[evlatests] C-band Sensitivity Oscillation

[Robert Hayward]

I made the following reply on Friday to Rick's evlatest message, only to 
find he was the only one to get it. It covers some of the same ground as 
Mert's message today but adds a few more things...

-Bob

-------- Original Message --------
Subject: Re: [evlatests] C-band Sensitivity Oscillation
Date: Fri, 25 Jul 2008 10:07:13 -0600
From: Robert Hayward <rhayward at aoc.nrao.edu>
Organization: NRAO
To: Rick Perley <rperley at nrao.edu>
References: <48891708.7030906 at aoc.nrao.edu>

Look at Figure 11 in the EVLA Memo 95 "Design, Prototyping and
Measurement of EVLA C-Band Feed Horn" which you can get from here...

http://www.aoc.nrao.edu/evla/geninfo/memoseries/evlamemo95.pdf

The return loss ripple with the ESCCOLAM-10 radome in place looks pretty
similar - about 11 to 12 cycles between 4.5-5.5 GHz. Note that it is at
its worst in the 4-5 GHz region. Still, the return loss is below -25 dB,
at least across 5-8 GHz, so the feed is not wildly out of spec. The
ripple might get worse if there's moisture on the membrane (or the
vacuum window).

We might get better results using a radome made from Goretex RA7906
material, similar to what we use at K, Ka & Q-Band, and which we intend
to use at X & Ku. Unfortunately, the last I heard was that W.L. Gore no
longer sells it and we bought the last of their stock (a roll 44 meters
long by 45 inches wide - I loved their mix of units - for just under
$10,000) to complete the EVLA Project with enough spare material to take
care of a reasonable amount of lossage (although a hail storm would
certainly put us out of business).

Before we get too worried, we should wait and see what the passbands
look like when we have a few more wideband OMT's on the Array.

The good news is that this ripple is a lot less of a problem then the
higher frequency ripple seen from reflections with the subreflector
(something like 10 MHz or so) and the old 3 MHz ripple from the circular
waveguide system which can cause confusion when looking for your typical
narrow, weak spectral line. What we'd need to be assured of is that this
90 MHz ripple is stable. For example, would a radome that isn't
stretched very taut cause the ripple frequency to change enough to
affect the data with changes in elevation or when it is windy?

-Bob


Rick Perley wrote:
>     We have long noted that IF#1 at C-band had slightly (~5%) poorer 
> sensitivity than IF#2 for EVLA antennas.   Recent tests showed that this 
> is not an effect due to the IF -- swapping frequencies caused the 
> sensitivity difference to change.  Furthermore, the effect is not seen 
> at X-band, nor for the VLA antennas at C-band. 
> 
>     Emmanuel ran a test last night to clarify the picture.  He tuned the 
> array from 4.80 through 5.05 GHz, in 5 MHz steps, using 6MHz continuum.  
> The sensitivity for each frequency was derived via the correlation 
> coefficients. 
> 
>     We find a remarkable sinusoid oscillation in sensitivity, for all 
> IFs, for all EVLA antennas, over the entire bandwidth surveyed.  The 
> amplitude of the oscillation is about 10% (in Tsys/effic), the period is 
> close to 90 MHz.  If due to a standing wave, the length is 1.7 meters -- 
> about the length of the C-band horn? 
> 
>     We stumbled upon this because the default frequencies -- 4885 and 
> 4835 MHz, just happen to lie on the peak and trough of the standing wave...
> 
>    
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