Somebody mentioned in passing that it is possible "buy"
coax-to-waveguide adaptors. Well, I suppose they are
buyable, but it's sometimes more fun to make them from
scratch.
In particular, here's a scenario that might provide a
nice opportunity to teach about wave mechanics, including
waveguides, antennas, polarization, and a bunch of other
details .... *AND* provide a memorable answer to the
question of what physics is good for.
Here's the motivational scenario: Really and truly, on
several occasions I've had computer data available in one
building that was needed in another building a couple hundred
yards away.
Nowadays 802.11 wireless networking stuff is getting
rather inexpensive (but be careful, because a lot of it
is also getting quite cheap, in the bad sense). So it
would be logical to set up a building-to-building wireless
link. The stuff will definitely not work over such
distances using the omnidirectional antennas that are
typically provided ... but it definitely will work if you
have some antenna gain.
I haven't tried it, but I could imagine making a project out
of it. Set up an 802.11 station a couple of hundred meters
away (line of sight) and ask students to build antennas for
talking to it. It will more-or-less automatically turn into
a contest, to see who can get the most antenna gain. There
will need to be some rules for the contest, for starters:
-- passive-only
-- within a size constraint (perhaps a 1'x1'x3' box) ... lest
somebody try to build an Arecibo-sized dish
-- within budget (perhaps $10.00 in parts, max)
Make sure the students calculate the cost of the 802.11 repeater
and compare it to the cost of digging up the street to install a
wired connection.