An Introduction
This document explores the concepts involved in radio
broadcasting relevant to the pirate radio operator on VHF FM. We will go a step
by step tour of a typical VHF FM transmitter system starting with the output
from the tape recorder or mixer, and finishing with a brief discussion of
aerials. At each stage we will discuss the pros and cons of various
alternatives, provide additional background information and introduce the use of
test equipment.
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This information originally
published in
The title "Radio is my Bomb" comes from a quote from Chantal
Paternostre, a Belgian anarchist, when being interrogated on trumped up
charges of arson and bombings for the CCC guerrillas. Chantal worked
for Radio Air Libre, the Brussels pirate prior to her arrest on August
15th 1985. After more than a year in prison, most of it in solitary
for resisting the system, the Belgian authorities finally believed her
statement and let her go. |
What we are trying to is get information from one place to lots of others.
I'm using information here in a wider sense, meaning speech, music, propaganda
etc, rather than phone numbers of local hairdressers or whatever. Now I'm going
to assume we're going to use radio broadcasting to achieve this, which
immediately rules out things like standing on top of tall buildings and shouting
really loud. We'll also assume we've got this information in the form of an
audio frequency signal, ie. what comes out of a tape recorder or an audio mixer.
You can't transmit audio frequency signals very easily so what we do is impart
the information in the audio frequency signal onto a higher frequency (radio
frequency) carrier signal.
Radio frequency (RF) signals have amplitude and
frequency. The frequency is how fast the signal is oscillating
from one extreme to the other and back again. Frequency is measured in cycles
per second (cp/s), which these days are known as Hertz (Hz).
1000Hz = 1KHz, 1,000,000Hz = 1MHz and 1,000,000,000 = 1GHz. The amplitude is to
what extent the signal is oscillating. Level or
Strength can be thought of as meaning the same as amplitude.
Amplitude can be measured in Volts (V) or Watts (W or dBm).
Two ways of imparting our wanted information onto the RF carrier are
Amplitude Modulation (AM) and Frequency Modulation
(FM).
In AM the amplitude of the carrier is determined at
every instant by the amplitude of the audio signal, the carrier frequency
remains constant. In FM the frequency of the carrier is
determined at every instant by the amplitude of the audio signal, and the
carrier amplitude remains constant.
Frequencies between 30 MHz and 300MHz are known as Very High
Frequencies or VHF. This corresponds to wavelengths
between 10metres and 1metre. To convert between wavelength and frequency use the
formula
wavelength (in metres) = 300
frequency (in MHz)
There are two sorts of FM, known as Narrow Band FM (NBFM)
and Wideband FM. They differ by the maximum allowable frequency
shift of the carrier when the transmitter is fully modulated. This frequency
shift is known as the Deviation. Legal CB radios use NBFM with
a maximum deviation of 8 kHz. Wideband FM is used by the BBC and IBA for radio
broadcasting and for studio to transmitter links. The standard maximum deviation
for FM radio broadcasting in the UK is 75KHz. There is no simple way to set the
deviation of a transmitter without a Deviation Meter which is an expensive piece
of test gear. Probably the best way to do this is to vary the level of the audio
signal going into the transmitter (TX) and listen on a receiver, until your
signal sounds about the same loudness as the other (legal) broadcast stations.
If you use too high a deviation you'll use a bigger than necessary chunk of the
radio spectrum and be more likely to cause interference to others, which will
make you even more unpopular with the DTI (Department of Trade & Industry,
responsible for policing UK airwaves).
If your first action is to reach for your receiver and tune through looking
for a blank space, think again, for a kickoff the UK FM broadcast band is 88 to
108 MHz. What stations you can receive is determined by
where you are, as well as by the type and positioning of your aerial. In the UK
if you get a current list of the frequencies and locations of all FM broadcast
stations from Maplin (big UK electronics
retailer). What it doesn't say, of course, is the frequency of existing pirates,
so you'll have to check this out yourself.
Armed with this information you should make a
list of all frequencies in use in, say, a 30 mile radius. In the UK, if you
write to the BBC or IBA's Engineering Information Offices they'll send you
service maps of where their TX's are meant to be able to be heard. Then its just
a question of finding a big enough gap between stations, with the proviso that
your station shouldn't be nearer than 200KHz (0.2 MHz) to the frequency of any
existing station.
