Ferrite antenna with amplifier SDR

Ferrite antenna with amplifier SDR

Ferrite antenna is a magnetic antenna with a ferrite core. Due to the high magnetic susceptibility of ferrites, the size of the ferrite antenna is much smaller than the frame antenna, given that the induced electromotive forces in the antenna are the same.

The receiving ferrite antenna is used when receiving electromagnetic waves. Conditions of reception – distance up to 300 km, progressive motion, in the presence of short stops reception is carried out duplex or simplex operation. Ferrite antenna is a rectangular block, which has a dielectric shell. On the inner side of the casing, two rods made of ferrite are attached to its base, which are collected from the rings, together with inductance coils. The coils are magnetic receivers in this design. Between the ferrite rods is a unit that adjusts the antenna with an amplifier. On the antenna casing there are connectors for connecting the antenna with the receiver, as well as for controlling the tuning and connecting power to the amplifier.

Receiving ferrite antenna has the form of a symmetrical frame antenna. Its antenna loop must be in resonance with a set of Capacitors. A symmetric amplifier amplifies the received signal and matches the high impedance loop to the low impedance input of the antenna receiver. High-frequency relays connect the capacitors to work.

In the ferrite antenna, the sub-bands can be adjusted either manually or automatically. Here is the simplest circuit.

Рис. 1. Source repeater for the magnetic antenna.


This option does not need a telecoil, because the source repeater has a very high resistance. When using an external full-size antenna field-effect transistor is desirable to use medium or high power (type KP601, KP901, KP903, KP907, of imported 2SK125). But the magnetic antenna signal size, and most importantly, the various interference and interference is much smaller, so it is quite suitable KP302, KP303, KP307, and from imported – 2SK241. The input of the emitter repeater on the bipolar transistor VT2 is directly connected with the output of the source repeater (any high-frequency n-p-n – KT315, KT316, KT368 will do). Supply voltage – 9V.

But we would like not only to save a useful signal, but also to amplify it. For this purpose, double-gate field-effect transistors are suitable, which allow you to easily adjust the amplification and have a small inherent noise. The disadvantage of the double shutters of domestic production (KP306, KP350) is their fear of static electricity. He himself killed more than one such transistor, until he learned how to work with them. It is much more convenient to work with KP327, which has protective diodes, and the level of its own noise is much lower. But I prefer the imported ones, like BF982 (enclosure cross, F<1.5 dB) BF998 (SOT143B, F<0.6 dB). These transistors have protective diodes and are easier to work with and the BF998 cost 80r per decade. In addition to their high input resistance, field-effect transistors have low input and throughput capacitance, which makes them similar to electronic lamps.


Рис. 2. Amplifier for the magnetic antenna on the double-gate transistor


First transistor gate can be directly connected to the magnetic antenna circuit by removing C1 and R2. The gain is controlled by changing the voltage at the second gate. Transistor load is T1 broadband transformer. When using BF998 it is possible to reduce the supply voltage to 5 V, and resistor resistance R3 to 100-150 Ohm.

Well, the logical conclusion is the differential amplifier cascade. The differential amplifier is a well-known circuit used to amplify the voltage difference between two input signals. Ideally, the output signal does not depend on the level of each input signal, but only on its difference. When the signal levels of both inputs are changed simultaneously, this change in the input signal is called in-phase. A differential or differential input signal is also called a normal or useful signal. A good differential amplifier has a high coefficient of in-phase attenuation (CFC). It is optimal to use a chip as such an amplifier because it achieves a high transistor identity such as the AD8129. But it is possible to try to build a differential amplifier on discrete components as well. I suggest this circuit.


Рис. 3. Differential amplifier circuit on double gate field-effect transistors


Balancing the cascade is carried out with a resistor R5 to minimize noise. To adjust the gain on the second shutter is fed an adjustable voltage to the resistor slider R9. Of course, due to the difference in control characteristics of VT1 and VT2, there will be imbalance of the stage and you will have to twist the R5 again. If it is not satisfactory, we leave R9 in the right position, balance the stage and do not touch the resistors anymore.

The output voltage is taken from the secondary of the broadband transformer Tr. It is possible, if the amplification is sufficient, not to put a second stage on the VT3, which is assembled according to the common base circuit (CT606, CT610).

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