[lavnkjop] easy 21cm radio astronomy

is the 21 cm (1420 MHz) hydrogen line the easiest radio astronomy "target"?  not really a target if observing the hydrogen radiation from "everywhere" with an omnidirectional (dipole) antenna.  I have seen such a simple antenna and software-defined radio successfully detect 21 cm radiation.  it was fun to see a peak right at 1420 MHz.  does 21 cm signal strength vary significantly by antenna orientation and what part of the sky (probably the center of the Milky Way) is overhead?  such space and time variance could make observing 21 cm with simple antenna more interesting than just a peak.

an even easier astronomical target to observe in radio frequencies might be the sun.  I think the sun emits broadband noise, not a clear peak.  by measuring intensity, how easy is it to detect active regions rotating into view, or track the sunspot cycle through radio?  for the former, you need to keep your radio calibrated through months; for the latter, through decades.

is 21 cm the easiest astronomical radio thing to observe not also observable in visible light?  it is fun to see the invisible.  (previously: easiest visible-light astronomy with binoculars.)

while not astronomical, lightning can be detected with radio, even from great distance.  the necessary equipment to triangulate lighting strikes is more complicated than a simple dipole antenna but seems gettable by amateurs.

is 21 cm the longest astronomically interesting intrinsic wavelength of electromagnetic radiation?

(the extrinsic effect of cosmological redshift stretches wavelength.  at cosmological distance z=9, 21 cm radiation becomes 21 * (1 + z) = 210 cm.)

the 21 cm line is the hyperfine electron transition of ground-state hydrogen.  I think hyperfine transitions in other elements have shorter wavelengths.  previously, cesium, 3 cm.

electron energy levels become closely spaced near ionization, so transitions between adjacent high energy levels correspond to longer wavelengths.  but do we care about radiation from transitions near but not quite at ionization?  do they happen often enough to observe?  what about hyperfine transitions of excited hydrogen?  the electron probably unexcites much faster (more frequently) than it spin-flips.