit circle and
>you can indentify the phase as it rotates. When the
>beat note is larger, the phasor is rotating so fast, it
>no longer makes sense to do that (in that case you
>hear the beat note as a steady tone rather than as
>a series of alternating peaks and troughs). In any case,
>there is a very simply relationship between frequency
>and phase (phase is the integral of frequency). I think
>you are making it more complicated than it needs to
>be.

I think your example (an excellent one) helps illustrate how time,
frequency, and phase are so tightly related that it is hard for us to
get our brains around the relationships between them. W0UN alluded to
this complexity, and he is right. The work he described in his email
suggests that he does have his head pretty well around it. 

My point, though, is that the differences (and relationships) between
frequency, phase, and time, are NOT semantic, but are fundamental to
how our stuff works. 
>
>
>> >Mathematically, flipping polarity is the same as adding a 180
>> >degree phase shift to one of the local oscillators. The two are
>> >indistinguishable. In other words if I have two phase locked
>> >SSB receivers, flipping the "polarity" of the audio output
>> >of 1 receiver has the same effect as shifting phase of one
>> >of the two LO's by 180 degrees.
>>
>> ONLY if we are talking about a sine wave of a single frequency.
>>
>
>I am not sure that I agree in this case. If I shift the phase of
>one of the LO's in the two receiver diversity system by 180
>degrees, the phase of that downconverted signal coming from
>the mixer fed by the phase-shifted LO will be inverted at all
>frequencies. This 180 degree phase shift of the LO, will in
>essence be translated into a polarity shift.  If we were talking
>about adding a 180 degree phase shift to the RF path
>(instead of the LO), then I would agree with you that baseband
>polarity shift and the 180 degree RF phase shift would
>produce different results.  

I've studied your example again and think you're correct.  What you're
saying is that a polarity reversal at RF survives the conversion
process.  The question though, is, what happens in your example if one
LO is shifted in phase by a few degrees? Isn't that the equivalent of
tuning the phase shift of one element of an array?  This is what W0UN
was describing, but in a far more complex system. 

Jim