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Technician Course, Module 7: Antennas and Feed Lines
Covers NCVEC 2026-2030 Technician Pool, Subelement T9 (Antennas and Feed Lines). Two questions on your exam come from this material.
Ray holds up two radios. "Fifty-watt mobile into the rubber duck antenna off your handheld. Five-watt handheld into the antenna on my roof. Which one talks farther?"
The roof wins, every time, by a lot. "Money spent on the antenna beats money spent on the radio," he says. "This module is why."
7.1How an Antenna Works
An antenna is a resonant structure, and Module 4's tuned circuit is hiding inside it: the wire's own inductance and capacitance make it resonate, and resonance depends on length.
The master relationship: antenna length and frequency are inversely related. A dipole cut for a lower frequency is longer; shortening a dipole increases its resonant frequency. When your antenna is resonant too low, you trim it. Too high, you add wire. Every antenna adjustment you ever make flows from that one rule.
The workhorse is the half-wave dipole: a wire split in the middle and fed at the center. Its radiation is not equal in all directions. A dipole radiates strongest broadside to the antenna, off the sides of the wire, and weakest off the ends. Point the side of your dipole at the station you want.
Polarization carries over from Module 2, and the antenna is where it comes from: polarization is described by the orientation of the electric field, which follows the orientation of the antenna's elements. Vertical element, vertical wave. That is why mobile and repeater antennas stand up straight and weak-signal VHF beams lie flat.
Sometimes a full-size antenna will not fit, especially on a vehicle. Antenna loading is the fix: electrically lengthening an antenna by inserting inductors in the radiating elements. The coil in the middle of a short mobile whip is doing exactly that, letting a four-foot antenna behave like a longer one. Loading is a compromise, trading efficiency for size, but it is the compromise that makes mobile HF possible at all.
Knowledge Check 7.1
Tap an answer. In your own words first, then check yourself.
1. Your dipole is resonant slightly below the frequency you want. What do you do?
2. Where does a half-wave dipole radiate its strongest signal?
3. The coil in the middle of a short mobile whip is there to:
T9A the easy way: all 11 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
T9A01 What is a beam antenna?
Answer An antenna that concentrates signals in one direction
Tip A beam focuses your power one way, like a flashlight instead of a bare bulb, so it hears and talks stronger in that direction.
T9A02 Which of the following describes a type of antenna loading?
Answer Electrically lengthening by inserting inductors in radiating elements
Tip Loading means adding an inductor (a coil) to an antenna so it acts electrically longer than it physically is.
T9A03 How is the polarization of an antenna described?
Answer By the orientation of the electric field
Tip Polarization follows the electric field. If the wire is vertical the signal is vertical, so match the other station's orientation.
T9A04 What is a disadvantage of a handheld radio transceiver’s short flexible antenna compared to a full-sized quarter-wave antenna?
Answer It has low efficiency
Tip A stubby rubber duck is a compromise. Making it tiny costs efficiency, so a full quarter-wave whip beats it.
T9A05 Which of the following increases the resonant frequency of a dipole antenna?
Answer Shortening it
Tip Length and frequency go opposite ways. Cut the dipole shorter and its resonant frequency goes up.
T9A06 Which of the following types of antennas offers the greatest gain?
Answer Yagi
Tip Of these, the Yagi wins on gain because it adds director and reflector elements to push power one direction.
T9A07 What is a potential drawback of using a handheld VHF transceiver inside a vehicle that lacks an externally mounted antenna?
Answer Signal strength is reduced due to the shielding effect of the vehicle
Tip A car body is a metal shield. Inside it blocks your HT signal, so an outside antenna helps a lot.
T9A08 Why is a 19-inch-long vertical antenna often used on 2 meters?
Answer It is a resonant quarter-wave
Tip A quarter wave on 2 meters is about 19 inches, so that length is resonant and works well.
T9A09 What is an advantage of a 5/8-wavelength whip antenna for VHF or UHF mobile service compared to a 1/4-wave antenna?
Answer It has more gain
Tip A 5/8-wave whip squashes more energy toward the horizon, giving more gain than a quarter-wave for mobile VHF or UHF.
