Antenna filter for combining (separating) VHF signals. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / VHF antennas

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If a radio amateur has two single-band antennas (144 and 430 MHz), but a dual-band radio station, then the antennas can be connected with one cable by installing a crossover filter on the roof. A similar filter is also suitable for solving the inverse problem - summing the input signals from two single-band radio stations when working on one dual-band antenna.

The scheme of the device is shown in fig. 1. It consists of a high pass filter with a cutoff frequency of 350MHz and a lowpass filter with a cutoff frequency of 200MHz. The high-pass filter is assembled on the elements L1-L3C1-C4, and the low-pass filter is assembled on the elements L4-L7C5-C7. The isolation between sockets XW2 and XW3 is at least 40 dB, and the filter passband loss is fractions of a decibel. In the 144 MHz band, the device provides additional suppression of harmonics of the main signal by 20...30 dB (100...1000 times in power).

The details of the antenna filter are placed on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in Fig. 2. The second side of the board is left metallized and connected along the contour with the metallization of the first side with foil strips. In addition, the sides are interconnected by segments of conductors passed through holes in the board. Between the low-pass filter and the high-pass filter, a shielding partition is soldered onto the board.

Capacitors of KD types can be used in the device. K10-17. All coils are wound with PEV-2 0,9 wire on a mandrel with a diameter of 4 mm and contain: L1 - L33 - 1,5 turns each with a step of 2 mm, L4 and L7 - 3,5 turns each with a winding length of 6 mm. L5 and L6 - 4,5 turns each, wound turn to turn. The length of the leads for the coils is 7...8 mm.

All capacitor connections and leads must be kept to a minimum length. One of the terminals of the elements L4, L7, C1, C4 is soldered directly to the sockets.

The board is placed in a metal case (or in a case soldered from foil fiberglass), on the walls of which high-frequency sockets XW1 - XW3 are installed.

In principle, the establishment of an antenna filter is not required, but before installation, it is imperative to check its parameters in order to be sure that the device is working. It is most convenient to do this on a panoramic frequency response indicator or using a generator and an RF voltmeter.

When operating on two separate antennas, the filter is placed on the roof. The drop cable is connected to XW1, the 144 MHz antenna to XW3, and the 430 MHz antenna to XW2. If two radio stations are used, then the filter is placed next to them, and instead of antennas, the outputs of radio stations of the corresponding range are connected to the sockets XW2, XW3.

When placing the device on the roof, it is necessary to cover the board and elements with a moisture-resistant varnish, ensuring the sealing of the case.

Author: I.Nechaev (UA3WIA)

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