Loop antennas for reception are very common because of their compact size, but I have never heard of a loop antenna for transmission. If they worked, that would be great for people with limited space.
Comments?
There certainly are loop antennas for transmission, but they are not useful for Part 15 AM. One reason (and not the only one) is that, most likely, the Section 15.219(b) 3 m length limit includes the length of the wire in the loop. The length of a single-turn loop is slightly more than three times the diameter. This makes a very inefficient use of the 3 m length limit; and therefore, the vertical monopole is a much more efficient antenna to use.
Ermi Roos is right, and I found a calculator which demonstrates how inefficient a Part 15 transmitting loop would be. You enter the numbers and the calculations automatically give you all the important numbers.
http://www.66pacific.com/calculators/small_tx_loop_calc.aspx
The difference as I understand it is that the calculations do not take into account an RF network, which would be necessary to tune it.
What's interesting to me about loops is that when using some of the rudimentary modeling software, a certain consequence of calculation can be observed.
We start with a common antenna form, the base loaded vertical. One interesting exercise is to reduce the length vertical element and increase the length of the base loading coil (while keeping the diameter of the coil the same) and watch the various factors change as the antenna becomes a helical radiator.
Another exercise, more relevant to the loop discussion, is to again start with a base loaded vertical and then shorten the vertical radiator and increase the diameter of the base loading coil while keeping its length constant, until you have a loop.
most likely, the Section 15.219(b) 3 m length limit includes the length of the wire in the loop
I would suggest that, for experimentation purposes, this is the kind of thinking to stay away from. The interpretation could just as easily be 3 Meters in diameter for a loop.
Furthermore, physics and all those dreary equations indicate that any configuration of a radiator that has an ultimate limiting dimension of 3 meters (even a cube of copper coils, 3 Meters on a side) will be so inefficient at the AM BCB that such antennas will cause the FCC very little in the way of worry about some kind of frightening range increase using any of the strategies so far discussed.
For us, it will always be a game of inches but something mildly helpful is always possible.
And don't forget the discontinous loop 🙂