A transmission line (also called a feed line) is the dedicated RF path between your transceiver and your antenna. It is not a ground wire, not a radial wire, and not a counterpoise -- it is the cable that actually carries the signal back and forth. Every amateur station has one, and choosing the right type matters enormously for performance.
Every transmission line has a characteristic impedance (Z0), measured in ohms. This is arguably the single most important concept in feed line theory. The characteristic impedance is determined entirely by the physical dimensions and relative positions of the conductors -- the size and spacing of the wires, and the dielectric material between them. It has nothing to do with the line's length, the operating frequency, or what load is connected at the far end.
The factors that set impedance differ slightly between line types. For an open-wire line, impedance is determined by the distance between the centres of the conductors and the diameter of the conductors. For a coaxial cable, it is the ratio of the inner diameter of the outer shield to the diameter of the inner conductor. In both cases, it is geometry that matters -- not length, not frequency.
| Line Type | Impedance Determined By |
|---|---|
| Open-wire line | Distance between centres of conductors and diameter of conductors |
| Coaxial cable | Ratio of inner diameter of outer shield to diameter of inner conductor |
When a transmission line is terminated by a load whose impedance exactly matches the line's characteristic impedance, all of the RF energy is absorbed by the load. None is reflected back. For instance, a 50-ohm coaxial cable terminated by a purely resistive 50-ohm load will absorb all the RF energy it receives -- this is the ideal matched condition.
If the termination differs significantly from the characteristic impedance, the impedance measured at the input of the line becomes a value influenced by line length. The mismatch creates standing waves, and the apparent impedance varies along the line depending on where you measure. This is why impedance matching is so critical in amateur radio.
Radio signals travel slower in a transmission line than in free space. The velocity factor is the ratio of signal speed in the line to the speed of light, typically ranging from about 0.66 to 0.95 depending on the cable. The major factor influencing the velocity factor of a coaxial cable is the dielectric material -- the insulation between the inner conductor and the shield. Foam dielectrics have a higher velocity factor (closer to 1.0) than solid dielectrics because foam is mostly air.
If you need to run a feed line underground, the commonly available transmission line that can be buried directly in the ground without adverse effects is coaxial cable. Its outer jacket and shield protect it from moisture and soil contact. Open-wire line and window line cannot be buried because their conductors are exposed or minimally insulated.
B-006-001-001: What connects your transceiver to your antenna?
B-006-001-002: The characteristic impedance of a transmission line is determined by the:
B-006-001-003: The characteristic impedance of a 20-metre piece of transmission line is 52 ohms. What would the impedance be if 10 metres were cut off?
B-006-001-004: Why can coaxial cables of different diameters have the same characteristic impedance?
B-006-001-005: What commonly available transmission line can be buried directly in the ground for some distance without adverse effects?
B-006-001-006: A transmitter is delivering RF energy into a coaxial cable with a characteristic impedance of 50 ohms. The cable is terminated by a purely resistive load. What value of load resistance will absorb all the RF energy it receives?
B-006-001-007: What is the major factor influencing the velocity factor of a coaxial cable?
B-006-001-008: The characteristic impedance of an open-wire transmission line depends, in part, on the diameter of its conductors. What other dimension determines its characteristic impedance?
B-006-001-009: A transmission line is terminated by an impedance that differs significantly from the characteristic impedance of the line. What impedance will be measured at the input of the line?
B-006-001-010: What factors determine the characteristic impedance of an open-wire transmission line?
B-006-001-011: What factors determine the characteristic impedance of a coaxial transmission line?
COAXIAL CABLE (Cross-section) OPEN-WIRE LINE WINDOW LINE (Ladder Line)
======================== =============== ========================
Outer Jacket Wire Wire Wire Wire
+------------+ | | | |
| ########## | Braided | | +------+--------+------+
| # ~~~~~~ # | Shield | | | Insulating | |
| # ~ ** ~ # | | | | spacer with | |
| # ~~~~~~ # | Dielectric | | | windows | |
| ########## | | | +------+--------+------+
+------------+ | | | |
Insulated Two wires embedded
* = Centre conductor spreaders in flat insulation
~ = Dielectric (rods) with cutout windows
# = Shield/braid
Coaxial cable is the most widely used transmission line in amateur radio. It has a centre wire inside an insulating material (the dielectric), surrounded by a metal shield or sleeve, all wrapped in a protective outer jacket. The shield confines the RF fields inside the cable and provides shielding from external interference. Because it is self-shielding, coax can be run along metal structures, through walls, and even buried underground without problems.
Window line (also called ladder line) has two wires side-by-side embedded in insulating material, with periodic cutouts ("windows") to reduce dielectric losses. Its typical impedance is 300 or 450 ohms. Open-wire line is made of two conductors held apart by insulated rods (called spreaders) at regular intervals. It has very low loss and a typical impedance of 450-600 ohms. The key distinction is that open-wire line uses rigid insulated spreaders to maintain a constant distance between the conductors.
A balanced transmission line is made of two parallel wires -- both conductors carry equal and opposite currents. Open-wire line, window line, and twin-lead are all balanced. Coaxial cable, by contrast, is inherently unbalanced because one conductor (the shield) is typically connected to ground. A transmission line becomes unbalanced when one conductor is connected to ground.
A balun (BALanced-to-UNbalanced) is a transformer device that bridges the gap between balanced and unbalanced systems. Its purpose is to connect balanced and unbalanced systems together properly. For example, connecting coaxial cable (unbalanced) to a dipole antenna (balanced) requires a balun at the junction.
