Building your own gear

Building is part of what the amateur service is for. Part 97 lists among its purposes “advancing skills in both the communication and technical phases of the art” and expanding the “reservoir within the amateur radio service of trained operators, technicians, and electronics experts” (47 CFR 97.1). This guide covers the bench skills behind a project: reading the schematic, soldering, protecting parts from static, measuring, and testing a transmitter before it goes on the air.

Your build, your responsibility

Whatever you build, your station must be operated “in accordance with good engineering and good amateur practice” (47 CFR 97.101(a)). Spurious emissions from a transmitter must be reduced to the greatest extent practicable, and if one causes harmful interference the licensee must fix it (47 CFR 97.307(c)). There are also numeric limits: below 30 MHz, a transmitter installed after January 1, 2003 must keep each spurious emission at least 43 dB below the mean power of the fundamental, and between 30 and 225 MHz the requirement is at least 60 dB, with separate figures for transmitters of 25 W or less (47 CFR 97.307(d) and (e)). The low-pass filter designer shows how a filter keeps harmonics down.

Read the schematic first

A schematic shows how components are connected, not where they sit or how long the wires are (Technician pool, T6C12). Learn the symbols with the schematic symbols drill, then identify each part before you fit it: the resistor and capacitor tool reads color bands and the printed codes on capacitors and chip resistors.

Soldering

Use rosin-core solder. Acid-core solder should not be used for radio and electronic work (Technician pool, T7D08).

Common alloys melt at different temperatures (Kester):

The 63/37 alloy melts at a single temperature, while 60/40 melts over a 7 °C range. Hold the parts still until the solder sets: NASA’s soldering standard defines a disturbed joint as one damaged by “relative motion between the conductor and termination during solidification” (NASA-STD-8739.3).

A good joint is smooth and nonporous, with solder wetting every part of the connection and the outline of the lead still visible (NASA-STD-8739.3, 13.6). A cold tin-lead joint looks rough or lumpy (Technician pool, T7D09); NASA attributes cold joints to too little heat, poor cleaning or impure solder.

Static electricity

Static builds up when two materials touch and separate. Walking across carpet can charge a person to about 35,000 V in dry air (10 to 25 % relative humidity) and about 1,500 V in humid air (65 to 90 %), and many electronic components can be damaged by discharges of less than 100 V (ESD Association, Part 1). The Association lists discrete MOSFETs, junction FETs and operational amplifiers among the sensitive parts. A MOSFET’s gate is separated from its channel by a thin insulating layer (General pool, G6A09), and Zener diodes connected between the gate and the source or drain are there to protect the gate from static damage (Extra pool, E6A12).

At the bench, a grounded wrist strap keeps you at the same potential as the work, so there is no discharge between you. Its cord has a current-limiting resistor, most commonly one megohm. Keep sensitive parts in their shielding packaging until you fit them (ESD Association, Part 3).

Test equipment

Multimeter. A voltmeter connects in parallel with the component, and a multimeter measuring current connects in series (Technician pool, T7D02 and T7D03).

An ohmmeter applies a small current and measures the resulting voltage, so measure resistance only with the circuit unpowered, and never measure voltage with the meter set to resistance: that can damage it (T7D05, T7D11 and T7D06). An ohmmeter across a large, discharged capacitor reads a resistance that increases with time (T7D10). For high voltages, the meter and its leads must be rated for the voltage being measured (Technician pool, T0A12).

Dummy load. A dummy load lets you test a transmitter without sending a signal over the air. It is typically a 50 Ω non-inductive resistor on a heat sink (Technician pool, T7C01 and T7C03).

Antenna analyzer. An antenna analyzer computes SWR and impedance automatically (Extra pool, E4A07). For SWR measurements connect it to the antenna and feed line, and be aware that strong signals from nearby transmitters can upset its readings (General pool, G4B11 and G4B12).

Oscilloscope and spectrum analyzer. An oscilloscope can show complex waveforms that a digital voltmeter cannot (General pool, G4B02). A spectrum analyzer displays spurious signals and intermodulation products (Extra pool, E4A03), which is how you check a transmitter against the limits in 97.307.

Power supply safety

Filter capacitors in a power supply can hold a dangerous charge after it is turned off (Technician pool, T0A11); a bleeder resistor is there to discharge them when power is removed (General pool, G7A01). A fuse removes power in an overload, and replacing a fuse with a larger one risks a fire because excessive current can flow (T0A04 and T0A05). In a 120 V AC circuit the fuse or breaker goes in series with the hot conductor only (T0A08). The electrical safety guide covers shock, grounding and batteries, and the DC power wiring tool sizes the wiring for a 12 V radio.

Before it goes on the air

1. Run the transmitter into a dummy load first (T7C01).
2. Measure output with an RF power meter installed in the feed line between the transmitter and the antenna (Technician pool, T4A05).
3. Check for spurious emissions with a spectrum analyzer, and filter the output if needed (E4A03, 47 CFR 97.307).
4. Only then connect the antenna.

Tools for builders

• Resistors and capacitors: color codes, marking codes, standard values, LED resistors and dividers.
• Attenuator pads: resistor values and heat for tee and pi pads.
• DC power wiring: voltage drop, wire size and battery run time.
• Schematic symbols: the symbols on the exam figures.
• Coil and toroid winder and low-pass filter designer: parts for RF projects.