Amateur G5RV wire antenna in the picture with a size of 31 meters, it is the transition between the antenna option with long arms and a 54-meter antenna with an arm length of 27 meters. By wire antenna connected two-wire power supply with an impedance of 300 ohms (balun). Length of the power line should be 12.90 meters. This line acts as a power transformer. Towards the bottom of the feed point of the transformer is connected coaxial cable (Figure).
In the range of 28 MHz each half G5RV wire antenna has a length 3L / 2. Diagram of a shape of the antenna length 3L. In-phase excitation power gain compared to a conventional dipole is 1.8. In this range, the antenna G5RV works well. Since the length of the line is transforming 5L / 4, the wire antenna is in excellent agreement with the coaxial cable. At the points of "BB" impedance of about 60 ohms.
In the range of 21 MHz antenna length is slightly longer wavelength (L) of the range and reach the transformer is also a little longer than 3L / 4 matching any worse. Sizes do not allow you to work in a resonant mode, in spite of this antenna has a gain over a dipole is 1.8 dB, due to the fact that half the vibrator excited in phase. Diagram of the wire antenna in the same range as the two wave vibrator.
In the range of 14 MHz antenna matching is even worse than 21 MHz. At the points of "AA" very little resistance from the fact that the antenna has a size of 1.5L and is approximately 70 ohms. At these points there is a maximum current. Transforming line transforms the antenna impedance (70 ohms) to 200 ohms at points "BB". It is clear that such resistance is not suitable for connecting a coaxial cable with an impedance of 50 ohms ... 75. Reinforcement wire antenna in the range of about 2 - 2.2 dB.
In the range of 7 MHz G5RV wire antenna has a length 3L / 4 and the input impedance of 600 ohms. Length of transforming the line has 300 ohms resistance and the length of the 0.3L, as a result of the partial transformation of the 600 ohm resistance at the points of "AA" to the point "BB" is obtained impedance 150 ohm. Since the directional properties of the antenna wire in this range are similar pattern in a half-wave dipole and the win is no gain.
In the range of 3.6 MHz is a resonance antenna of its length in this range is 0.36 ... 0.47L. Antenna impedance is 150 ohms, the length of the transformer is equal to 0.15 ... 0.16L at points "BB" resistance is 100 ohm. In this range, the directional properties of even worse and the winning wire antenna gain, however.
From the above it follows that the written wire antenna (classical) English G5RV amateur radio should be used in conjunction with the tuner (for the transceiver). Changing the length of the line transformer and its impedance leads to improved coordination in some ranges and a deterioration in others.