CB antenna to a stationary, so. magnetic (magnetic loop) is the optimal output in the absence of conditions for a typical installation of the antenna fixed at a high altitude, outside the building. The preferred ratio of the dimensions of effectiveness, low sensitivity to the height placement above ground, wąskopasmowość and interference causing, that this solution is often used in the mobile military equipment. During World War II, the magnetic antenna was used on German submarines.
Advantages:
- size range for the CB - about 90 cm in diameter,
- efficiency - comparable to the dipole półfalowym,
- radiation pattern - almost omnidirectional,
- a very narrow band - "sharp tuning " which results in cutting out interference from adjacent channels,
- ability to install an antenna on the premises at a relatively low level (attic, balcony, etc),
- for the place of installation complete "burzo - wind - rainy - freezing" immunity,
- antenna "is not circle "in the eyes of neighbors,
- marginal cost of elements
- ideal for use in field conditions as mobile - portable.
Disadvantages:
- need for frequent tuning (away for 2 - 3 channels in the side - requires re-tuning),
- tuning capacitor need for remote main loop,
- very strong magnetic component can be harmful to health (in broadcasting should be in the safe - a few dozen meters - the distance from the antenna).
IMPLEMENTATION
main loop:
loop is made of a 270 cm section of Al-pex pipe, pipes used in heating. Alu-pex is constructed of two layers of plastic separated by a layer of aluminum. It is very light and malleable - can be perfectly formed into a circle and holds a predetermined shape. At the ends, which connect the variable capacitor, remove the top layer of plastic (hot gas burner) and clean the rest of the aluminum tube with plastic fine sandpaper. The ends of the loop, we combine the Teflon rod with a diameter equal to the inner diameter of the pipe loop. Drill holes 4 mm bolt loop to Teflon rod. For these bolts connect the capacitor - as soon as possible and as the thickest cable may be flat copper.
Possible modifications - a loop can be made of copper pipe installation (15 - 20 mm) with copper (aluminum) flat or coaxial cable. The diameter of the tube does not play a major role. Please note, however, with a diameter of the loop - the loop of pipe length should not exceed 0.25 wavelength.
loop coupling:
coupling loop made of copper wire with a diameter of 2 - 3 - 4 mm and connect the tip of the loop with a diameter of 220 mm to the tip of the antenna cable. The loop can be connected directly to a cable or through the appropriate socket and plug. Diameter coupling loop should be about 20% of the diameter of the main loop. The loop can also perform the coupling of coaxial cable.
Capacitor:
In the case of CB-band antenna tuning on all channels of basic 40th provides a variable capacitor with a capacity of 3 to 20 - 30pF. Capacitor should be called. butterfly. Such capacitors can be found in an old military equipment, you can also - quite easily - do it yourself. When choosing a capacitor, remember that at the time of broadcast on the cover of a capacitor, the higher power, there is tension of thousands of volts! Hence, these plates must remain relative to each other at a distance. In practice, for up to 4 watts CB just 0.5 mm. Tunable capacitor must be remotely using an electric motor with gearbox ensures circulation of about 0.5 per minute. Motor power supply wire run plastic pipe in the middle of the superstructure, which is embedded antenna. Working motor does not cause interference.
issues mounting the individual components, motor and gearbox housing constructors leave creativity. In the present embodiment were applied:
- motor with gear - old servo Model with damaged electronics, the two wheels of an electric timing clock and tape deck,
- motor housing and a capacitor - plastic mounting box (Castorama, a few zł)
- coupling loop fastening - plastic case (stores of electronic parts),
- mast - 150 cm section of plastic pipe with a diameter of 25 mm,
- alu-pex pipe - Castorama 2 - 3 zł / mb,
- capacitor butterfly - with the surplus.
The experience of life:
antenna is mounted in the attic storey house covered with ceramic tile. Proven communication coverage - 45 km from the base station. It is a performance comparable to the seventh meter antenna 5 / 8 lambda I have been using before, and he dismantled for safety reasons - frequent and severe storms in my area. The antenna stroimy "by ear" - when the alignment of the channel is a sharp increase in noise, SWR drops to 1.2 - 1.5. Motor power - battery 1.2 V, 1.5 times for 2 minutes.
