The Alpha Antenna (USA) Magnetic Loop is a portable antenna for a maximum transmit power of 30W. The antenna comes in a handy transport bag and includes a tripod which has an extended height of about 200cm. With the help of this tripod the antenna can be installed nearly anywhere in the field. The fully assembled antenna can be tuned from 40 to 15m. With an additional inner coupling loop (included) and a small modification to the outer loop the 12 and 10m bands can be used as well. Before operation the antenna must be assembled from the supplied aluminium strips, all required hardware is included.
The tuning of the antenna is done by means of a butterfly variable capacitor in the lower tuning box. The drive of the capacitor uses a 6:1 reduction and allows very precise tuning. To minimize the effects of capacitive influence of the body and to reduce the risk of HF burns the drive shaft is made from non-conductive nylon. The antenna has a comparatively high efficiency by using only one butterfly capacitor and by placing the tuning components inside the large loop.
The tuning process is very simple: first tune for maximum noise while listening on or near the desired frequency, then you transmit with low power and adjust the capacitor for minimum SWR by watching the built-in SWR meter of the radio. As an additional help the manufacturer has provided a light in the tuning box, which lights up when the SWR is below 3:1 (and the power is at about 5W). With a little practice an SWR of 2:1 or better can be achieved.
By their very nature magnetic loops are quite narrow banded by the hi Q of the resonant circuit. The manufacturer claims a usable bandwidth of 20 to 100% of the ham radio band without retuning: 200kHz on 10m, 40kHz on 12m, 150kHz on 15m, 80kHz on 17m, 70kHz on 20m, 50kHz on 30m and 40 kHz on 40m (all values approximatively). The narrow bandwidth is also beneficial for the receiver, the antenna works like a preselector!
The diameter of the assembled loop is 85cm, the large loop forms slight ellipse. The HF feed is done on the upper box, a PL socket is provided, you need to provide a coaxial cable. Included in shipment is a very useful nylon carrying bag with a length of 87cm. The low weight of only 3kg makes the Alpha Antennas Loop a perfect companion for portable operations, field day use or vacations.
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