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Transmission lines are used the convey RF signals from transmitters to antennas or vice versa. Different types of transmission lines carry signals in different ways. Signals may be balanced, unbalanced or guided.
Balanced signals are conveyed in two separate wires where the current in one wire is always traveling in the opposite direction to the current in the second wire.
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| A balanced signal in a pair of wires. |
This causes the magnetic fields in the two wires to cancel each other, resulting in no electromagnetic radiation from the line. Also, any outside interference (such as electromagnetic interference [EMI]) has currents traveling in the same direction in both wires.
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| Noise induced from outside sources affects the wires equally. The difference between the wires remains the same. It is the difference that constitutes the signal carried from one end of the cable to the other. |
Noise induced from outside sources affects the wires equally. The difference between the wires remains the same. It is the difference that constitutes the signal carried from one end of the cable to the other.
The termination of a transmission line with a balanced signal is always a transformer or antenna that cancels any signal that is the same in both wires. This results in any outside interference to be eliminated at the termination of the line.
Unbalanced signals are conveyed with the signal in one wire and the other wire acting as a common connection, usually ground. Radiation from the transmission line and immunity to outside interference is achieved by surrounding the signal wire with the ground wire (coaxial cable).
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| Coaxial cable |
A guided signal is conveyed with the signal contained in a tube which also shields the signal from outside interference. This tube is called a waveguide.
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| A cross section of a waveguide (split to show internal wave components of the wave). |
A transmission line of any type acts like a series of inductors and capacitors. An inductor is a coil of wire, but even a straight wire has some inductance. A capacitor is two conductors separated by an insulator. A transmission line is two wires separated by insulation and therefore has capacitance. The capacitance and inductance of a transmission line combine to act as the following circuit.
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| The electric model of a transmission line |
In the transmission line, the voltages and currents balance in such a way that the line exhibits a characteristic impedance. This impedance is the ratio of voltages and currents traveling along the line. It is determined by the diameters of the wires and the distances between them. This cannot be measured with an ohm meter; the resistance between the two conductors should measure as infinite ohms. If an ohmmeter placed between the inner and outer conductors of a coaxial cable reads anything other than infinite ohms, the transmission line is faulty.
Assuming a transmission line is terminated with an antenna that has the same impedance, the impedance of the transmission line does not change with the length of the line. The transmission line and antenna constitute a continuous system that balances to a given impedance. For example, a 50-ohm transmission line terminated with a 50-ohm antenna presents 50 ohms to the transmitter, regardless of the length of the transmission line.
Although there is voltage, current and impedance in a transmission line, this impedance is reactive (inductance and capacitance). Therefore, like a transformer, transmission lines produce no heat, mechanical motion or electromagnetic radiation and therefore don't dissipate power.[1] If the impedance of the transmission line matches the impedance of the antenna, the product of voltage and current inserted into the transmission line passes through to the end where it is dissipated as electromagnetic radiation from the antenna.
Recall that to get the maximum power transfer from a circuit to a load, the
load impedance must match the output impedance of the driving circuit. With a
radio transmitter, you want to transfer the maximum power from the transmitter
to the antenna, where the power is dissipated as radio waves. For maximum
efficiency, the antenna impedance must match the transmitter output impedance.
As mentioned above, if the transmission line impedance matches the antenna
impedance, the transmitter sees an impedance equal to the antenna impedance. For
example, if you have a 50-ohm antenna attached to a 50-ohm transmission line,
the transmitter sees an impedance of 50 ohms.
If the antenna impedance
doesn't match the transmission line impedance, some of the power is unable to
leave the antenna as radiation. This energy returns to the transmitter as a
"reflected" wave.
A standing wave is a wave that is not moving. If you have a taut string, such as on a violin, guitar or piano, and pluck the string, a wave travels down the string. When the wave hits the end of the string, it reflects back. This wave continues back and forth along the string. The original wave and reflected waves combine in such a way that you see the string vibrate from side to side. The combined waves moving back and forth along the string appear as a stationary wave. This is a standing wave.
In a vibrating string, you might see a single wave where the middle of the string moves from side to side and the ends are not moving. This happens when the length of the string is equal to ½ of the wavelength. If the length of the string is equal to one wavelength you get two standing waves; the ends of the string are stationary and there is a stationary point in the middle of the string. The standing waves take up each half of the string (the point where the string is not moving is called a node). If the length of the string is 1½ times the wavelength, there are three standing waves with two nodes. If you look along the string, you find repeated areas of high vibration and no vibration.
