ANTENTOP- 01- 2006, # 008

Experimenters with the VHF Magloop

By: Igor Grigorov, rk3zk

Credit Line: Igor Grigorov: Practical Antenna Design. Moscow, DMK, 2000 (in Russian)

Some experiments on use of "printed" VHF magloop (i.e., magloop made on a PCB) were made at the 2- meters band. Loop has square shape and wide of printed wire in 5 millimeter. 50-Ohm coax was matched with the magloop by a coupling loop. Figure 1 shows the design.

Design of the Magloop: Coupling loop (item 1) is located in the corner of the magloop (item 2). Rigid coax (item 3) has 8 centimeters length and fastened to the PCB by a clamp (item 6). Treads (that hold the coupling loop) (item 5) go through holes (item 4) in the PCB.

Matching: The magloop is connected directly to a VHF hand- held turned on to Low Power (100 mW). Firstly the perimeter of the coupling loop consists of 1/3 from perimeter of the magloop. Do matching of the magloop by decreasing of the perimeter of the coupling loop. Do tuning of the magloop capacitor every time when you decreased the perimeter of the coupling loop. Field Strength Meter shows you how coupling loop work. I used FSM described at Reference 1.

Passband of the Magloop. Passband of the magloop was near 2.0 -MHz at the 2- meters. The passband was determined by FSM by decreasing of the RF level in twice compare to central frequency.

Theory and Practice. MMANA (see Reference 2) allows to suggest that the magloop has gain minus 5... 7 dBi. Practical test of this magloop have shown that on the open area the antenna was equal to hand- held helical antenna in 14 centimeter length. However the magloop do the same job as a lambda/4 vertical (50 cm at 144 MHz) when communication was inside and from concrete buildings.

Caution: Sometimes (at some cheap hand- helds) the magloop (working at transmitting mode) could be seriously worsen the quality of the signal because of the strong magnetic inducing to the transmitter.

Figure 1 Printed VHF Magnetic Loop

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