Now let's take a little stroll through the whole system. Click on the
part in the picture you're interested in to go straight there.
What are we going to feed into our TX? Possibilities include:
- A tape or cassette player
- Live, direct from the mixer
- Live, via some kind of link from studio to TX site (highly recommended)
TAPE
This is the safest approach in that you can put a tape on and then retire to
a safe distance. Links are now being traced and studios busted, and some of the
biggest pirates are going back to taped broadcasts. If the DTI trace your
transmission and turn up all they can do is confiscate your tape player, TX and
aerial, ie. no arrests (unless they catch you changing the tape!) It's also the
most inflexible alternative as tapes will have to be prepared in advance. Time
checks, if you're into that, will be difficult and live phone ins are right
out.
Give a little thought to your choice of tape recorder, as it will probably be
the weakest link in terms of sound quality. In an old clapped out one the heads
will be worn flat. Maybe you can use a 'Walkman' type of player, which are
small, can be battery powered, have ok quality and are cheap. The latest and
greatest is to use 'Stack machines' which will change the tapes for you.
Whatever you use get one that can be battery powered as you may not always have
access to mains power.
To reduce 'noise' or 'tape hiss' on such recorders, if you're doing
programmes with quiet passages, you can use a circuit known as a Dynamic
Noise Limiter (DNL), which is placed on the output and cuts off the
'noise' just in quiet pauses. DNLs are sometimes used in the soundtracks of old
films.
The advantages of mono are that the TX is kept as simple and cheap as
possible, and you don't need as much power as on stereo to get the same result.
The disadvantages are you don't sound as professional. Quite small pirates are
now using Stereo Encoders, and maybe people might dial past when the red stereo
light on their receivers doesn't flash. With stereo the listener can get quality
the same as legal stations. Weigh against this the extra cost, extra circuitry
and more output power needed for the same signal.
What you need is a Stereo Encoder, which combines the left and right stereo
signals into a single composite stereo signal which is then fed into your
TX.
A brief technical description follows. The left (L) and Right (R) signals are
fed into a summing and differential amp to get a L+R and L-R signal
respectively. The L-R signal is mixed in a balanced modulator with a 38KHz
sub-carrier to produce an amplitude modulated double sideband suppressed carrier
signal. The 38KHz signal is derived from the same source as the 19KHz pilot
tone. The composite output is formed by mixing the L+R signal, the sidebands
containing the information on the L-R signal, and a bit of 19KHz pilot tone. The
pilot tone switches on the Stereo Decoder in peoples' receivers.
Back in the receiver, once the Stereo Decoder has extracted the L+R and L-R
signal the original left and right signals are easily got by (L+R) + (L-R) = 2L
and (L+R) - (L-R) = 2R.
The reason L+R and L-R signals are encoded rather than L and R is so that a
mono receiver can just demodulate the L+R bit and ignore the rest of the signal.
If L and R were encoded a mono receiver would only be able to hear the left
channel. The 19KHz pilot tone is usually got from a crystal oscillator, to be
quite accurate and stable. A crystal resonating on 4.864 MHz is convenient as
4.864 MHz divided by 2 eight times is 19 KHz. This can easily be done by digital
logic chips. But its highly unlikely you'll be able to buy a 4.864MHz crystal
off the shelf, so you'll have to have one made to order.
It doesn't matter if you didn't understand all of the above but one thing is
important. The standard FM broadcast audio bandwidth extends only to 15KHz and
Stereo Encoders are designed to assume this figure. If you put signals into them
with frequencies above that the L+R signal and the lower side band of the L-R
signal could spread into each other and you will get a right bloody mess. With a
tape recorder you can't really get over 15KHz, but if you're live it is quite
possible. In that case you need a low pass filter on each input
to a stereo encoder. This is an audio low pass filter, not to be confused with
an RF low pass filter, to be discussed later. If you're using a link between
studio and TX and you want stereo you will have to know the bandwidth of the
link. If it's 53KHz (= 38+15) or more you can use it after the encoder.
Otherwise you'll need two links and have to encode at the TX end.
