[evlatests] The 128 MHz comb zoo ...

[Rick Perley]

    I've labored hard to try and understand the characteristics of the 
128 MHz comb -- largely without success.  The characteristics vary by 
band and even by frequency within a band. 
    But a few general characteristics seem clear. 

    A)  Harmonics of 128 MHz are seen in L, S, and C bands.  Only a 
single harmonic is seen in X-band -- at 8192 MHz.  This one probably has 
a separate origin within the T304. 

    B)  The odd harmonics are usually, but not always, the strongest.  
Not all odd harmonics are visible, and some even harmonics are very 
strong. 

    C) There is no spatial correlations for these harmonics -- the 
'loudest' antennas (which vary by band -- see below) are randomly 
distributed.  This clearly indicates an internal origin. 

    D) At L-band, antennas 9, 10, 14, 20, 24 and 26 appear to have no 
128 MHz harmonics.  All the others have varying degrees of the  tone. 

    E)  As S-band, the harmonics are generally much stronger than in 
either L or C bands.  But the situation varies remarkably (or 
depressingly, depending on your POV) depending on which harmonic we look 
at.  For example, for the 21st harmonic (2688), antennas 6 and 7 are 
'clean'.  But for the 23rd harmonic (2944), antennas 16 and 26 are 
'clean', while 7, 8, 9, 24, and 28 are strong.  For the 25th harmonic 
(3200 MHz), the situation is again different:  antennas 14 and 26 are 
completely clean, antennas 16 and 24 give only very weak birdies.  For 
the 27th harmonic, antenna 10 and 26 are now 'clean', while 8 is 
especially bad.  (I gave up trying to characterize the harmonics  higher 
than this at this stage ...)


    F)  At C-band, the situation is again different ... <audible 
sigh>.     The 32nd harmonic (4096 MHz) is generally quite weak, but is 
strong on antennas 10 and 12.   The same pair gives a strong harmonic at 
4992 (39th).   But at 5504 MHz, (43rd harmonic), only antenna 10 is 
bad.   But (sadly), the situation deteriorates at 5888 MHz, where 
antennas 10, 14, 20 and 24 are all very bad, while 15, 27 and 28 are good. 

    In summary, multiples of 128 MHz show up everywhere at the lowest 
three bands, but it is very difficult to identify particular 
commonalities.