Käferlein Loop AMA-21DH 10-20m 500W

AMA Magnetic Loops - Principle of construction

The main feature of a magnetic loop is it's comparably small size in relation to the wavelength. This makes this type of antenna very attractive to many radio amateurs with little space for aerials. The tuning is done with a high voltage capacitor in the loop, which is made from aluminium tubing. For best convenience the capacitor is tuned remotely by an electric motor.

• A magnetic loop antenna must be weatherproof for operation outside a building. The selection of materials and construction of the joints are of high importance. By many practical experiments the best suitable materials have been selected.
  
  

• The tube of the loop and the tuning capacitor must be constructed very robust, to avoid fluctuations of the inductivity and capacity and therefore fluctuations of the resonant frequency. Besides the mechanical stability the tuning capacitor must be built with thick plates, a solid axle and large electrical contacts to prevent heating from the high electrical currents. The also keeps losses at a minimum.
  
  

• For a precise tuning an accurate and continues mechanical drive, free of play, should be used. This mechanical drive must work reliably under all weather influences.

• The magnetic loop antenna should withstand excess power without permanent damage. This requires that the insulating material of the capacitor is resistive against high voltages, leak currents and arcing. When the antenna is accidentially overloaded there will only be a flash arc between the capacitor plates, but the antenna itself will not be damaged.

From July 2017 on, all antennas with a loop diameter larger than 1.2m are shipped with a support tube. This support tube varies in size and material (aluminium, steel) depending on the size of the antenna. It runs vertically through the antenna. The support is fixed to the antenna mount at the bottom, and to the protective cover of the capacitor at the top. It relieves the loop from strong side loads at high windspeeds.

The tube visible at smaller loops just serves to guide and protect the cable and is not designed to support the loop structurally, such an additional support is usually not required for these models.

The difference in level between the main direction and 90° to the side is approx. 25 to 35 dB, depending on the model and band. This means that a rotor would be appropriate here to use the nulls to suppress QRM.

The models of the AMA Loop Antennas

For operation of the AMA loop antennas a DC voltage of 13.5V, max. 250mA is required, a supply cable is included in the shipment, but no power supply. Further a two wire cntrol cable is required, a wire gauge of 0.75mm² is sufficient. See below. The control cable connector is supplied. For HF a PL socket is used.

AMA loop-antennas are partially manufactured after order. The current delivery time can be found in the listing below.