Fundamentals

Part 1: Radio

Bob Harper

Issue 13, July 2018

Although you may have all grown up knowing about radio, humans have only had the benefit of radio technology for a little more than the last century.

So what is radio, how does radio work, and why do you need to know more about it?

Any electric current radiates energy as a magnetic field, and any potential difference as an electrostatic field; in fact, both exist even at DC. The term “radiates” means to spread out in a radius from the origin of the fields, but since about 1904 the term “radio” has been coined to describe the intentional radiation of electromagnetic energy.

My grandfather CW Littleford, built radio receivers as a young boy around 1920, almost 100 years ago, and I guess that’s the seed for my interest; but times and interests have changed. Today, few young people are interested in “radio”, yet almost all know what WiFi is, and many want to fly a drone or quadcopter, and never pass a thought of how computers communicate with one another, or drones with the controller.

Radio hasn’t changed, but the methods of using it have – dramatically. Digital electronics used to mean Morse code; ‘ - • – • • – – ’, but now in the WiFi age, 2.4GHz is nothing to be concerned about. It’s just there, and data itself is now transmitted at higher frequencies (bit rates) than an old AM radio used.

author
Half a life ago this author climbed Mt. Beerwah in Queensland with an equally old radio, motorcycle battery and army tank antenna, and talked to South Australia.

RADIO CIRCUIT

Radio is a huge subject, and is a major part of modern electrical engineering. Lets get started with the following diagram, and explain a few terms.

diagram

Just as an electric circuit has a source, path and sink, radio circuits require the same processes, although we would not normally refer to them that way. Instead, let us use the terms “transmitter”, “path”, and “receiver”. We only mean to introduce these terms at the moment, and will concentrate on each of them individually later in this series.

TRANSMITTER

The transmitter is the means of generating the radio energy, and although early transmitters were simple and crude, modern transmitters are required to generate radio frequency (RF) energy, which is free of noise and harmonics, and locked to a set frequency.

Note: Transmitters must be licensed to be permitted to operate within Australia, and in fact, worldwide. WiFi and such devices are licensed under a scheme that doesn’t require the user to pay a license fee, but there are standards to follow (e.g. ieee-802.11).

FREQUENCY

Transmitters operate at a fixed, yet changeable, frequency, meaning that it shouldn’t change while transmitting; but it’s more complex than that. WiFi has a number of channels within two bands, on which it can transmit. The term “bands” mean a range of frequencies. WiFi can operate on the 2.4GHz band or the 5.8GHz band, which has more channels.

The whole radio spectrum is split into bands — let's take a look at a simple radio spectrum plan.

frequency

Frequency literally means how often a wave repeats; from DC which has a frequency of zero waves per second, to AM Broadcast, which has a frequency from ~500,000 waves per second, written as 500 kiloHertz (500 kHz), to 1.7 MegaHertz, (1.7MHz).

FM radio operates around 100MHz, while TV operates from 45MHz through to around 600MHz with other radio allocations between the TV channels.

WiFi is much higher in frequency at 2,400,000,000Hz, or 2.4GHz.

Interestingly, we use the terms “low frequency” and “high frequency”, and other similar terms, but all are relevant to the technology we are working with. Anything above 50Hz is high frequency to an electrician, while an audio tech thinks it refers to the top third of the audio spectrum. In amateur radio, “HF bands” refer to high frequency as up to 30MHz, yet amateur radio licenses allow operation into the high GHz bands. These frequencies are all a part of the radio spectrum, of which the spectrum of human vision is only a small part.

The applications we have for radio must be matched to the characteristics of frequencies, and that is the job of the government-controlled ACMA, which allocate frequencies to license holders based on their needs, and with the aim of reducing the risk of interference with one another. As radio can travel around the world, and even off of this planet, radio frequency allocation is coordinated by the ITU — International Telecommunications Union. If you think Air Traffic Control has a problem, imagine trying to coordinate all the mobile phones in the world!

