Coaxial dummy load. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Coaxial dummy load. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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It is known that the setting of the transmitting equipment must be done on the equivalent of the antenna. At the same time, it is desirable that its parameters (input impedance, SWR) be identical to the parameters of the antenna-feeder device that the radio amateur will use.

Coaxial dummy load

The described dummy antenna (see figure) is a coaxial type load designed to operate in a 50-ohm coaxial path. It is made on the basis of MLT-2 resistors.

This load ensures the absorption of electromagnetic energy in a wide frequency range: from direct current to hundreds of megahertz.

The equivalent is connected to the output connector of the transmitter using a piece of coaxial cable equipped with high-frequency connectors.

The energy absorber consists of three sections, each of which contains six resistors 1 MLT-2 with a resistance of 100 Ohm. The resistor leads are soldered in the holes of the bushing 3 made of brass LS59-1 and washers 4-6 made of double-sided foil fiberglass 1,5 mm thick (SF2-35-1.5). The resistors in the sections are connected in parallel, and the sections are connected in series with each other. In this case, their total resistance should be 45 ... 50 ohms. On one side of the absorber, there is a conical contact 9 made of sheet (0,8 mm thick) brass LS59-1. The absorber is placed in housing 2 made of AMG-6 alloy (possibly made of duralumin D16) and fixed with an M4X12 screw. The housing is closed with cover 7 (AMG-6), in which the female part of the coaxial connector SR-50-165F is mounted (to simplify the drawing, the coaxial connector is not shown in the side view).

The assembly of the equivalent begins with soldering the leads of the resistors in a brass sleeve 3. Before this, the leads of the resistors are shortened to 8 ... 10 mm, the surface of the sleeve is covered with POS-61 solder (tinned). The leads are soldered in the inner groove of the bushing. Solder should not protrude beyond the dimensions of this part. Then, a fiberglass washer 4 of the smallest diameter is put on the free terminals of the resistors. The leads are distributed through one hole and soldered to the outside of the washer so that they protrude no more than 3 mm.

The leads of the resistors of the second section are inserted into the free holes and soldered in the same way. The second, middle, washer 5 is put on the free terminals of the resistors of the second section and the resistors are soldered. Also collect and the third section.

After assembly, the absorber is a fairly rigid structure that can not only retain its shape, but also withstand a small load.

In the process of assembling the absorber, it is necessary to ensure that the resistors form a kind of drum, and the washers are located perpendicular to its axis. In addition, it is necessary to pay attention to the fact that the total length of the drum is 80 ± 0,5 mm.

Then, a conical contact 6 is soldered to the outer metallized surface of the washer 9. It must be located coaxially with the absorber. The final conical contact is soldered after installation, in the assembled load of the coaxial connector. The absorber is lowered into the load case and fixed with an M4 screw. After that, cover 7 with a connector is screwed onto the body. The output of the latter must enter the hole of the conical contact. The cover is screwed up to the stop, and through the hole in it, the connector output is soldered to the conical contact. Then, a hole is drilled in the cover and the body and an M2 thread is cut, the relative position of the body parts is fixed with the M2 screw.

For a stable position of the load on the table, a corner 3 (AMG-8) is attached to the end of the case with two M6 screws. The outer surface of the body parts is coated with nitro-enamel paint.

The equivalent assembled by the author provided a dissipation power (maximum) of 15 watts. The standing wave ratio (SWR) at frequencies of 80 ... 600 MHz did not exceed 1,2. The DC resistance at the load connector was about 50 ohms.

The power dissipation can be increased by drilling holes in the case with a diameter of 8 ... 10 mm to ensure air convection. At the same time, their number should not exceed 15 - 20. It is better to arrange holes on the conical surface of the housing evenly, since otherwise the SWR deteriorates by about 0,1. The deterioration of the SWR is explained by the fact that the presence of holes in the load housing leads to an increase in the reactive component of the load impedance. It can be improved by additional adjustment - by gradually cutting off the metallization layer on the washers and slightly shifting the resistor drum.

To increase the load power by 2 ... 2,5 times, it is necessary to apply forced cooling with a fan. An equivalent with a maximum dissipation power of 50 W or more can be made of a similar design, but it is necessary to increase the number of sections and the number of resistors in the sections, but the total resistance of the equivalent direct current should be 45 ... 50 Ohm.

The possibilities of setting up the transmitter using this equivalent can be expanded by its simple refinement, which will allow, using a conventional avometer, to control the output power and adjust the transmission path to the maximum high-frequency voltage taken from the resistor section farthest from the input.

When modernizing the load on a fiberglass washer of the "smallest diameter, it is necessary to additionally place the parts of the diode section. For example, they are soldered to metallized contact pads measuring 3x4 mm in the peripheral zone of the washer, leaving the foil there. The diode section is made according to a scheme similar to that by which the diode section is assembled in high frequency voltmeters.

Two single-pole sockets are installed on the body of the equivalent. One of them is connected to the housing, the other with a shielded conductor - to the output of the diode section. An avometer operating in the DC voltage measurement mode is connected to these sockets during the transmitter setup process.

It should be noted that the temperature regime of the diode section will depend on the absorbed power, and, therefore, the device cannot be accurately calibrated in terms of power. And yet, the use of an internal diode section in conjunction with an avometer will greatly facilitate the process of setting up the transmitter and its interface with the load.

Author: S. Rumyantsev (RA3DQA) Pushkino, Moscow Region; Publication: cxem.net

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