T9A10 In which direction does a half-wave dipole antenna radiate the strongest signal?
Answer Broadside to the antenna
Tip A dipole throws its best signal out the sides, broadside, not off the ends of the wire.
T9A11 What is antenna gain?
Answer The increase in signal strength in a specified direction compared to a reference antenna
Tip Gain is how much stronger your signal is in one direction compared to a reference antenna. It is relative, not free power.
7.2Gain, Beams, and the Antennas You Will Actually Own
Antenna gain is the increase in signal strength in a specified direction compared to a reference antenna. Read that carefully: gain is not free power added to your transmitter. An antenna with gain takes the same energy and concentrates it, like squeezing a balloon: more in the favored direction, less everywhere else.
An antenna that concentrates signals in one direction is a beam antenna, and the classic beam is the Yagi, the TV-antenna-looking array of parallel elements. Of the common antenna types, the Yagi offers the greatest gain. Ray's tape-measure foxhunt antenna from Module 6 is a Yagi built from a hardware store.
The verticals ladder, from weakest to strongest:
- The handheld's stock rubber duck is the bottom rung: compared to a full-size quarter-wave, its disadvantage is low efficiency. It hears less and talks less, and it is the first thing worth upgrading. Related trap: using a handheld inside a vehicle without an external antenna means your signal is reduced by the shielding effect of the vehicle's metal body.
- The quarter-wave vertical is the honest standard. On 2 meters it is about 19 inches long, and 19 inches is popular for exactly that reason: it is a resonant quarter wavelength at 146 MHz. Check it against Module 2's math: 300/146 is about 2 meters, a quarter of that is about half a meter, and half a meter is about 19 inches. The formula keeps paying rent.
- The 5/8-wave whip is the mobile upgrade: compared to a quarter-wave, its advantage is more gain, flattening the radiation pattern toward the horizon where the repeaters live.
Knowledge Check 7.2
Tap an answer. In your own words first, then check yourself.
1. What is antenna gain?
2. Which antenna type offers the greatest gain?
3. Why is a 19-inch whip the standard 2-meter mobile antenna?
7.3The Feed Line's Job
Between radio and antenna runs the feed line, and its whole job is to deliver power without losing or radiating it along the way.
Coaxial cable dominates amateur stations because it is easy to use and requires few special installation considerations: run it along a wall, tape it to a mast, bury it in conduit, and it does not care. The standard impedance of amateur coax is 50 ohms, matching what your transceiver expects.
But coax charges rent, and the rent goes up with frequency: as the frequency of a signal in coax increases, the loss increases. Cable choice therefore matters more at VHF and UHF than at HF:
- RG-213 has less loss than skinny RG-58 at any given frequency. Thin cable is for short jumpers, not fifty-foot runs.
- Foam-dielectric coax has less loss per foot than solid-dielectric.
- Air-insulated hardline is the lowest-loss feed line available, the choice for long runs to serious VHF/UHF antennas.
Connectors are chosen by frequency and sealed against weather:
- The PL-259 is the standard connector at HF and VHF. It is not waterproof, and neither is anything else: all coax connectors used outdoors, PL-259, BNC, and Type N alike, should be carefully taped for weather protection, because Module 5 already taught you what moisture does to coax.
- Above 400 MHz, the Type N connector is the right choice, built for UHF and beyond.
SWR returns one more time for its definition: a measure of how well a load is matched to a transmission line. When the match is poor, an antenna tuner (antenna coupler) can help, and its major function is matching the antenna system impedance to the transceiver's 50-ohm output. Be clear about what that means: the tuner makes the radio happy at the radio end. It does not fix the antenna, and any loss in the mismatched line is still being paid. A tuner is a diplomat, not a repairman.
Knowledge Check 7.3
Tap an answer. In your own words first, then check yourself.
1. What happens to coax loss as frequency increases?
2. Which connector is most suitable above 400 MHz?
3. What does an antenna tuner actually do?
T9B the easy way: all 12 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
T9B01 Which of the following connectors should be carefully taped for weather protection when used outdoors?
