Have you ever thought about the importance of the transmission line between a radio and antenna? It plays a more important part than you might think.
Last issue we looked all too briefly at radio paths and antennas. We explained that transmitters generate RF energy at a required frequency and amplitude, that is fed via a cable called a transmission line, to an antenna. The RF passes through the signal path to the second antenna, down the transmission line and into the receiver. Transmitters, receivers, transceivers, transmission lines, and antennas all have a characteristic impedance at their connections for maximum power transfer. The characteristic impedance for free space is 120πΩ, a dipole is ~72Ω, and common transmission lines are 50Ω or 75Ω.
Radio is something I and many others have studied for most of our lives. Rest assured that there are a lot of Amateur Radio types that only barely understand radio, and of course many others who live and breathe radio!
In this issue we intend to continue on with just the radio path, and the transmission of RF energy from the transmitter to the receiver. That path includes the Transmission Lines and the Antennas, which will continue to be our main focus.
If you can understand a garden hose that carries water from a tap to the sprinkler, or water gun, then you have the basic concept of a transmission line. Modern transmission lines may even look like a garden hose, although transmission lines that you might see are not as thick!
What you may never have considered however is how the garden hose actually works. Even if it is full of water, the sprinkler won't work. The sprinkler requires both pressure and flow, as does the transmission line. RF energy with a suitable pressure will flow to the Antenna, or whatever load we present to the transmission line.
The garden hose must contain the water, as leaks are generally not appreciated. The RF transmission line must similarly contain the RF energy, which is much more difficult to contain as RF can pass through anything, although metals only allow a shallow depth before reflection or absorption occurs.
In transmission lines it is the electromagnetic field that helps keep the RF inside the cable. Old TV transmission lines were a flat ribbon with two wires parallel to one another. They have long since gone, but they make a particular point that the transmission line does not need to be coaxial, as in coaxial cable.
In fact, RF will travel across the ground or water using the surfaces as their transmission lines.
What is a Transmission Line?
The easiest transmission line to understand is the parallel wire line, also called an open-wire line when not moulded into plastic. The open-wire line is simply two conductors of a consistent diameter, a fixed distance apart, and away from any metallic surfaces. Many Physics classes have used such a simple pair of wires, 20mm apart and 4mm diameter which provides a 276Ω impedance, chosen for a particular 'Physics' and 'Maths' purpose. A similar and very common open-wire line of the day was 300Ω, which would require 24.43mm distance between the 4mm wires. Old TV 300Ω ribbon by comparison used maybe 0.5mm diameter wires about 10mm apart.
We will give you the formula later on. The important information for now is that the two essential dimensions are the diameter of the wire and the distance between them.
Transmission Line as an “Un-tuned Circuit”
Any wire is an inductor, which seems obvious to anybody who knows what an inductor is; an electromagnet when made into a coil, having inductance and minimal resistance. However, most of us do not stop to realise that every conductor also has capacitance, between its surface and the surface of another conductor, even the Earth itself.
Those long wires, that in theory can go on to infinity, have both inductance in the conductors, and capacitance between the conductors. Both are consistent per unit length. One metre, or one foot, or one kilometre, will have the same inductance and capacitance as any other length of the same unit, e.g. per metre.
If the inductance and capacitance were in parallel, or in series, then a tuned frequency could be calculated. However, the inductance is in series with the eventual load, and the capacitance is in parallel with the eventual load. This results in an “Un-tuned Circuit” which in theory is always the same impedance from the first connection point to the final connection point, as long as it too is a matched impedance e.g. a 50Ω coaxial transmission line terminated with a 50Ω pure impedance (meaning purely resistive and not reactive) will appear as an infinitely long cable.
RF energy sent into the transmission line will continue to the far end, with no losses in a perfect cable, but we will talk about reality in a moment. At the far end the RF, energy will dissipate into the matched load, preferably a perfectly matched antenna, and never be seen again.
Any physical conductor has resistance that increases with its length, and any pair of physical conductors have a high resistance between them that reduces as the length increases. These two resistance values may be insignificant if the cable is suitably short, but become an obstacle when the cable is long, causing unacceptable losses of RF energy.
Therefore, larger diameter cables with thicker conductors result in lower losses, but their dimensions become an issue as the frequency increases, and other losses become another issue.
Dielectric losses in the insulation can result in an upper frequency limit for transmission lines and for that reason, higher frequency transmission lines tend to be smaller in cross-section dimensions.
The dielectric materials used in a cable can result in a lower transmission speed for the RF energy within the cables. Rather than 95% of the speed of light that is sometimes given as the speed of electricity in a wire, the dielectric material can cause the speed of RF in a cable to drop to 66%, for example, in conventional RG-58 coaxial cable.
