This page is part of the N0NJY General Class self-study course for Technician operators upgrading to General.
Your antenna system is the most important part of your station. A modest transceiver running 100 watts into an excellent antenna will consistently outperform a high-power station feeding a mediocre antenna. This is not theory — it is the practical experience of every operator who has ever upgraded their antenna and discovered that the bands suddenly seemed louder and their signal reports improved dramatically. Get the antenna right, and everything else follows.
This module covers the antennas you will actually build and use, what makes each one work, how height and feed line choices affect real performance, how to interpret SWR correctly, and what to do when things are not working as they should.
This is one of the most common sources of confusion among new HF operators. Resonance and impedance match are related but distinct concepts. Confusing them leads to incorrect antenna adjustments and misinterpreted SWR readings.
Antenna resonance is the condition where the antenna's inductive and capacitive reactances cancel, leaving only radiation resistance. A resonant antenna has no net reactance at its feed point — the impedance is purely resistive. For a half-wave dipole in free space, resonance occurs when the antenna is approximately one-half wavelength long at the operating frequency. At resonance, the feed-point impedance is approximately 72 ohms, purely resistive.
Impedance match is the condition where the antenna's feed-point impedance equals the characteristic impedance of the feedline. Since most amateur coax is 50 ohms and a free-space dipole has a feed-point impedance of 72 ohms, a resonant dipole is not a perfect impedance match to 50-ohm coax, even though it is resonant. The SWR on 50-ohm coax feeding a resonant dipole is approximately 1.5:1 — not 1:1. This is perfectly acceptable and causes very little loss in practice.
What happens when the antenna is not resonant: If the antenna length is not correct for the operating frequency, the feed-point impedance includes reactance in addition to resistance. This reactance causes reflected power on the feedline, increasing SWR above what the resistive mismatch alone would produce. An antenna tuner can cancel this reactance at the transmitter end, but the SWR on the feedline between the tuner and the antenna remains high, which increases feedline loss.
The practical conclusion: A resonant antenna on a low-loss feedline with a modest resistive mismatch (1.5:1 to 2:1 SWR) is a better system than a non-resonant antenna with a tuner compensating for high reactance on a lossy feedline. Build antennas close to resonance. Use low-loss feedline. Then use a tuner only for fine trimming.
These are the formulas you will use every time you build a wire antenna:
| Antenna Type | Formula (feet) | Formula (meters) | Example at 7.2 MHz |
|---|---|---|---|
| Half-wave dipole (total length) | 468 / f(MHz) | 142.6 / f(MHz) | 65 feet total (32.5 ft each side) |
| Quarter-wave vertical | 234 / f(MHz) | 71.3 / f(MHz) | 32.5 feet |
| 5/8-wave vertical | 585 / f(MHz) | 178.3 / f(MHz) | 81.25 feet |
| Full-wave loop (circumference) | 1005 / f(MHz) | 306.3 / f(MHz) | 139.6 feet total perimeter |
The factor 468 (rather than the theoretical 492 for a half-wavelength in free space) accounts for the velocity factor of real wire and the end effect that shortens the electrical length relative to the physical length. For very thick conductors or antennas very close to other objects, the actual resonant length may differ from these formulas — always trim to resonance using an antenna analyzer or SWR meter.
The half-wave dipole is the reference antenna against which all others are measured. It is cheap, easy to build, performs well, and is understood by every operator worldwide. If you have never built an antenna before, start here.
Construction: Cut two equal lengths of wire totaling 468/f(MHz) feet. Any insulated or bare copper wire works — #14 to #18 AWG is common for lightweight portability; #12 is better for permanent installations. Connect the two halves at the center through a 1:1 current balun (choke balun) and a coax connector. The balun prevents common-mode current on the coax shield, which causes RF in the shack and pattern distortion. Without a balun, the coax becomes part of the antenna and behaves unpredictably.
The feed-point impedance of a half-wave dipole in free space is approximately 72 ohms. When installed at a height of one-half wavelength or higher, SWR on 50-ohm coax will be approximately 1.5:1 — perfectly acceptable. At lower heights, ground proximity reduces the feed-point impedance, which may actually improve the match to 50 ohms.
