[evlatests] the P band polarization saga

[Rick Perley]

     Finally, some clarity ... (except that we are quite certain the
various antennas are connected differently, which means that
they all have cross-wiring, but in different places).

     I agree with Bryan here.  We should pick a standard, and cable
and label *everything* according to this standard.

     The intended standard:

     V dipole connected to RCP electronics, which becomes X on
the astronomy side,

     and

     H dipole connected to LCP electronics -> Y in the astronomy side

     is logical to me.

    This would mean tagging those cables with H
and V at each end, and ensuring that H is connected to L, and V to R.

     Also:  The T301 (4P Converter) should be labelled *only* with H and V,
and the T302 (LSC Converter) should be labelled 'bilingually' on the input.


     By doing this, we should ensure nobody else following us need be 
confused (as we have been, for so long ...).

     And all this should be written up and recorded in a logical place.

     Rick

On 03/12/2014 02:39 PM, Ken Sowinski wrote:
> I report on work done by Paul and Dan to sort out the polarization
> mysteries at P band.  The intent is to clearly establish the relation
> between RCP and LCP signal paths in the antenna with the dipole
> orientation considered, as H and V, or X and Y.  The former is the
> natural physical interpretation, H is provided by the dipole element
> parallel to the horizon when the antenna is pointed at the horizon;
> the latter is the convention established by the IAU in which the
> element providing X is the one which points to the North Celestial
> Pole.  The (H, V) system is natural and for an alt-az antenna and is
> easy for technical personal to relate to the hardware.  The (X, Y)
> system is appropriate for polarization post processing.
>
> The method was to monitor changes in power measurements along the
> signal path in the T302, T304 and samplers as cables between the
> dipoles and the T302 were disconnected, one at a time. all the tests
> were carried out at ea22.
>
> The wiring  scheme, as designed,  was to connect  the H dipole to the
> LCP signal path in the antenna, and the V dipole to the RCP path.
> Thus:
>
>      RCP path =~ V =~ X
>      LCP path =~ H =~ Y
>
> We found that when H dipole was disconnected from the receiver
> that power was decreased in the RCP signal path.  Similarly
> when the receiver H output was disconnected.  And again, when the
> V dipole was disconneceted, the power in the LCP signal path decreased.
> Everything was recnnected as normally and the input labelled "H"
> at the T301 was disconnected.  Now the power in the LCP path decreased,
> as expected.  Clearly there is a swap between the receiver output
> connector and the T301 input connecter.  Paul and Dan laboriously
> traced cables after this test and found that the color coded tags at
> the ends one pair of cables were mis-colored, but connected as if they
> were correct creating the signal swap we observed.  It is likely that
> this error was made in many antennas.
>
> So, what we really have is:
>
>      RCP path =~ H =~ Y
>      LCP path =~ V =~ X
>
> It is proposed that we simply define this as the connectivity and
> do not try to make the system agree with previous documentation by
> swapping cables in all the antennas.
>
> The final link in the chain is labelling the output of the correlator
> in the BDF.  The claim is made that if in the VCI document for a P band
> observation we replace R with Y and L with X, the CBE will respect
> those labels and order all the polarization products  properly in the
> BDF.  To achieve this a change is needed to model2script.
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