There is an antenna based on a H pattern fed at the center of the horizontal part of the H. Supposed to be resonant at all frequencies?
Also the theory goes that the tube of a satellite receiver antenna operates like a co-axial cable. What signal enters goes down the tube and is received by the antenna at the end of the tube exactly as it came in atthe top. I wondered if this is the case , can a hollow metal tube be used to point to an rf emmitting source picking up the signal from what it is pointed at strongly but picking up little of what is transmitting off to any side of it.
While looking at this ,vertical 1/4 wavelength antennas can be made directional by placing another at approx .25 wavelength spacing.
In fact all of the antenna theory including the loop antennas discussed can be put down to a length of metal in any shape picks up radio waves as a voltage . To read a particular frequency you must couple to that metal . A capacitor does that job.
Whatever the shape and mass of the metal affects which frequencies it favours . A thin wire of 170mm favours 433Mhz ,bend it how you like except back on itself. A thicker wire has a larger spread of favoured frequencies.
To increase the signal voltage more metal must be placed approx .25 wavelength away in the desired direction . RF hits the first bit of metal slows and then re-emits a burst of RF. The coupled metal receives this re-emitted burst in phase with the next wave coming through thus increasing the voltage. That is how you get gain .
The front metal also gets that next wave and re-emits it and the next wave is then received in phase with the third wave by the coupled metal etc.etc. I used to reckon I could see the yagi antenna shivering with exciting vibrations !