The inverted-V is a dipole fed at the apex (top) with the two wire elements sloping downward at an angle rather than running horizontally. It requires only one central support point (a tree, a pushup mast, a vehicle-mounted pole, or a communications shelter) rather than two support points needed for a flat dipole. This makes it the most practical antenna for field deployment, portable operation, and NVIS work.
The inverted-V is the antenna used most frequently in military field communications for exactly this reason: one person, one tall support, and about 20 minutes of work produces a functional HF antenna covering 500+ miles via NVIS. In the 3rd Combat Communications Group, and in portable HF operations worldwide, the inverted-V cut to frequency is the standard first choice for rapid deployment.
Construction notes:
Inverted-V dipole — single central support, elements slope to ground stakes. Apex angle should be at least 90 degrees.
The end-fed half-wave antenna has become extremely popular for portable operation because it requires only one elevated support point (at the far end of the wire) and the feed point is at ground level where it is convenient. A single wire cut to a half-wavelength is fed at one end through a 49:1 or 64:1 impedance transformer.
The end of a half-wave antenna is a high-impedance point — approximately 2,000 to 5,000 ohms depending on height and surroundings. The transformer steps this down to something closer to 50 ohms. A short counterpoise wire (typically 0.05 wavelength, or a few feet at HF) connected to the coax shield at the feed point improves the match and reduces RF on the coax shield.
EFHW antennas work on multiple bands if cut for a fundamental frequency that has harmonics on other desired bands. A wire cut for 40 meters (approximately 66 feet) also works on 20, 15, and 10 meters with the same transformer. This multiband capability at a low cost and small footprint makes the EFHW popular among portable and SOTA (Summits on the Air) operators.
A quarter-wave vertical radiates in all horizontal directions (omnidirectional), which makes it useful when you want to work stations in multiple directions without rotating an antenna. The feed-point impedance of a quarter-wave vertical over a perfect ground plane is approximately 36 ohms. Over real ground, the impedance is typically 50 ohms or close to it, which makes a direct 50-ohm coax feed possible with good SWR.
The ground plane is critical. A quarter-wave vertical operating without a proper ground plane is severely compromised — efficiency drops dramatically and the feed-point impedance shifts unpredictably. At minimum, four quarter-wave radials laid on or just above the ground surface are required for acceptable performance. More radials (16, 32, or more) improve efficiency progressively. The radials do not need to be elevated — laying them on the ground surface is effective and far easier to install.
For portable and field use, a vertical with four to eight elevated radials (horizontal, at the base of the antenna) provides good performance with a manageable setup time. Elevated radials must be horizontal and at least 5 to 10 feet above ground for best efficiency.
A Yagi (Yagi-Uda) antenna uses one or more parasitic elements (reflector behind the driven element, directors in front) to concentrate the radiation pattern in a preferred direction. A 3-element Yagi provides approximately 7 to 8 dBi of forward gain (5 to 6 dBd) with a front-to-back ratio of 20 to 25 dB. On a DX-targeted beam, that gain is the equivalent of quadrupling your transmit power and dramatically improving your receive signal-to-noise ratio from the beam heading direction.
Most HF Yagis are rotatable, mounted on a tower or mast with a rotator. Fixed Yagis are used when the target is always in the same direction (Europe from the US East Coast, for example). For portable satcom work and field directional communications, a simple fixed Yagi cut for the operating frequency and physically aimed at the target is a quick and effective solution.
A folded dipole is a half-wave dipole with an additional conductor connecting the two ends, forming a narrow loop. The feed-point impedance is four times that of a simple dipole — approximately 288 ohms in free space. A 4:1 balun transforms this to 72 ohms for a near match to coax (or to 50 ohms with a slightly higher SWR).
The folded dipole has approximately twice the bandwidth of a simple dipole, meaning it maintains low SWR across a wider frequency range. This makes it useful for multi-frequency operation without retuning. TV antenna "rabbit ears" are folded dipoles. A 40-meter folded dipole built from 300-ohm TV twin-lead (the wire is already doubled for you) with the ends shorted and the feedline connected at the center provides a practical, wide-bandwidth antenna with very simple construction.
A radiation pattern shows how an antenna distributes its radiated power in different directions. Understanding radiation patterns tells you what your antenna is actually doing and why height, antenna type, and installation choices matter.
