AMA Magnetic Loops Design DK5CZ 'Käferlein'

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

AMA41 AMA 41 Magnetic Loop

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.
  • For best radiation symmetry we will use a symmetric coupling with a secondary loop.

  • 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.

The models of the AMA Loop Antennas

AMA 45 Loop

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.


Käferlein AMA Magnetic Loops
AMA21DH AMA31D AMA41 AMA42 AMA45 AMA42DH AMA85 AMA87 AMA82 AMA84DH AMA163 AMA164
0.8 0.8 0.8 0.8 1.3 1.7 0.8 1.3 1.7 3.4 3.4 3.4 m
Potenza mass. 500 250 100 150 250 500 100 100 100 500 100 100 W
Banda 160 m 160 160
80 m 80 80 80 80 80 80
40 m 40 40 40 40 40 40 40 40 40 40
30 m 30 30 30 30 30 30 30 30 30
20 m 20 20 20 20 20 20 20 20 20
17 m 17 17 17 17 17 17 17
15 m 15 15 15 15 15 15
12 m 12 12 12 12
10 m 10 10 10
Guadagno
[1]
160 m -10.9 -5.8 dBi
80 m -13.5 -6.9 -4.7 0.2 -4.8 0.2 dBi
40 m -4.9 -4.9 -1.0 0.4 -4.9 -1.0 0.4 1.6 0.4 1.6 dBi
30 m -1.3 -1.3 -1.3 0.8 1.3 -1.3 0.8 1.3 1.3 dBi
20 m 0.6 0.6 0.6 0.6 1.4 1.6 0.6 1.4 1.6 dBi
17 m 1.2 1.2 1.2 1.2 1.6 1.2 1.6 dBi
15 m 1.4 1.4 1.4 1.4 1.7 1.4 dBi
12 m 1.6 1.6 1.6 1.6 dBi
10 m 1.7 1.7 1.7 dBi
Efficacia
[1]
160 m 2 17 %
80 m 2 10 22 69 22 69 %
40 m 21 21 53 72 21 53 72 95 72 95 %
30 m 49 49 49 79 90 49 79 90 89 %
20 m 76 76 76 76 92 96 76 92 96 %
17 m 88 88 88 88 96 88 96 %
15 m 92 92 92 92 97 92 %
12 m 95 95 95 95 %
10 m 97 97 97 %
Larghezza di banda
(VSWR 2:1) [2]
160 m 3 4 kHz
80 m 4 3 4 9 4 9 kHz
40 m 7 7 11 12 10 11 12 28 15 28 kHz
30 m 11 11 11 22 26 17 23 26 33 kHz
20 m 20 20 24 24 41 38 33 46 45 kHz
17 m 32 32 36 36 65 49 75 kHz
15 m 42 42 53 53 87 68 kHz
12 m 62 62 80 80 kHz
10 m 80 80 97 kHz
Ordine No. 11255.21DH 11255.31D 11255.41 11255.42 11255.45 11255.42DH 11255.85 11255.87 11255.82 11255.84DH 11255.163 11255.164
Disponibilità Autunno 2016 Fino 2016 Disponibile Fino 2016 Autunno 2016 Fino 2016 Fine di ottobre 2016 Fino 2016 Autunno 2016 Fino 2016 Fino 2016 Fino 2016
Prezzo 1280.00 € 1090.00 € 1290.00 € 1250.00 € 1430.00 €
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[1]: Theoretical values by Ch. Käferlein DK5CZ, based on Magnetic Loop calculations DL2FA
[2]: empirical mean value

AMA Loop Antennen Steuergerät AMA Loop Controller

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 teh AMA magnetic loop antennas
  • Antenna
  • Mast mount for vertical installation, max. 60 mm mast diameter
  • Controller
  • DC cable for controller
  • Connector for control cable (Hirschmann STAS200 to the antenna, 3.5 mm mono phone plug to the controller)
  • Printed instructions
Ordine No.
11255.CONTRO AMA Loop Steuergerät einzeln 129.00 € Ordina
Info sui costi di spedizione

Which AMA Loop Antenna models should be build?

Formerly OM Käferlein and Funktechnik Beese offered a larger selction of AMA Magnetic Loop antennas, you can see the entire choice below. If you assume that the missing models will have a price in roughly the same range as the models currently offered – which antenna should we produce for you? Please send us a short email, just click on the link. We will treat these mails just as information, not as a an order :)

The we list the previously manufactured models here does not imply that we will offer these models eventually. We have to learn which demand exists. Many thanks for your comprehension.

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Modello Diametro Gamma di frequenze Potenza mass. Prezzo
AMA 21 0.8m 14 – 30 MHz 150W 950.00 € I would choose this model!
AMA 21DH 0.8m 14 – 30 MHz 500W 1280.00 € I would choose this model!
AMA 31D 0.8m 10 – 30 MHz 250W 1080.00 € I would choose this model!
AMA 42 0.8m 7 – 25 MHz 150W 1090.00 € I would choose this model!
AMA 41 0.8m 7 – 29.7 MHz 100W 1090.00 € Available from stock. 1090.00 € Ordina
AMA 85 0.8m 3.5 – 21.5 MHz 100W 1250.00 € I would choose this model!
AMA 45D 1.3m 7 – 22 MHz 250W 1270.00 € I would choose this model!
AMA 87 1.3m 3.5 – 18.2 MHz 100W 1270.00 € I would choose this model!
AMA 42DH 1.7m 6.6 – 15 MHz 500W 1490.00 € I would choose this model!
AMA 82 1.7m 3.5 – 14.5 MHz 100W 1430.00 € I would choose this model!
AMA 163 1.7m 1.8 – 11 MHz 100W 1690.00 € I would choose this model!
AMA 84DH 3.4m 3.5 – 7.5 MHz 500W 1850.00 € I would choose this model!
AMA 164 3.4m 1.8 – 7.5 MHz 100W 1750.00 € I would choose this model!