When feeding a dipole antenna with 50-ohm coaxial cable, the balun should be installed between the coaxial cable and the antenna -- right at the feed point where the transition from unbalanced to balanced occurs. A balun can also be used to connect coaxial cable to window line (since coax is unbalanced and window line is balanced), or any time you need to bridge between these two worlds.
If your antenna tuner does not have a balanced output and you wish to use window line to feed an HF antenna, place a balun between the tuner and the transmission line to make the conversion.
B-006-002-001: What kind of transmission line has a centre wire inside an insulating material that is covered by a metal shield or sleeve?
B-006-002-002: What kind of transmission line has two wires side-by-side embedded in insulating material?
B-006-002-003: What kind of transmission line is made of two conductors held apart by insulated rods?
B-006-002-004: What is the purpose of a balun?
B-006-002-005: Where would you install a balun to feed a dipole antenna with 50-ohm coaxial cable?
B-006-002-006: What causes a transmission line to be unbalanced?
B-006-002-007: What device can be installed to feed a balanced antenna with an unbalanced transmission line?
B-006-002-008: What device should you use to connect a coaxial cable to window line?
B-006-002-009: A balanced transmission line:
B-006-002-010: Your antenna tuner does not have a balanced output and you wish to use window line to feed an HF antenna. What device should you use between the tuner and the transmission line?
B-006-002-011: What kind of transmission line has two conductors maintained side by side, a constant distance apart, using insulated spreaders?
There are four main RF connector types you need to know for the exam. Each is designed for a different use case, and they differ in size, weather resistance, and frequency range.
| Connector | Also Known As | Primary Use | Key Feature |
|---|---|---|---|
| PL-259 / SO-239 | UHF connector | HF transceivers, RG-213 cable | Most common HF connector; PL-259 is plug, SO-239 is socket |
| SMA | SubMiniature-A | Hand-held transceivers (HTs) | Small, threaded; standard on modern HTs |
| BNC | Bayonet Neill-Concelman | Low-power transceivers, test instruments | Quick-connect bayonet locking; reliable at VHF |
| N-type | N connector | VHF/UHF, outdoor/tower installations | Moisture resistant; best weatherproofing |
If a transmission line must be supported by attaching it to a metal fence for several metres, coaxial cable will NOT be adversely affected by proximity to the fence because its shield confines the fields inside the cable. Open-wire and window line would have their impedance disrupted by nearby metal. In locations frequently affected by icing, coaxial cable is again the most suitable transmission line because ice cannot bridge its conductors (they are sealed inside). Open-wire and window lines can short out when ice forms between the conductors.
A common-mode current choke can be made by winding coaxial cable on a ferrite toroid. Cable with solid dielectric is preferred over foam dielectric for this purpose because there is less risk of a short due to centre conductor movement when the cable is bent tightly around the toroid.
Why do most amateur radio antenna systems use coaxial cable rather than other types of transmission line? Because coax is more usable in a wide variety of settings -- it can be buried, run along metal structures, bent around corners, and easily terminated with standard connectors.
The type of coaxial outer conductor that offers the best shielding is a solid shield (sometimes called solid copper tube or hardline). Double-braid is second best, followed by single-braid. The primary advantage of coaxial cable with a foam dielectric (vs. solid) is lower loss, because foam has a lower dielectric constant and a higher velocity factor.
B-006-003-001: A transmission line must be supported for several metres by attaching it to a metal fence. What type of transmission line will NOT be adversely affected by proximity to the fence?
B-006-003-002: A common-mode current choke can be made by winding coaxial cable on a ferrite toroid. Why is cable with solid dielectric preferred over foam dielectric?
B-006-003-003: Why do most amateur radio antenna systems use coaxial cable, rather than other types of transmission line?
B-006-003-004: What type of connector is commonly installed on RG-213 coaxial cable for connection to an HF transceiver?
B-006-003-005: What type of connector usually joins a modern hand-held transceiver to its antenna?
B-006-003-006: Which popular RF connector is designed to be moisture resistant?
B-006-003-007: What type of RF connector is commonly used for low-power transceivers and test instruments?
B-006-003-008: Why should you regularly clean and tighten all antenna connectors?
B-006-003-009: What type of coaxial outer conductor offers the best shielding?
B-006-003-010: If your location is frequently affected by icing conditions, which type of transmission line would be the most suitable?
B-006-003-011: What is the primary advantage of choosing a coaxial cable with a foam dielectric instead of a solid dielectric?
RF transmission line losses are expressed in decibels per unit length (e.g., dB per 100 feet or dB per 100 metres). This makes it easy to calculate total loss for any cable run: simply multiply the loss-per-unit-length by the actual length.
Signal loss increases as the length increases -- the relationship is directly proportional. If you double the length of a coaxial transmission line from 20 metres to 40 metres, the loss increases by 100% (it doubles).
Signal loss in a transmission line increases with increasing frequency. This is why cable that works fine on HF may have unacceptable loss at UHF. As the operating frequency increases, loss increases due to internal line losses -- specifically the skin effect (current crowds onto the surface of the conductor at high frequencies) and dielectric losses.