The radiation pattern of a horizontal dipole changes dramatically with height above ground. This is one of the most important practical concepts in HF antenna work:
Elevation pattern comparison: a high dipole (½λ AGL) radiates at low angles for DX; a low dipole (⅛λ AGL) radiates nearly straight up for NVIS regional coverage. Both are useful — for different purposes.
A horizontal dipole is not omnidirectional in azimuth (horizontal direction). It has a figure-eight pattern with maximum radiation broadside (perpendicular) to the wire and minimum radiation off the ends. When you orient a dipole, you are aiming it — the strongest signal leaves the antenna at right angles to the wire. Orient your 40-meter dipole east-west if you want maximum signal to the north and south (Europe and South America from the US). Orient it north-south for east-west coverage.
Azimuth (top-view) pattern of a horizontal dipole oriented East-West. Maximum radiation is broadside (North and South). Deep nulls off the ends (East and West). Orient the dipole so the broadside directions point toward your target areas.
Your feed line connects the transceiver to the antenna. The wrong choice of feedline can silently waste a significant fraction of your transmitted power as heat in the cable before it ever reaches the antenna. This section gives you the practical knowledge to make the right choice.
Coaxial cable has a characteristic impedance determined by the ratio of the inner and outer conductor diameters and the dielectric material. Amateur radio uses 50-ohm coax for antenna systems. (75-ohm cable is used for TV and cable applications; while it can be used with an antenna tuner, it is not the standard and introduces a mismatch.)
The most important coax specification for HF operators is loss per 100 feet at the operating frequency. Loss increases with frequency and with poor-quality cable. Here is what you actually need to know:
| Cable | Diameter | Loss at 7 MHz | Loss at 14 MHz | Loss at 30 MHz | Verdict |
|---|---|---|---|---|---|
| RG-58/U | 0.195" | 1.1 dB/100ft | 1.6 dB/100ft | 2.4 dB/100ft | Marginal — short runs only. 100 ft at 14 MHz loses 30% of your power. |
| RG-8/U or RG-213 | 0.405" | 0.5 dB/100ft | 0.7 dB/100ft | 1.1 dB/100ft | Good — standard choice for HF up to 100 ft runs. |
| LMR-400 | 0.405" | 0.22 dB/100ft | 0.32 dB/100ft | 0.47 dB/100ft | Excellent — best practical choice for HF, especially longer runs. |
| RG-174 | 0.100" | 2.0 dB/100ft | 2.9 dB/100ft | 4.5 dB/100ft | Do not use for HF antenna feedlines. Acceptable only for short patch cables inside equipment. |
This is the most important practical concept in the feedline section. High SWR on a lossy feedline is far worse than high SWR on a low-loss feedline.
When SWR is 1:1, the feedline loss is just the base loss for the cable at that frequency. When SWR is elevated, reflected power makes additional trips through the cable, each trip losing more power. The total additional loss depends on both the SWR and the base cable loss.
Examples at 14 MHz with 100 feet of cable:
The lesson: use low-loss cable and keep SWR reasonable. If you must use an antenna that presents a significant mismatch (end-fed long wire, for example), use ladder line to the antenna and add a tuner at the shack — ladder line handles high SWR with very low loss.
Ladder line (also called twin-lead, window line, or open-wire feedline) consists of two parallel conductors held at a fixed spacing by insulating spacers. The characteristic impedance is determined by the conductor spacing and diameter — commonly 300, 450, or 600 ohms.
The critical advantage of ladder line is its very low loss even at high SWR. While coax loss rises sharply with SWR, ladder line loss at high SWR remains nearly as low as at matched conditions. A 100-foot run of 450-ohm ladder line at 10:1 SWR loses only a fraction of a dB. The same SWR on RG-58 would lose most of your power.
This is why the classic multiband HF antenna system consists of a center-fed doublet (a dipole slightly longer than needed for the lowest frequency of operation), fed with ladder line, connected to a balanced antenna tuner at the shack. This system covers multiple bands efficiently even though the antenna is non-resonant on most of them — because the ladder line handles the high SWR without significant loss.
