A simple 80m transmitter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple 80m transmitter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transmitters

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For experimental work and beginners, the simple transmitter with quartz stabilization presented here will be of interest (it can also be used in sports radio orientation). The transmitter circuit contains quite a lot - about 50 parts, but practically does not require adjustment, which is important when there is no measuring equipment at hand. Tell me why, for example, a beginner needs a transmitter consisting of 20 parts that he cannot tune?

One of the main problems with transmitters is their frequency stability. In this transmitter, the problem is solved simply: a quartz master oscillator is used. This fact means that the transmitter is designed to operate at one frequency, although who is stopping you from introducing a set of quartz resonators designed for the most popular frequencies, switched or replaced in the transmitter (you can also provide a slight frequency drift by connecting a variable capacitor in series with the resonator, a coil inductance or series circuit). I used quartz resonators at 3650 kHz QRP and TV - at 3579 kHz. The transmitter circuit is shown in fig. 1.

Rice. 1. Schematic diagram (click to enlarge)

Transistor TR1 operates in a widely used crystal oscillator circuit with manipulation of the emitter circuit using a telegraph key. The base of the transistor TR2 is connected to a common wire through a resistor R5 (180 ohms). Since no open bias is applied to the base of this transistor, the transistor turns on only at the peaks of the input manipulated RF voltage. The output stage of the transmitter is assembled on a field-effect transistor TR3. No current will flow through it until a positive potential appears on the gate, relative to the source. The circuit has a diode D1 connected by the cathode to the gate of TR3, and by the anode to the common wire. If now a RF voltage of sufficient amplitude is applied to the TR3 gate from the previous stage, then the positive wave will charge the supply capacitor without attenuation, create a bias that will control the transistor (open it), and the negative wave, opening the diode, will close to the common wire. This stage will amplify the existing signal to practical power: it is rated to produce 3 watts of RF output power.

Since the transmitter operates in class B, its output signal is replete with harmonics, which are easily suppressed by the applied low-pass filter (LPF). The low-pass filter passes almost without attenuation all frequencies below the cutoff frequency and suppresses all frequencies above the cutoff frequency. Of course, nothing in this world is perfect, but by measuring, I found that when the low-pass filter was connected, the harmonics, in relation to the main signal, were suppressed by 50 dB. To switch "reception-transmission" I used a small relay with two groups of contacts for switching. Switching to "transmission" is carried out automatically, each time the key is pressed, the receiver input, at the same time, is closed to the case, in order to avoid overloading the transmitter signal. The transmitter is powered by a voltage of 12 ... 18 V, which is constantly supplied to the transmitter. Why turn off the voltage if, for example, in the receive mode, the cascades on the transistors TR2 and TR3 are locked, and TR1 is not working (not being manipulated).

Simple 80m transmitter
Rice. 2. Location of parts on the transmitter circuit board

The design of the transmitter is very simple: All parts that are connected to a common wire are soldered directly to the foil foil on one side of the fiberglass board. Parts that are not connected to a common wire are attached to the terminals of the first. Installation is shown in Fig. 2 and corresponds to the circuit wiring, almost completely.

This helps to quickly find errors in the installation. Transistors TR1 and TR2 are mounted on the parts adjacent to them with the leads up. TR3 is fixed above the very surface of the foil by soldering the output of its source in the shortest way possible physically. The gate and drain leads are bent at the foil itself. At first I thought a heatsink would be required, but as it turns out, in normal CW operation, a heatsink is not needed. Conclusions of parts for better contact before soldering should be twisted.

A little time has passed since the development of the transmitter, and I "dared" to modify it: I introduced a switch for two quartz resonators in order to quickly change the frequency (see photo in Fig. 3).

Simple 80m transmitter
Rice. 3. General view of the assembled transmitter with quartz stabilization at 80 m

If any further clarification is needed, please contact PA3AAF, DL1NF and 2E0AGP. I think the transmitter will please you as much as I do.

Specification for parts for the transmitter

R1 - 22 kOhm

R2 - 10 kOhm

R3, R6 - 220 Ohm

R4 - 100 Ohm

R5, R7 - 180 Ohm

C1, C4, C5, C6, C7, C9, C10, C14 - ceramic 0,01 uF

C2, C3 - 220 pF

C8 - 4,7uF x 25V (minimum)

C11, C13 - 750 pF x 25 V (minimum)

C12 - 1500 pF x 25 V (minimum)

L1 - 2 x FX1115 with the maximum possible number of turns, wire 0 mm

L2, L3 - T37-2 core 23 turns (see Fig. 1 and text), 0,5 mm wire

D1, D2 - 1N4148

TR1, TR2 - BC183

TR3-VN46

X1 - 3560 kHz or/and 3579 kHz or any desired frequency in the range

J1 - 3mm jack socket

12-volt relay with two groups of contacts for switching

Single-sided fiberglass board measuring 10 x 6 cm or more.

Author: V.Besedin

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