DIY low-power transmitter projects: 50 mW VFO-Controlled AM Transmitter for 530-1710 kHz

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The design and construction of a simple AM transmitter that can be operated in the AM broadcast band and the 160-meter amateur band for QRP (low power) experiments are discussed in this section. The transmitter uses five transistors and one volt age regulator IC and can be built on perfboard or in "ugly bug" style, or a PC board can be designed for it. It is a basic AM transmitter based on traditional "old technology," and the parts should be easy to find for it. The circuit can also be built with miniature or even surface-mount components if desired, and it will run from a 9- or 12-volt battery supply.


FIG. 1 Audio Preamplifier Circuit

The variable-frequency oscillator (VFO)-controlled transmitter will output 50-75 mW of RF power into a 50-ohm load, is capable of 80 percent modulation, and can be driven with line level audio. VFO control has the advantage of not requiring crystals. Crystals commit you to just one frequency and are not readily available in this frequency range in standard AM broadcast frequencies. 1.000 MHz and 1.8432 MHz crystals are sometimes available because they are commonly used in microprocessor circuitry, but this choice is fairly limited. Unless frequency synthesis is used, VFO control is advantageous in this frequency range. A frequency-synthesized AM transmitter is described elsewhere in this guide but is much more complex in design than this VFO-controlled unit. For AM broadcast frequencies, VFO stability is not too difficult to obtain using standard components.

A simple audio stage can be added to boost the output of an electret or dynamic microphone to a sufficient level (1 volt peak-to-peak) if desired, but this step was not done on this design. A schematic of a typical audio amplifier for this use is shown in FIG. 1. It consists of a low-noise transistor and bias and coupling capacitors. One precaution must be taken with transmitters in general. It is possible for the transmitter signal to be picked up by the microphone or audio input device, and this RF signal can be strong enough to be detected in the base-emitter junction of the first audio stage.

This problem is especially noticeable in AM transmitters and can cause a feedback howl or squeal. It also can occur with SSB transmitters, and in TV transmitters, it may show up as severe buzzing in the audio. FM transmitters are not as bad in this respect, but the stray RF pickup can upset the bias in the first audio amplifier, causing possible audio distortion. The cure for this problem is to RF bypass the audio input and/or place a series RF choke or high resistance in series with the base lead of the first audio stage. In the microphone amplifier circuit shown, C4 serves this function. C4 can be typically 470 pf to as much as .01 uf. Chip capacitors are effective and are preferred for their lower inductance. Ferrite beads placed on the base lead of the audio transistor act as RF chokes and work well in some cases. Every case is different, though, and some experimentation may be needed to find the best solution for each circuit.

The schematic of the AM transmitter is shown in FIG. 2. Q1 is a series-tuned Colpitts oscillator circuit. This circuit features relatively high-frequency stability compared to most other oscillator circuits. The stability arises from the fact that the active device is loosely coupled to the oscillator frequency-determining circuit. The frequency is determined by C1 and C2, L1, and to a very small degree, C4 and C5.

Ideally, these components should be low-loss, high-stability components, but we used an ordinary NPO ceramic at C2, a polyethylene trimmer for C1, and mylar capacitors for C4 and C5. A low Q RF choke was used for L1 for small size, although a high-Q air wound or a stable ferrite toroidal inductor would be tradition ally used in this spot. After construction, however, excellent stability with time and applied voltage was observed, the circuit drifting less than 500 Hz over 24 hours in a room-temperature environment. This result is more than adequate for AM use and in a simple hobby application such as this one.


FIG. 2 Schematic of 5mW AM Transmitter Operating at 1650kHz

By proper choice of components and suitable mechanical construction, this project could be improved by an order of magnitude. Considering that the transmitter was constructed on a 2-by-3-inch piece of perfboard using off-the-shelf components, it is not doing badly at all. R1, R2, and R3 bias Q1 and IC1 supplies a regulated 5 volts, aiding stability. RF output is taken from the emitter of Q1 and fed to buffer stage Q2, which acts as a rudimentary op amp, and the "virtual ground" seen at the base of Q2 acts to minimize changes in loading on the RF oscillator circuit. Q2 has a voltage gain of about 10, determined by R4 and R5. Q2 drives the base of RF amplifier Q3 via C7 and self bias resistor R8. Q3 operates in class C and delivers about 50-75 mW RF output. L2, C9, C10, L3, and C11 act as a matching and filtering network optimized for 1650 kHz.

If operation on other frequencies is desired, the L and C values can be scaled appropriately in both the Q1 and Q3 circuits. Operation at the high end of the AM band was chosen for two reasons: (1) there are fewer stations in the newly allocated 1600- to 1700-kHz portion of the band, and (2) the radiating antennas are likely to be more efficient, and better range can be expected. A 10-foot antenna and 100 mW is allowed by the FCC for Part 15 operation, however, the circuit can be operated anywhere between 150 and 2000 kHz with the appropriate components in the RF circuits. This covers the entire long- and medium-wave broadcast bands, and the 160-meter amateur band, on which some AM activity takes place.