Using RG-58 (thin coax) on the 70 cm band (UHF, ~430 MHz) leads to excess RF loss in the transmission line. Thinner cable has inherently higher loss, and this becomes severe at high frequencies. Changing from RG-213 to RG-58 (same length) means less RF power is radiated from the antenna. RG-58 has roughly 2-3 times the loss per unit length of RG-213.
| Cable Type | Impedance | Diameter | Relative Loss | Best For |
|---|---|---|---|---|
| RG-58 | 50 ohms | ~5 mm | High | Short runs, low power, HF only |
| RG-213 | 50 ohms | ~10 mm | Moderate | General HF/VHF station use |
| LMR-400 | 50 ohms | ~10 mm | Low | VHF/UHF, longer runs |
| Twin-lead | 300 ohms | Flat ribbon | Lower than coax | TV antennas (legacy), balanced feeds |
| Window line | 300-450 ohms | ~25 mm wide | Very low | HF balanced antenna feeds |
| Open-wire | 450-600 ohms | Variable | Lowest | HF antenna feeds, high-SWR operation |
B-006-004-001: What is the major adverse consequence of using RG-58 coaxial cable for a transmission line operating on the 70 cm band?
B-006-004-002: What is the major advantage of open-wire transmission line?
B-006-004-003: If your transmitter and antenna are 15 metres apart, but are connected by 60 metres of RG-58 coaxial cable, what should be done to reduce transmission line loss?
B-006-004-004: As the length of a transmission line is changed, what happens to signal loss?
B-006-004-005: As the frequency of a signal is changed, what happens to signal loss in a transmission line?
B-006-004-006: Assuming the same transmitter and RF output power are used, what is the effect of changing the transmission line from RG-213 coaxial cable to RG-58?
B-006-004-007: The lowest loss transmission line on HF is:
B-006-004-008: In what values are RF transmission line losses expressed?
B-006-004-009: If the length of a coaxial transmission line is increased from 20 metres to 40 metres, how would this affect the line loss?
B-006-004-010: If the operating frequency is increased, how does the transmission line loss change?
An SWR reading of 1:1 means the best impedance match has been attained. All forward power is absorbed by the antenna; no power is reflected. This is the ideal. An SWR reading of less than 1.5:1 means a fairly good impedance match -- most transceivers operate happily at or below this level.
An SWR meter measures forward and reflected voltage to determine the SWR. It is placed in series with the transmission line, usually between the transceiver and the antenna feed line. The meter compares the voltage of the signal going toward the antenna with the voltage of the signal reflected back.
If the characteristic impedance of the transmission line does not match the antenna input impedance, standing waves are produced in the transmission line. A high SWR reading can be caused by an open or short circuit in the antenna system -- broken connections, corroded connectors, or a damaged antenna element. Erratic SWR readings (jumping around unpredictably) are most likely caused by an intermittent connection in the antenna system -- a loose connector, corroded joint, or broken wire that makes and breaks contact in the wind.
The main adverse effect of high SWR is increased transmission line loss. Reflected power travels back and forth in the cable, being attenuated each time -- more power is wasted as heat in the cable. Standing waves on a transmission line result in reduced transfer of RF energy to the antenna.
An antenna analyzer is the instrument most useful for adjusting the physical length of an antenna. It shows SWR, impedance, and resonant frequency without needing to transmit. An antenna analyzer provides the SWR of the antenna system over a range of frequencies, helping you find the resonant point and evaluate bandwidth.
B-006-005-001: What does an SWR reading of 1:1 mean?
B-006-005-002: What does an SWR reading of less than 1.5:1 mean?
B-006-005-003: What is the most likely cause of erratic readings on an SWR meter?
B-006-005-004: Which of the following can cause a high SWR reading?
B-006-005-005: What is the main adverse effect due to operating with high SWR?
B-006-005-006: What instrument is useful in adjusting the physical length of an antenna?
B-006-005-007: If the characteristic impedance of the transmission line does not match the antenna input impedance then:
B-006-005-008: The result of the presence of standing waves on a transmission line is:
B-006-005-009: What does an SWR meter measure to determine the SWR?
B-006-005-010: What information can be obtained with an antenna analyzer?
B-006-005-011: What is the effect of line loss on the SWR reading at the station?
An antenna tuner matches a transceiver to a mismatched antenna system. It transforms the impedance seen at the end of the feed line to the 50 ohms the transceiver expects. Without a tuner, excessive impedance mismatch between transceiver and transmission line will cause a modern solid-state HF transceiver to automatically reduce power (fold-back) to protect its output transistors.
An antenna tuner compensates for impedance mismatch by adding capacitive or inductive reactance using variable capacitors and inductors. These components cancel out the reactive component of the impedance and transform the resistive component to 50 ohms. For maximum power delivery when both source and load impedances are purely resistive, the load impedance should equal the source impedance -- this is the maximum power transfer theorem.
An end-fed half-wave antenna (EFHW) has a very high feed point impedance. A transformer can provide a good match to 50-ohm coaxial cable. A transformer for impedance matching at radio frequencies can be designed to work over a wide bandwidth, making it versatile for multi-band operation.
The advantage of locating an antenna tuner near the antenna feed point (rather than at the transceiver) is less transmission line loss. When the tuner is at the antenna, the coax between the tuner and transceiver carries matched (low SWR) power, minimizing loss. Impedance matching should be done at the junction between the transmission line and antenna to minimize transmission line losses.
An antenna tuner (external or internal) is frequently used with modern solid-state transceivers because it enables the transceiver to deliver rated power to a mismatched antenna system without triggering the protective power foldback.
If a 50-ohm line feeds a folded dipole with a feed point impedance close to 300 ohms, the impedance transformation ratio needed is 6:1 (300 / 50 = 6).
Problem: A transmission line with characteristic impedance of 50 ohms feeds a folded dipole with a feed point impedance close to 300 ohms. What impedance transformation ratio is needed?
Step 1: Identify the values: Zload = 300 ohms, Zline = 50 ohms.
Step 2: Calculate the ratio:
$$\frac{300}{50} = 6$$Answer: A 6:1 impedance transformation ratio is needed.
