Topband: Polarity and Phase

[Michael Tope]

----- Original Message -----
From: "Jim Brown" <jim at audiosystemsgroup.com>
> >Phasing the receiver audio output is absolutely no different that phasing
> >two antennas, and flipping one speaker lead is no different than flipping
an
> >antenna phase 180 degrees.
>
> You are absolutely incorrect! First, you don't understand (or are
> unwilling to admit) that phase and polarity are different. When you
> reverse the wires that connect elements of a system (e.g., a receiver,
> transmitter, loudspeaker, microphone, or a dipole), you are changing
> the polarity of the system. When you change the spacing between
> antennas, or the length of a feedline between elements of an array, you
> are changing the phase, but you are not (usually) changing the
> polarity.  What's the difference?  Well, obviously, the feedline will
> be a different electrical length, and thus produce different phase
> shift, at every frequency. We know, of course, that the phase
> relationships are a big part of what produces the patterns of our
> antenna arrays.
>

I think this last statement is a red herring, Jim. While it is true
that a feedline will have a different phase shift at every
frequency, that doesn't mean that this phase vs. frequency
behavior is a prerequisite for synthesizing an antenna pattern.
It simply means that real antennas will in most cases have finite
pattern bandwidth.


> Example -- the 180 degree out of phase azimuth(s) for a an array of
> vertical antennas ARE, indeed, where the null(s) will be. But the
> radiation from the elements of the array will ALSO be out of phase with
> each other in every other direction, and to a varying degree. In these
> directions, the pattern will be some intermediate value between its
> peak and its null -- as the phase between the signals from the array
> elements varies CONTINUOUSLY. And we know that we can change the
> azimuths where the nulls are by varying the phase shift -- the length
> of the feedlines, the spacing between the antennas, and the azimuth
> between the array elements.  In this paragraph, every use of the word
> "phase" is correct. In your sentence, quoted above, the first use of
> "phasing" appears to mean "polarity," the second use is not clear, and
> "flipping one loudspeaker lead"  means inverting the polarity.
>

Actually if you want to split hairs Jim, by your own argument
below, the use of phase shift to describe the length of the
feedlines or the spacing between the elements in an antenna
is technically incorrect. These are time delays. The phase shift
associated with these delays varies as a function of frequency.
For an ideal matched non-dispersive line, the phase will
vary linearly with frequency. In the real world, the phase vs.
frequency response of a "delay line" will be non-linear.


> When you write carelessly like this, you confuse people, and you also
> tend to muddy the thinking of folks who are trying to learn. More
> important, the sentence quoted is technically incorrect, even if we
> were to rewrite it substituting "polarity" where you mean it.  This
> difference is at the crux of why it is so difficult to get good
> broadband performance from a directional array. I learned this as a kid
> when I drove through the nulls of a 4-tower AM directional antenna
> system a few miles from my home. The null for the carrier was deeper at
> a different azimuth than for the sidebands. The phase shift WAS 180
> degrees at the null for the carrier frequency at some point, but it was
> a different number of degrees at the frequency of the audio sidebands,
> and their cancellation was far less.  Those phase relationships were
> NOT created by "flipping" wires -- they were carefully created by the
> antenna spacings, their physical locations, the length of the feedlines
> between them, and even phasing networks.

Yes, everything you say is correct, but as Tom points out, you
can still null two "in-phase" signals by flipping wires at the
point where the baseband signals are combined the same
as you can null the signals by adding a 1/2 wavelength long
transmission line in front of one of the receivers or at the
output of one of the two LO's.

> This is also incorrect. When you the oscillators (in a CW or SSB radio)
> are on different frequencies, you are "uncorrelating" the signals, and
> "phase" has no meaning. When they are free running but fairly stable,
> you may, by some happy accident, be able to get them close (even rather
> precisely) to the same frequency.
>

Not really, Jim. Take an old rig like a Drake R-4C, put it in
SSB mode, and then tune it to an AM signal. If you are
careful, you can adjust the VFO for an almost perfect zero
beat. As the phase/frequency of the PTO in the receiver starts
to drift from this point (which it invariably will do), you'll be able
to hear the carrier phase start to rotate relative to the PTO
frequency. At first it starts out very slow when the frequency
offset is sub-hertz then gradually speeds up as the PTO drifts
off frequency. When the offset gets large enough, you'll no
longer be able to count peaks and troughs in the detected
audio and you'll start to hear a steady beat note (probably at
10 to 20 Hz). I find it helpful to picture a rotating vector. The
position of the vector is the phase of the signal, and the
angular velocity of the vector is the frequency of the signal.
In any case, that slow beating you hear when the offset is
very small is the phase difference between the AM carrier
and the PTO rotating around the unit circle. In this case, the
phase definitely has meaning. At large offsets, the phase
is rotating so quickly that all you hear is the "beat note".

> The use of "phase" when we mean "polarity" is an unfortunate
> anachronism, and laziness, and it confuses people. Yes, when you and I
> grew up, it was common usage. But as engineering has matured, we have
> learned to use the right word to describe polarity separately from
> phase.

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.

> There is also the issue of time. Add some delay, and you have added
> phase shift that is directly proportional both to time and frequency.
> This is not polarity either, nor is it phase. It is time.

Yes, you are correct that a constant time delay produces a
phase shift that varies linearly with frequency. But, if the
bandwidth of the signal is small relative to the carrier
frequency, then a time delay element (such as a matched
transmission line) produces - to a good approximation -
a fixed phase shift.

>
> Polarity is not a function of space, or of time, or of frequency. These
> are fundamental concepts, and understanding how they interrelate is key
> to our understanding of how antennas (and lots of other systems) work.
> It is unprofessional to make that understanding more difficult by using
> the wrong words to describe what's going on.
>

Yes, I agree. The use of precise language in engineering is
very important, especially when large sums of money are
involved :)

73 de Mike, W4EF................................