T-contour. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Filters and matching devices

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To match a powerful output transistor with an antenna and filter the signal, in most cases, insufficiently perfect circuit solutions are used:

- ferrite transformers, which do not have filtering properties and are not easily accessible to radio amateurs;

- P-circuit, but it poorly matches the low output impedance of the transistor stage (2 ... 15 Ohms) with the output impedance of the antenna, while the losses are large.

One of the effective solutions to the problem that has arisen is the T-loop, the principle of which is clear from Fig. one.

Ris.1

It can be seen from the diagram that this is a two-circuit system. The input impedance of the antenna (Ra) is introduced into the K2 circuit. In this case, the reactive component of the input impedance of the antenna can be easily compensated by changing the value of Lc, which is very important when tuning the antenna. The input impedance of the K2 circuit at point "c" when tuned to resonance.

It has a value of the order of 100 ... 2000 Ohm, depending on the value of Lc, which, in turn, is selected depending on the operating frequency and output power of the cascade.

The transistor collector, operating as a current generator, is connected in series to the K1 circuit. The load of the circuit K1 (at point "b") is the input resistance of the circuit K1 - Rin2. To obtain a T-contour, it is enough to connect the points "b" and "c". In this case, the total capacity is C \u1d C2 + CXNUMX.

The input resistance of the K1 circuit at point "a" has the value

The presence of two circuits in the circuit allows you to qualitatively filter out unnecessary components in the signal, there is no need for additional filters.

In practice, the elements are selected from the following ratios:

С [pf]=(8...20)L,

where L is the average wavelength in m.
C - the more, the greater the power, i.e. at P=10 W C=8L, at P=200 W C=20L. Cp-separating capacitance for direct voltage between Ek and the antenna.

where Rin1, is the load resistance of the transistor.

A practically working cascade circuit for the ranges of 160, 80 and 40 m is shown in Fig. 2.

<Fig.2>

Coil L1 contains 16 turns of PESHO-0,5 wire, frame diameter - 16 mm, winding length - 16 mm, tuning - with a ferrite core with a diameter of 8 mm, a length of 20 mm. L2 contains 37 turns of PESHO-0,3, taps - from 25, 29 and 33 turns. Frame diameter - 10 mm, winding length - 12 mm. The coils are arranged so that there is no mutual inductance between them: shielding, perpendicularity of the axes of the coils.

S1 - range switch. The setting is as follows: using switch S2, a connection is established with the antenna, and the L1 core is used to tune the circuit to resonance at the maximum of the antenna indicator and the maximum current of the transistor collector. In general, the tuning resembles the tuning of a P-loop, but instead of variable capacitors, there are variable inductances and tuning for the maximum current of the transistor. Instead of L2 and S2, you can use a variometer with an inductance of 8...12 µH. It is possible to tune into resonance using a variable capacitor, then the inductance L1 is constant and is 2,1 μH, but the author's inductance tuning showed an excellent result, and the device turned out to be smaller than when using a variable capacitor.

When operating in the receive mode, the T-loop is an additional input band filter of the receiving device. Switching "reception-transmission" is carried out using a relay K1 type RES-55A or the like. Transformer T1 is wound on a ring measuring 8x4x2 mm with a permeability of 100 ... 1000 and contains 20 turns of PESHO-0,3 wire with a tap from the 5th turn, read from the grounded end (with the input resistance of the receiver 75 Ohm).

Literature

1. Red E. Reference manual for high-frequency circuitry.
2. Rivanenkov L. Output stages of HF transmitters on transistors. Radio amateur KB and VHF. - 1996. - No. 2.

Author: L. Rivanenkov (UA3LDW), Smolensk; Publication: N. Bolshakov, rf.atnn.ru

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