Small transmitting loop antennas "magnetic loops" have become very popular. There is no doubt that they can provide good performance for their small size. While they are easy to build if you have (or can make) a suitable variable capacitor, many people opt for a commercial product. This usually means that the first opportunity to try it is after you have made a purchase. One way to experience operating under "loop conditions" is to use a known simple antenna type such as a half-wave dipole or quarter-wave monopole and reduce your power to simulate a loop.

Loop manufacturers seldom provide the efficiency figures for their antennas but fortunately they are easy to calculate. Loop efficiency will vary depending on the loop design and on the band of operation. A number of online calculators can be used to give an estimate of loop efficiency. I favour the calculator provided by Owen Duffy as this seems to give a reasonable worst-case.

Simulating the performance of a loop is simply a matter of reducing your power into your known antenna to take into account the loop efficiency. Your known antenna will have an efficiency of close to 100% (all the power you apply to it will be radiated). Bear in mind that the maximum power rating of most loops is less than 100 Watts so you will need to factor that into your experiment.

The following table shows the power levels to use.

The method is simple. Using an online calculator, calculate the likely efficiency of the loop that you are considering. Set up your dipole/monopole in an open area. Reduce the power of your radio according to that table above and go on the air with your "loop simulator". This simple test will quickly give you an impression of what life might be like with a loop. It's not perfect but it will be close!

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Example: 1 m diameter loop constructed from LMR400 co-axial cable can be expected to give the following efficiency figures: