HF bridge VHF band. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, setup and matching
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Figure 1 shows the RF bridge circuit, developed on the basis of the UA9AA design [1].
Ris.1
As a rule, the hinged mounting used in the manufacture of the bridge limits the operating frequency range of such devices to 140 ... 150 MHz. To ensure operation in the 430 MHz range, it is advisable to manufacture the device on a double-sided foil textolite. One of the successful installation options is shown in Figures 2 and 3.
Ris.2
On the upper side of the board (Fig. 2) there are two non-inductive resistors R1, R2 with compensation capacitors C4, C5. On the underside (Fig. 3) the remaining parts of the bridge are placed. The installation was done on the "spots".
Ris.3
The distances between the "patch" are determined by the dimensions of the parts used. The circles indicated in the figures by dashed lines are interconnected through holes in the board.
In the manufacture of the bridge, special attention should be paid to the quality of the parts used. Capacitors C1, C2 - ceramic, leadless, type K10-42, K10-52 or similar. Reference capacitor C3 - KDO-2. Trimmer capacitors C4, C5-type KT4-21, KT4-25; the rest of the capacitors - KM, KTs. Resistors R1, R2 must be of the type MON, C2-10, C2-33 with a power of 0,5 W and have the same resistance within 20 ... 150 Ohms. If resistors of the MON type are used, then their conclusions are bitten off to the base, which is cleaned and tinned, and then soldered to the desired "patch". Resistor R3 - type SP4-1, SP2-36, non-inductive, with a graphite track. This resistor is mounted on the side wall of foil textolite, however, the foil at the place of its attachment is removed. The body of the resistor is not connected to a common wire, otherwise the bridge cannot be balanced. The handle fixed on the axis of the resistor must be made of insulating material. In addition to the resistor R3, CP-50 connectors are attached to the side walls. The joints (joints) between the side walls and the main board are carefully soldered.
The signal power from the generator should be about 1 watt. For example, IC-706MK2G, varactor tripler, etc. can be used as a generator.
When checking the balancing of the RF bridge in the VHF and UHF bands, only non-inductive resistors are used. Fine tuning of compensation capacitors (with the same load resistance) corresponds to a constant balance on several ranges (for example, 7 ... 430 MHz). If it is not possible to select a sufficient number of non-inductive resistors to calibrate the bridge, intermediate scale values of the device can be calibrated on the low-frequency ranges using common resistors, for example, MLT or MT.
To measure the reactivity of the load, it will be necessary to replace the C5 capacitor with a variable one (with an air dielectric and a maximum capacitance of about 20 pF), however, the upper frequency measurement limit is limited to a range of 144 MHz, because fails to fully compensate for mounting capacitance.
If chokes with an inductance of 200 μH are used in the device, the frequency range of the bridge will be 0,1 ... 200 MHz.
The proposed design has a very good repeatability, in contrast to devices made using surface mounting.
Literature
- Y. Selevko (UA9AA). Device for tuning antennas. Radio amateur, 1991, N5, S.32...34.
Author: V.Kiselev (RA4UF), Saransk; Publication: N. Bolshakov, rf.atnn.ru
See other articles Section Antennas. Measurements, setup and matching.
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