Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The development of personal communications (pagers, radiotelephones, etc.) has led to the appearance of interference in amateur radio communications from the transmitters of these systems, which operate at frequencies close to those of the amateur bands. You can eliminate this kind of interference by installing a bandpass filter at the input of the VHF radio station receiver. One of the options for such a filter is described in the published article.

The antenna filter is designed to increase the selectivity of the receiving path of VHF radio stations in the 144 MHz band. It is a high-quality two-loop lumped selection bandpass filter with inductive coupling between the loops. To reduce power losses during the operation of a VHF radio station for transmission, the filter turns off the high-frequency VOX.

The filter made by the author has the following characteristics:

• input and output resistance - 50 Ohm;
• bandwidth at the level of 0 dB with unevenness less than 0,2 dB - 2 MHz (144 ... 146 MHz);
• level bandwidth - 6 dB - 4 MHz (143 ... 147 MHz);
• level bandwidth - 20 dB - 6 MHz (142 ... 148 MHz);
• insertion attenuation in the operating frequency band - no more than 1,5 dB.

The filter circuit (L1C1L2C2) with the control unit (VT1-VT3) is shown in fig. 1. The control unit does not have any features compared to similar devices described in the literature. LEDs HL1 and HL2 are set only if necessary to indicate the "Bypass" and "Filter" modes. Relay K1 of any type with an operating voltage of the winding of 10 ... 12 V. By selecting the mode of the transistor VT1, when setting up the device, they achieve VOX operation in the entire power range of the radio station used (from minimum to maximum).

Let us dwell in more detail on the design of the filter itself.

Its body (Fig. 2) is made of double-sided foil fiberglass with a thickness of 1,5 ... 2,0 mm and is a rectangular box with two compartments. The partition between the filter compartments (Fig. 3) is made of fiberglass laminated on one side with a thickness of 1,0 ... 1,5 mm. Two holes with a diameter of 8 mm are designed to install trimmer capacitors. The elements of the housing are interconnected by soldering. Before assembling the body parts, it is advisable to clean with fine sandpaper from traces of oxidation and other damage, and then polish the foil, preferably to a shine.

Inductive coupling between the circuits is carried out through a "window" with dimensions of 25x15 mm in the partition. The location of the "window" on the partition relative to the filter coils is shown in fig. 4.

To ensure the required filter characteristics and obtain the highest possible quality factor, the circuits are made of XNUMX mm silver-plated copper wire.

Coil data: number of turns - 6, diameter - 13,4 mm, winding length - 15 mm. Approximate inductance - 0,31 μH. The taps of the coils for communication with the antenna and the radio station are made from the side of the "cold" end and are selected when adjusting the input and output impedances. In the author's copy, the tap is made from 0,5 turns.

The coils are soldered directly to the trimmer capacitors of the KTPK type (Fig. 5) and when adjusting the filter, turning together with the capacitors (see Fig. 4), they can approach the "window" in the partition or move away from it. In this way, the inductive coupling between the circuits is regulated. The top of the filter is covered with a lid made of tinned copper (brass) foil.

The filter is switched by high-frequency relays K1 and K2 of the RPD12 type for an operating voltage of 13 V. The outputs from the coils L1 and L2, the connections between the relay contacts, as well as connections to external devices are made with a 50-ohm coaxial cable.

The filter was tuned using an X1-48 instrument for studying the amplitude-frequency characteristics and an MFJ-259 antenna analyzer.

The tuned filter, together with the relay and the HF VOX board, is placed in a metal case, which is a common screen for the entire device. The device is powered by a VHF radio power supply.

The author expresses his gratitude to UA3AAX and RA3ACC for valuable advice and consultations in the manufacture and adjustment of the filter.

Author: Vladimir Gordienko (RU3BE)

See other articles Section Civil radio communications.

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