[time-nuts] Re: The Collapse of Puerto Rico’s Iconic Telescope [April 5th, 2021 New Yorker]

[Lux, Jim]

On 3/31/21 11:33 AM, Wes wrote:
> On 3/30/2021 4:33 PM, Lux, Jim wrote:
>> On 3/30/21 2:56 PM, Wes wrote:
>>> You would know better than I, but I was thinking of physical size; 
>>> 100m v. 70m.
>>>
>>> Obviously a BIG difference in TX power.
>>>
>>> Wes
>>
>>
>> It's all about EIRP, baby.
>>
>> I know they're talking about half a megawatt for GB, but I don't see 
>> it happening. They've spent so much time making it "radio quiet", 
>> putting a big honkin transmitter there seems odd.   Just think, a 
>> return loss from the feed of -30dB (which is pretty good) is good 
>> fraction of a kilowatt. 
>
>
> But AFAIK the system is bistatic (pseudo-monostatic) so there's no 
> local receiver to be subjected to transmitter leakage.  We worried 
> about leakage in the pulse doppler radars I'm familiar with; AMRAAM 
> and Phoenix missiles, but even they were bistatic for most of their 
> flights, tracking off the aircraft fire control radar which had 
> significantly higher ERP.  Only when close to the targets did they go 
> active. 

In the airborne radar environment, typically the other receivers already 
are designed to take a fairly high signal without damage (after all, you 
get painted by someone else's radar).  They may have a limiter on the 
input (there are low capacitance back to back schottky diodes for this), 
and you accept the loss in signal or raised noise floor from the 
protection circuit

But for radio astronomy, they don't want to give up anything - these 
receivers have noise temperatures in single digit K and they are 
optimized for "small signal" performance.

All those wideband cryogenic receivers for other frequencies are what 
people would be nervous about. There are other receivers at GB, not on 
the 100m GBT, too. You'd worry about a stray reflection from some 
structural member shooting a few hundred watts toward your sensitive 
radiometer which burns out at femtowatts or something like that.  The 
feed would be designed with some illumination taper, so there's still 
going to be spillover and scattering. If the taper is -17dB, the power 
density at the *edge* of the aperture is down 17 dB from the center. 
100m is about 7800 square meters, so the average power density with a 
500 kW transmitter is about 63 W/square meter. That's a pretty big power 
density.

http://www.naic.edu/~astro/sdss5/talks/ReceiverSystem.pdf

In some ways it's like high power laser labs. It's not the direct beam 
you worry about - nobody is going to put their hand in the beam path. 
It's the stray reflection when something gets bumped and falls across 
the optical bench and reflects a stray beam at 0.01% power into your eyes.