RECEIVERS

All radio transmissions (i.e. the energy from a transmitter), share the same radio space. Radio receivers must, therefore, select those frequencies that the user wants to receive and filter out all others, including selecting a low signal strength from amongst strong signals, where possible. That received signal then has to be converted back into voice, music, video, data and other forms of intelligence.

The process of encoding intelligence onto transmitted signals is called “modulation”, and recovering it is called “de-modulation”. We won’t get into modulation yet, but you already know of two or maybe three types: AM, FM, and Morse code. There are many others, and we will eventually get to some of those that you may need for data links and RC links.

TRANSCEIVERS

When a radio contains both a transmitter and a receiver, and it’s allowed to do both tasks (although generally not at the same time), it’s called a “transceiver”, which you might notice is a conjunction of the two words: “transmitter” and “receiver”.

Transmitters and receivers are usually very complex electronic circuits, and we may look at some of that technology in good time, even if most of you will never make or repair either.

the path

THE PATH

Many of you will have some need to understand the radio path in order to select appropriate antennas, and hopefully the remainder of you technophiles will find it interesting as well. This column will concentrate on how radio energy is conveyed from the transmitter to the receiver, starting at the socket on the back of the transmitter, and ending at the socket on the back of the receiver.

The path involves the antenna cables, antennas and the space between them, which is known as the signal path.

SIGNAL PATH

We mentioned earlier that different frequencies behave differently, especially between antennas. Thinking about the path, and everything that might be between the antennas, you might be surprised what radio waves have to contend with.

To begin with, let’s assume that the RF travels from one antenna to the other in a clean and direct path, known as the “line-of-sight” path.

The RF would travel straight (i.e. directly from transmitter to receiver). While that might occur between the Moon Lander and Apollo in open space, within the earth’s atmosphere there is air, perhaps clouds, rain or dust, all of which cause some signal losses. Rain increases losses, and aircraft passing through the signal can cause reflections and deflections.

As most of our conversation will be about line of sight paths, we’ll leave it to that for brevity, but if you’re interested, many amateur radio clubs give free or minimal cost tuition in radio theory to anyone who’s interested. Some may ask for membership, but check with your local club.

ANTENNA THEORY

That brings us to the freaky science, which is sometimes also called the “magic” or “black art” of antennas and transmission lines. We can only begin to describe the basics of antennas, as a great many volumes have been written on the subject. However, you should end up with about the same knowledge as a person requires for the amateur radio exam – at the Foundation License level, at least.

ANTENNAS AND TRANSMISSION LINES

You might be convinced that antennas are connected directly to the transceiver/receiver, having seen lots of telescopic whips on FM radios and even walkie talkies; or, you may believe the antenna belongs inside as with most mobile phones. This column aims, partly at least, to explore better WiFi coverage, and drone and FPV links. Of course we might also look into LoRa links, and other services on 433MHz, 898MHz, 915MHz.

THE SINE WAVE

I am sure that you will have seen material on sine waves before, either in DIYODE, or during high school maths or physics; but, a quick recap may be helpful. Below we have a sine wave, being the naturally occurring oscillation of electricity. It is often called a "pure sine wave" meaning that it has no deflections from the mathematically drawn sine wave.

sine wave

The diagram above shows two periods of a sine wave (i.e. two complete cycles). One cycle is one complete oscillation, or the distance from one point on the waveform to the next point of the same value and direction. The diagram shows from one peak to the next peak, but in reality this is difficult to measure on a CRO screen. So instead, it is often measured from where the rising trace crosses a horizontal grid line, to the next time the trace crosses the same grid line in a rising direction.

The term “period” refers to the time period of one cycle and on a calibrated oscilloscope, the period is the inverse of the frequency (e.g. the period of a 50Hz waveform would be 1/50 = 20mS).

At 27.125MHz, an old CB radio channel, the period is 1/27.125MHz = 36.87 nanoSeconds, which can be handy to know, but for antenna work it is much more important to know the wavelength.