B-006-006-001: Which of the following antenna system conditions will cause a modern solid-state HF transceiver to automatically reduce power?
B-006-006-002: What does an antenna tuner do?
B-006-006-003: An end-fed half-wave antenna (EFHW) has a very high feed point impedance. What device could be used to provide a good match to 50-ohm coaxial cable?
B-006-006-004: If both source and load impedances are purely resistive, what value of load impedance will result in maximum power delivery to the load?
B-006-006-005: What is the advantage of locating an antenna tuner near the antenna feed point, over locating it near the transceiver?
B-006-006-006: How does an antenna tuner compensate for an impedance mismatch in an antenna system?
B-006-006-007: What advantage does a transformer present when used for impedance matching at radio frequencies?
B-006-006-008: Where does impedance matching need to be done to minimize transmission line losses in an antenna system?
B-006-006-009: If an antenna is correctly matched to a transmission line, the length of the transmission line:
B-006-006-010: Why is an antenna tuner (external or internal) frequently used with modern solid-state transceivers?
B-006-006-011: If a transmission line with a characteristic impedance of 50 ohms feeds a folded dipole with a feed point impedance close to 300 ohms, what impedance transformation ratio is needed to match the two?
Polarization refers to the orientation of the electric field of a radio wave relative to the Earth's surface. Horizontal polarization means the electric lines of force are parallel to the Earth's surface. Vertical polarization means the electric lines of force are perpendicular to the Earth's surface. The polarization of an antenna is determined by the orientation of the electric field relative to the Earth's surface.
HORIZONTAL POLARIZATION VERTICAL POLARIZATION
Electric field (E) Electric field (E)
-------------------> ^
-------------------> |
-------------------> |
===================== Ground ===================== Ground
(E-field parallel to ground) (E-field perpendicular to ground)
Antenna orientation: Antenna orientation:
|==================| |
(horizontal dipole) |
|
(vertical whip)
A horizontal dipole produces horizontally polarized waves. A vertical antenna produces vertically polarized waves. A dipole antenna will emit a vertically polarized wave if it is mounted vertically. A Yagi antenna with its elements parallel to the Earth's surface produces horizontal electromagnetic wave polarization. A half-wavelength antenna perpendicular to the Earth's surface has vertical electromagnetic wave polarization.
An isotropic antenna (isotropic radiator) is a hypothetical point source that radiates equally in all directions. It does not exist in practice but serves as a reference for measuring antenna gain. The three-dimensional radiation pattern of an isotropic radiator is a sphere -- equal radiation intensity in every direction.
VHF signals from a mobile station using a vertical whip antenna will normally be best received using a vertical ground-plane antenna. Matching polarization maximizes signal transfer. Compared with a horizontal antenna, a vertical antenna will receive a vertically polarized radio wave at higher strength -- matching polarization can provide 20 dB or more advantage.
B-006-007-001: What does horizontal wave polarization mean?
B-006-007-002: What does vertical wave polarization mean?
B-006-007-003: What electromagnetic wave polarization does a Yagi antenna have when its elements are parallel to the Earth's surface?
B-006-007-004: What electromagnetic wave polarization does a half-wavelength antenna have when it is perpendicular to the Earth's surface?
B-006-007-005: Polarization of an antenna is determined by:
B-006-007-006: An isotropic antenna is:
B-006-007-007: What is the three-dimensional radiation pattern of an isotropic radiator?
B-006-007-008: VHF signals from a mobile station using a vertical whip antenna will normally be best received using a:
B-006-007-009: A dipole antenna will emit a vertically polarized wave if it is:
B-006-007-010: If an electromagnetic wave leaves an antenna vertically polarized and reaches the receiving location by ground wave, what will be its final polarization?
B-006-007-011: Compared with a horizontal antenna, a vertical antenna will receive a vertically polarized radio wave:
Radio waves travel at the speed of light: 300,000 kilometres per second. The speed of a radio wave is the same as the speed of light -- this is a fundamental property of all electromagnetic waves, and it is constant regardless of frequency. The relationship between wavelength and frequency is:
Problem: What is the wavelength in free space of a 25 MHz signal?
$$\lambda = \frac{300}{25} = 12 \text{ metres}$$Answer: 12 metres. (B-006-008-003)
Problem: What is the wavelength in free space of a 2 MHz signal?
$$\lambda = \frac{300}{2} = 150 \text{ metres}$$Answer: 150 metres. (B-006-008-011)
An antenna's resonant frequency is determined by its physical (electrical) length. There is an inverse relationship: longer antenna = lower frequency; shorter antenna = higher frequency. To change a wire dipole's resonant frequency from 3900 kHz down to 3600 kHz (lower), make it longer. To change from 3600 kHz up to 3900 kHz (higher), make it shorter. The resonant frequency of an antenna may be increased by shortening the radiating element, and lowered by lengthening it.
Adding a series inductance (loading coil) to an antenna decreases the resonant frequency. The coil makes the antenna electrically longer without being physically longer. This is particularly useful for mobile antennas where physical length is limited.
A trap is a coil and capacitor in parallel (a parallel LC circuit). Traps are inserted in antenna elements to block current at specific frequencies, allowing one antenna to operate on multiple bands. At the trap's resonant frequency, it presents high impedance, effectively "disconnecting" the wire beyond it. Below the trap frequency, current flows through to the full length.
TRAP IN AN ANTENNA ELEMENT
<---- Wire ----| TRAP |---- Wire ---->
+--||--+
Detail: -----+ +-----
+-)()(--+
C L
(Parallel LC circuit)
At the trap's resonant frequency, it presents high impedance,
effectively "disconnecting" the wire beyond it.