In a typical audio signal the high frequency sounds have less energy than the
low ones and so produce less deviation of the carrier. This in turn makes them
susceptible to 'noise' when received. To avoid this high frequencies are boosted
before being transmitted by pre-emphasis. In the receiver the
frequencies are cut by the same amount by de-emphasis So the
overall frequency response of TX to receiver stays flat, but the level of
background noise is reduced a lot.
Pre and Deemphasis networks are characterised by their time
constant. In the USA the standard is 75 uS, but in UK it is 50uS so
anything designed or bought from there needs slight modification. In a mono TX
the pre-emphasis network can be built into the front end of the exciter. For a
stereo TX such a network must NOT be in the exciter or it'll play hell with the
composite stereo signal from the encoder. Instead you need two networks, one for
each channel, on the inputs of the stereo encoder, They're actually often built
into the stereo encoder.
Compressors and Limiters operate on the same principles, but their effects
and the reasons for using them are completely different.
A compressor compresses, it reduces the dynamic range of its
input signal. This means as the input amplitude varies over a certain range, the
output amplitude varies over only a fraction of that range. The graph shows a
2:1 compression characteristic. In this case with every change in the input
amplitude the output changes only half as much. The dotted line shows a 1:1 non
compressed characteristic.
A Limiter passes its signal unaffected till the input amplitude reaches its
threshold. At this point the limiter prevents the output
increasing much by compressing its input much more strongly than in compressors
eg 10:1.
Some American music stations and some pirates compress their programmes to
make it seem 'louder' and more 'upfront' than other stations, This occurs
because the compressor keeps the average level of the signal high, even in quiet
parts of the programme. The flip side of this is listeners can soon get
'listener fatigue' as constant compression can become boring and irritating to
the ear, as if the music were rammed into it!
Compression has other uses, you might compress your programme as you transfer
it to tape to stop quieter bits fading into background tape hiss when played.
The process of recording and replaying does this to some extent anyway. Don't
compress the output of a tape recorder as it'll make tape noise worse. Guitar
effect units, labelled compressors, are unlikely to be much use. Compressors
intended for use in home studio recording are worth experimenting with. A stereo
compressor with a 2:1 characteristic can be simply constructed around a NE570 or
NE571 IC. Get the data sheet in PDF format from Philips
Semiconductors.
Limiters are used to stop a signal's amplitude going over a certain level.
Eg. when cutting a master disc in record manufacture, large PA systems at gigs
to stop loudspeakers blowing every time someone burps in a mike and, in
broadcasting. In FM particularly, as the signal level increases so also does the
bandwidth of the transmitted signal, risking interfering with other stations.
With tape input to your TX the output is inherently limited by the recording
process, so no limiter is needed. With live input to the TX it's different.
Though you might set the levels right to start, along comes a loud record or
voice and you could be interfering with the next station. Use a limiter.
Any limiter based on 2 back to back diodes is little more than a guitar fuzz
box and will sound like one.
At the heart of everything is the Oscillator that generates
the VHF signal. The frequency of this is modulated by applying an audio signal
to it. The most common way of doing this is by using one or two
varicap diodes. When a varicap diode is operated with a reverse
bias the capacitance of the diode varies with that bias. The diode(s) is/are
connected to a frequency determining part of the oscillator. The audio signal is
connected across the diode to achieve frequency modulation. Also by varying the
DC reverse bias the oscillator can be fine tuned. The higher the voltage, the
lower the capacitance, the higher the frequency.
The VHF signal can either be generated directly, or the oscillator can
oscillate on a lower frequency eg a third or half that desired and then followed
by a tripler or doubler stage. There are three
main types of oscillator:
These are simple oscillators which can be built round a single transistor.
This can be a Bipolar Junction Transistor (BJT) or a Field Effect Transistor
(FET).
The problem with these simple oscillators is that the frequency is too
dependent on the temperature of the transistor - a few degrees temperature
change will result in a significant change in transmitting frequency. For this
reason these oscillators are unsuitable for serious use as a
TX.
