[evlatests] Bandpass Ripple in ea28, RCP, L-band

[Rick Perley]

     I'd like to return to this topic, with some more information.

     There is no doubt that the W8 RFI monitor is the guilty party.  The 
new information is this:

     The data came in three configurations:  For the first, ea24 was on 
W8.  For the second and third, ea28 was on W8.  In reviewing the 
calibration, I find:F

     a)The rapid oscillation noted before is on these antennas -- and 
only these antennas.

     b) The cross-polarization signals (RL and LR correlations) are 
*completely ruined* on all baselines connected to ea24 for the first 
configuration, and ea28 for the other two.

     The reason for this cross-polarization problem is that the 
fine-scale ripple is both extremely large, and extremely time variable.  
So there is no way we can realistically 'calibrate it out'.      No 
doubt the parallel-hand signal (for the polarization used for that 
monitor) is similarly affected by the time variability.

     The fact that this problem was easily visible on two different 
antennas (which occupied the same pad -- W8), on three observations 
spanning many months, strongly suggests that many other observations 
have been affected.

     Hence my questions:

     A) Has the current problem (with ea28) been repaired?

     B) Do we really need this monitor?  Unless the reflection 
associated with this monitor is reduced by at least 20dB, the 
observational data cannot be used.


On 9/3/19 2:42 PM, jjackson via evlatests wrote:
> Wes and I just had a chat about this and our chief suspect would be 
> the W8 RFI monitor cable that is connected to L-band on this antenna. 
> The length is just too long for anything else to make sense on a 
> single polarization.  30 meters would be about right for the length of 
> the cable running down the antenna.  We'll have someone take a look at 
> it.
>
> Jim
>
>
>
>
>
> On 9/3/2019 11:01 AM, Rick Perley via evlatests wrote:
>>      Bandpass solutions from my Cygnus A observations, taken in April 
>> and August this year show a strong bandpass ripple in RCP only, for 
>> antenna 28 and L-band.  The period of the ripple is about 4.6 MHz, 
>> corresponding to 30 meters if the cause is a reflection.  The problem 
>> was not visible in data taken in December last year.
>>
>>      The ripple pattern is strongly time dependent (both in its 
>> amplitude and phase), so is very difficult to calibrate out.
>>
>>
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