Topband: Polarity and Phase

[W0UN -- John Brosnahan]

>If the receivers are tuned to different frequencies, the resulting
>audio signals are of different frequencies, and phase has no meaning!
>Phase is only defined for sine waves of the same frequency!

Actually this is NOT true.  While there is no FIXED phase value, there
is a time varying phase relationship between each audio component.
While the resulting signal may not be easy to process with two ears
and the human brain, the results can be very useful.

Let me walk you though a quick example, FYI.

Say you have two identical receivers, tuned to the same frequency,
using separate antennas.  If you insert a fixed phase shift in one
antenna lead and combine the outputs of the two receivers you
get a beam formed by the physical relationship of the antennas
and the amount of phase shift inserted into one antenna.

Now if this phase shifter is changed to a new value the beam that
is formed will change positions.  And if this phase shift is varied with
TIME the beam will be continuously scanned--creating electronic
beam scanning.

I am sure we can agree on the above scenario.

You can accomplish the SAME thing by placing a fixed phase-shift
in one of the Local Oscillator lines to one of the receivers.   (This is
easier to grasp when you think in terms of a direct conversion receiver.)

If this phase shifter is then made to be time varying you once again
get the same electronic beam scanning.

Now since this is a time-varying phase shift you may obtain the mathematical
equivalent by OFFSETTING the two local oscillators by the frequency that
equals the value of the phase shift rate.   This then provides a constantly
varying phase shift and therefore a continuously-swept antenna pattern.
All without a phase-shifter per se, but rather with just two receivers
with offset local oscillators.

In fact this was the basis of the hardware I developed in the late 1970s
to do lightning interferometry.   For a reference to this system you can
check this reference.

Warwick, J. W., C. O. Hayenga, and J. W. Brosnahan, 1979:
Interferometric directions of lightning sources at 34 MHz.
J. Geophys.Res., 84, 245--2468.

To save you looking this reference up, the system consisted of two
34 MHz receivers that had BW of about 2 MHz.  The LOs were offset
by 200 KHz, thereby sweeping the lobes at this 200 KHz rate.

(W8JI's subsequent note talks about a difference of a few Hz which
would provide a "whossh whoosh" sound to the noise.  In the case
of the lightning noise at low VHF this "whoosh whoosh" is centered
at 200 KHz.  This "noise" was our signal and contains all of the
positional information needed in order to track the position of the VHF
emission of the lightning's stepped-leader--something that had never
been done up to that time.)

Of course this was before the advent of modern, high-speed data acquisition
(modern means something that a university could afford!).  Now this can
more easily be done by sampling both receivers and doing any phase-shift
type of processing in the digital world.  The following reference provides a
look at a more modern system using multiple receivers to locate multiple
targets in a radar system--in this instance it was for doing atmospheric and
ionospheric sounding.

Brosnahan, J. W. and G. W. Adams, The MAPSTAR imaging Doppler
interferometer (IDI) radar: Description and first results, J. Atmos. Terr. 
Phys.,
55(3), 203-228, 1993.

Other results that incorporate this type of multiple receiver signal
processing can be found in the following paper.

Hines, C.O., Adams, G.W., Brosnahan, J.W., Djuth, F.T., Sulzer, M.P.,
Tepley, C.A. and Van Baelen, J.S., Multi-Instrument Observations of
Mesospheric Motions Over Arecibo: Comparisons and Interpretations,
J. Atmos. Terr. Phys., 55, 241, 1993.

The following two patents by Adams and Brosnahan provide additional insight
into the mathematical formation of beams in the simplistic case and provide
greater insight into the formation of beams (or more precisely the position 
location)
of individual spectral components.  In other words if you have multiple 
"targets"
that have different frequencies or Doppler shifts you can locate each target
by doing interferometry on each spectral component.  In other words the 
phase relationship is maintained through the receiver chain as well as 
through the
complex FFT that is used to convert the signals from the time domain to the
frequency domain.

Patent Number    4,717,916   High resolution imaging doppler interferometer

Patent Number    4,630,051   Imaging doppler interferometer

Clearly this is way beyond the scope of a discussion on the Topband
reflector but it does indicate that there is MUCH that can be done to
improve performance on 160M (and the other bands) with multiple
receivers.  It only needs someone who is motivated to take up the task!
    ;-)

One thing that is possible (for example) is to locate the direction of
arrival of each audio component and eliminate all of the components
that are NOT from the desired direction.  This could be quite useful
both for CW and SSB QRM elimination.   Now that Sound Blasters
and 3 GHz P4 processors are widely available it should be relatively
easy!    ;-)

73--John  W0UN
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