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.
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