It's still around in different forms. I once worked on an accoustic direction finder for the blind. There's also the rooftop mounted gunshot direction finder system (which works on different principles). I used to play with a dish antenna and microphone, which was great for listening to the neighbors argue across the road.
I like his collection of weird railroad locomotives.
These are much better, although not perfect.
Horn antennas, being based on aperture,
That's an understatement. Methinks yagi antennas doth suck at microwave frequencies. Except for the size, horns are probably the best compromise of gain, bandwidth, and beamwidth.
Yep. Think of the horn as a transformer between the waveguide input impedance (same an the probe impedance or about 50 ohms), and the impedance of free space (about 377 ohms).
Nice, but that spreadsheet only covers the water hole frequencies (1.4GHz) and is only useful for SETI.
Bigger is better.
Yech. Try it and you'll probably hate it.
The problem with the periscope antenna is that the beamwidth the feed (dish) antenna has to be small enough to illuminate the reflector, or most of the RF goes away behind the reflector. It has to be much narrower than the -3dB beamwidth because that only guarantees that half your power will hit the reflector. There are also reflection losses (same as a dish) involved. It might work in the near field, or with large billboard size reflectors, but not for small antennas. Incidentally, by order of the FCC, that style of antenna is illegal for commercial use on licensed microwave frequencies.