Below the trap frequency, current flows through to the full length.
Insulators at the ends of a suspended wire antenna serve to limit the electrical length of the antenna. They define where the antenna element ends and the support rope begins, preventing current from flowing onto the support structure.
B-006-008-001: A wire dipole has a resonant frequency of 3900 kHz. How can you change its resonant frequency to 3600 kHz?
B-006-008-002: A wire dipole has a resonant frequency of 3600 kHz. How can you change its resonant frequency to 3900 kHz?
B-006-008-003: What is the wavelength in free space of a 25 MHz signal?
B-006-008-004: The velocity of propagation of radio frequency energy in free space is:
B-006-008-005: Adding a series inductance to an antenna would:
B-006-008-006: The resonant frequency of an antenna may be increased by:
B-006-008-007: The speed of a radio wave:
B-006-008-008: Why are insulators used at the ends of a suspended wire antenna?
B-006-008-009: To lower the resonant frequency of an antenna, the operator should:
B-006-008-010: Some antennas are constructed with traps. What is a trap?
B-006-008-011: What is the wavelength in free space of a 2 MHz signal?
Antenna gain is the ratio of the radiated signal strength of an antenna to that of a reference antenna (usually an isotropic radiator or a dipole). Gain does not create power -- it focuses existing power in a preferred direction. Antenna gain, especially on VHF and above, is quoted in dBi. The "i" stands for isotropic -- gain relative to an isotropic radiator.
Problem: An antenna has a gain of 4.1 dBi. How much gain is this over a half-wave dipole?
Step 1: A half-wave dipole has 2.1 dBi gain.
Step 2: Subtract the dipole's gain from the antenna's gain:
$$4.1 \text{ dBi} - 2.1 \text{ dBi} = 2.0 \text{ dB over a dipole (dBd)}$$Answer: 2.0 dB gain over a dipole. (B-006-009-011)
A parasitic antenna element is energized by induction or radiation from a driven element. It has no direct electrical connection to the transmission line. The directivity of a half-wave dipole can be increased by adding one or more parasitic elements -- this is how a Yagi-Uda antenna is created.
Adding a slightly shorter parasitic element (a director) to a dipole increases radiation strength from the dipole towards the new element. The director "pulls" the pattern toward itself. Adding a slightly longer parasitic element (a reflector) increases radiation strength from the new element towards the dipole. The reflector "pushes" the pattern away from itself.
The property of an antenna that defines the range of frequencies to which it will respond is called its bandwidth. Antenna bandwidth is the frequency range over which the antenna may be expected to perform well (typically defined as SWR below 2:1).
In free space, the radiation pattern of a half-wave dipole has maximum radiation broadside from the antenna (perpendicular to the wire), with nulls off the ends. The 3D pattern is a torus (donut shape).
HALF-WAVE DIPOLE RADIATION PATTERN (Top View)
^ Maximum radiation
/ \
/ \
/ \
/ \
Null <-----------=====-----------> Null
(off end) \ DIPOLE / (off end)
\ /
\ /
\ /
\ /
v Maximum radiation
3D: Rotate this pattern around the dipole axis
to get the "donut" (torus) shape.
B-006-009-001: How is a parasitic antenna element energized?
B-006-009-002: How can the directivity of a half-wave dipole be increased?
B-006-009-003: If a half-wave dipole is converted to a Yagi by adding a slightly shorter parasitic element, in what direction(s) does the radiation strength increase?
B-006-009-004: If a half-wave dipole is converted to a Yagi by adding a slightly longer element, in what direction(s) does the radiation strength increase?
B-006-009-005: The property of an antenna that defines the range of frequencies to which it will respond, is called its:
B-006-009-006: What is the approximate gain of a half-wave dipole in free space relative to an isotropic radiator?
B-006-009-007: What is meant by antenna gain?
B-006-009-008: What is meant by antenna bandwidth?
B-006-009-009: In free space, what is the radiation pattern of a half-wave dipole?
B-006-009-010: The gain of an antenna, especially on VHF and above, is quoted in dBi. The "i" in this expression stands for:
B-006-009-011: An antenna is said to have a gain of 4.1 dBi. How much gain is this over a half-wave dipole antenna?
QUARTER-WAVE VERTICAL WITH GROUND PLANE
^
|
| Vertical radiator
| (quarter wavelength)
|
|
-------------+------------- <-- Feed point
/ / | \ \
/ / | \ \ Radials (quarter wavelength)
/ / v \ \
(ground)
Coax: Centre conductor --> vertical element
Shield --> radials
ELEVATED GROUND PLANE MOBILE VERTICAL WITH LOADING COIL
(Downward-sloping radials)
^ ^
| | Whip
| Radiator |
| +---------+
-----+----- Feed | Loading |
\ | / | Coil |
\ | / Radials slope +---------+
\ | / ~45 degrees |
\|/ downward | Lower section
v ======+====== Car roof
(roof acts as ground plane)
To calculate the approximate length in metres of a quarter-wavelength antenna for frequencies below 30 MHz, divide 71.3 by the operating frequency in MHz. For a half-wave vertical at VHF, use 143/f instead.
