Antenna for operation in the field
To work in the field over a range of 10 m was made whip antenna from dural tubes with a total length of 2600 mm. It consists of four sections with a diameter of from about 8 (bottom) to 3.5 mm (top). The antenna was mounted on a magnetic base, made of plastic jars and the magnetic system of the old speaker.
To attach a 50-Ohm cable used standard instrument connector, center pin, which is connected to the pin. External contact connected to the disk with a diameter of about 100 mm, cut from a thin foil fiberglass or brass sheet. Disk 'glues' between the base of the antenna and the roof of the car, thus providing a sufficiently large receptacle to connect the cable with the mass of the antenna. It weighs just 230 g, and the bulk of this meager weight falls on the magnetic base. CWS turned out to be less than 1.2 at any frequency. The antenna can be adjusted even more precisely, ndwiga and telescoping sections.
Very effective for a 10-meter range was the so-called J-antenna with the length of the matching line of 0.24 l. and height of the radiator 0,47 l. For its production tubing was used a gradually decreasing diameter from bottom to top from 22 to 10 mm). They were inserted one into the other and secured by screw M4, which zawinuls into the threaded hole on the side wall of the tube of larger diameter. The distance between the tubes of a two-wire line about 100 mm (it's not critical). From below they are bonded dural plates, and in the middle and top (for rigidity of the whole structure) - pads made of organic glass. The pads are compressed with M4 screws, firmly locking the tube. It is necessary to find the point of connection cable for minimum SWR.
J-antenna height of 10 m range was about 7.5 m, is attached to the base of the upper trunk of the car and fix the stretch marks from fishing lines. They tied at the top right to the middle of the half-wave vibrator. High-frequency voltage there is minimal, and bending the antenna wind loads about this point largely compensated. J-antenna while links provided winnings of about 0.5...1 point compared with the quarter-wavelength pin as at close and long-distance routes.
Were tested and wire antennas for 40 and 80 meters To install them used the pole (see figure). It was raised "antenna" quarter-wave wire cut, as opposed to the same length stretched along the ground. Better not to raise the very end of the antenna wire, and the point where some two thirds of its length. The same end of the wire increased stretching from the fishing line, twine or cord. Even better to use a pair of stretch marks, razvitii wire per pole. This will move higher up the peak current in the antenna and increase its effectiveness. The diameter of the wire does not matter - the wire diameter of 0.4 mm works just as well as thicker.
A few words about the choice of location of the station. It depends on what connection you are going to spend. For earthly ties wave, it is desirable to choose the open hill.
As for the relationships of ionospheric waves seat height does not matter. Moreover, shallow open lowlands, river valleys and lakes can be preferable because of interference coming terrestrial wave, there is less, and the conductivity of the soil above that improves the performance of the antennas.
In any case, it should be placed away from objects associated with large electrical currents and usually disturbing. Only then you can feel what a truly clean the air and what opportunities it provides.
The author's first impression of the work "in the field" was like the shock of the noise level of the receiver in the range of 10 m when connecting the antenna almost not increased. For objective evaluations of winning in the actual sensitivity was manufactured with a low noise preamplifier, S-meter receiver with preamp was graduated with HSU in microvolts. Noise in the range of 10 m and a the bandwidth of 2.4 kHz and hiring above the whip antenna amounted away from the settlements of 0.07 0.1 mV.... On electric garden site 50 km from Moscow, he rose to 0.14...0.2 mV, and in the city on a permanent QTH author - 0.7 μv or more (depending on how much interference sources is included at this point in the grid).
Thus, the outing allowed "to increase the sensitivity of a receive path of the transceiver 20 dB (or more precisely, to fully realize its capabilities). Try to produce and supply the antenna with a gain of 20 dB to get the same results in the city! •
Author: Vladimir Polyakov (RA3AAE); Publication: N. Bolshakov, rf.atnn.ru