Wavelength is the distance the wave will travel in space during one cycle. To know that distance we need to know the speed of light, which is also the speed of radio waves.

Known as “C” which I believe came from the Latin word “Cito” meaning “fast”, C is approximated as 300,000,000 m/sec. Yes, I know that’s not 100% accurate but I can’t measure any better myself!

So the wavelength of 27.125MHz is almost exactly 11.06m, which is why CB radio was once called the “11m band”.

Referring back to the radio spectrum, note that the measurements along the bottom are given in metres. Thankfully nobody decided to measure it in feet!

It is common in calculations to use the lower case Greek letter Lambda (λ) when referring to wavelength.

λ = C/f, and when the frequency is given in MHz; λ = 300/f.

AMPLITUDE

The amplitude of the sine wave may refer to either the voltage or current of a signal. The maximum amplitude is the peak value, which can be the positive peak measured above the centre line, but the peak could be measured in the negative direction and should yield the same value. As the centre line isn’t available on an oscilloscope screen, we usually refer to the peak-to-peak value, which is the voltage (or current) value measured from the lowest point on the waveform to the highest point on the waveform.

MAXIMUM POWER TRANSFER THEOREM

Before we jump into transmission lines and antennas, we must first explain the “maximum power transfer theorem”. You should first imagine that we can connect the receiver directly on to the transmitter. What would internal resistance of either have to do with getting the best transfer of power from the transmitter to the receiver?

description

The transmitter is a voltage source with an internal resistance (impedance), the source being an AC voltage source of the appropriate frequency. The typical internal resistance in modern radios is 50Ω. We won’t explain why except to say it is intentionally designed to be 50Ω.

The receiver is a black box as well, with an internal resistance across the terminals also of 50Ω, by design. To avoid another long story, the only other item inside the black box receiver, is a perfect amplifier having infinite resistance, connected across the terminals as well.

So to illustrate my point, let’s assume that the transmitter generates 1V of RF at an unimportant frequency, or unknown one if that works better for you. The receiver appears as a 50Ω load across the transmitter terminals, making a series circuit with the transmitter source voltage and internal resistance. The current in the load will therefore be found from:

I = V/R = 1/(50 + 50) = 0.01 Amps or 10mA

The power conveyed to the receiver’s internal resistance will be:

P = I2R = 0.012 × 50 = 5mW

That doesn’t sound like much but would be a lot for a receiver.

Now try making the receiver internal resistance 40Ω or 60Ω, or even 49Ω or 51Ω, and calculate the power delivered to the receiver. Remember to calculate the transmitter current for each resistance first.

Power in the 40Ω load is 4.938mW, and for the 60Ω load is 4.959mW. All values above or below 50Ωs will receive less power.

This is known as the “maximum power transfer theorem” as the maximum power is transferred when the load resistance is equal to the internal resistance of the source.

load in ohms

HOW MAXIMUM POWER TRANSFER AFFECTS THE PATH

The actual path will probably include at least two antennas, two cables, and an open space between the antennas. So matching all of these can become a big issue.

Coaxial cable is designed to have what is known as a “characteristic impedance”, which can be almost anything; but hopefully you’re screaming out “50Ω would be good!”, and therefore it would be good if the antennas are also 50Ω.

maximum power transfer

Once again, we’re leaving for a future issue, the explanation of why transmission lines have a characteristic impedance.

Antennas have a characteristic impedance as well, based on their design and dimensions, so often they must be matched to the cable, by an appropriate “antenna matching” circuit.

The antennas themselves are matching circuits that match the cable to what we call “free space”. The characteristic impedance of free space is a scary value of 120πΩ = ~377Ω (sigh in relief now because it isn’t quick to explain!).

Just remember, if the parts are not matched you will be losing signal strength and wasting power. In higher power transmitters, that wasted power can cook the power transistors of the final (main) amplifier.

