Magnetic Loop Antennas...
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Roger Johnson's Magnetic Loop
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Magnetic Loop Antennas...
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Roger Johnson's Magnetic Loop
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The Magnetic Loop presented at last months GVARC Meeting has been upgraded for better performance and durability. Changes are as follows:
See Photos below for more detail.
Close up of Butterfly Capacitor and DC Motor Drive.
Slot cut into 7/8 heliax ends using grinder with cut off wheel.
Copper Strap inserted into slot in Heliax Cable. Center Conductor and
shield were soldered to copper strap. Other end of copper strap was bolted to Butterfly Capacitor.
How well does it work?
The Magnetic Loop has a figure 8 Radiation Pattern in the plain of the loop with very sharp nulls when turned perpendicular to station being worked. In a recent test a local station was receiving me 15db over S9 when turned in the plane of the loop and when turned perpendicular I was received at noise level and unreadable. When turned to favor a station the loop receives and transmit equal to my Gap Titan DX Vertical. Stations in Europe, South and Central America and throughout the US have been worked with gratifying signal reports. |
Would YOU like help building YOUR own Magnetic Loop Antenna
Understanding Pattern and Polarization of Electrically Small Loop Antennas
by Gary, KD8ZWS The radiation pattern of a small loop is the same as that of a small dipole positioned perpendicular to the plane of the loop and located at the center of the loop as indicated in Fig. 1a. The resulting 3-D radiation pattern is shown in Fig. 2. The loop antenna displayed at the March GVARC meeting was in a vertical plane, so the pattern in Fig. 2 would need to be rotated such that the z-axis in Fig. 2 lies in the horizontal (i.e., x-y) plane. In that orientation, the azimuthal pattern will be directional with deep nulls in two opposing directions. The polarization of the small loop is determined by the orientation of the radiated electric field vector traveling in the direction of pattern maxima, which is any point in the horizontal (i.e., x-y) plane in Fig. 2. (The pattern in Fig. 2 is omni-directional in azimuth.) What, then, is the orientation of the electric field vector in Fig. 2 if the pattern is that of a small loop? Answer: the electric field vector will be horizontal and thus, the polarization will be horizontal. But, the dipole in Fig. 2 is vertical, so shouldn’t the polarization be vertical? No, because the electric field vector must be in the same plane as the loop. Why? For several reasons: First, electric field lines begin and end on electric charges on the loop. So, in the horizontal plane, the electric field vector is in the horizontal plane, Secondly, there is a constitutive relationship in electromagnetic theory, J = σ E where J and E are vectors representing the current density J on the loop and E representing the electric fieldjust outside the surface of the loop conductor, and σ is the conductivity of the loop conductor. Since J and E are vectors (the bold indicates they are vectors), we conclude that E must be in the same direction (i.e., orientation) as J and thus the polarization must be horizontal in Fig. 2. Thirdly, the correct dipole to use in Fig. 2 to represent the loop performance is not an electric dipole but a magnetic dipole. In Fig. 1a, note that the magnetic current is denoted Im and has two arrow heads, not one as in Fig. 1b where the current is an electric current, Ie. Note also that the electric and magnetic fields of the electric and magnetic dipoles are interchanged as Figs. 1a, 1b and Table 3-1 indicate. In the electric case of Fig. 1b, the E field is in the plane of the paper whereas in the magnetic case of Fig. 1a the electric field is perpendicular to the plane of the paper. Thus the polarization in Fig. 2 must be horizontal. However, the polarization of the loop antenna displayed at the March GVARC meeting was vertical since the plane of the loop was vertical. Do magnetic dipoles actually exist.? No, and neither do magnetic currents and magnetic charges. But the small loop of electric current replicates the performance of the small magnetic dipole making the magnetic dipole a good model for the small loop of electric current. 
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Magnetic Loop project members Roger, W5IP, John, KL7CY, and project leader Bruce, K3BAT. A TV tripod and a wood mast were used for support. For low electrical resistance a 13 foot piece of hardline was used for the loop. A piece of plywood cut out to reduce wind resistance was used for the mount with the loop tie wrapped to it.
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The butterfly capacitor Bruce located. This type of capacitor increases the voltage rating to 13,000 volts by using large spacing and structured as two capacitors in series.
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The very high voltages require adequate capacitor isolation.
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Very large currents in the neighborhood of 60 amps require large low resistance connections.
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Bruce with the 3rd iteration of the loop showing the current motor drive in place. The coupling loop is also visible and is presently bent out of shape to reduce the amount of coupling,
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The home brewed bias T unit used in early versions of the loop to transmit DC to the tuning motor. The final remote tuning system is still being trialed.
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The proposed tuning controller under construction. A revised drive system that removes most of the mechanical slop and a new pulsing DC driver should create more useful tuning steps.
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The results of an AIM 4170 scan showing SWR in red, impedance differential in purple and impedance magnitude in green. The 2db bandwidth is about 50kHz as was expected.
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Prototype loop made from 7/8" heliax
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AIM scan of K3BAT's loop on 20 meters. The red line is SWR
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"Here's a group of GVARC members lead by Bruce, K7BAT discussing loop antennas. Several of the members will be building loops with the assistance of Bruce and Roger, K5IP. The meeting was held February 5th at K7BAT's QTH.
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