Problem: What is the length of a quarter-wave vertical for 21.125 MHz?
$$L = \frac{71.3}{21.125} = 3.37 \text{ metres}$$Answer: 3.37 metres. (B-006-010-002)
Problem: If you made a half-wavelength vertical antenna for 223 MHz, approximately how long would it be?
$$L = \frac{143}{223} = 0.641 \text{ metres} = 64.1 \text{ cm} \approx 67 \text{ cm}$$(Using 143/f for a half-wave vertical at VHF.) (B-006-010-003)
A quarter-wavelength vertical antenna on the roof of a car sends out energy equally well in all horizontal directions (omnidirectional in the horizontal plane). This makes verticals ideal for mobile use where you do not know where the other station is. A five-eighths wavelength vertical antenna is better than a quarter-wavelength vertical for VHF or UHF mobile operations because it has more gain -- the 5/8-wave design concentrates more radiation toward the horizon.
Downward sloping radials on a ground plane antenna bring the feed point impedance closer to 50 ohms, providing a better match to standard coaxial cable. Horizontal radials give about 36 ohms; drooping them at approximately 45 degrees raises the impedance to about 50 ohms. For an elevated quarter-wave vertical to match a 50-ohm coaxial cable, use downward sloping quarter-wave radials. The best match to the base of a quarter-wave ground-plane antenna comes from 50-ohm coaxial cable (when using drooping radials).
A vertical antenna that is only 2 metres long can be made to resonate in the 80-metre band by installing an inductor in series with the antenna. The loading coil adds the missing electrical length. A loading coil is often used with an HF mobile vertical antenna to tune out capacitive reactance -- a short antenna looks capacitive, and the loading coil (inductance) cancels this.
B-006-010-001: How do you calculate the approximate length in metres of a quarter-wavelength antenna for use on frequencies below 30 MHz?
B-006-010-002: If you made a quarter-wavelength vertical antenna for 21.125 MHz, approximately how long would it be?
B-006-010-003: If you made a half-wavelength vertical antenna for 223 MHz, approximately how long would it be?
B-006-010-004: Why is a five-eighths wavelength vertical antenna better than a quarter-wavelength vertical antenna for VHF or UHF mobile operations?
B-006-010-005: If a quarter-wavelength vertical antenna is placed on the roof of a car, in what direction does it send out radio energy?
B-006-010-006: What is an advantage of downward sloping radials on a ground plane antenna?
B-006-010-007: What configuration of radials will match an elevated quarter-wave vertical antenna to a 50-ohm coaxial cable?
B-006-010-008: Which of the following transmission lines will give the best match to the base of a quarter-wave ground-plane antenna?
B-006-010-009: How can a vertical antenna, 2 metres in length, be made to resonate in the 80-metre band for mobile use?
B-006-010-010: Why is a loading coil often used with an HF mobile vertical antenna?
B-006-010-011: When using a ground mounted vertical HF antenna, what can you do to reduce ground losses?
3-ELEMENT YAGI-UDA ANTENNA (Top View)
Direction of maximum radiation ------------------->
REFLECTOR DRIVEN ELEMENT DIRECTOR
(longest) (fed by coax) (shortest)
| | |
| | |
|==+================| | |=============| | |==========|
| |==+=================| |
| | |
| | |
| | |
===+====================+====================+=== <-- Boom
| | |
|
Feed point
(coax attaches)
Element lengths for a given frequency f:
+------------------+------------------------------+
| Reflector | ~5% LONGER than driven |
| Driven element | ~half wavelength = 143/f(MHz)|
| Director | ~5% SHORTER than driven |
+------------------+------------------------------+
Element spacing: ~0.15 to 0.25 wavelength between elements
Optimal compromise for 3 elements: 0.20 wavelength spacing
The driven element of a Yagi is approximately a half wavelength long: \(L = \frac{143}{f_{\text{MHz}}}\) metres. The director is about 5% shorter, and the reflector is about 5% longer.
Problem: What is the approximate length of the driven element of a Yagi antenna for 14.0 MHz?
$$L_{\text{driven}} = \frac{143}{14.0} = 10.21 \text{ metres}$$Answer: 10.21 metres. (B-006-011-002)
Problem: What is the approximate length of the director element of a Yagi antenna for 21.1 MHz?
Step 1: Calculate the driven element length:
$$L_{\text{driven}} = \frac{143}{21.1} = 6.78 \text{ m}$$Step 2: The director is ~5% shorter:
$$L_{\text{director}} \approx 6.78 \times 0.95 = 6.44 \text{ metres}$$Answer: Approximately 6.44 metres. (B-006-011-003)
Problem: What is the approximate length of the reflector element of a Yagi antenna for 28.1 MHz?
Step 1: Calculate the driven element length:
$$L_{\text{driven}} = \frac{143}{28.1} = 5.09 \text{ m}$$Step 2: The reflector is ~5% longer:
$$L_{\text{reflector}} \approx 5.09 \times 1.05 = 5.34 \text{ metres}$$Answer: Approximately 5.34 metres. (B-006-011-004)
Increasing the boom length and adding directors to a Yagi antenna causes gain to increase. More directors means more signal focusing in the forward direction. The major advantage of increasing element spacing on a Yagi antenna is higher gain -- wider spacing (up to about 0.2-0.25 wavelength) allows each element to interact more effectively.
The bandwidth of a Yagi antenna can be increased by increasing the diameter of the elements. Fatter elements have a broader frequency response.
Antenna front-to-back ratio is the ratio of the power radiated in the forward direction to the power radiated in the opposite direction. A high front-to-back ratio means the antenna strongly rejects signals from behind.
Yagi antennas are often used on HF bands from 20 metres to 10 metres because rotatable high-gain antennas become feasible due to shorter element lengths. At lower frequencies (40m, 80m), the elements become impractically large for rotatable installations.
A single Yagi antenna can function on the 20-metre, 15-metre, and 10-metre bands by using element traps. Traps electrically shorten the elements at higher frequencies. If the forward gain of a six-element Yagi is about 10 dBi, stacking two of them yields approximately 13 dBi. Stacking adds about 3 dB (doubling the effective aperture).
