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The end of the line for the Arecibo radio telescope

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MathWizard:
Anyone remember the early X-files episode where they went to Arecibo ?

coppercone2:
is reflector effiency and everything constant when dish size decreases?

I.e. 5MHz is 60 meters, the dish is 300 meters, so it is large in comparison to the wavelength.

If your dish is too small for the wavelength what happens? But what is the deal with that telescope, it transmits on 5Mhz but only picks up a minimum of 300MHz?

CatalinaWOW:

--- Quote from: coppercone2 on March 07, 2021, 09:16:36 pm ---is reflector effiency and everything constant when dish size decreases?

I.e. 5MHz is 60 meters, the dish is 300 meters, so it is large in comparison to the wavelength.

If your dish is too small for the wavelength what happens? But what is the deal with that telescope, it transmits on 5Mhz but only picks up a minimum of 300MHz?

--- End quote ---

There is no simple answer for these questions.   Reflector efficiency (a term that needs some definition if you want to get down to details) tends to drop as reflector size gets small wrt wavelength.  Small also requires some definition.  For a visible optics person a reflector that is only five wavelengths across is extremely tiny.  But at 60 meters it is actually huge.  The simplest way I know to think of it is this.  The beam reflected from a parabolic dish goes to a point in the ideal case.  Reflectors hundreds or thousands of wavelengths across approach this ideal, with the differences from ideal often being dominated by imperfections in the geometry of the reflector (remember the Hubble kefluffle).   As reflector to wavelength ratio drops the reflected wave is diffracted and departs significantly from the ideal case, even if the reflector has perfect geometry.  Soon that point from the ideal case becomes a blob approaching the size of the reflector.  You start to have to trade collecting the reflected energy against blocking incoming energy from getting to the reflector.

I suspect you have mis-interpreted some of the publicity sheets.  I am sure the active radar function of Arecibo sent and received at the same frequencies.  Perhaps as low as 5 MHz, but it seems likely to me that higher frequencies were used, if for no other reason than to penetrate the Heaviside layers.  Arecibo was also operated as a passive receiver for radio astronomy.  No reason for that function not to have different operating bands than the active radar.

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