Antenna amplifier range 2 meters. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception
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It is known that in amateur radio practice it is necessary to receive very weak signals, comparable in level with the intrinsic noise of the receiving device. In the VHF bands, additional problems are created by the feeder, the losses in which worsen the signal-to-noise ratio at the receiver input. That is why radio amateurs on VHF often use low-noise amplifiers located at the antenna, which provide a noticeable predominance of the signal over the noise at the input of the receiving device. The antenna amplifier, the circuit of which is shown in the figure, was developed by K. Britain (WA5VJB). It was successfully repeated by radio amateurs in different countries, in particular, in Bulgaria (it was described in the magazine "Radio, Television, Electronics").
The noise figure of this amplifier is about 0,3 dB, but it is possible to achieve lower values - about 0,15 dB. Experimental verification showed that the noise factor when replacing the MGF1601 transistor with the ZP602G transistor changes slightly (by no more than 0,2 dB). The amplifier works steadily.
(click to enlarge)
Installation of elements is volumetric (hinged), and each element must have the minimum possible length of the leads. The source of the transistor is high frequency blocked by surface mount capacitors (SMD type). The input circuit of the amplifier should have negligible losses. Here it is necessary to use tuned capacitors with an air dielectric (ceramic ones are not suitable). Coil L1 frameless, contains 5 turns of bare silver-plated wire with a diameter of 0,7 mm. The inner diameter of the coil is 8 mm. Transformer T 1 has 2x3 turns of PEL wire 0,3mm. It is wound on an annular ferrite magnetic core M50VCh-2, size K5x3x2mm.
A ferrite bead or ring is put on the drain terminal of the transistor. Before mounting the transistor, its terminals are interconnected with a thin wire, which is removed after the amplifier is mounted. Only low-temperature solders should be used for soldering transistor leads. In addition, it is necessary to solder quickly and carefully: overheating worsens the noise parameters of the transistor, and an electrostatic breakdown of the channel is also possible. Soldering should be carried out with a soldering iron with a grounded tip disconnected from the mains.
It is not advisable to use tube noise generators for tuning an amplifier on a GaAs field-effect transistor, since their output signal sometimes contains voltage surges, from which an expensive MGF1601 can fail. Here it is better to use a modern semiconductor noise generator.
The attenuation coefficient of the output circuit attenuator is chosen depending on the specific conditions (cable attenuation coefficient and receiver noise figure). By optimizing it, you can remove the excessive gain of the cascade and increase the upper limit of the dynamic range of the receiver.
The supply voltage is stabilized and does not turn off when the amplifier is already installed on the antenna. This reduces the risk of breakdown of the transistor by static pickups.
The operation of the amplifier in the conditions of the city of Sofia showed that the reception of weak signals is limited by industrial interference and such an amplifier does not provide a significant improvement when receiving very weak signals. Under such conditions, a good amplifier based on a MOS-FET transistor (for example, BF966S, KP350, etc.) gives approximately the same results as on a GaAs field-effect transistor. The effect of using the latter can only be felt in reception conditions without urban QRM.
Author: Andrzej Gaidarjchev (LZ3UF), Sofia, Bulgaria; Publication: N. Bolshakov, rf.atnn.ru
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