3 - Small Magnetic Loop Characteristics
Small Relative To Wavelength
- In recieving mode, the small loop antenna responds to the magnetic lines of force that are at a 90 degree angle to the electrical signal. The
antenna must be oriented perpendicular to the arriving signal to take full advantage of the antenna's area being cut by the maximum
lines of force.
- In transmit mode, there is a large magnetic component in the near field. The far field signal is the same as
any other antenna.
Height Is Not Importrant
- Receiving loops will generally be less than 1/10th of the desired wavelength in total circumference. In practicality, they can be much smaller
than 1/10th wavelength and still be effective receiving antennas because of the sensitivity of modern receivers.
- Loops designed for optimum transmission qualities will be greater than 1/8th wavelength but less than 1/4th of the desired wavelength
in total circumference. At a smaller circumference than 1/8th wavelength the efficiency goes down rapidly, while circumferences greater
than 1/4 wavelength results in the antenna being self resonant because of the inherent capacitance of the loop itself.
- At 1/10th wavelength the small loop antenna will have a tight figure-eight pattern, with sharp nulls perpendicular to the plain of the loop.
Azimuth Pattern 1/10 Wavelength
Nothing Is Free
- Small loop antennas, even those mounted only a few feet off the ground, are good for local, regional and DX contacts, since they radiate equally well in all
Elevation Pattern of Loop @ 5 ft.
- Raising the loop antenna will result in increased radiation at a lower angle, but is really not necessary for DX operation with this antenna,
since even at a low height it radiates at a low angle (see above pattern).
Elevation Pattern of Loop @ 33 ft.
- Local noise and signals coming in perpendicular to the plain of the loop and at relatively low angles are greatly dimminished.
- At increasing elevation angles of received or transmitted signals, the sharp nulls disappear. Incoming sky waves are unaffected by the nulls.
Loop Azimuth Pattern at Various Elevation Angles
- A loop antenna designed for optimum transmission qualities (about 1/4 wavelength) will not have the sharp nulls of a 1/10th wavelength receiving antenna, but will have
an azimuth pattern similar to a dipole mounted at 1/4 wavelength above the ground.
Dipole Azimuth & Elevation at 1/4 WL
- A good resonant circuit, like a well constructed small loop antenna, will have a reasonably high Q. The high Q will give
a "boost" at the received target frequency. This results in quiet reception because signals and noise outside of the
selected frequency do not get augmented.
- But a high Q comes at the price of a narrow bandwidth. Some means of tuning becomes a necessity.
Narrow Loop Responce
- If the Q becomes too high the bandwidth can become so narrow as to be unuseable (less than a few kilo-hertz).
- A high Q also means that the resonant circuit will generate very high voltages across the tuning capacitor. Even
relatively low power levels (less than 100 watts) can and will produce voltages in the thousands. Higher power levels
will produce high currents and voltages in the tens of thousands.
Result From Heating and Arcing
- A properly designed and constructed small loop antenna can have an effective efficiency of 90 percent or greater.
- The efficiency is determined by the following relationship between the radiation reistance and the resistive impedance.
Loop Efficiency Formula
Other Loop Equations
- Since the radiation resistance is typically very small (typically 100 to 200 milli-ohms), loop loss resistance
added by skin effect, poor connections or capacitor wiper contacts results in a 1% loss in efficiency for every milli-ohm
of added resistance.
- A small loop antenna operated outside of it's design parameter or poorly constructed will only transmit a small
portion of transmitter power. The rest of the power will be consumed as HEAT!