What's wrong with ladder line: It cannot be routed through walls, bent sharply around corners, or run parallel to metal surfaces or other conductors. It must be kept away from metal objects by at least several inches. In tight quarters or buildings with limited routing options, coax is more practical despite its higher loss.
Electromagnetic waves travel more slowly in a dielectric medium than in free space. The velocity factor (VF) of a feedline is the ratio of the wave's speed in the cable to the speed of light in a vacuum. A coax with VF = 0.66 means signals travel at 66% of the speed of light in that cable. Velocity factor matters when:
For general HF antenna use, velocity factor is not usually critical. Typical velocity factors: foam-dielectric coax (RG-8X, LMR-400): 0.83 to 0.85. Solid polyethylene coax (RG-8, RG-213): 0.66. Ladder line: 0.90 to 0.97.
A dipole is a balanced antenna — equal and opposite currents flow in its two halves. Coaxial cable is an unbalanced feedline — the center conductor carries signal; the outer shield is a ground reference. When you connect an unbalanced feedline directly to a balanced antenna without a balun, the imbalance drives current on the outside of the coax shield. That current makes the coax act as an additional antenna element, distorts the radiation pattern, and carries RF back into the shack.
A 1:1 current balun (choke balun) presents a high impedance to common-mode current while passing differential (antenna) current freely. It forces the antenna to operate as a balanced system. For a center-fed dipole to 50-ohm coax, the 1:1 current choke balun is the correct choice. The G5 module covered the electrical principles; here is the practical application.
A 4:1 balun transforms 200 ohms to 50 ohms (or 300 ohms to 75 ohms). Use a 4:1 balun when:
An antenna tuner (impedance matching network, antenna coupler) matches the impedance at the feedline input to 50 ohms, satisfying the transmitter. It does not:
What it does do: present the transmitter with a 50-ohm load so the transmitter delivers full power to the feedline. The transmitter is satisfied. Whether that power reaches the antenna efficiently depends entirely on feedline quality and SWR on the feedline, which the tuner does not affect.
SWR (Standing Wave Ratio) is the ratio of maximum to minimum voltage along a transmission line that has a mismatched load. SWR of 1:1 is a perfect match. As the impedance mismatch between feedline and antenna increases, SWR increases.
SWR is not a measure of antenna efficiency. SWR is a measure of impedance match. You can have:
Practical SWR guidelines:
| SWR Reading | What It Means | Action |
|---|---|---|
| 1.0:1 to 1.5:1 | Excellent match. Minimal reflected power. | Operate normally. |
| 1.5:1 to 2.0:1 | Good match. Less than 10% power reflected. | Acceptable. Optional tuner for transmitter protection. |
| 2.0:1 to 3.0:1 | Moderate mismatch. Some power reflected. Transmitter may reduce power. | Use a tuner. Investigate antenna if loss of performance noted. |
| Above 3:1 | Significant mismatch. Modern transceivers will fold back power to protect finals. | Retune or fix the antenna before operating at full power. |
| Very high or infinite | Open or short circuit in the antenna system. | Stop transmitting. Find and fix the fault before operating. |
For a horizontal dipole, higher is almost always better for DX work. The radiation angle decreases as height increases, directing more energy at the low angles needed for long-distance skip propagation. A dipole at 60 feet will consistently outperform the same dipole at 30 feet for DX contacts on 40 meters.
The exception is NVIS: for regional coverage on 40 and 80 meters, the ideal height is one-eighth to one-quarter wavelength above ground (about 15 to 30 feet for 40 meters). Going higher for a NVIS antenna actually degrades its performance for the intended purpose.
Trees make excellent antenna supports and are free. A wire antenna supported by two trees 50 to 100 feet apart can be installed with minimal hardware. Use end insulators on both ends to prevent the wire from being shorted to the tree itself, and leave slack in the support rope to allow for tree movement in wind. A rigid connection between a swaying tree and a taut wire will eventually break the wire at the connection point.
If trees are not available, telescoping fiberglass masts (often called "crappie poles" from their original fishing application) in 20 to 40-foot lengths are popular for portable and temporary installations. They are lightweight, inexpensive, and can be guyed at the base with three or four lines to provide stability in wind.