While 66% of the speed of light may not seem an issue to those of us who cannot imagine such a speed, it becomes an issue when using multiple cables in an installation, and in antenna matching uses of coaxial cables. Next issue we will look into other ways transmission lines are used.
One particular type of transmission line was originally used for telephone lines, and then data lines, until what we now call Cat6 data cable. The twisted pair refers to two insulated wires twisted together to form a transmission line pair. Old phone lines had twisted pair cables a century ago, but as the frequency of data has increased way past the intended use of phone lines, the cables simply had to change. However, audio and low frequency RF, and even video can be sent, at low power levels, over twisted pair cables.
One of the reasons for twisting the cables is to keep a constant distance between the conductors. However, another reason is that the signals in one pair of wires may interfere with another pair of wires. Twisting them together means that every time they are twisted, the wires are swapped, and as the signal travels in different directions in the twisted pair wires, an alternating polarity will cancel out any cross talk over a suitable length.
In higher category cabling, Cat4, Cat5 and Cat6, and even Cat8 to come, the wire diameter and insulation thickness is kept to tighter tolerances, and multiple pairs of cables are twisted at different twist rates (the length of one complete 360° twist.)
We have spoken about the characteristic impedance of a transmission line being important in matching a transmitter to an antenna, and we have said that each transmission line has an impedance based on its dimensions, so you might be asking why use so many different impedance cables? For example, 50Ω is popular for transmitters using coax, but 300Ω and even 450Ω and 600Ω open-wire lines have been used. TV uses 75Ω coaxial cable, and older computer network cables were 65Ω and 95Ω, and yet others no doubt exist.
The short answer is some cables are better for handling power and others for giving low losses. A power cable will probably have higher voltages generated, and a higher impedance cable can handle these higher peak voltages.
I have seen a transmission line for Radio Australia that had 5 × 50mm galvanised pipes as the conductors, one in the centre, and the other four in a square formation around it forming an open coaxial line. So, power does require bigger conductors!
In last month’s issue we defined the term 'radiates' as meaning to spread out in a radius from the origin of the fields. The term 'Radio' refers to the intentional radiation of electromagnetic energy, usually for commercial purposes. Let’s consider the basic physics of radiation.
Electromagnetic radiation is a combination of the Electromagnetic and Electrostatic Fields. Although there are no physical lines of force either in magnetic or static fields, the concept helps us visualise the relationship between the fields. The figure below shows a magnetic field being generated as 'lines of force' around a conductor, with the field strongest at the middle of an antenna where the current is strongest. The static field lines are shown as inline with the antenna with the strongest being generated by the highest potential difference occurring at the ends of the dipole.
At a distance from the antenna, known as 'Far Field', and usually said to be 10 wavelengths or greater from the antenna, the two fields are simply two fields at right angles to one another. This is represented as the electrostatic field lines being aligned with the dipole antenna, and typically any linear conductor working as an antenna. Electromagnetic fields are shown at right angles with the dipole, or antenna wire.
Therefore, an antenna mounted vertically, that is, with the wire of the dipole vertical, is called a ‘Vertically Polarised' antenna, and as you might expect, a horizontally mounted antenna, a horizontal dipole for example, is 'Horizontally Polarised'.
There are also 'Circular Polarised' antennas where one or the other end may be moving and changing polarisation. Quadcopter devotees might be familiar with Clover Leaf Antennas, or Helix Beams. Most circular polarised antennas were developed for rocket and space technology. We won't be going into them yet but intend to discuss them in future issues.
Antennas are not amplifiers. A common mistake is made by assuming that an antenna with a high gain will deliver a high-power output. Not so! One of my lecturers gave me a good example that I have passed on many times. Let us imagine that I have a Parabolic Dish Antenna that has a very high gain, say 30dB and I want to cook an egg with it.
First, a reminder about decibels. The term Bel (named in honour of Alexander Graham Bell) is a logarithmic relationship between two values of power, but as the relationship is measured in decibels (dB), meaning one tenth (deci) of the unit “Bel”, the formula is 10xLog(Power Out / Power In).
Note: Log always refers to base10 for decibels.
- Gain(dB) = 10Log(Pout/Pin)
If power is lost then the gain value will be a negative number.
A power gain of 2:1 gives us 3dB gain, a gain of 4:1 gives us 6dB gain, 8:1 gives us 9dB and 10:1 gives us 10dB. Every decade gain results in an increase in the 'tens' column of 1. So 36dB becomes 30dB + 6dB which breaks into 3x10dB, 10^3, or 1,000:1, and the remaining 6dB means 4:1, or 4 times 1000:1 = 4,000:1. Note that while gain ratios must be multiplied, decibels may be added, which is why they are so valuable to us.
