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.

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 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
Diamètre 0.8 0.8 0.8 1.3 1.7 0.8 1.3 1.7 3.4 1.7 3.4 m
Puissance maxi 500 250 100 250 500 100 100 100 500 100 100 W
Max. Vitesse du vent [3] 120 120 120 120 120 120 120 120 120 120 120 km/h
Charge de vent 135 135 135 180 280 135 180 280 490 280 490 Nm
Bandes 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
Gain
[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
Efficacité
[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 %
Largeur de bande
(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
Référence 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
[3]: Max. allowed windspeed 120 km/h, with momentary gusts up to 160 km/h.

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. The control cable connector is supplied. For HF a PL socket is used.

  • 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.

Référence
11255.21DH AMA21 1280.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.31D AMA31D 1090.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.41 AMA41 1090.00 € Acheter Pas de stock, livraison en 25 jours
11255.45D AMA45 1370.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.42DH AMA42DH 1490.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.85 AMA85 1250.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.87 AMA87 1350.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.82 AMA82 1430.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.84DH AMA84DH 1850.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.163 AMA163 1690.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.164 AMA164 1750.00 € Acheter Pas de stock, uniquement sur commande, livraison en 25 jours.
11255.CONTRO AMA Loop Controller only 149.00 € Acheter En stock, expedié sous 1 à 2 jours.
Info frais de port
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).

Référence
40059.50 50m Double wire control cable, 0.75mm² cross section, black 28.00 € Désolé, ce produit n'est plus disponible.
Info frais de port