Build a Tri-Band Transceiver
Inexpensive & Battery Powered & Low Voltage!
During QRP operations there is a question of the power supply durability. It is not always possible to use large batteries and the radio equipment must be powered with the common dry batteries. Of course those batteries will drain rather quickly during any extended QRP operations. At the same time, the batteries sometimes cannot provide adequate power even during short operating periods of operation. Also, solar batteries often cannot power the equipment and charge batteries at the same time.
So a decision was made to build a power supply for the experimental transceiver that would operate on low power using only 3 volts. This would enable operation from two batteries, such as the R20 (D or C cells in the USA). This type of power supply ensures operation for 5-8 days at 3 to 4 hours a day of operation. The transceiver is stable and works well at the reduced supply voltage of only 2.2 volts and this allows the usage of the transceiver almost to full discharge of the batteries. Before going on a new QRP expedition, one can buy two new batteries much more cheaply rather than having to purchase 10 batteries for a 12-volt powered transceiver.
Experiments were conducted with two NiCad batteries rated at 1.5A/h. At their fixed charge rate, it proved possible to operate the transceiver for 10 hours at 3 to 4 hours per day. All summer the transceiver was operated with one cell from an automobile lead acid battery that produced 2.6v without ever fully discharging. The transceiver is lightweight and overall dimensions are only 240w x 140d x 70h mm (click for metrics conversion chart). Including the battery power supply, the weight is still only 900 grams. The transceiver is lightweight because of being assembled in a chassis made of printed circuit board material and joints carefully soldered.
Characteristics
Bands:7, 14, 21 MHz
Operation: CW
Supply voltage: 2,2 … 4,5 Volt
Current in a condition “reception”: 70 mA (at a supply voltage = 3 V)
Current in a condition “transmission”: 800 mA (at a supply voltage = 3 V)
Sensitivity of the receiver: 2 micro
Output power of the transmitter, measured on an equivalent of 50 Ohm
– not less than 0,7 watts on 21 MHz
– no more than 1,2 watts on 7 MHz
Antennas, used by the transceiver: 15-100 Ohm
In reviewing the construction of the transceiver, the circuitry is shown in Figure 1a and Figure 1b. The oscillator works on a frequency range of 6.9-7.2 MHz, and uses two transistors, VT3-VT4. For best results of the oscillator, these transistors should be bought as a complimentary pair. The design generates a maximum of HF voltage at coil L1 and capacitors C6 and C7 to maintain maximum thermal stability and minimise oscillator frequency drift. Voltage to the oscillator is stabilised at 1.9v by diodes VD3-VD5 (Zener diodes 0.65v rating). This ensures a stable voltage from input of2.2v up to 4.5v.
The analog of the lambda-diode used here is more stable at lower voltages. In this case it is less than 1.9 volts. On this unit, this requires the use of a dropping resistor, R1. VD6 causes the transmit frequency to offset when transmitting. When the desired station is turned in, R7 is used for fine-tuning the station to zero beat. Irrespective of the band being used, the receiving station is either above or below this frequency. The transmit offset will allow the receiving station to hear the sending station. The buffer transistor, VT5 should not be a high gain transistor as it could cause oscillation and other problems with the transmitter on the 7 MHz band. The VFO was assembled in its own enclosure separate from the other components. This box has a dimension of 70w x 70d x 60h mm. The mounting of the components utilized the “dead bug” method and the copper foil was cut away as required for component mounting. After setup and calibration of the oscillator, the upper cover was then sealed in place. A Japanese-type variable capacitor, C6 was used along with a built in vernier. The end bearings were lubricated with silicone and the capacitor was sealed in the VFO with the shaft sticking through the panel of the box.
Transistors VT1 and VT2 are used in the power converter which converts 3 volts to 12 volts for the output stages of the transceiver. The converter has adequate parameters for usage in QRP equipment. Current drain at no load is no more than 10ma. At a heavier load, the efficiency reaches 80%. The output may be increased by adding more turns to the secondary. However, this could turn out to be a bad idea if an improper load is hooked up to the transmitter causing the final transistor to fail. The transformer was wound on a toroid form (with a µ = 2000) with physical diameter of 17mm OD, 8 mm ID, and 5mm thick. The primary winding for power voltage from 2.2 up 3.2v consists of a total of 170 turns ( 80 + 10+ 80 ) tapped at 80 turns from each end with a wire diameter of 0.12 mm. The secondary consists of 12+12 turns or 24 turns total, center tapped, of wire with a diameter of 0.5mm. This is wound over the primary, with both windings evenly distributed on the toroidal form. If the voltage that will power the transceiver is in the range of 4.2 to 4.5 volts, then the primary windings must be reduced (for example to 48+12+48 or 108 turns tapped at 48 turns from each end). In the case, the secondary consists of 10+10 turns or 20 turns total, center tapped, of wire with a diameter of 0.5mm.This is necessary for reliable operation of the converter. When the converter is finished but it does not function, it will be required to swap the leads from the secondary winding to get the correct phasing. When operating correctly, there will be no interference to reception. Also, it is not required to increase the size of capacitor C2 because it will not improve operation.