The transistor in the oscillator will heat itself up slightly, and other bits
of the TX, like the power amps, will be fair old chucking heat out, and are
usually built into the same case as the oscillator. The frequency will drift
most when the TX is first switched on as all the components will be at the same
temperature as the air outside the TXs case, this is known as the
ambient temperature. After the TX is turned on the heat from
the power amplifiers will warm the air in the case directly or indirectly. As
the transistor warms the frequency will drift a bit. When heat loss from the TX
equals the heat gain (generated internally) you get thermal
equilibrium and the TX frequency should not drift so much. Keep your TX
out of drafts to avoid messing this up. If you have a frequency counter plug in
to a dummy load and see how long it takes for the frequency displayed to settle
down, maybe about 15 minutes. If you have time you can arrive at the TX site
early and run your TX for the warm up time with no input into a dummy load. This
avoids listeners who tune in immediately having to retune as your frequency
drifts.
This is also a simple oscillator but incorporates a crystal into the
frequency determining network. There are various types of crystal (fundamental,
3rd overtone, 5th overtone etc) and various ways of using them (series mode,
parallel mode) but their basic properties are the same. They are resonant on one
frequency which is determined by the crystals characteristics when it's made.
This is their problem, whereas VFOs are not very stable, crystal oscillators are
too bloody stable and it's a job to get enough deviation. You'll probably lose
the higher frequencies of your programme and stereo is right out. Also chances
are you'll have to get a crystal made to order for your desired frequency so if
you want to change it you'll need a new one. There are places which make
crystals to order.
The way it's done properly is with the Phase Locked Loop oscillator. This
combines the ease of tuning and wide deviation of a VFO with the frequency
stability of a crystal oscillator. It works thus: A crystal oscillator is used
to provide a reference frequency. This is digitally divided by logic chips to a
relatively low frequency, say 25KHz. A VFO provides the wanted output frequency,
which is also digitally divided to give another relatively low frequency. These
two low frequencies are presented to a phase comparator which
basically decides which frequency is higher by comparing the phases of the two
signals. The phase comparator generates an error voltage which
is connected back to the input of the VFO through a low pass
filter (LPF). This is the loop bit.
If the VFO is running too fast the phase comparator decreases the error
voltage so as to slow it down till the phases at its input are the same. If it's
running too slow the error voltage is increased to speed it 'til the phases are
the same. All this happens instantaneously, of course so the output frequency
remains constant. In this way the temperature stability of the VFO isn't
important and it can be built round a BJT, as its output frequency is phase
locked to the crystal oscillator, and the frequency stability of this is very
good. Clever eh?
Two more things to explain. How do you change the output frequency? By making
the VFO's divider programmable. Say it's set to divide by the number N. The
phase comparator is a simple minded sort of soul, concerned only with equalising
the phases at its inputs, it doesn't know what's really coming out of the VFO,
which is N times the divided reference signal. Because this signal is so low
compared to the VFO frequency N can be made to have hundreds of different
values, giving hundreds of different output frequencies from the VFO. So
changing the frequencies is just a matter of clicking some little switches.
Hang on a sec, the VFO is being frequency modulated by the audio input, so
its frequency at any given instant depends on the voltage of the audio output.
We don't want this variation of the VFO's frequency to be
ironed out by the PLL system so we 'iron out' the error voltage from the phase
comparator, so it just contains the underlying trend rather than what's
happening at any split second. This is the purpose of the low pass filter.
This system can be simplified by leaving out the dividers. If this is done
you end up with an output frequency determined solely by the crystal. You've
still got the wide deviation capability of course, which distinguishes this
system from one based on a simple crystal oscillator. This sort of fixed
frequency oscillator is used for things like wireless mikes and could be used
for studio to TX links. Programmable PLL oscillators are used in all manner of
professional communications equipment, including broadcast TXs.
If you can get hold of one, a frequency counter will make setting up an
exciter a lot easier, as you can see what frequency you're on and whether you're
drifting. You don't need to make a physical connection between the frequency
counter and the circuit, in fact it's better not to. Either connect a single
piece of wire to the input of the frequency counter, or use a BNC lead ending in
a pair of crocodile clips clipped together to form a little loop. Frequency
counters are normally sensitive enough to get a reading when the wire or loop is
placed near a coil. You might get lots of other readings as you move it about.
