The ARRL calls antenna choice “the single biggest factor” in whether you can communicate effectively (ARRL, Choosing a Ham Radio). This guide covers the common antenna types, how to size them, and how to get the power from the radio to the antenna without wasting it.
The half-wave dipole
The half-wave dipole is the classic wire antenna: two equal wires, each a quarter wavelength long, fed in the center with coax (ARRL). It is also the reference for gain figures in dBd (ARRL Radio Mathematics). The ARRL gives its total length as:
Length (feet) = 468 / f (MHz)
| Frequency | Total length | Each side |
|---|---|---|
| 3.55 MHz | 468 / 3.55 = 131.8 ft | 65.9 ft |
| 7.15 MHz | 468 / 7.15 = 65.5 ft | 32.7 ft |
| 14.25 MHz | 468 / 14.25 = 32.8 ft | 16.4 ft |
A half wavelength in free space is 492 / f feet (ARRL Radio Mathematics), so the 468 formula gives a wire about 5 percent shorter than a free-space half wavelength (468 / 492 = 0.95). Treat it as a starting point: the ARRL suggests adding about six inches, then trimming for the lowest SWR. If the SWR falls as you tune lower in frequency, the antenna is too long (ARRL); shortening a dipole raises its resonant frequency (Technician pool, T9A05). The antenna calculator does the arithmetic.
A dipole radiates most strongly broadside to the wire (Technician pool, T9A10). In free space its pattern is a figure eight at right angles to the wire. Mounted less than half a wavelength high, its pattern at elevation angles above about 45 degrees is almost the same in every direction. A dipole with a single center support is called an inverted V (General pool, G9B04, G9B05 and G9D12).
Quarter-wave verticals
In an electrical sense, a vertical is a dipole with half of its length “mirrored” in the ground or in a set of radial wires (ARRL). Half the dipole figure gives the length of a quarter-wave vertical element:
Length (feet) = 234 / f (MHz)
That makes a 2 meter quarter-wave whip 234 / 146 = 1.6 feet, about 19 inches, and a 10 meter vertical 234 / 28.5 = 8.2 feet, the same lengths the question pools give (Technician pool, T9A08; General pool, G9B12). A quarter-wave ground-plane vertical is omnidirectional in azimuth, sending signals equally in all horizontal directions (General pool, G9B03). Some operators favor verticals for long-distance work because they radiate at a low angle to the horizon, and low angles often mean longer paths (ARRL). A vertical needs a good radial system: the ARRL notes that a ground-mounted vertical may need anywhere from a dozen to 100 radials depending on the soil, laid on the surface or buried a few inches deep, and that without them “the vertical is a poor cousin to the dipole” (ARRL).
Use vertical antennas for FM and repeater work, since those stations are vertically polarized (ARRL, Choosing a Ham Radio). The radio waves guide explains polarization.
Gain and directivity
An antenna cannot create power: a lossless antenna with gain radiates the same total power as an isotropic antenna fed with the same power (Extra pool, E9B07). Gain is the increase in signal strength in a specified direction compared with a reference antenna (Technician pool, T9A11), achieved by concentrating signals in a preferred direction. Every 3 dB of gain doubles your signal strength in that direction (ARRL, Choosing a Ham Radio).
Two references are used (ARRL Radio Mathematics):
- dBi compares the antenna with an isotropic antenna, a hypothetical, lossless antenna that radiates equally in all directions (Extra pool, E9A01).
- dBd compares it with a half-wave dipole.
A dipole has 1.64 times the gain of an isotropic antenna; Part 97 builds this into its definitions of effective radiated power (ERP, relative to a dipole) and EIRP (relative to isotropic), where EIRP = ERP x 1.64 (47 CFR 97.3). In decibels, 10 log(1.64) = 2.15 dB, so dBi = dBd + 2.15 (ARRL Radio Mathematics; General pool, G9C04). A beam rated 6 dBd is 8.15 dBi. Watch for this when comparing advertised figures.
Worked example: 100 W into an antenna with 6 dBd of gain (a ratio of 3.98) gives an ERP of about 398 W in the favored direction, or an EIRP of about 653 W. Both are calculated from the power supplied to the antenna (47 CFR 97.3), so feed line loss, covered below, reduces both.
Yagis and other beams
A beam antenna concentrates signals in one direction (Technician pool, T9A01). The Yagi, a rotatable beam, has a driven element about a half wavelength long, a longer reflector and one or more shorter directors. Making the boom longer and adding directors increases gain. The front-to-back ratio compares the power radiated in the main lobe with the power radiated in the opposite direction (General pool, G9C02, G9C03, G9C05 and G9C07). You need a rotator to point a Yagi (ARRL, Choosing a Ham Radio). Another directional design, the log-periodic, varies its element lengths and spacing logarithmically along the boom, and its advantage is wide bandwidth (General pool, G9D06 and G9D07).