Transistors VT6-VT7 are RF amplifiers for heterodyning with the incoming received signal. This amplifier increases the RF- voltage up to a level permitting transistor VT9 either to amplify or to multiply the heterodyne signal, depending on the range of operation of the transceiver. The collector resonant circuit VT9 consists of L2 and a proper capacitor (C20 or C21 or C22) which is switched by S3. Using this collector resonant circuit and to select the desired range of operation, according to the required harmonic, it is amplified to a level to drive a proper transmitter operation and proper receiver operation. Again, for correct tuning of this stage this is very important for proper operation of the receiver and to attain maximum transmitter power. The tuning capacitors C20, C21, and C22 each consist of two capacitors: one small variable and another for fixed tuning capacity. The tuning of the circuit is to obtain maximum RF – voltage on the gate of VT11 and the exact tuning can be for maximum output on transmission or for maximum received signal.
The power amplifier of the transceiver VT11, is a field effect transistor and serves as the receive mixer. The experiments conducted show that the highest power output and best receive sensitivity came from using the ??902 (USA transistor – nearest specification to a BFL 522) in place of the IRF510. The final amplifier output from VT11 feeds the network formed by L4 and C27. The coupling coil L5 is selected for maximum power out on the 14 MHz band into a 50-ohm load. On the 7 and 21 MHz bands, acceptable output is obtained with this network, which will work over a 50-75 ohm antenna impedance range.
The tuning of the circuit is done with the capacitor C27, which has a capacity of 12-495 pF. One section of a triple variable capacitor with a built-in vernier was used. The application of the capacitor with a vernier drive enables tuning the output circuit precisely on a receiving frequency. Thus, tuning can be done by tuning for maximum volume level in the receive mode. It is possible to tune to maximum output by using a field strength meter or an antenna tuner and tuning for maximum antenna current.
Connected to the final amplifier DC power input is a 1kHz oscillator, consisting of transistors VT12-VT14. This is the keying monitor and feeds the 1 kHz tone to the audio output headphone jack. When the transmitter is keyed, transistor VT10 turns on and power is supplied to the final transistor VT11. Diode VD7 is switched on and in turn activates transistor VT9 since the full voltage from the power supply is now present. Resistor R23 limits the current to a minimum consistent with adequate performance. Since VT11 has less gain, it is necessary for VT9 to have full voltage on receive.
The transceiver uses a direct conversion receiver and VT11 is the mixer stage as well as the transmitter final. VT8 (hfe more than 1000) is the audio preamplifier, but it is possible to use a transistor with a smaller amplification factor. Resistor R12 sets the voltage on the collector of VT8 at 1.7 volts with a supply input to the transceiver of 3 volts. With a power supply of 4.5v, it is possible to use the higher collector voltage. But, it is essential to determine the point of maximum amplification while preventing feedback oscillations of the audio amplifier. The output audio amplification is done by chip A1, such as the TDA7050, which will work with supply voltages from1.6 to 6v. This chip can be purchased from ads in radio magazines for as little as $1.00 USD. They come in several configurations of DIP and flat pack. Depending on mounting considerations it is possible to use a chip in either configuration. T2, the audio output transformer was salvaged from an old transistor radio and the low impedance headphones cam from an old tape player. Other such device can be used depending on what is readily available. The headphones used here have an impedance of 32 ohms and can be directly connected to the amplifier. However, there is a possibility of shorting the amplifier or inadvertently connecting the output of the amplifier to the negative terminal of the power supply. Therefore, the use of the transformer is definitely recommended!
Coil data of the transceiver is shown in Table 1.
Coils | Inductance (microHenrys) | Diameter of Spool (mm) | Length of Windings (mm) | Number of Turns | Diameter of Enamalled Wire (mm) |
---|---|---|---|---|---|
L1 | 1.5 | 10 | 10 | 15 | 0.5 |
L2 | 1.2 | 10 | 10 | 12 tap @ 4 | 0.5 |
L3 | 10 | 8 | 10 | Turn to turn | 0.1 |
L4 | 0.9 | 30 | 45 | 6.5 tap @ 3 | 1.5 |
L5 | – | Over ground end L4 | 4 | 3 | 0.8 |
The layout of construction is shown in Figures 2a and 2b. In the summer of 1999, the transceiver was tested in both field and stationary conditions. The transceiver worked on all bands and showed stability. All receiving stations gave excellent signal reports.
Originally posted on the AntennaX Online Magazine by Igor Grigorov, RK32K
Last Updated : 21st May 2024