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| Standing waves on a string. As the frequency doubles, then triples, the wavelength becomes ½ then ⅓ of the original (this makes the string ½, 1 then 1½ wavelengths). This causes one, then two, then three standing waves on the string. |
If a transmission line does not match the impedance of the transmitter, receiver or antenna, waves reflect from the ends. This causes standing waves in the transmission line. A standing wave in a transmission line is manifested by repeated areas of high and low voltage along the cable.
Just as the standing wave on the guitar string imparts soundwaves to the air, standing waves in a transmission line impart radio waves to spacetime. The problem is, you don't want radio waves emanating from the transmission line. You want the waves to transfer entirely to the antenna, which is designed to radiate radio waves; you want your antenna to radiate, not your transmission line. Standing waves in the transmission line also return power to the transmitter. This can cause components to overheat or develop too much voltage. Overheating can damage components and overvoltage can result in signal distortion as well as component damage.
The distance between the conductors of a transmission line must remain constant throughout its run. This maintains an even impedance along the line. Any deviation in the geometry causes reflected waves with their concurrent problems.
The velocity factor is the speed of signals in a transmission line compared to the speed of light in a vacuum. A typical velocity factor is 60. This means that the speed of the signal in the transmission line is 60% of the speed of light in a vacuum.
The relative permittivity (often imprecisely called the dielectric constant) of the insulating material is primarily responsible for the velocity factor. For example, polyethylene insulated twin-lead has a velocity factor of approximately 80 and ladder line (which has air as the insulator) has a velocity factor of 90 to 99.
Twin-lead consists of two wires that are held a specific distance apart. The most common type of twin-lead uses 20 gauge or 22 gauge wire that is held about 7.5mm apart by a rigid continuous spacer of high-density polyethylene. This is your typical "TV" twin lead. It has an impedance of 300 ohms. High-quality twin-lead of this type has a significant portion of the spacer cut-out, leaving "windows" in the flat part of the line. This is to reduce loses caused by heating in the polyethylene as its molecules are polarized by the alternating electromagnetic fields (the dielectric effect).
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| Typical 300-ohm twin lead attached to a TV balun. |
Another type of twin-lead is called ladder line. It consists of two wires that are held apart by plastic or ceramic spacers. Ladder line is usually about one-inch across and has spacers about every six to eight inches. The impedance is usually 600 ohms. Ladder line has less dielectric loss than twin-lead or window line.
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| 600-ohm ladder line leading to a dipole antenna |
600-ohm ladder line leading to a dipole antenna
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| 450-ohm window line. This is a hybrid made from twin lead with cutouts. |
Coaxial cable consists of one wire that is surrounded by a cylinder of braided wire. The cylinder may also be augmented with aluminum foil or may even be a complete tube of copper or aluminum.
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| Coaxiallcablecable |
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A. Outer jacket |
Most coaxial cable has an insulating material between the outer conductor and inner conductor. Coaxial cable that has a rigid tube for the outer conductor may have plastic spacers to maintain proper spacing. This air-spaced cable has less dielectric loss (see twin-lead above) than cable with solid polyethylene or polyethylene foam insulation.
Coaxial cable used for cable television has a copper clad steel core. The skin effect pushes the electric currents away from the center of the core. A steel core with a thick copper cladding doesn't affect the conductivity. This allows a stiff cable that resists kinking.
The most common types of coaxial cable are:
Type |
Physical Size |
Impedance |
Use |
| RG-6 | 6.9mm | 75 ohms | Cable TV |
| RG-8 | 10.3mm | 50 | CB and Ham radio (base stations), Ethernet (Thicknet).[2] |
| RG-58 | 5mm | 50 | CB and Ham radio (mobile stations), Ethernet (Thinnet). |
| RG-59 | 6.1mm | 75 | Closed-circuit TV |
A waveguide is a derivative of twin lead. It is typically a tube with a rectangular cross-section where the small dimension is one-quarter wavelength and the long dimension is one-half wavelength. Electromagnetic waves travel as an interaction between the air inside the waveguide and the metallic surface. This is much like the light in single-mode fiber optics.
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| A J-Band (10 GHz to 20 GHz) radar waveguide |
Waveguides are only suitable for high frequencies where the wavelengths keep dimensions manageable. For example, a waveguide for a frequency of 100MHz would have a cross-section of approximately 1.5 meters by .75 meters, large enough to crawl into. Typical waveguides work at frequencies above 1GHz where the cross-sectional size of the large dimension is less than 10cm.
1. In computer networking, you may read about the effects of standing waves in network cables as if standing waves are expected there. They're not. Standing wave are just as bad in network cables
2. In the 1960s and 1970s, there was a myth in citizens band and amateur radio that your transmission line must have a length that is some multiple of 1/2 of the wavelength of the operating frequency. It was untrue.
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