[evlatests] C-band Sensitivity Oscillation

[Bob Dickman]

I wonder whether Gore makes a similar fabric these days.  When I worked with
them, there were a variety of essentially transparent fabrics.

Bob DIckman


On 7/28/08 5:20 PM, "Robert Hayward" <rhayward at nrao.edu> wrote:

> 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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>> 
> 
> 
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