[evlatests] WIDAR P-band status update: 13SEP2014

[Huib Intema (NRAO)]

Hi Paul,

The P-value is the measured relative power with respect to the median. 
So 0.35 means that the measured antenna power is 35 percent of the 
median, or 65 percent lower than the median.

In principle, healthy antennas with a moderately lower power level are 
not a problem in themselves. Over the commissioning period, I have seen 
that healthy antennas have P-values in the approximate range 0.8 - 1.2. 
I have also seen that antennas with P-values < 0.7 tend to have issues, 
like strong bandpass ripples, polarization issues, variable gains, or 
other issues. Antennas with P-values < 0.5 are almost all problem cases.

-- Huib



On 17-Sep-14 12:09, Paul Harden wrote:
>
> On 9/16/2014 2:38 PM, Huib Intema (NRAO) wrote:
>
>> * Antenna power levels (determined from gain calibration):
>> Most antennas have similar (healthy) power levels, within ~30 percent of
>> the median, except:
>> - EA04 RCP: P = 0.05
>> - EA04 LCP: P = 0.40
>> - EA14 RCP: P = 0.35
>> - EA16 RCP: P = 0.35
>> - EA18 RCP: P = 0.45
>> - EA19 RCP: P = 0.45
>> (P = power level relative to median)
>
> I want to ensure I understand this correctly.  For example, on EA16 
> P=.35, this means the power is 35% lower than your mean/median?  That 
> would equate to about 4.5dB less than the median (probably marginally 
> OK - see below).  On EA04 P=.05 would be -13dB (clearly bad).
>
> Specs for most of the modules calls for LCP and RCP powers to be 
> within 2dB of each other.  I know the low band receiver output powers 
> never exceed 2dB with 1-1.5dB typical (when they leave the lab).  
> However, this 1-2dB LCP-RCP difference is also probably typical of the 
> T301 4/P Converter and the T302 LSC Converter.  If the LCP channels 
> (for example) in these modules are all low by 1-2dB, this could easily 
> accumulate to 4-5dB difference by the time it gets to the T304s ... 
> but shouldn't the T304 automatic leveling make up for that?  (WIDAR 
> path, not VLITE). Thus, I would think 4-5dB accumulative difference 
> might be the norm, and within the EVLA system specs.
>
> On some of the reported low antennas, Dan and I can check the power as 
> it trickles through the IF system (once it stops raining) to see where 
> this difference is originating, or if an accumulative effect.
>
> Bottom line: What would be the maximum power difference between LCP 
> and RCP in the system you'd like to see?  Then, we can try to figure 
> out how to equalize the powers if ~ 4dB difference is not acceptable.  
> Might require adding coaxial attenuators between modules to equalize 
> the power (at the expense of a few dB loss of sensitivity in one 
> channel).
>
> Paul
>
>
>
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