Modulation is obtained by modulating the collector supply for Q3 with the input audio. Q4 and Q5 are a feedback pair consisting of an NPN-PNP direct-coupled audio amplifier. C14 is an RF bypass capacitor, and R13, R14, and C13 make up the feedback network. A gain of around six is set because this allows full modulation with a 1- to 2-volt p-p audio signal. R12 provides bias for Q4 collector and Q5 base.

The collector of Q5 has as its load the collector circuit of RF amplifier Q3. R8 and R11 bias Q4, and the exact Q point is set with R9. R9 is adjusted for symmetrical modulation and is typically set so there are 5-6 volts at the collector of Q5, assuming a 12-volt supply. Good modulation up to about 80-85 percent can be obtained with this circuit. Input audio is coupled to the base of Q4 via C12. The frequency response is 3 dB at 7000 Hz, which is adequate for most AM audio work, but this level can be changed by changing C13 as required. C8 serves as a bypass capacitor for the 12-volt supply line. With appropriate adjustment of R9, the circuit can also be operated from a 9-volt supply with slightly less (30-50 mW) RF output.

The transmitter can be built in almost any reasonable mechanical configuration to suit your needs. Because the frequencies are relatively low, layout is not too critical, but it is wise to keep outputs away from inputs. A layout in which the components are arranged similarly to how the schematic is drawn is a good idea if you can arrange it.

The circuit should easily fit on a 2-by-3-inch (5-by-7.5-cm) piece of perfboard. The thickness is limited by component height, but 1/2 inch (1.25 cm) should be attainable. It can be enclosed in a plastic experimenters' project box, but a metal case is a good idea because it provides RF shielding and protects the VFO components from detuning, with resulting unwanted frequency shifts. RCA phono connectors are fine for audio and RF at AM frequencies, and the power connector can be anything you wish to use.

One common application of transmitters of this type is in "talking signs." A transmitter such as this is set up with an 8- or 10-foot antenna and fed a message from a tape deck or a solid-state audio recorder chip. The message continuously repeats, and a sign is placed in an appropriate location telling people where to tune their AM radios to hear the message. Real estate brokers, for example, set up a transmitter in a house for sale, and a sign is placed outside the house announcing the details. Prospective buyers can tune their car radios to the indicated frequency to hear the details. This is commonly called a "talking house."

Another application is in surveillance, or monitoring sounds in a room. This transmitter can legally be connected to a transmitting antenna and used by licensed amateurs as a low-power AM transmitter, for experimental work or just for fun, to see how far the signal can be heard. Ranges of 10 miles or more on 160 meters (1800-2000 kHz) can be obtained with this transmitter if good antennas are used.

This phase of ham radio is known as QRP operating. QRP stands for low power, and originally QRP was used as a signal to signify "shall I decrease power?" by radio operators in the early days of radio. This signal is one of a whole series of abbreviations known as "Q" signals that were invented for brevity and to standardize messages and are still used today by hams who prefer CW (Morse code) operation.

Today, QRP is a popular mode of ham radio operation, and thousands of miles are often covered by low-power transmitters operating at 1 watt or less.

Range solely depends on the antenna used. For most applications, a length of wire 3-10 feet is sufficient, but do not exceed 10 feet. A good earth ground connected to the negative side of the power supply will help. Range with this length of wire is 50- 200 feet, and more if some effort is made in the area of antenna matching.

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Parts List

  • Resistors (1/8 watt or larger, 10 percent tolerance)
  • R1, R2, R5, R10 10K
  • R3 330 ohms
  • R4, R12, R13 1K
  • R6 2.2K
  • R7, R8 470 ohms
  • R9 50K pot, linear taper
  • R11 4.7K
  • R14 220 ohms
  • Capacitors (*values for 1650 kHz shown)
  • C1 3-40 pf trimmer *
  • C2 220 pf NPO or silver mica*
  • C3, C14 .1 mfd mylar
  • C4, C5 .0033 mfd mylar or silver mica
  • C6, C7 .01 disc ceramic
  • C8, C12 10 ufd 16 volt electrolytic
  • C9, C13 2200 pf mylar or silver mica*
  • C10 3900 pf mylar or silver mica*
  • C11 1000 pf mylar or silver mica*
  • RF inductors (values for 1650 kHz shown)
  • L1 47 microhenry
  • L2,L3 6.8 microhenry
  • Transistors and ICs
  • Q1 through Q4 2N3904 or ECG123
  • Q5 2N3906 or ECG153
  • IC1 78L05 5V regulator

Also, hardware as needed, perfboard or PC board to suit a 9- to 12-volt battery pack, connectors and jacks as needed, and case as required.

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