B-006-011-001: What design feature allows a single Yagi antenna to function on the 20-metre, 15-metre and 10-metre bands?
B-006-011-002: What is the approximate length of the driven element of a Yagi antenna for 14.0 MHz?
B-006-011-003: What is the approximate length of the director element of a Yagi antenna for 21.1 MHz?
B-006-011-004: What is the approximate length of the reflector element of a Yagi antenna for 28.1 MHz?
B-006-011-005: What is one effect of increasing the boom length and adding directors to a Yagi antenna?
B-006-011-006: What is the major advantage of increasing element spacing on a Yagi antenna?
B-006-011-007: Why are Yagi antennas often used on HF bands from 20 metres to 10 metres?
B-006-011-008: What does "antenna front-to-back ratio" mean in reference to a Yagi antenna?
B-006-011-009: How can the bandwidth of a Yagi antenna be increased?
B-006-011-010: For a three-element Yagi antenna, what approximate element spacing (in wavelengths) provides the best compromise between gain and front-to-back ratio?
B-006-011-011: If the forward gain of a six-element Yagi is about 10 dBi, what would the gain of two of these antennas be if they were "stacked"?
HALF-WAVE DIPOLE ANTENNA
Insulator Feed point Insulator
| (centre-fed) |
| | |
Support--+<--- L/4 -------->|<----- L/4 -------->+--Support
| | |
Rope Coax (via Rope
balun ideally)
Total length L = 143 / f(MHz) metres
Current distribution:
------/\-----------/ \-----------/\------
| \ / \ / |
| \ / Max \ / |
| \ / current \ / |
| \ / at centre \ / |
Zero| \/ \/ |Zero
current current
at ends at ends
Feed point impedance: ~73 ohms (free space)
The factor 143 (rather than 150, which would be exactly half a wavelength in free space) accounts for the "end effect" -- the antenna is slightly shorter than a free-space half wavelength due to the capacitance at the wire ends.
Problem: What is the length of a half-wavelength dipole antenna for 28.150 MHz?
$$L = \frac{143}{28.150} = 5.08 \text{ metres}$$Answer: Approximately 5.08 metres. (B-006-012-001)
Problem: If you were to cut a half-wave dipole for 3.75 MHz, what would be its approximate length?
$$L = \frac{143}{3.75} = 38.13 \text{ metres}$$Answer: Approximately 38.13 metres. (B-006-012-009)
The impedance at the feed point of a half-wave dipole in free space is approximately 73 ohms. This is a good natural match to 50-ohm or 75-ohm coaxial cable -- close enough that the SWR stays low without additional matching.
The three-dimensional radiation pattern of a half-wave dipole in free space is a torus (donut shape) around the antenna. Maximum radiation is broadside (perpendicular to the wire); minimum (nulls) are off the ends. A horizontal half-wave dipole with its ends pointing North/South (ignoring ground effects) radiates mostly to the East and West -- broadside to the wire.
HALF-WAVE DIPOLE RADIATION PATTERN (Top View)
^ Maximum radiation
/ \
/ \
/ \
/ \
Null <-----------=====-----------> Null
(off end) \ DIPOLE / (off end)
\ /
\ /
\ /
\ /
v Maximum radiation
3D: Rotate this pattern around the dipole axis
to get the "donut" (torus) shape.
A disadvantage of a random wire antenna is that you may experience RF feedback in your station (RF in the shack). Because the feedpoint impedance is unpredictable, current can flow on the outside of the coax shield or on equipment chassis.
A major advantage of an end-fed half-wave antenna (EFHW) is that it is capable of multi-band operation. The EFHW is resonant on all harmonic frequencies (e.g., a 40m EFHW also works on 20m, 15m, and 10m).
A disadvantage of using an antenna equipped with traps is that it may radiate harmonics more readily. The traps can allow harmonic energy to pass at certain frequencies. However, an advantage of using a trap antenna is that it may be used for multi-band operation.
B-006-012-001: If you made a half-wavelength dipole antenna for 28.150 MHz, approximately how long would it be?
B-006-012-002: What is one disadvantage of a random wire antenna?
B-006-012-003: What is the three-dimensional radiation pattern of a half-wavelength dipole in free space?
B-006-012-004: What is the impedance at the feed point of a half-wave dipole in free space?
B-006-012-005: Ignoring ground effects, what is the radiation pattern of a horizontal half-wave dipole installed with the ends pointing North/South?
B-006-012-006: What is a major advantage of an end-fed half-wave antenna (EFHW)?
B-006-012-007: What is a disadvantage of using an antenna equipped with traps?
B-006-012-008: What is an advantage of using a trap antenna?
B-006-012-009: If you were to cut a half-wave dipole for 3.75 MHz, what would be its approximate length?
QUAD ANTENNA DELTA LOOP ANTENNA
(Square Loop) (Triangular Loop)
+-------------------+ /\
| | / \
| | / \
| | / \
| ~L/4 | / ~L/3 \
| per side | / \
| | / \
| | / \
+---------+---------+ /________________\
| |
Feed point Feed point
(bottom centre (bottom centre
= horizontal = horizontal
polarization) polarization)
Total wire = 1 wavelength Total wire = 1 wavelength
QUAD WITH PARASITIC ELEMENT -- Side View
Driven Reflector
element (~5% larger)
+--+ +--+
| | | |
| | | |
| | | |
+--+ +--+
| |
+-- 0.15-0.2L --+
spacing
Direction of maximum radiation: --->
(from reflector toward driven element)
A quad antenna consists of two or more parallel four-sided wire loops, each approximately one wavelength long. It typically has a driven element and one or more parasitic elements (reflector, directors). The driven element of a quad antenna has an approximate overall length of one wavelength.
