On the matching of plate antenna amplifiers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers

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In the published article, the author, considering the amplifiers used in Polish-made antennas, offers his own way of using them in the antenna-feeder path.

Plate Antenna Amplifiers (PAA) are part of popular Polish TV antennas ASP-4WA. ASP-8WA (CX-8WA). They are a 60x40 mm printed circuit board with surface-mounted microelements. During normal switching on, such an amplifier is installed directly on the antenna: its input is connected to its collecting line, and the output is connected to a drop cable, through which it receives the supply voltage.

Currently, many interchangeable PAH brands such as SWA, GPS are being produced. RA. RAE and others. Their circuitry, characteristics, features of a particular amplifier and its repair have already been considered in the journal 11. [ 2]. An unusual use of PAHs is proposed here, which refers to any inclusion of them separately from the ASP antennas.

It should be noted that PAHs are "ready-to-use" electronic components and, given their relatively low price (1.5 ... 2 dollars), it becomes clear that radio amateurs want to use PAHs in their antenna designs. And they are often more efficient and better adapted to specific reception conditions than ASR antennas. The only problem that needs to be solved is matching the input impedance of the antenna to the input impedance of the amplifier.

Let us first consider how the matching is done in the ASR antenna. At the input of the PAH, a balun-matching transformer on a K 10x6x2,5 ferrite ring is turned on. It has two windings - two-wire long lines containing three turns each. Such a transformer on long lines (LTL) has a high efficiency (up to 98%), small dimensions and a wide range of operating frequencies [3]. When the received oscillations propagate in it, the currents flowing through the conductors of the lines are equal and opposite in direction, which means that the magnetic circuit is not magnetized and there are practically no losses in it. On the other hand, the presence of a magnetic circuit significantly increases the inductance of the windings, eliminating their shunting effect on the antenna and load. The magnetic circuit does not affect the propagation of oscillations, since the traveling wave mode is provided.

The conditional scheme for switching on the TDL of the ASP antenna is shown in fig. 1. It has a balanced input (points a. b, c) and an unbalanced output. It satisfies the relations indicated in [3] Rr = n2RH; U1 = nU2. p = nRn, where R, is the resistance of the generator equivalent to the antenna, Ohm; RH - load resistance, Ohm; n - transformation ratio; p - wave resistance of a two-wire line, Ohm.

Some values ​​in the formulas require explanation. So, the transformation ratio is numerically equal to the number of TDL windings, the generator resistance in the receiving antenna is equal to its input resistance, and the load resistance is equal to the PAH input resistance. The input impedance of the antenna is about 300 ohms, and the number of TDL windings is two. Substituting into the formulas, we get: U2=0,5U„ RH=75 Ohm, p=150Ohm.

Consequently, as a result of matching, the voltage of the useful signal is halved, and the input resistance is four times, and the latter is transformed into close to standard (75 ohms). It follows from this that the active component of the input impedance of the PAH is close to 75 ohms, i.e., its input is actually matched with the characteristic impedance of the coaxial cable. The output of the amplifier is also designed for such a load. As a result, we can conclude that a PAH without TDL is able to work effectively when it is included in a coaxial cable break without additional coordination.

To assess how the removal of the TDL will affect the properties, let us dwell in more detail on the frequency characteristics of the latter. Although theoretically TDL does not have cutoff frequencies, in reality its lower operating frequency f "is limited by the inductance of two-wire lines, which is determined by the formula given in (3]: Ll \u2d w250μS / 2dcp (μH), where w is the number of turns on the magnetic circuit; μ is the relative magnetic permeability of the magnetic circuit, S is the cross-sectional area of ​​the magnetic circuit, cm3, dcp is the average diameter of the ring, cm.

The calculations lead to the following results: Ll = 0.68 μH, fn = 220 MHz. This frequency value indicates that in almost the entire MB range, the transformer does not operate in the optimal mode. This means a decrease in the KBV and the transmission coefficient, which is especially noticeable at the frequencies of the 1st - 5th television channels, as indicated in [4].

