Matching devices in the 144 MHz range. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Matching devices in the 144 MHz band. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Filters and matching devices

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When working on VHF, matching devices are used extremely rarely. This is because a matched resonant antenna is usually used.

But there is a situation when the radio amateur does not have a cable with the necessary wave impedance. Not always, even in the VHF range, the impedance of the antenna is equal to its theoretical value, especially when it is installed irrationally. Matching devices help to fix this, which in the VHF range is structurally quite simple. The simplest matching device (SU) is an ordinary P-loop (Fig. 1).

144 MHz band matching devices

With a capacitance of variable capacitors of 2 ... 15 pF and a coil containing 2 turns of wire with a diameter of 1 mm, wound on a mandrel with a diameter of 10 mm, and with a distance between the turns of 5 mm and a length of the coil leads of 15 mm (no more), this control system allows you to match active load from 30 to 300 ohms with a transmitter having an output impedance of 50 ohms or 75 ohms. At the same time, when matching a load from 50 to 30 Ohms with a 100 Ohm cable, the efficiency of the device was at least 80%, when matching a load from 100 to 300 Ohms - no worse than 60%, when matching a load from 10 to 30 Ohms - no worse than 50%. This control system has in the KB bands from 1,8 to 50 MHz SWR not worse than 1,2.

Thus, using this SU directly on an antenna operating in the HF band, it is possible to match this antenna for operation at 144 MHz, while practically not changing the matching mode in the HF band. This makes it possible to use the existing HF antennas for VHF operation, which significantly saves the cable. Particularly good matching can be obtained by using two matching devices: one directly at the antenna and the other at the output of the transmitter.

When using the control system (Fig. 1) to match the output of the transmitter with the antenna power cable, it is possible to match the transmitter with the cable, but it is far from always possible to get the cable to match the antenna when using the control system directly on the antenna (of course, on the one that is not designed to work on 144 MHz). This is explained by the fact that the antenna impedance can have a large reactive component in the VHF band, and the control system (Fig. 1) effectively matches only the active components.

This problem can be solved quite simply. The first thing to do is to slightly change the number of turns in the SU coil - it can be from 1 to 5, its diameter is from 5 to 15 mm and the distance between the turns is from 1 to 7 mm.

144 MHz band matching devices

It is also very effective to turn on the inductance at the output of the SU (Fig. 2). An additional coil contains from 2 to 5 turns of wire with a diameter of 0,8 ... 1,5 mm. Its diameter is from 5 to 15 mm, the distance between the turns is 2...7 mm. Usually, by varying this coil and the P-loop coil, almost any antenna can be tuned to operate in the 144 MHz band. Sometimes, in the most difficult cases of matching, it may be necessary to turn on a capacitance of 10 ... 50 pF at the output of the P-loop (it can be variable) (Fig. 3).

144 MHz band matching devices

In parallel with the capacitance, a choke is connected (10 turns of PEL-1 wire on a mandrel of 10 mm, the distance between the turns is 1 mm), which does not resonate with this capacitance in the 144 MHz range and has low resistance at HF. When tuning a HF antenna for operation in the VHF band, you can get a false tuning when the SWR of the system is 1, but the antenna will not radiate. It is good if the radio amateur has a simple field strength indicator. Using it, it is desirable to compare the field strength generated by the antenna with a control VHF antenna - a 50 cm long pin connected to the output of this matching device instead of a tuned HF antenna.

It should be noted that long HF dipole antennas, slopers, and especially antennas having a shape other than a linear configuration, such as loops, create a rather complex VHF radiation pattern with numerous lobes, but the difference between the maxima and minima of the field strength usually does not exceed 10 dB.

All control systems were made in a carefully soldered double-sided foil fiberglass case, air capacitors from old VHF radios were used, all connections were of a minimum length. When using the SU directly on the antenna, it is necessary to take measures for its moisture insulation. Although the SWR of the control system (Fig. 1...Fig. 3) in the HF band is small, their use can still shift the resonant frequencies of antennas operating in the upper KB bands, especially short ones.

This will require additional tuning of such antennas in the HF band. You should also pay attention to the fact that some KB antennas, due to their design features, cannot be used to operate on 144 MHz, due to the fact that their input is fundamentally inconsistent in the VHF band, for example, antennas with gamma matching with an extension coil and some other types of antennas.

The matching devices shown here are operational in the entire range of 144 ... 146 MHz.

Author: I. Grigorov (RK3ZK), Belgorod; Publication: N. Bolshakov, rf.atnn.ru

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