The Simplest Antenna – The Dipole

The word “dipole” simply means “two poles”, or “two ends”. The dipole is the easiest to understand and one of the quickest to make, and in the process we intend to explain some of the dark magic of antennas (then it will no longer be dark or magic).

wavelength

The drawing above depicts a one wavelength (1λ) length of plain copper wire, and many antennas have been built using stranded electricians building wire, 1.5mm2 or up to 6mm2, depending on the frequency and power; although most often it’s based on what is cheaply available.

The two sine waves shown represent the voltage and current density curves for the antenna. That’s not the same as current flow because in an antenna, especially a resonant antenna (i.e. an antenna cut to a resonant length), the current reduces towards the ends of the antenna until there is almost nothing at the ends.

The voltage is greatest at the ends, and there is very little in the centre — 0V in a perfect antenna — but remember, there is matching and some voltage can be present due to slight mismatch.

For a straight antenna, the full wave does not work. The reason can be seen if you know what to look for. The electromagnetic field generated due to the current will cancel out because the current in one half of the wire opposes the current in the other half. It’s like trying to push a car when there’s somebody also pushing at the other end.

The voltage is actually 90° out of phase with the current, so the sine wave for the voltage has a peak voltage of the same polarity, at each end. Therefore, the electrostatic fields oppose each other as well. A full wave antenna is possible and we’ll show you how that works in future issues.

The diagram below shows the fields on a half-wave antenna, which we’ve have already called a “dipole”. Note: the current distribution shown on the diagram above is greatest in the centre, and least at the ends, and all of the same polarity. The voltage distribution is zero in the centre, and at maximum on the two ends; but importantly, of opposite polarity so they generate the strongest possible electrostatic field.

fields on a half-wave antenna

Now we’re in business – except we have to connect power to it and match it. The diagram shows a dipole antenna with a centre feed point. The impedance for a textbook dipole is 72Ω, but reducing as the diameter of the conductor is increased. Increasing the diameter also means that the dipole needs to be a shorter length for the same resonant frequency, due to the increased capacitance caused by it’s thickness.

In effect, we can cut the dipole at the centre, connect it to some 50Ω coax, and it will work connected to a 50Ω impedance receiver. Purists will argue with me, but in practice it works successfully. It may have enough mismatch to cause issues on a powerful transmitter, but let’s stick to receivers for now.

practical example

Practical Example:

Dipole For FM Radio

Broadcast FM radio operates from 88MHz to 108MHz band, with the centre frequency at 98MHz.

A half wavelength for 98MHz will be λ/2 = (300/98)/2 = 1531mm. There are two quarter waves in a half wave so ~765mm each side of the centre connection. All you need to do is find some coax – perhaps an old 75Ω TV coax lead* – with connectors to fit to the back of your radio. Make sure it does fit and it is your property to destroy, then cut the unused end off. Bare the cable sheath and inner conductor making sure they can’t short out.

* TV coax is intended for 75Ω, which is a pretty good match for a 72Ω antenna.

Now solder a length of at least 765mm wire to the inner conductor, and another 765mm wire to the outer sheath. Double-check your connections and isolation of the two conductors. You can cover it all with tape when you’re done, or hot glue is also good, perhaps also gluing a piece of plastic sheet over it as a mechanical support.

dipole antenna

Finally, assuming you’re happy with 98MHz, trim the two conductors to exactly 765mm. Mount it vertically, as FM is vertical polarised (we’ll also explain that next month), and away from metallic items like furniture or window frames. Plug it into your radio or stereo (i.e. FM input socket). You should get an increase in signal strength, but not a change in sound levels.

Hopefully you just made your first dipole antenna!

Note: You can calculate for other frequencies (e.g. 92.7MHz requires 809mm either side).

SUMMARY

Radio Transmitters generate RF energy at a required frequency and amplitude, which is fed via cable to an antenna. The RF passes through the signal path to the second antenna, down the transmission line and into the receiver. Transmitters, receivers, transmission lines, and antennas all have a characteristic impedance at their connections for maximum power transfer.

PART 2