Transmitter Receiver Circuit for 80 Meter Amateur Radio

This small and simple transmitter receiver kit allows you to communicate over 100 miles around the world by tuning to 80 meter amateur ham radio stations.

What is the 80 meter band?

80 meters using the 3.5 MHz band for radio communications, authorized under amateur radio use

IARU Region 2 (mostly North and South America) is between 3.5 and 4.0 MHz, and Regions 1 and 3 (accommodating the rest of the world) are typically between 3.5 and 3.8 or 3.9 MHz, respectively.

This upper portion of the radio frequency band, commonly used for telephone (voice) communications, is often referred to as 75 meters. In Europe, 75-m is actually a shortwave broadcast band that may include several national broadcast stations transmitting in the 3.9 to 4.0 MHz spectrum.

If not for amateur radio, the designs in question could simply be tuned to each other to act as a personalized long-range walkie-talkie circuit.

Working principle of the transmitter

The transmitter circuit is very simple, using only 3 low-cost BJTs. The input power specification (and thus the output power specification) is determined by the drive voltage. A 6-volt input supply might allow for 1.2 watts of total input power.

The output power is expected to be about 50% of the input power. If you operate the unit from a 12 V supply, you will get an output of 4 Watts of power; using 24 Volts, it will increase to an impressive 10 Watts; if 40 Volts is applied, the maximum voltage that can be used with this design will allow the unit to operate at a power output of 20 Watts. The transmitter circuit is really easy.

It is a crystal oscillator configured to operate at 3725 kHz and is paired with a Class C output amplifier. The transistor does not require a heat sink. You will only find a couple of adjustment controls: Oscillator Tuning and Output Tuning. Each of them essentially has to be adjusted to get the best reading on the meter.

How to Set Up

The transmitter is almost as easy to use as the receiver circuit. You must connect the resonant antenna to the antenna connector.

A suitable antenna would be any half-wave dipole (12.5 feet long, separated in the center by a dielectric) connected via 50 ohm coaxial cable (such as HG-58).

Connect a 6 V to 40 V DC power supply rated at 500 mA to the power terminals, observing polarity.

Next, a crystal for the 80 meter amateur band (between 3705 and 3745 kHz) is plugged in. Turning the power on and quickly fine-tuning a pair of controls to get the highest meter reading possible will only get you on the air, transmitting your sound across the 80 meter band, across the miles.

The least difficult strategy for using both a transmitter and a receiver is to use separate antennas. The transmitting antenna should be resonant at the frequency it is operating at, but the receiving antenna does not need to be too critical in its specifications.

Once built and set up, you can have a lot of fun using this amateur receiver and transmitter kit to communicate with friends from miles away.

How the Receiver Works

The receiver circuit covers the portion of the 80-meter band commonly used for transmitting Morse code signals, such as the 3700 to 3750 kHz amateur band.

It actually works quite well, and can easily capture ham stations 100 km away using any standard amateur antenna. It isolates the signal very effectively, but obviously not on par with pricier receivers.

The original author used this tiny receiver and its partner transmitter to check with stations 200 miles away. Both are built entirely using only BJTs for minimal power consumption, high consistency, low cost, smaller size, and ease of use.

The receiver is an advanced design model that was very popular in the old days of Hamming. It basically consists of a regenerative detector and an audio amplifier.

RFC is a 2.5 mH choke

The detector stage has very sensitive and selective characteristics. This stage will not only receive code pulses, but also SSB and AM telephone calls. Anyone accustomed to complex receiver systems will be surprised at the working capabilities of this simple receiver circuit.

How to Set Up

The receiver unit is also very easy to use. We recommend using magnetic headphones with an impedance of 500 to 10,000 ohms. Low impedance headphones made for pocket radios and crystal sets may not do the job effectively.

A wire of about 30, 90, or 125 feet in length will work as a very good antenna. Although not found in the wiring diagram, it is recommended that a proper ground be connected to the box for the most effective results.

You can tune the receiver by turning the regeneration switch control potentiometer clockwise just before the switch "clicks" open until you hear a small howling noise.

Now, keep tuning in until you start catching up with some ham stations. You will get the best results if you do this at night, and you will probably start listening to many of them pretty quickly.

The code station picks up the most common sounds, adjusts the regeneration control to the point where the detector stage barely oscillates, and every dit and dah comes through loud and clear.

You should be able to pick up many very slow stations near the middle of the tuning range.

These stations will be primarily in the amateur radio bands. For those who have crystals in the amateur band range, it is possible to track your own personal signal by broadcasting from the transmitter in question. Voice stations on 80 meters are generally of two types: AM and SSB.

Ideally, AM is received by adjusting the regeneration control potentiometer until the detector stage jitter begins to oscillate, and allows you to tune to SSB like a code station, with the detector stage oscillating.

If not tuned correctly, the SSB signal may sound funny, almost like a quacking duck.

The small adjustable capacitor or trimmer connected between the antenna and the tuning coil can be anything from 2 to 13 to 3 to 40 pF. The value is certainly not very important, but if you find that your receiver is not oscillating correctly, a slight adjustment may be necessary.

Another simple 3.5 MHz receiver circuit

This 3.5 MHz regenerative receiver is designed to self-optimize and adjust the LC tank circuit ratio to enhance tuning and selectivity.