[AR] Re: SDR Radio's was "infrastructure" (was Re: Amateur solid rockets for ...)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 16 Dec 2017 14:40:15 -0500 (EST)
On Sat, 16 Dec 2017, Marcus D. Leech wrote:
Moving up in frequency means the path-loss gets worse...
This is something that often confuses people; it's not that higher
frequencies actually incur more losses en route. The real issue is that
the way path loss is defined, it includes a separate question: what's the
aperture (effective collecting-area diameter) of an omni receive antenna?
The answer is that a radio photon has to pass within a fraction of a
wavelength of the antenna to interact with it... so an omni antenna's
aperture shrinks as the frequency rises.
Overall, omni-to-omni communications work better at lower frequencies.
Omni-to-dish (or vice versa; substitute other high-gain antennas for
the dish if desired) is pretty much frequency-agnostic. Dish-to-dish
works better as the frequency rises. (For all three, this assumes other
things are equal... which they often aren't.)
In VHF, it's perfectly reasonable to do omni-to-omni comms from orbit (or
to orbit), if data rates are modest or transmitter power is generous.
Which is just as well, because VHF high-gain antennas are big clumsy
things. (Our NORSAT-2 has a deployable VHF Yagi, but it's rather above
the cubesat class.)
In S-band or higher, well, not so much, unless you've got a *seriously*
muscular transmitter.
Henry
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