Coax connectors at the antenna feed point are exposed to weather. Moisture in a coax connector significantly increases loss and eventually causes the connector to fail. Protect outdoor connectors with self-amalgamating (self-fusing) tape wrapped tightly over the connector and several inches of coax on each side. Self-amalgamating tape fuses to itself and creates a waterproof seal that standard electrical tape does not. Inspect and re-tape outdoor connectors annually.
The G9 subelement covers antennas and feed lines as tested in the 2023–2027 FCC General Class question pool. All pool questions are covered below.
Q1 (G9A01) — What is the relationship between SWR and the characteristic impedance of a transmission line?
Q2 (G9A02) — What is the characteristic impedance of flat ribbon TV twin-lead?
Q3 (G9A03) — What is the characteristic impedance of most coaxial cable used in amateur radio?
Q4 (G9A04) — What is the primary purpose of an antenna tuner?
Q5 (G9A05) — What is the feed-point impedance of a half-wave dipole antenna?
Q6 (G9A06) — How does antenna height affect the feed-point impedance of a horizontal dipole when the antenna is lowered from a half-wavelength to a quarter-wavelength above ground?
Q7 (G9A07) — What is the approximate feed-point impedance of a quarter-wave vertical antenna over ideal ground?
Q8 (G9A08) — What is the SWR when the characteristic impedance of a transmission line is 50 ohms and the connected antenna has a feed-point impedance of 200 ohms?
Q9 (G9A09) — What is the SWR when the characteristic impedance of a transmission line is 50 ohms and the connected antenna has a feed-point impedance of 10 ohms?
Q10 (G9A10) — What is the SWR when the characteristic impedance of a transmission line is 50 ohms and the connected antenna has a feed-point impedance of 50 ohms but includes a reactance of 50 ohms?
Q11 (G9A11) — What is the effect of an antenna system with a high SWR on the coaxial cable feeding the antenna?
Q12 (G9B01) — What is the approximate length of a resonant half-wave dipole at 14.250 MHz?
Q13 (G9B02) — What is the approximate length of a resonant half-wave dipole at 3.550 MHz?
Q14 (G9B03) — What happens when the feed-point of a dipole previously fed at the center is moved to a point one-quarter the way along the antenna from one end?
Q15 (G9B04) — What is the radiation pattern of a dipole antenna in the plane perpendicular to the antenna?
Q16 (G9B05) — How does the radiation pattern of a horizontally polarized antenna change when the antenna height is increased to one wavelength above ground?
Q17 (G9B06) — Where should the radials of a ground-mounted vertical antenna system be placed?
Q18 (G9B07) — How does the feed-point impedance of a ¼-wavelength vertical antenna over a real, imperfect ground compare to the impedance over perfect ground?
Q19 (G9B08) — What is the approximate length of a resonant quarter-wave vertical antenna at 7.1 MHz?
Q20 (G9B09) — What is the radiation pattern of a vertically polarized antenna in the horizontal plane?
Q21 (G9B10) — In what direction is the pattern of a half-wave dipole antenna most sensitive?
Q22 (G9C01) — Which of the following would increase the gain of a Yagi antenna?
Q23 (G9C02) — What is a beta or hairpin match?
Q24 (G9C03) — Which statement about a three-element, single-band Yagi antenna is true?
Q25 (G9C04) — How does the gain of two antennas stacked vertically compare to the gain of a single antenna?
Q26 (G9C05) — What does "front-to-back ratio" mean in a Yagi antenna?
Q27 (G9C06) — What is the reason a Yagi antenna radiates more in the forward direction than in the rearward direction?
Q28 (G9C07) — What is the approximate gain of a 5-element Yagi antenna versus a half-wave dipole?
Q29 (G9D01) — What does the term "antenna gain" mean?
Q30 (G9D02) — What is a log-periodic antenna?
Q31 (G9D03) — Which of the following describes a "screwdriver" antenna?
Q32 (G9D04) — What is the advantage of a trapped antenna?
Q33 (G9D05) — What is the advantage of a 5/8 wavelength vertical antenna compared to a 1/4 wavelength vertical?
Q34 (G9D06) — What is the feed-point impedance of a 5/8 wavelength vertical antenna?
Q35 (G9D07) — Which of the following describes a J-pole antenna?
Q36 (G9D08) — What is the primary purpose of a phasing line when used with a fed array?