A natural question arises: why did the designers not lower the frequency fn by simply increasing the number of turns of two-wire lines? The fact is that this is prevented by the maximum length of two-wire lines Lmax, which should not exceed λ/8 [3]. For the upper operating frequency of the UHF range, we obtain Lmax \u4d 1 cm. It is this length that the lines of the transformer TXNUMX have. An increase in the number of turns will inevitably lead to an excess of Lmax, WHICH WILL worsen the parameters of the transformer at the upper frequencies of the UHF range. Therefore, it will not be possible to ensure the optimal operation of the TDL on all television channels. Therefore, the designers preferred to obtain the maximum efficiency and transmission coefficient in the UHF range. Such a transformer and the entire antenna can be called decimeter.

Obviously, the matching TDL always degrades the parameters of the ASP antenna at one of the edges of the full television range. However, other matching devices are even less broadband and unsuitable for such antennas. It is impossible to do without matching and balancing in an antenna similar to an ASP antenna with an asymmetric PAH. Although, of course, antenna amplifiers are known that do not require matching devices [4], but this is a topic for another conversation...

Of the various known antennas [5] listed in the table, the input impedance close to the same parameter of the ASP antennas is possessed by a half-wave loop vibrator. PAHs (with TDL) are connected to it without any alterations. And the often practiced connection of a conventional half-wave vibrator to the points aiv TDL is inefficient, since their input impedances differ by a factor of four.

The above conclusion allows us to propose for almost any antenna a simple way to include a PAH-without TDL in a coaxial cable break. At the same time, a balancing-matching device, necessary for it, is mounted at the antenna input, described in [5y. U-elbow, half-wave loop, short-circuited loop, RF transformer or adder (when using two antennas), and the amplifier is turned on behind it like this. as shown in fig. 2 (for example, the "wave channel" antenna is conventionally shown).

Alteration of the amplifier itself is minimal: TDL is removed from it. It is even enough to unsolder its leads from the contact pad, to which the lead of the capacitor C1 is soldered [1.2]. without deleting TDL. Then, with a segment of the input coaxial cable of the required length, the antenna is connected to the PAH. The central conductor of one end of the cable is soldered to the contact pad, to which the output of the capacitor C1 is connected, and the braid to the common wire of the amplifier. The second end of the cable is connected to the matching-balancing device of the antenna. After that, the amplifier is attached to the mast (with screws or a clamp) and a drop cable is connected to its output with a clamping device on the board. The amplifier is carefully sealed, especially in places of soldering and cable connection. The way of supplying voltage to it is usual, more than once described on the pages of the magazine.

Removing the TDL ensures equalization of the frequency response of the amplifier in the MB interval and increases its transmission coefficient. Most PAHs work stably in this mode. If, nevertheless, the amplifier is excited (in models with a large gain, as noted in [1]), the supply voltage should be slightly reduced.

An additional advantage of this method of using PAHs is the possibility of varying the location of its installation: from close to the antenna to placement in a closed room. In the first case, the maximum signal-to-noise ratio is obtained, in the second - reliable protection of the amplifier from atmospheric influences, which prolongs its service life. With a small length of the input cable segment (up to 5 m) or the use of the PK-75-9-13 cable. having a low linear attenuation, the amplifier can be installed under the roof of the house (in the attic). The real deterioration in signal quality in this case will be insignificant and noticeable only at the upper frequencies of the UHF.

In conclusion, I would like to note that this method of incorporating PAHs is simple, versatile, and gives good results. Of course, another way is also possible - calculation (according to the method proposed in [3]) and production of a new TDP. matching a specific antenna with the amplifier input.

Literature

1. Pakhomov A. Antenna amplifiers SWA. - Radio. 1999. No. 1. p. 10-12.
2. Pakhomov A. New antenna amplifiers. - Radio. 2000. No. 7. p. 6 - 8.
3. Zakharov V. Matching devices on ferric magnetic circuits - Radio. 1987. No. 6. p. 26-29..
4. Volkov E. Active antennas and antenna amplifiers. - Radio hobby. 1999. No. 2. p. 52-54.
5. Onishchenko IP Receiving television antennas. - M.. DOSAAF. 1989.

Author: A.Pakhomov

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