# RE: commutating antennas (was Re: Canted S-band antennas on satellite)

```At 08:11 AM 5/3/2002, Tom Clark \(W3IWI\) wrote:
>Historically, one of the serious problems with this approach is spin-induced
>phase modulation. The antenna phase center -- the point (or set of points)
>from which the energy appears to start -- is not coincident with the spin
>axis. If the radius of the antenna element is R, then you can get a phase
>shift of a significant fraction of R wavelengths as the satellite spins; a
>50 cm sized satellite would have multiple wavelengths of phase shift at a
>wavelength like 13cm.

Yes, the geometry does create phase modulation.  If we think of a patch
antenna (or small array of patches) in the middle of each face of a
hexagonal satellite.  Lets assume the phase center is right at the surface
of the patch.  Every 60 degrees of rotation, there is a new
antenna.  Maximum phase shift occurs between the two extremes of an antenna
pointed directly at you, and a rotation 30 degrees from this condition,
where you are in between two antennas.  Lets compute the phase shift
between those two conditions.  The phase shift is due to the distance
change between you and the antenna's phase center as the satellite
rotates.  If the distance from the rotational axis to the face, and
therefore the antenna, is R, then I believe the distance change due to a 30
degree rotation is (1/(cos 30 degrees) -1)R = 0.155R  If R were, say 25 cm,
(presuming that your "50 cm" number was a diameter) then the distance
change would be 3 cm.  Now,  suppose a 13 cm band downlink frequency as you
did.  3 cm / 13 cm x 360 degrees = 83 degrees of phase shift.  That's less
than "multiple wavelengths" as you stated.  Did I compute this correctly?

Note also that this is a nice gentle nearly sinusoidal modulation.  It is
also very slow.  If the satellite were to rotate at 1 rev/second then the
modulation frequency would be 6 Hz.

>If you are using a mode where phase is of little importance (SSB, CW, FM)
>then you will hear a whoompa whoompa modulation.

If I put 6 Hz 83 degree modulation on an FM signal, and feed it to your
ordinary FM receiver, you think you'll hear the modulation on the audio
output?  Won't that come out as an additive 6 Hz signal in the audio?  That
is outside the audio passband of the receiver, and also your ears.

For SSB it is does seem likely that you would hear it.  I haven't
considered that question carefully.  I was considering digital modulation
carrying voice when I initially thought about commutating antennas, so SSB
didn't really enter into my consciousness.

Note however that our EXISTING spin-stabilized satellites have spin
modulation that you can hear, so this would be nothing new.

>If you are using a digital mode where phase is of importance, the PLL will
>be pulled with this phase modulation. The demod PLL's will have to be
>designed to track thru this modulation.

Absolutely, and I think it is doable.

Depends of course on other parameters.  The loop bandwidth would naturally
be wider in a high data rate demodulator, such as one would have in a TDMA
system, such as the one that Phil Karn proposed awhile back, and thus
probably naturally wide enough to handle this modulation.  Other schemes
might naturally want to have narrower loops, which would not naturally be
able to track this modulation.  In that case one might need to do something
a little special.

One could also ask the satellite transponder to remove the phase
modulation, although I am not yet proposing this.  The satellite software
will know the times that the antennas switch, so it knows the rotational
phase of the spacecraft.  Look that up thru a little table, and drive a D/A
to a varactor phase shifter?  Doesn't sound terribly difficult.

>I'm sure Karn will suggest that proper application of coding/interleaving
>can make the data losses be tolerable.

I do not believe that one need rely on coding/interleaving to deal with
this little phase modulation.  Of course, no one should build digital
communications today without good application of coding/interleaving.  No
one should ignore a 40 year old technology that produces such great
benefits.  To do so would be a stupid as designing without transistors.

Tom: Do you have a reference on the location of phase center for different
kinds of antenna?  I know you're into this, as I heard you give a talk on
errors due to various GPS antenna characteristics once. :-), including the
fact that the phase centers of many antennas at the 2 GPS frequencies are
not in the same location.

Someone somewhere has probably written a survey paper or book chapter that
shows the phase center of different kinds of antennas, eh?

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