Look for a stable one near the frequency you expect (use a receiver or wavemeter
if you're not sure). It's a bit of an art but you'll soon get the hang of it.
For our purposes a frequency counter must have:
- A bandwidth of at least 150MHz
- A display resolution of at least 6 digits, preferably 8.
Any oscillator, regardless of its type, is followed by a buffer. This is
usually one or two transistors operating in what is known as class A mode. Its
function is to protect the oscillator from what is going on further along the
circuit, especially from changes in its 'load' as the following stage is tuned.
The combination of oscillator and buffer together is called the
exciter and is a small but fully fledged TX. Small in respect
to its output power. Typical values are in the region of 100 - 500 mW (1000mW =
1W).
To increase the power output of our fledgling TX we need to add an amplifier.
Here we are talking about Radio Frequency (RF) amps, not audio amps. RF amps
have certain important characteristics:
Here's a picture
of a 100W amplifier stage Graphic is 24KB.
This is the range of frequencies the amplifier will amplify properly. The
bandwidth is ultimately limited by the characteristics of the active devices in
the amp (ie the transistors or valves) but more specifically by its type,
linear/broadband or a tuned amplifier.
A linear or broadband amplifier will amplify quite a large range of
frequencies and they have a good bandwidth, commonly 1.8 - 30MHz which covers
all of the amateur shortwave broadcast bands - no good for a VHF pirate, but
could be useful for a medium wave pirate. They operate in class A, AB or B mode
and have the advantage that they don't need adjusting when the frequency is
changed. Their disadvantage are they're more complex and dearer then tuned amps
and are harder to design, requiring extensive knowledge of the transistors round
which the amp is constructed.
Tuned amplifiers only amplify a narrow band of frequencies - they have a
small bandwidth which is centred on one frequency. This frequency is determined
by the tuned circuits in the input and output networks of the
amplifier. Tuned circuit have a resonant frequency. This can be
usually adjusted by variable capacitors known as trimmers, to the desired
frequency. The amplifier will produce maximum output when the tuned circuits
resonant frequency is the same as the input frequency from the exciter. Tuned
amplifiers often operate in the class C mode, which is more efficient than class
A or B. This means more of the power being drawn from the battery or whatever
turns into watts up the aerial rather than heat in the amplifier.
Class C amplifiers are relatively simple circuits, and are easier to design.
The bandwidth is a trade-off with gain, the wider the bandwidth the less the
gain. The disadvantages of a tuned amp is, of course, you have to tune it to the
frequency you're using, and if you change the frequency you'll have to retune to
maintain the gain of the amp. Also class C amplifiers have a tendency for
instability (oscillation). A little bit of bias can improve the stability
substantially whilst retaining class C levels of efficiency
The power gain (as opposed to a voltage or current gain
which is different) of an amplifier is defined as a ratio:
power gain = output power
input power
and is a measure of the amplifiers ability to make its input 'bigger'. Power
gains are often expressed in decibels (dB) which are
defined:
power gain(dB) = 10 log10 output power
input power
Amplifiers also have a maximum output power. When this is reached increasing
the input power will not result in more output power and may damage the
amplifier. This condition is known as saturation.
In the case of single stage (ie one transistor) class C tuned amplifier the
gain and maximum output power of the amplifier is basically the gain and maximum
output power of the transistor. Knowing these we can calculate the input power
necessary to produce the maximum output power.
For example, lets consider the formerly popular MRF237 transistor. According
to the manufacturers data sheet this has a maximum output power of 4W and a gain
of 12dB. First we've to convert the gain in dB to ordinary gain using this
equation:
gain = 10(gain(dB)/10)
so for our example
gain = 10(12/10) = 101.2 = 15.85
input power = output power = 4 = 0.25W = 250mW
gain 15.85
So for 4W output power we need 250mW of input power. Most exciters can manage
this, hence the former popularity of the MRF237 in the 1st amplifier after the
exciter. The joker in the pack is that all these figures quoted are for a
frequency of 175MHz, as this is the frequency for which this transistor was
designed. You can't predict what happens at 100MHz and have to experiment.
