6. Antennas and Feed Lines on The HamStudy Amateur Extra Class HamBook (2024 - 2028)

6.1 Radiation and Efficiency

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.1: Radiation and Efficiency #

A dummy load can show a perfect 1:1 SWR while radiating almost nothing, because it turns the power it accepts into heat. An antenna also turns some power into heat, in its conductors, any loading coils, and the nearby ground. The rest leaves as radio waves. Comparing those two shares shows how well the antenna uses the power it accepts.

Radiated Power and Lost Power #

You can picture each share as power in a resistance that carries the antenna current. Radiation resistance, $R_r$ , stands for the power leaving as radio waves. Loss resistance, $R_l$ , stands for the power that becomes heat. Efficiency is the radiated fraction:

6.2 Gain and Antenna Patterns

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.2: Gain and Antenna Patterns #

An antenna’s quoted gain tells only part of the story. A large number is little comfort if the other station sits in a null, or if a strong noise source falls in a side lobe. The radiation pattern shows where the antenna’s gain will help and where it will not.

The pattern also shows where that gain comes from: an antenna concentrates power in some directions by taking it from others. Like a squeezed balloon, it can only bulge out where it’s pinched in somewhere else.

6.3 Feed Line Behavior

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.3: Feed Line Behavior #

On a mismatched feed line, forward and reflected waves combine into a standing wave, with voltage peaks and dips at fixed points along the line. SWR sums up that pattern in one number, the ratio of the highest voltage to the lowest. Voltage and current vary from point to point, so the impedance does too, and a line cut to a chosen length can become a matching component, built from nothing but coax.

6.4 Wire Antennas and Ground

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.4: Wire Antennas and Ground #

The common $468/f$ rule cuts a dipole about 5% short of a free-space half wave, because a wire’s thickness makes it act electrically longer, as Section 2.5 showed. Feed that dipole at its center and it presents roughly 73 Ω. Feed the same wire at an end and it presents thousands of ohms, because current is highest at the center and voltage is highest at the ends. Many wire antennas reshape that dipole to get a different impedance, more bands, or a different pattern.

6.5 Electrically Short Antennas

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.5: Electrically Short Antennas #

A full-size quarter-wave vertical for 40 meters stands about 10 meters (33 feet) tall. A loading coil lets a mobile whip a fraction of that height resonate on the same band, yet the whip may turn most of the power it accepts into heat. The efficiency ratio from Section 6.1 shows why, and what helps.

Resonance Without Full Size #

A base-fed whip used below its natural resonant frequency is electrically short and has capacitive reactance. The lower the frequency, the smaller a fraction of a wavelength the same whip becomes, and the less it radiates for a given current. Loss resistance then makes up a larger share of the total.

6.6 Arrays and Reflectors

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.6: Arrays and Reflectors #

A Yagi favors one direction, and changing that direction means turning the antenna. Two fixed verticals can favor a direction too, then point another way at the flip of a switch in the shack; only their feed changes. In the pair, as in the Yagi, fields from separate elements meet at a distant point. Like ripples spreading from two pebbles dropped in a pond, they add in some directions and cancel in others. Spacing and the phase of each element’s current decide which directions are which.

6.7 Antenna Matching Methods

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.7: Antenna Matching Methods #

Put an antenna tuner beside the radio and the transmitter sees a 50-ohm load, but the feed line beyond the tuner still carries the antenna’s mismatch. The radio is happy; the coax isn’t. A match made at the antenna clears the mismatch from the line. The methods here make that match with sections of transmission line or with connections to the radiating element itself.

6.8 Reading the Smith Chart

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.8: Reading the Smith Chart #

The stub at the end of the previous section needs two numbers: its distance $d$ from the antenna and its length $\ell$ . Both depend on how impedance changes along the line, and calculating that change point by point is slow. On a Smith chart, each point represents one complex impedance, $R+jX$ , and moving along a line moves that point in a predictable way. At first glance, the chart looks like a tangle of circles. Learn its two families of curves, and the tangle becomes a map.

6.9 Receiving Antennas

Mon, 01 Jan 0001 00:00:00 +0000

Section 6.9: Receiving Antennas #

For transmitting, losing most of the accepted power is disappointing. For receiving on a noisy low band, a lossy antenna can still be excellent if it rejects more noise than wanted signal. Switch to it and the S-meter may fall while the words become easier to copy. For this job, signal-to-noise ratio matters more than a large meter reading.

Listen in Fewer Directions #

On 160 and 80 meters, atmospheric noise usually far exceeds the receiver’s own noise. Antenna loss weakens the wanted signal and that noise together, so their ratio stays the same. That holds until the loss grows large enough for the receiver’s own noise to matter. Directivity can improve the ratio by favoring the wanted station while rejecting noise from other directions.