Magnetic Antennas

The electromagnetic antenna is one that has been around for a while and is generally not well understood. An example of this type of antenna is the DDRR, the Directional Discontinuity Ring Radiator. Another is the hairpin which was developed by General Dynamics. The small loop is another one. There are others, but these are the ones that have been used to some extent on the ham bands.

This type of antenna is characterized by being electrically very short for the frequency that it is being used on, with very, very low impedance, very high currents, narrow bandwidths, and large conductors. Tuning is usually accomplished by series or parallel vacuum capacitors or a combination of series and parallel capacitors that both match and tune the antenna

Now even though mobile whip antennas are very short and present a low impedance on their operating frequency, nearly all of them are resonated by loading coils or other means. The magnetic antenna uses no loading coils or other means to obtain resonance, and loading coils contribute to losses in a magnetic antenna.

An examination of a conventional dipole, with an impedance of 72Ω being fed by a 100 watt transmitter, will show that a current of 1.17 amps will flow in the antenna. If a magnetic antenna with an impedance of .1Ω is fed with the same 100 watts, an antenna current of 31.6 Amps will flow in the antenna. Quite a difference! This also will explain why a lot of ham that tried the DDRR met with dismal results for the most part.

The reason is simple. At such low impedance, skin effect becomes really important and if you have a high resistance anywhere in the antenna, your power will be partially absorbed in heat due to this resistance. Losses can be as much as 90%, which isn’t doing anyone any good. The military DDRR antennas used very large conductors with silver or gold plating and welded construction. Large conductors connected the capacitor to the open end of the antenna where a vacuum variable tuned the antenna to resonance. This type of construction was also used in the hairpin antenna.

Now if you remember from basic transformer theory, if you have a stepdown transformer, the secondary voltage goes down as the current goes up, and the magnetic field around the transformer increases as the load current increases. A soldering gun is a good example of this known by anyone holding a pair of pliers or a screw driver next to one and pulled the trigger. Anyone who has done this can tell you the pliers or whatever vibrated a lot due to the intense magnetic field around the coils.

The one turn in the soldering gun the heating element is connected to is most definitely a low impedance load and the current is very high. If the nuts holding the tip in the gun get a little loose, the tip will not get as hot or heat up as fast due to the resistance at the connectors. A twist of the wrench or screwdriver and the ole gun is ready to go. The same theory applies to the magnetic antenna. Any resistance in the antenna circuit and your rf goes up in heat and the antenna does not work properly.

This is the reason so many hams have not had any success with this type of antenna. Losses in the antenna due to heating and comparatively high overall resistance as compared to the antenna impedance. The dc resistance of the antenna as well as the rf skin resistance must be lower than the rf impedance or the antenna efficiency falls way off. The success of this type of antenna depends entirely on keeping the losses due to resistance as close to none as possible.

Another interesting thing about this type of antenna is the noise rejecting ability it has. The magnetic antenna is a very quiet antenna. This is due to the natural makeup of static. Studies through the years have proven that natural static, such as that caused by lighting, precipitation static, static electricity from the carpet and hair dryers consists of 75% electrostatic discharge and 25% electromagnetic discharge. Since the electromagnetic antenna is just the opposite makeup in its field, 75% electromagnetic and 25% electrostatic, which component of the signal do you think it will receive and radiate the best?

Since the amount of voltage induced in the receiving antenna is going to be proportional to the strength of the magnetic lines of flux that pass through the elements of that antenna, the received signal will be a lot stronger than one from a conventional antenna for the same transmitter power. So if a magnetic type antenna is correctly constructed and tuned up, the resulting field strength at a given location should be greater than another omni-directional antenna.

There is a lot of experimentation to be done in this field as far as amateur radio operators are concerned. And, as long as the basic construction principals of extremely low resistance joints and large conductors are kept in mind, success will be within the reach of any amateur that wants to play with these interesting antennas.

The small loop sold by MFJ and by AEA are the only commercial antennas most hams can afford at this time and they work extremely well. Other antennas are on the market, but are much more expensive. Be sure to let us know what you learn, there are others that are interested in these antennas.

Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF

Last Updated : 12th March 2024