ISS setup 27 March 2022

Cloudy and cool in the coastal region with light mist at times.  Clear and windy inland of the Santa Ynez Mountains and at Sedgwick, with a high in the mid-60's

Spent the day testing the 915 MHz antenna ISS site #3 (which happens to be Antenna #3).   When I first started up the portable VNA, the cursors remained at their last locations, and indicated that the last patch that I measured had a better input return loss (IRL) at 909 MHz.  The VNA was calibrated from 900-930 MHz to get a wider look at any trends in the IRL measurements.   The IRLs for each beam were measured looking into the main antenna port.   There were no clear resonances or frequencies across the frequency range where the antenna's IRL was better than 16.7 dB, with most of the beams producing IRL numbers in the 6-14 dB range.   A video of the measurements was taken but is too large to be uploaded on the Wiki, so I uploaded to the shared APG_team_drive Google Drive (https://drive.google.com/file/d/1NZghXvnDb9MmcyK15yRBfzkZDmG4tLk2/view?usp=sharing).   These IRLs are quit poor when compared to the IRLs of the 'good' antenna (at ISS site #2) which were better than 22 dB for all the off-vertical beams.  This is indicative that something is wrong with the antenna.

I then pulled off the access panel on the bottom of the antenna and then measured the IRLs looking into the eight 1:4 way splitters that feed the patch antennas, both in the X and Y polarities.  Measurements taken at these points are after the input connector and relay banks and just examine the splitters and the patch antennas themselves.  Here are images showing how the splitters for the X polarization are numbered (Y polarization splitters numbered in the same manner) and an image of how the measurements were taken, with the VNA plugged directly into the splitter inputs.

The IRL's looking into the splitters did not look good either, i.e. no sharp/deep resonances where the IRL exceeded 20 dB.   Since the IRL of a combiner can be no better than the worse IRL of the patches that are connected to the splitter's outputs, the poor IRL issue is with the antenna patches themselves.  The issue also appears across all the splitters, so it appears to be affecting the entire antenna.   Here are the IRL images looking into the X1-X4 and Y1-Y4 splitters and a summary of the measurements.

Splitter       IRL at 915 MHz     Frequency with best IRL & IRL value

   X1                 15.0 dB                          909.9 MHz, 17.8 dB

   X2                 12.5 dB                          911.7 MHz, 13.0 dB

   X3                 18.9 dB                          914.2 MHz, 19.3 dB

   X4                 14.2 dB                          911.9 MHz, 15.1 dB

   Y1                 13.5 dB                          909.6 MHz, 15.9 dB

   Y2                 16.0 dB                          913.0 MHz, 16.4 dB

   Y3                 18.5 dB                          914.2 MHz, 17.8 dB

   Y4                 15.1 dB                          912.4 MHz, 15.7 dB

It is noted that X3 and Y3 feeds are close to having their best IRL at 915 MHz and also have the maximum IRL values.  Unfortunately, the four elements that each of these splitters feed are on different subpanels, as is true for the other splitters, so whatever is going on with the antenna elements is not isolated to one of the subpanels.

In summary, the antenna is not resonating well at any frequency between 900-930 MHz.    The issue has been tracked to the output splitters and antenna elements, though these findings don't rule out other potential problems with the relays as we did find at least one 'sticky' relay during earlier testing on the 3/13/22.  However, fixing the relay(s) will not solve the problem with the elements not radiating well, which must be fixed before the antenna will work with the system.

There is a coating of dirt/dust on the surface of the antenna, under the radome, that could be causing the degraded performance, especially if the dust has an ferrous material in it.  However, this is a long shot and I suspect the issue is with the antenna substrate material, i.e. it has degraded in some way(s) over the years, such as material has dried out, or has developed cracking, or has otherwise become lossy.   Tomorrow the plan is to remove the clutter fence and radome, sweep off the dirt/dust and then wipe the antenna surface clean to see if that has any effect.   I have talked to Steve and it sounds like the surface group can help me in the morning to remove the clutter fence and radome.  

More fun (and measurements/data) tomorrow!