The MRF238 had 30W output power and a gain of 9dB, so it needs 3.8W input
power. This can be conveniently had from the MRF237. That's how the makers
(Motorola Corpse) planned it.
Here's a picture of a 5W driver with 37dB of gain
Graphic is
25KB.
Impedance is the alternating current (AC) version of resistance. It, too, is
measured in Ohms. The standard impedance outputs of exciters and outputs and
inputs of amplifiers is 50R. The impedance of the input and the output networks
of an amplifier is altered by the tuned circuits, which you recall also tune the
circuit in a tuned amplifier. The input impedance is important
as it effects the load the amplifier has on the stage before
it. Maximum power is transferred between stages when the impedance of the output
is equal to the impedance of the input. If the impedances aren't equal a
mismatch is said to occur, and in this case some energy is
reflected back from the input of a stage back into the output of the preceding
stage, where it is wasted as heat.
The amplifier should first be tuned with reduced input power and supply
voltage. Adjust the input network trimmers Cl & C2 for the best input match
(lowest reading on a VSWR meter connected to the input side) and adjust the
output trimmers for maximum output power. Be sure the extra power is in the
frequency you want and not in the harmonics or
spurious. Check with a wave meter (more of this coming up).
Another VSWR meter can be used for a relative indication of the output power, or
an RF Probe, will give an absolute indication. The pairs of
trimmers are interdependant, adjust one and you'll have to adjust the other, and
so on.
Another thing to keep a weather eye on is efficiency. This
is defined as:
efficiency = RF output power
DC input power
where
DC input power = DC supply voltage * DC supply current to output stage
For a class C amplifier of more than a few watts, you'd expect an efficiency
of at least 50%, preferably more like 75%. If your efficiency is less than 50%
the amplifier is either badly tuned; badly designed; you're not hitting it with
enough power; or something else is wrong.
Having done this and if things seem to be OK, increase the input power (by
increasing the voltage supply to the previous stage) and the voltage supply
slightly and repeat the tuning. Do all of this a few times until you reach the
required levels. Listen in on a nearby (but not too near) receiver. The signal
should be in just one place on the dial with no funny noises or modulations
going on. Check with a wavemeter. Altering the trimmers and varying the input
power and supply voltage should result in smooth variations of the supply
current and output power with no steps or jumps. The exception is, as the input
power is reduced at some point the amplifier will switch off, a characteristic
of Class C amplifiers.
To vary the supply voltage you need a Variable Stabilised Power
Supply Unit. If you can't get hold of one you could build one. They're
not expensive and are well handy, and give you some experience, if needed, of
electronic construction.
Each amplifier bumps up the power some more, because the transistor in each
one can only supply so much gain. To get more power needs more gain - more
transistors in more amplifiers. So if you're the proud owner of a 5 watter and
you're offered a 1000W output stage, it's no good to you as you'd probably need
50W of power to drive it.
If your TX has a series of amplifiers together, you can break in between the
stages to tune each one at a time. Do this by unsoldering components and
soldering in short bits of co-ax with suitable plugs to connect to dummy load
and VSWR meter.
Harmonics are multiples of the transmitting frequency. For a frequency of
100MHz, the 1st harmonic, known as the fundamental is 100MHz,
The second is 200MHz, the 3rd is 300MHz etc. They are produced as side effects
in various parts of the circuit, and will interfere with other users of these
frequencies if they escape from the TX. This will produce Radio
Frequency Interference (RFI), and won't do you any favours with the
relevant authorities. Tuned class C amps don't amplify harmonics, as the
harmonic frequencies are out of the range the amplifier will amplify. However
the use of Class C means that harmonics are generated by the amplifier along
with the desired frequency. The strongest harmonics (apart from the fundamental)
from such amps are usually the 3rd, then the 5th etc. The amplitude of harmonics
is minimised if the output networks are tuned properly, but they're still there.
Oscillators and Buffers can also generate harmonics if they're not set up
right.
