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
AMA-82

Käferlein AMA Magnetic Loops
AMA21DH AMA31D AMA41 AMA45D AMA42DH AMA85 AMA87 AMA82 AMA84DH AMA163 AMA164
Diametro 0.8 0.8 0.8 1.3 1.7 0.8 1.3 1.7 3.4 1.7 3.4 m
Potenza mass. 500 250 100 250 500 100 100 100 500 100 100 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
30 m 30 30 30 30 30 30 30 30
20 m 20 20 20 20 20 20 20 20
17 m 17 17 17 17 17 17
15 m 15 15 15 15 15
12 m 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 -1.0 0.4 -4.9 -1.0 0.4 1.6 0.4 1.6 dBi
30 m -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 1.4 1.6 0.6 1.4 1.6 dBi
17 m 1.2 1.2 1.2 1.6 1.2 1.6 dBi
15 m 1.4 1.4 1.4 1.7 1.4 dBi
12 m 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 53 72 21 53 72 95 72 95 %
30 m 49 49 79 90 49 79 90 89 %
20 m 76 76 76 92 96 76 92 96 %
17 m 88 88 88 96 88 96 %
15 m 92 92 92 97 92 %
12 m 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 11 12 10 11 12 28 15 28 kHz
30 m 11 11 22 26 17 23 26 33 kHz
20 m 20 20 24 41 38 33 46 45 kHz
17 m 32 32 36 65 49 75 kHz
15 m 42 42 53 87 68 kHz
12 m 62 62 80 kHz
10 m 80 80 97 kHz
Ordine No. 11255.21DH 11255.31D 11255.41 11255.45 11255.42DH 11255.85 11255.87 11255.82 11255.84DH 11255.163 11255.164

[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. See below.

  • Included in shipment of the 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

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

Ordine No.
11255.21DH AMA21 1280.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.31D AMA31D 1090.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.41 AMA41 1090.00 € Ordina In magazzino, spedizione in 1 o 2 giorni.
11255.45D AMA45 1290.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.42DH AMA42DH 1490.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.85 AMA85 1250.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.87 AMA87 1270.00 € Ordina In magazzino, spedizione in 1 o 2 giorni.
11255.82 AMA82 1430.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.84DH AMA84DH 1850.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.163 AMA163 1690.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.164 AMA164 1750.00 € Ordina Non in magazzino, verrà ordinato solo su richiesta, tempi di consegna c. 25 giorni.
11255.CONTRO AMA Loop Controller only 149.00 € Ordina In magazzino, spedizione in 1 o 2 giorni.
Info sui costi di spedizione
Download: AMA assembly instructions and manual (PDF, DE, 530KB)
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).

Ordine No.
40059.25 25m Double wire control cable, 0.75mm² cross section, black 15.00 € Ordina Non in magazzino, di nuovo disponibile en 10 giorni
40059.50 50m Double wire control cable, 0.75mm² cross section, black 28.00 € Ordina In magazzino, spedizione in 1 o 2 giorni.
Info sui costi di spedizione