The good results obtained with the antenna «Magnetic Loop», led I1ARZ try to build an antenna on the low bands.
At first he intended to build a loop antenna circular shape (Figure 1) with a perimeter of about 10.5 m, which is a quarter wavelength at 7 MHz. For this purpose we made a loop of copper tube 40 mm in diameter with thin walls, however, in the course of the work it was found that flexion and extension tubes of this size - quite a difficult task, and the shape of the antenna was changed from round to square. A decrease in efficiency is balanced by a considerable simplification of manufacturing. For the range of 1.8 ... 7.2 MHz can be used copper tube diameter of 25 ... 40 mm. One can also use a duralumin tube, but not all have the possibility of welding in argon. After assembling all the antenna frame is covered with several layers of protective lacquer. For proper operation of the antenna tuning capacitor is very important. He must be of good quality, with a large gap between the plates using a vacuum capacitor 7 ... 1000 pF with permissible voltage of 7 kV He stands antenna power exceeding 100 W, which is sufficient. In the case where the range is 160 m, the vessel should reach 1600 pF. Hinge square shape is assembled from four copper tubing 2.5 m long and 40 mm diameter tubes are connected together with four water knees of copper. The tubes are welded to the tribes. Opposing sides of the frame to be parallel to each other. At the top of the tube in the middle is cut out a piece of length 100 mm, is inserted into the cut Teflon spindle and is secured on both sides of clamps and screws. Diagonal loop is 3.4 m, the total length - 10.67 m (with copper plates 50 mm wide, which are attached to the ends of the tube that connects the tuning capacitor). To ensure proper contact after their attachment plate should be welded to the ends of trubki.Na Figure 2 shows the frame design with the base and the carrier truck. The mast should be a dielectric, such as steklvolokonnogo rod. One can also use plastic pipe. At the bottom frame is fixed to the carrier mast steel clamps. For strengthening the lower horizontal frame piece on it stretches over a length of about 300 mm heated copper tube slightly larger diameter. Motor torque capacitor strengthened by a steel pipe at a height above the roof about 2 m. Stiffen the entire structure below the motor is installed at least three rastyazhek.Prosche all agree on the antenna frame and the power line using a loop type coaxial cable RG8 or RG213 diameter coil is determined empirically (approximately about 0.5 mm). Connection of the inner core and sheath in accordance with Figure 3. After matching coil is set to the lowest SWR, for protection from rain over the connection area stretches corrugated plastic tube. At the end of the matching coil to install coaxial connector. At the point of attachment of the lower coil matching the fixing clamp is passed duralumin piece of copper tape, which after folding is soldered to the shield of the cable. It is necessary for good electrical contact with the grounded duralumin tube (Figure 4). The upper part of the matching coil is attached to the dielectric mast rubber ties. If the antenna is located on the roof, for remote control of the tuning capacitor requires a drive unit DC motor. Placing the tuning capacitor. As already mentioned, the fixed and movable parts tuning capacitor attached to the upper cut parts of the frame with two copper plates about 0.5 mm thick, 50 mm wide and 300 mm long each. Tuning capacitor is placed in a plastic tube, which is attached to the vertical mast of fiberglass carrier. The upper part of the frame is connected to a Teflon spindle and fixed to the supporting fiberglass pole with U-bolts. Adjust the TRX to a dummy load, switch the output to the antenna TRX. Antenna tuner in this experiment did not use. At a lower power output capacitor start to rotate until a minimum SWR achieve low VSWR If this method does not work, try a few deform matching coil. If the SWR is not improving, it is necessary to turn or to lengthen or shorten. With a little patience, it is possible in the range 1.8 ... 7 MHz to achieve SWR 1 ... 1.5 achieved the following SWR 1.5 - 40 m, 1.2 - 80, 1.1 -160 m results. Adjusting the antenna is "acute." In the range of 160 m bandwidth of the antenna is a unit of kilohertz. Diagram (RP) - almost circular. Antenna gives the best results in the range of 40 m. 50W author established many links with the east coast of the United States with a report of 59 at distances up to 500 km a day reports were 59 + 20 ... 25 dB. The antenna is also very good at the reception, because it is quite "sharp" setting reduces noise and signals are working a number of strong stations antenna works surprisingly well, and in the range of 160 m. From the first attempts to link has been established at a distance of over 500 km with a report of 59 + 20 dB. From a fundamental point of view, in this range the efficiency of the antenna is much lower than in the range of 40 m.
The antenna should be placed as far as possible from large metal objects such as fences, metal poles, gutters, etc. Antenna is not recommended to place indoors as frame antenna transmission emits a strong magnetic field, which is harmful to health. When working with powers above 100 W frame is heated by application of high voltage. At the upper range of the horizontal polarization antenna.
Same antenna can be constructed on the higher frequencies, thus reducing the dimensions of the frame and capacity of the tuning capacitor.
article is taken from a Russian amateur radio site