[evlatests] C-band Sensitivity Oscillation

[Dan Mertely]

I've gone through our C-band feedhorn return loss testing
data & found the following information that may be of use
in determining the cause (or not) of the 10% Tsys variation
with frequency.

Without the Escolam 10 radome in-place, the return loss from the
feeds tended to be in the -35 to -45 dB region.  With the radome
in-place, the RL was in the -20 to -35 dB region.

The radome clearly introduces a 82 MHz ripple in the bandpass
(20 peaks in 1640 MHz from around 4 GHz to 5.6 GHz). At the
low end of this range, the peak to null difference in return
loss is from 15 to 20 dB.  However, even the peaks are below
20 dB RL, except at the very bottom end of the band (typically
in the -25 to -30 dB range, as mentioned earlier).

Sri mentioned this effect in his report on feedhorn return
loss testing (discussing the L-band feed in this case):

"Measurements were taken with and without a radome. The radome
material is Escolam 10 with a thickness of 25 mils.  When the
radome is present, return loss is higher by about 10 dB at
frequencies above 1.18 GHz.  The ripple frequency corresponds
to the round-trip distance from the calibration plane to the feed
aperture plane ..." (From EVLA Memo 87)

Whether these effects are significant enough to produce the
10% effect found in Rick & Emmanuel's test data, I leave others
to determine.  It seems to me, however, that the effect should
have less than a 1% effect on the data, since all the feeds
we tested showed better than 20 dB return loss between 4.5
and 5.0 GHz.

(Data from the latest 3 C-band feedhorns tested attached as a .pdf)

-Mert



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...
> 
>    
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
-------------- next part --------------
A non-text attachment was scrubbed...
Name: EVLA-FH-RL.pdf
Type: application/pdf
Size: 49628 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20080728/17e59c50/attachment.pdf>