Answer All these choices are correct
Tip Water is the enemy of connectors. Tape any of them when outdoors, so all of the choices are correct.
T9B02 What is the most common impedance of coaxial cables used in amateur radio?
Answer 50 ohms
Tip Ham coax is almost always 50 ohms, so radios and antennas are built around that number.
T9B03 Why is coaxial cable the most common feed line for amateur radio antenna systems?
Answer It is easy to use and requires few special installation considerations
Tip Coax is popular because it is simple. You can route it near metal and it just works with little fuss.
T9B04 What is the major function of an antenna tuner (antenna coupler)?
Answer It matches the antenna system impedance to the transceiver's output impedance
Tip A tuner is a matchmaker. It makes the antenna system look like the impedance the radio wants to see.
T9B05 What happens as the frequency of a signal in coaxial cable is increased?
Answer The loss increases
Tip Higher frequency, more loss in coax. The same cable eats more of your signal as you go up in frequency.
T9B06 Which of the following connector types is most suitable as an RF connector for frequencies above 400 MHz?
Answer Type N
Tip Above 400 MHz, reach for Type N. It is built to handle those higher frequencies cleanly.
T9B07 Which of the following is true of PL-259 type coax connectors?
Answer They are commonly used at HF and VHF frequencies
Tip PL-259 (the UHF connector) is the everyday choice for HF and VHF gear.
T9B08 Which of the following is a source of loss in coaxial feed line?
Answer All these choices are correct
Tip Coax loses signal to conductor, dielectric, and radiation, so all of the choices are correct.
T9B09 What can cause erratic changes in SWR?
Answer Loose connection in the antenna or feed line
Tip A loose connection makes readings jump. Erratic SWR usually points to a bad joint in the antenna or feed line.
T9B10 What is the electrical difference between RG-58 and RG-213 coaxial cable?
Answer RG-213 cable has less loss at a given frequency
Tip Fatter cable, less loss. RG-213 is bigger than RG-58, so it loses less at the same frequency.
T9B11 Which of the following types of feed line has the lowest loss?
Answer Air-insulated hardline
Tip Air-insulated hardline leaks the least signal, giving the lowest loss of these feed lines.
T9B12 What is standing wave ratio (SWR)?
Answer A measure of how well a load is matched to a transmission line
Tip SWR tells you how well the load matches the line. A perfect match reads 1 to 1.
Review: every Exam Anchor from this module
T9A, part 1 Antenna length and resonant frequency are inversely related; shortening a dipole raises its resonant frequency. A half-wave dipole radiates strongest broadside to the antenna. Polarization is described by the orientation of the electric field. Antenna loading: electrically lengthening by inserting inductors in radiating elements.
T9A, part 2 Antenna gain: increased signal strength in a specified direction versus a reference antenna. Beam antenna: concentrates signals in one direction; the Yagi has the greatest gain of the common types. Handheld flexible antennas have low efficiency; a handheld used inside a vehicle loses signal to the vehicle's shielding. A 19-inch vertical is a resonant quarter-wave on 2 meters. A 5/8-wave mobile whip's advantage over a quarter-wave is more gain.
T9B Coax is the most common feed line because it is easy to use with few installation considerations; standard impedance 50 ohms. Coax loss increases as frequency increases. RG-213 has less loss than RG-58; foam dielectric has less loss than solid; air-insulated hardline has the lowest loss of all. PL-259: common at HF and VHF. Type N: best above 400 MHz. Tape all outdoor connectors for weather protection. SWR: measure of how well a load is matched to a transmission line. Antenna tuner: matches the antenna system impedance to the transceiver's output impedance.
Before you move on
You now know why the roof antenna beat the fifty-watt mobile: resonance, pattern, gain, and a feed line that delivered the power instead of eating it. You also know the upgrade ladder for your own station: replace the rubber duck first, use fat coax for long runs, and seal every outdoor connector like the weather is personal.
Module 8 takes everything so far and bolts it together into a working station, at the home bench and in a vehicle.
© 2026 Anthony Pelfanio, K4DIA. All rights reserved.