Section 1.2: Electrical Units and Quantities #

To understand how our radios work, we need to familiarize ourselves with some basic electrical quantities. Let’s start with the “big three”: voltage, current, and resistance.

Voltage, Current, and Resistance #

While we often compare electricity to water flowing through pipes, this analogy isn’t perfect. Let’s look at what’s really happening:

Voltage ( $E$ or $V$ ) is like electrical “pressure” - it’s the force that causes electron flow in a circuit, and we measure it in volts (V).

• Voltage is also sometimes called electromotive force (EMF), because it creates the conditions that allow current to flow.
• Voltage is always measured between two points. When we say a car battery is 12V, we mean there’s a 12-volt difference between its positive and negative terminals.

Current ( $I$ ) is the flow of electrons in an electric circuit, measured in amperes (A). Here’s what’s interesting:

While individual electrons move quite slowly through a conductor (about half an inch per minute!), the movement of electrical charges is nearly instantaneous - like a long tube filled with marbles. Push one in at one end, and one pops out the other end almost immediately.

In amateur radio, you might deal with:

• Milliamps (mA) in low-power circuits
• A few amps powering a mobile transceiver
• 15-20 amps for a high-power HF amplifier

Resistance ( $R$ ) is how much a material opposes current flow by converting electrical energy into heat, measured in ohms ( $\Omega$ ). Think of resistance as a fundamental property that affects how electrical energy moves through a circuit:

• Conductors like copper wire have very low resistance, allowing current to flow easily
• Insulators like rubber or glass have very high resistance, blocking current flow
• Resistors are components with specific, controlled resistance values used to:
  • Limit current in LED circuits to prevent burnout
  • Divide voltage in measurement circuits
  • Convert electrical energy to heat in applications like dummy loads

These three quantities are fundamentally linked as explained by Ohm’s Law, which we’ll cover in Section 1.3.

Voltage Drop #

When current flows through a resistive component or wire, some voltage is “used up” in the process. This decrease in voltage is called voltage drop.

It’s similar to water pressure decreasing as water flows through a pipe - the longer or narrower the pipe, the more pressure is lost. In electrical terms, when current flows through resistance, voltage decreases along the path.

This concept is important in amateur radio because:

• Long power cables to your radio can result in lower voltage at the radio than at the power source
• Higher current draw (like during transmission) increases voltage drop

This is why most mobile ham installations use thick, short power cables - to minimize voltage drop when operating equipment that draws significant current. This is a direct application of how resistance affects current and voltage in real-world amateur radio setups.

Frequency #

Now that we understand these basic DC electrical concepts, let’s look at alternating current and frequency.

In amateur radio, we often deal with alternating current (AC), which smoothly changes direction in a sine wave pattern, unlike DC which flows constantly in one direction.

AC occurs naturally when a magnet rotates near a wire. As the magnet’s north pole approaches the wire, current flows one way. When the south pole approaches, current flows the opposite way. This continuous rotation creates the smooth sine wave pattern shown in the diagram. This is the basic principle behind generators that produce the electricity powering our homes and the alternating currents in our radio circuits.

The diagram shows two complete cycles of AC - each starting at zero, rising to a positive peak, falling through zero to a negative peak, then returning to zero. The number of these cycles completed per second is called frequency, measured in Hertz (Hz).

Some examples:

• Household power: 60 Hz (60 cycles per second)
• AM radio stations: around 1000 kHz (1,000,000 cycles per second)
• FM radio and many ham bands: MHz range (millions of cycles per second)

Understanding frequency is crucial in amateur radio because it determines which bands you can use and how far your signals can travel.

Power #

Power is the rate at which electrical energy is used or generated in a circuit. It’s measured in watts (W) and is calculated using the Power Law, which we’ll cover in Section 1.3.

In amateur radio, power is important because:

• More power = Greater transmission range. A stronger signal reaches farther.
• More power = More heat. High-power radios need proper cooling.
• More power = Higher energy demands. Your power supply must handle your radio’s needs.

Decibels #

Instead of always talking in watts, hams often use decibels (dB) to express power changes. The decibel is a logarithmic unit that makes it much easier to express and work with very large or very small ratios. Instead of multiplying power levels, we can simply add or subtract dB values, which is particularly useful in radio systems where signals might be amplified and attenuated multiple times.

Here’s what decibels mean in practice:

• +3 dB means power doubles.
• -3 dB means power is cut in half.
• -6 dB means power is cut in half twice (divided by four).
• +10 dB means power increases by ten times.
• +20 dB means power increases by one hundred times.

Some real-world examples:

• A power increase from 5W to 10W is a +3 dB gain.
• A decrease from 12W to 3W is a -6 dB loss.
• Going from 20W to 200W? That’s a +10 dB boost.

The beauty of the decibel scale is that it compresses large numbers into a more manageable range, making it easier to understand and discuss signal gains and losses throughout a radio system.

AC and DC #

Electricity comes in two types:

• Direct Current (DC) flows in one direction, like from a battery or power supply. Most radios run on 12V DC.
• Alternating Current (AC) constantly reverses direction, alternating between positive and negative directions, like household power.

Most ham gear runs on DC power, but radio signals themselves are AC—they alternate at radio frequencies. Radio frequency (RF) signals are simply AC signals at very high frequencies used for wireless communication.

Resistance opposes all types of current flow, including direct current, alternating current, and RF current.

Impedance #

Key Information: Impedance is the opposition to AC current flow and, like resistance, it’s measured in ohms ( $\Omega$ ).

In a DC circuit, resistance simply opposes the flow of electricity. But in an AC circuit, the story gets more complex.

Capacitance describes the ability to store energy in an electric field and is measured in farads (F).

Inductance describes the ability to store energy in a magnetic field and is measured in henrys (H).

In electrical circuits that include capacitors and/or inductors, the effect these components have on alternating current will vary with frequency. This frequency-dependent opposition to current flow is called reactance.

• Inductive reactance increases as frequency increases
• Capacitive reactance decreases as frequency increases

Impedance combines both resistance and reactance, giving us the total opposition to AC current flow in a circuit.

A resonant circuit consists of an inductor and a capacitor connected either in series (one after another) or parallel (side by side). At a specific frequency, the inductive and capacitive reactances are equal in magnitude but opposite in effect, effectively canceling each other out. This creates an electrical balance that allows signals at that frequency to pass easily (in series) or be blocked (in parallel). That frequency is called the resonant frequency, and this phenomenon is known as resonance.

Amateur radio equipment is typically designed to work with specific impedance values. For example, most transceivers are designed to connect to a 50-ohm system. When impedances aren’t properly matched:

• Power transfer becomes less efficient
• Some energy may be reflected back toward the source
• Equipment may need to reduce power output to protect itself

These principles apply to many parts of your radio system, including antennas, which exhibit properties of resistance, capacitance, and inductance.

Metric Prefixes and Electrical Units #

In amateur radio, we often deal with very large or very small numbers. Instead of writing out all the zeros, we use metric prefixes:

Key Information:

• The abbreviation kHz stands for “kilohertz”.
• The abbreviation MHz stands for “megahertz”.

Note: The F (farad) and H (henry) units refer to capacitance and inductance as discussed above, which we’ll explore in more detail in Section 2.1.

Here are some common conversions that may appear on the exam:

All of the conversions in this table are worth memorizing for the exam!

Wrapping It All Up #

Understanding these electrical quantities will help you make sense of your radio gear and pass the exam. As you continue your ham radio journey, these ideas will come up again and again, so keep this knowledge handy!