Antenna equivalent. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination
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To establish the transmission path of the transceiver (HF / VHF or CB), objective measurements of its characteristics and their control during operation, an antenna equivalent is required. "Branded" non-inductive resistors with a resistance of 50 or 75 ohms and a power dissipation of tens of watts are not available to most radio amateurs, so they usually use chains of MLT-2 resistors to make an antenna equivalent. It is widely believed that the resistors used should not have structural spiral grooves that are applied to the body of the resistor to increase its resistance. In other words, the resistors should be relatively low resistance. This recommendation assumes that the parasitic inductance due to the resistor design is then eliminated. The use of low-resistance resistors complicates the design of the antenna equivalent, since in order to achieve dissipated powers of tens of watts, it is necessary to use parallel-series circuits.
An experimental test carried out in the laboratory of the journal "Radio" showed that the concerns mentioned above about the effect of the parasitic inductance of the resistor on the characteristics are somewhat exaggerated. From 24 MLT-2 resistors connected in parallel with a resistance of 1,2 kOhm, the equivalent of an antenna with a resistance of 50 Ohms and a power dissipation of about 50 W was made. Structurally, it was made in the form of a "squirrel wheel" (see photo), which allows minimizing the influence of the inductance of the leads and creating close to optimal conditions for cooling each resistor. Previously, resistors for the equivalent of the antenna in terms of resistance were not selected. Measured at direct current, its resistance was 51 ohms. The calculated value of the antenna's own SWR equivalent (1,02) is beyond the accuracy of its measurement in amateur conditions.
MLT-2 resistors with a resistance of 1,2 kOhm have spiral grooves that form five "turns" on the resistor body. However, a test of this equivalent HF/VHF antenna with an MFJ-209 SWR analyzer showed that its SWR=1 in the frequency band from 1,8 to 60 MHz. It rises to 1,1 at 90 MHz, to 1,2 at 120 MHz, and to 1,3 at 150 MHz. In other words, this equivalent antenna is suitable for use on the HF bands without any reservations. It can also be safely used on the 144 MHz VHF band, since SWR values up to 2 are quite safe for even transistor equipment to work.
This dummy load is capable of dissipating up to 50W of power over a long period of time, and 100W of power for a minute (longer with forced fan cooling).
Author: B.Stepanov (RU3AX)
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