What is the most efficient transmitting antenna configuration...other than a dish?

Question

I had thought it was the quad, but I dimly recall a magazine article that said NASA now uses some bizarre bent-wire shape

Answer 1

Nearly all antennae are 95% efficient or more, in terms of the electrical power at the input and the power radiated.

The key issues are: directivity pattern &/or directivity index, its input impedance, and the frequency response of the previous two.

A transmit system can be inefficient with a poor antenna, but the energy is lost inside the amplifier and cable, not the antenna.

A high directivity allows the same amount of radiated power to be focused into a smaller region of space.

Answer 2

Here is the link to the bent wire antenna you are asking about.
http://www.boingboing.net/2006/02/22/web_20_in_space_borg.html

Answer 3

What is it you are trying to transmit?
In ham radio. I get the best transmission results using a 1/4 wave inverted V.

Answer 4

A quad is probably best, but they are very bulky and difficult to build and maintain.

I would choose a log-periodic antenna.