AMA Magnetic Loops Design DK5CZ 'Käferlein'
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The magnetic loop antennas by OM C. Käferlein (DK5CZ, Darmstadt) are well known and popular not only in Germany. The production was later continued by Funktechnik Beese (Darmstadt). In 2016 WiMo acquired the right to the name and all design documents of these fine antennas.
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.
Due to the physical principle of the loop antennas, these antennas have a very low radiation resistance. To achieve a good efficiency the losses in the antenna must be reduced as much as possible. This leads to the following construction requirements for AMA magnetic loop antennas:
- 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.
For best radiation symmetry we will use a symmetric coupling with a secondary loop.
The models of the AMA Loop Antennas
Originally there was a large selection ofAMA loops on offer. After taking over of the construction documents and the rights to the name, we (WiMo) have to gain experience in which models are in most demand, which models pose special challenges for production etc. So we will start with a rather small selection of antennas, which will be extended after a while.
(VSWR 2:1) 
: Theoretical values by Ch. Käferlein DK5CZ, based on Magnetic Loop calculations DL2FA
: empirical mean value
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. Unfortunately WiMo does not offer such a cable at the moment.
- Included in shipment of the AMA magnetic loop antennas
- Mast mount for vertical installation, max. 60 mm mast diameter
- DC cable for controller
- Connector for control cable (Hirschmann STAS200 to the antenna, 3.5 mm mono phone plug to the controller)
- Printed instructions
AMA loop-antennas are manufactured after order. The current delivery time can be found in the listing below.
|11255.21DH||AMA21 Delivery time 2 weeks||1280.00 €||Buy now|
|11255.31D||AMA31D Delivery time 2 weeks||1090.00 €||Buy now|
|11255.41||AMA41 Delivery time 2 weeks||1090.00 €||Buy now|
|11255.45D||AMA45 Delivery time 2 weeks||1290.00 €||Buy now|
|11255.42DH||AMA42DH Delivery time 2 weeks||1490.00 €||Buy now|
|11255.85||AMA85 Delivery time 2 weeks||1250.00 €||Buy now|
|11255.87||AMA87 Delivery time 2 weeks||1270.00 €||Buy now|
|11255.82||AMA82 Delivery time 2 weeks||1430.00 €||Buy now|
|11255.84DH||AMA84DH Delivery time 4 weeks||1850.00 €||Buy now|
|11255.163||AMA163 Delivery time 2 weeks||1690.00 €||Buy now|
|11255.164||AMA164 Delivery time 4 weeks||1750.00 €||Buy now|
|11255.CONTRO||AMA Loop Controller only||149.00 €||Buy now|
|Info freight charges|
Download: AMA brochure (PDF, DE, 1.8MB)
Double wire control cable
Double wire control cable, 0.75mm² cross section, black PVC coating. Suitable for the control of magnetic loops that require only two wires (AMA Käferlein Loops or similar).
|40059.25||25m Double wire control cable, 0.75mm² cross section, black||15.00 €||Buy now|
|40059.50||50m Double wire control cable, 0.75mm² cross section, black||28.00 €||Buy now|
|Info freight charges|