Over the years, I have successfully built several small transmitting loops, sometimes called magnetic loops. At an equal height, a compact loop is capable of performance similar to a dipole, but is usually very much smaller. The loop radiates like a dipole through the center axis of the loop, but more omni-directionally. Close to the ground they are best mounted vertically. At greater heights they may be mounted horizontally..

The compact loop is like a less than 1/10 wavelength dipole folded into a circle, with its ends insulated. It is brought to resonance by a capacitor across the ends. It is essenitally a large one-turn LC parallel tank circuitm but as such it is large enough to radiate.

Multi-band operation is practical by using a variable capacitor which, however, must be capable of withstanding very high voltages.

As a general principle, as an antenna is made smaller in proportion to its wavelength, such as with a loading coil or by bending it into a loop, the antenna's radiation resistance drops. Radiation resistance is what we want in an antenna. It is tthe loading of space on the antenna and it is where we want our transmitter's power to go.

But all real antennas also have conductor resistance, which is in series with the radiation resistance. Conductor resistance is generally small, but if radiation resistance also becomes small, again by making the antenna small while keeping it at resonance, a significant portion of transmitter's power will now be lost in conductor resistance.

Said mathematically, the effeciency of an antenna is proportional to radiation resistance divided by the sum of radiation resistance and conductor resistance This is one of the reasons why mobile HF whips traditionally have low efficiency.. Similarly, in a compact loop, where radiation resistance can easily be of the same magnitude as the conductor resistance or less, efficiency can suffer significantly.

To minimize this, compact loops are usually made of large diameter tubing of good conductivity. Copper is the best common material; aluminum is poorer. Steel or stainless steel are totally unsatisfactory. Flat metal srapping is also not a good idea. Due to skin effect, the RF avoids the center of the stapping, thereby increasing the conductor resistance.

| This simple little DOS program | from AA5TB and | his much better version | in Excel, both calculate all the necessary characteristics for round compact loops. For loops of other shapes, such as a square, calculate the non-circular loop's area and convert it to an equivalent circle. Efficiency will be slightly less, in inverse proportion to the difference in circumference as compared to a circle.

Here are | the equations| used to create the above programs. I have successfully run the simple DOS version on all versions of Windows including XP. AA5TB's Excel version is far better if you have Mictosoft Excel on your computer. | HIS WEB SITE | is also an excellent source of information on compact loops.