A delta loop antenna with parasitic elements is an antenna consisting of multiple elements, each a triangular loop whose total length is approximately one wavelength. The approximate overall length of a delta loop antenna is also one wavelength.
The driven element of a loop antenna (quad or delta) is one full wavelength. The formula uses 306 (rather than 300) to account for velocity effects in the wire:
Problem: What is the approximate length of the driven element of a quad antenna designed for 21.4 MHz?
$$L = \frac{306}{21.4} = 14.30 \text{ metres}$$Answer: 14.30 metres. (B-006-013-003)
Problem: What is the approximate length of the driven element of a quad antenna designed for 14.3 MHz?
$$L = \frac{306}{14.3} = 21.40 \text{ metres}$$Answer: 21.40 metres. (B-006-013-004)
Problem: What is the approximate length of a delta loop antenna designed for 28.7 MHz?
$$L = \frac{306}{28.7} = 10.66 \text{ metres}$$Answer: 10.66 metres. (B-006-013-005)
Problem: What is the approximate length of the wire for a horizontal loop tuned to 7.15 MHz?
$$L = \frac{306}{7.15} = 42.80 \text{ metres}$$Answer: 42.80 metres. (B-006-013-009)
The feed point location on a loop antenna determines its polarization. For horizontal polarization with an HF delta loop (bottom element parallel to ground), locate the feed point in the centre of the bottom element. Moving the feed point of a quad antenna from a side parallel to the ground to a side perpendicular to the ground will change the antenna polarization from horizontal to vertical.
QUAD LOOP: Feed Point Determines Polarization
HORIZONTAL POLARIZATION: VERTICAL POLARIZATION:
+-------------------+ +-------------------+
| | | |
| | +-- Feed |
| | | point |
| | | (side) |
| | | |
+---------+---------+ +-------------------+
|
Feed point
(bottom centre)
B-006-013-001: What is a quad antenna?
B-006-013-002: What is a delta loop antenna with parasitic elements?
B-006-013-003: What is the approximate length of the driven element of a quad antenna designed for 21.4 MHz?
B-006-013-004: What is the approximate length of the driven element of a quad antenna designed for 14.3 MHz?
B-006-013-005: What is the approximate length of a delta loop antenna designed for 28.7 MHz?
B-006-013-006: What is a major disadvantage of a quad antenna, as compared to a Yagi antenna with the same number of elements and boom length?
B-006-013-007: You are constructing an HF delta loop antenna. It is oriented with the bottom element parallel to the ground. Where should you locate the feed point for horizontal polarization?
B-006-013-008: Moving the feed point of a quad antenna from a side parallel to the ground to a side perpendicular to the ground will have what effect?
B-006-013-009: What is the approximate length of the wire for a horizontal loop tuned to 7.15 MHz?
B-006-013-010: The quad antenna consists of two or more square loops of wire. The driven element has an approximate overall length of:
B-006-013-011: What is the approximate overall length of a delta loop antenna?
| What | Formula | Example |
|---|---|---|
| Wavelength | \(\lambda = \frac{300}{f_{\text{MHz}}}\) | 25 MHz: 300/25 = 12 m |
| Half-wave dipole | \(L = \frac{143}{f_{\text{MHz}}}\) metres | 28.15 MHz: 143/28.15 = 5.08 m |
| Quarter-wave vertical | \(L = \frac{71.3}{f_{\text{MHz}}}\) metres | 21.125 MHz: 71.3/21.125 = 3.37 m |
| Full-wave loop (quad/delta) | \(L = \frac{306}{f_{\text{MHz}}}\) metres | 14.3 MHz: 306/14.3 = 21.40 m |
| Yagi driven element | \(L \approx \frac{143}{f_{\text{MHz}}}\) metres | 14.0 MHz: 143/14.0 = 10.21 m |
| Speed of radio waves | 300,000 km/s = speed of light | -- |
| Connector | Use | Remember |
|---|---|---|
| PL-259/SO-239 | HF transceivers | Most common HF connector |
| SMA | Handheld radios | Small = handheld |
| BNC | Test equipment, low-power | Bayonet = quick bench use |
| N-type | Outdoor/tower | Moisture resistant |
| Type | Z0 | Loss | Balanced? | Key Fact |
|---|---|---|---|---|
| RG-58 | 50 ohms | High | No | Thin coax, short runs only |
| RG-213 | 50 ohms | Moderate | No | Standard station coax |
| Twin-lead | 300 ohms | Low | Yes | Flat ribbon, TV use |
| Window line | 300-450 ohms | Very low | Yes | Needs balun to coax |
| Open-wire | 450-600 ohms | Lowest | Yes | Can run at high SWR |
| Antenna | Gain (dBi) | Pattern | Polarization | Feed Z |
|---|---|---|---|---|
| Isotropic | 0 | Sphere | N/A (theoretical) | N/A |
| Half-wave dipole | 2.1 | Torus (donut) | Depends on mounting | 73 ohms |
| Quarter-wave vertical | ~2-3 | Omnidirectional (horiz.) | Vertical | 36-50 ohms |
| 5/8-wave vertical | ~3-4 | Omnidirectional, lower angle | Vertical | Needs matching |
| 3-element Yagi | ~7-8 | Directional | Element orientation | ~25 ohms |
| Quad loop | ~7-8 | Directional | Feed point position | ~100 ohms |