Our 30dB gain parabolic dish will have a gain value in ratio terms of:
- 30dB = 10 × Log(Pout/Pin)
- 30/10 = Log(Pout/Pin)
- 103 = 1000 = Pout/Pin
- i.e. Gain = 1000:1
So, if we put in 1W, does that mean we get 1000W out?
I do hope you say “Of course not!”, but then what does it give us?
Antenna gain is a measure of how much stronger the signal is at the measured point than what would be expected from an isotropic source. Isotropic means equal values in every direction, and although some antennas come close to isotropic, it is still merely a mathematic concept.
So, if we put a 1W signal into a true isotropic source, 1W would be shared throughout every angle originating from that source.
If we put that same 1W into the feed line of a 30dB parabolic dish, then that 1W would be shared about into a much smaller area, but the total power can never exceed 1W, and our egg will remain cold.
Antenna as transmission line
If you have grasped the concept of the transmission line, then an antenna acts as a transmission line matching the RF energy from the coaxial cable to open space. As the RF energy conducts from the feed point of a dipole to the extreme ends, in a perfectly matched antenna, the RF is radiated into open space resulting in a combined electrical and magnetic wave travelling away from the antenna at the speed of light. The antenna simply matches the transmission line to free space as another form of transmission line.
Last month we introduced and made a dipole for FM Radio, operating from 88-108MHz. This frequency was chosen as both an easy size to make and as an antenna that is useful to those who listen to FM radio.
I like AM radio, and living away from the city, I could use an outside antenna to capture the signal better, but a dipole for my favourite 693kHz AM station would be 216m long! Luckily, dipoles are not the only type of antenna we can use, but before moving on to other antennas, let us provide you with a few more antenna types that you might come across, that are based upon dipoles.
The FM antenna was made to be a vertically polarised antenna which we earlier described as having the main element vertical, and when you think about car antennas for FM radio, and most other uses including police radio, they are all vertical.
Dipoles are not particularly suited to vertical mounting in their basic form unless they are mounted off the side of a tower or mast. Dipoles are also prone to picking up electrostatic noise.
Commercial communications antennas and data links often use an antenna called a Folded Dipole and no doubt you will have seen many if you were looking!
The folded dipole is popular for higher frequencies above 50MHz, and typically made from aluminium tube. Although formed as a dipole, the tube is actually almost a wavelength long, before it was folded. A half wavelength centre section has the quarter wave ends folded to form a loop with the ends almost touching.
This is in fact working as two dipoles close together, and the spacing acts as a transmission line as well. Without getting into the maths, the two dipoles share the current and the voltage resulting not in double the impedance, but four times, so the ~75Ω dipole becomes the ~300Ω folded dipole.
As the dipole ends continue back to one another, the ends do not attract the electrostatic noise that a simple dipole does, so they are much quieter RF wise than a standard dipole. They are often mounted by the middle of the continuous side directly to an earthed pole, so any noise on the dipole also has a good chance of being grounded, although some will remain.
Feeding a 300Ω antenna requires matching, and TV antennas always used 300Ω ribbon cable, or had a 4:1 Balun transformer to match the 300Ω ribbon to the 75Ω cable. The Balun also converted the “Balanced” antenna to the “Unbalanced” coaxial cable, hence the term “Balun”.
Folded dipoles are useful on VHF and UHF but begin to get fiddly at GHz frequencies. A folded dipole on 2.4GHz for example would be just 61mm long!
Coaxial or End Fed Dipole
As a mobile antenna, a dipole is inconvenient as it needs to be fed in the middle, and nobody wants a coaxial cable hanging off the side of their auto antenna. To avoid this, half-wave antennas can either be end fed, and matched to the very high impedance found there, up to 10kΩ, or centre fed by a coaxial cable passing up the centre of the antenna tube.
The end fed type has a coil of heavy wire at the bottom between the antenna and the earthed body of the car. The centre of the coaxial cable is connected at some point on the coil where the correct impedance is found (Mathematically or by trial and error!).
For WiFi and similar technology on 2.4GHz, the 61mm length becomes an advantage, and a small coil at the base allows those tiny antennas that we all see from time to time on WiFi boxes. You may have noticed those 2.4GHz and 5.8GHz WiFi antennas, as well as 800MHz or 900MHz telephone antennas, and even 433MHz antennas, are all about the same size and appearance. They are usually poorly labelled too, so WiFi problems could occur if you screw on an incompatible antenna. Your favourite drone may appear to have a signal when up close, but quickly loses interest in you when airborne!
Either coaxial or end-fed dipoles will have an almost flat angle of radiation with quite a bit of tolerance when the drone is tilted, but remember to make sure it is indeed intended for the correct frequency.
A transmission line is an 'Untuned' circuit resulting in a fixed characteristic impedance. Transmission lines convey energy reasonably efficiently from radio transmitters to antennas and back to receivers. Dipoles make excellent antennas, but many variations exist on this basic theme.