Loading and shortened antennas
When a full-size antenna will not fit, it can be made electrically longer than it is physically by inserting inductors in its radiating elements (Technician pool, T9A02). A loading coil resonates a short antenna by canceling its capacitive reactance (Extra pool, E9D09). Mobile HF “screwdriver” antennas vary the base loading inductance (General pool, G9D08) to tune nearly any HF frequency (ARRL, Choosing a Ham Radio). Traps let one antenna work on several bands (ARRL; General pool, G9D04).
Shortening has costs. Loading coils reduce the SWR bandwidth (Extra pool, E9D06), and a handheld’s short flexible antenna has low efficiency compared with a full-sized quarter-wave antenna (Technician pool, T9A04). The ARRL calls the “rubber duck” “not very efficient” and suggests a longer, more efficient replacement (ARRL, Choosing a Ham Radio). A handheld used inside a vehicle without an outside antenna also loses signal strength to the shielding effect of the vehicle (Technician pool, T9A07).
Coax and other feed lines
Coaxial cable has a center conductor surrounded by a shield, with insulating material between them. Amateur coax and transceivers use 50 ohms; ladder line, two parallel wires, runs from 300 to 600 ohms (ARRL). Coax is the most common feed line because it is easy to use and requires few special installation considerations (Technician pool, T9B03).
Feed line loss is rated in decibels per 100 feet (ARRL), and all cables lose more as frequency rises. RG-58 suits only short runs (the ARRL says 50 feet or less) because of its higher loss (ARRL, Choosing a Ham Radio), and the larger RG-213 has less loss at a given frequency (Technician pool, T9B10). The ARRL’s advice is to pick “the feed line that has the lowest loss at the highest frequency you want to operate” (ARRL). The effect is real: the ARRL gives an example where 100 W into 80 feet of RG-8X at 146.52 MHz, with a 1.4:1 SWR, delivers only 46 W to the antenna (ARRL).
SWR: what it does and does not tell you
Standing wave ratio (SWR) is a measure of how well a load, usually the antenna, is matched to a transmission line (Technician pool, T9B12). It is the ratio of the maximum to the minimum voltage along the line, from 1:1 (an exact match) upward (Microwaves101). For a purely resistive load, SWR is the larger impedance divided by the smaller, which gives the pool answers for 200 ohm and 10 ohm loads (General pool, G9A09 and G9A10):
- 100 ohm load on 50 ohm coax: 100 / 50 = 2:1
- 200 ohm load: 200 / 50 = 4:1
- 10 ohm load: 50 / 10 = 5:1
The magnitude of the reflection coefficient is (SWR - 1) / (SWR + 1), and its square is the fraction of power reflected at the mismatch (Microwaves101). At 2:1 that is (1/3) squared, about 11 percent; at 3:1 it is 25 percent. The SWR calculator does these conversions.
What SWR does tell you: whether the antenna system presents the 50 ohm impedance your radio is designed for. Radios reduce power to protect themselves when it does not (ARRL), and high SWR increases the loss in a lossy feed line (General pool, G9A02). Erratic changes in SWR can mean a loose connection in the antenna or feed line (Technician pool, T9B09).
What it does not tell you: whether the antenna radiates well. A dummy load is a 50 ohm non-inductive resistor used so that test signals are not transmitted over the air (Technician pool, T7C01 and T7C03); on 50 ohm coax it shows 1:1 (50 / 50) while sending almost nothing into the air. Feed line loss also flatters the reading: higher loss reduces the SWR measured at the input to the line (General pool, G9A11), so a long, lossy cable can show a low SWR at the radio over a poor antenna.
Antenna tuners
An antenna tuner, also called an antenna coupler, impedance matching unit or transmatch, converts whatever impedance the antenna system presents into the 50 ohm load your radio wants. It does not actually tune the antenna (ARRL, Choosing a Ham Radio). A tuner at the radio leaves the mismatch, and the SWR, unchanged on the feed line between tuner and antenna (General pool, G9A08). With coax at high SWR, that means extra loss. With ladder line at HF, the loss is so low that a tuner works well even when the SWR is very high (ARRL).
Connectors and weatherproofing
- PL-259 and SO-239 (the “UHF” family) are the standard connectors at HF (ARRL, Choosing a Ham Radio) and are commonly used at VHF. Despite the name, the Technician pool names Type N, not PL-259, as the connector for frequencies above 400 MHz (Technician pool, T9B06 and T9B07).
- Type N is a moisture-resistant RF connector useful to 10 GHz (General pool, G6B07).
- BNC connectors in 50 ohm versions have a typical upper limit of about 4 GHz for low SWR operation (General pool, G6B04).
- SMA is a small threaded connector suitable for signals up to several GHz (General pool, G6B11).