To detect harmonics we need an Absorption Wavemeter usually
called just a Wavemeter. Or we can use a Grid Dip Oscillator
(GDO) or a Gate Dip Oscillator, both of which are known as Dip
Meters. Most dipmeters have a
switch which turns them into wavemeters. A wavemeter has a tuning knob,
calibrated in frequency, a meter showing signal strength, and some kind of short
aerial. You hold the aerial near a coil in the bit of the circuit you're
interested in, and tune the wavemeter. It shows how much signal is present on
the frequencies shown on the scale. So you can see what frequencies are being
generated in that part of the circuit. Ideally you'll just find the fundamental,
unless the circuit is a frequency tripler or something.
If you buy a wavemeter be sure it covers the right range, from below 100MHz
to get the fundamental to above 300MHz to get the 3rd harmonic.
Some of you may know that we can use a VSWR meter (also known as
Voltage Standing Wave Ratio meter, SWR meter
or a Reflectometer) to detect mismatch between TX and the
aerial, but the VSWR meter is just as much at home doing this between amplifier
stages. VSWR is the ratio of the forward (or incident) and reflected power.
Except for the expensive ones, the majority of VSWR meters work the same way.
The switch is set to Forward or the set button
is pressed. The knob is then adjusted to make the meter read full scale. The
switch is then set to Reverse or the button is released. It now
indicates the VSWR. A VSWR of 1:1 is perfect (no reflected power) and so
unlikely. A VSWR of infinity:1 shows all the power is reflected back into the
amplifier, you'll get a reading like this if the VSWR meter is connected to an
amplifier output, and nothing is connected to the VSWR meters output (unless its
got a built in dummy load). You'll also get this if there's a short circuit
somewhere after the VSWR meter. In either case switch off
immediately or you can wave goodbye to your power
transistor.
The point of all this VSWR business is to get the maximum amount of power out
from the amplifier into the aerial, instead of a hot TX and a bad signal.
So how do we go about tuning such an amplifier? We've got to tune it with
something connected to it's output, a load, otherwise the
tuning won't be right and besides the transistor will probably blow up. We could
use an aerial, but this introduces an extra unknown quantity - the
characteristics of the aerial. As well as the fact that we'd be broadcasting.
What we need is a dummy load.
This is basically a resistor, but constructed in such a way that it presents
a load to the amplifiers output independent of frequency (unlike the aerial).
The three characteristics of a dummy load we're interested in are:
- It should be suitable for the frequency we're interested in, about 100MHz,
- It should be rated to take the power we're trying to produce,
- It should have a resistance of 50R (to match the output network of the
amplifier).
Obviously when you walk into the shop you might not want to say "Hello mate,
I'm building a pirate radio transmitter, can I have a dummy load"? so ask for
one for the 2 meter band. This is a common amateur radio band, centred on
145MHz. Most test gear for this band (dummy loads, VSWR meters, power meters,
wavemeters, RF voltmeters, frequency counters etc.) will work on the frequencies
we're interested in.
Even if you tune everything correctly you're still going to have some
harmonics generated by the last stage. A sensible pirate won't let these
harmonics reach the aerial. For example if you're using a frequency of
102.35MHz, the third harmonic is 307.05MHz which happens to be that used in the
UK by USAF Upper Heyford's Control Tower. You might think this is funny but you
won't stay on the air for long. To stop harmonics reaching the aerial we need
either a Low Pass Filter or a Band Pass
Filter.
A Bandpass Filter (BPF) only allows through a narrow band of
frequencies, ie it has a narrow bandwidth, a good one would be less than 1MHz.
It needs standard 50R input and output impedance and be able to take the power
you want to use. It also has to be tuned to the frequency you want let through.
Other frequencies are not stopped completely, but reduced to a much lower level.
It also reduces the level of the desired frequency slightly, by an amount known
as the Insertion Loss. To keep this loss low, bandpass filters
for high output powers are usually pretty chunky numbers.
Pirate gear doesn't usually have this filter built into the final stages, so
if you need one you have to add it on. It needs to be in a well screened case to
stop harmonics leaking out. In fact your whole TX should be well screened to
stop any funny waveforms inside the TX finding their way out. Say for instance
you used a shoebox and had your oscillator running at a third of the
transmitting frequency of 92.25MHz. You could be interfering with the pagers of
a local hospital as they use 31.75MHz, a third of 92.25MHz. Proper screening and
a bandpass filter will eliminate such possibilities.
As an alternative to a Band Pass Filter, you could use a Low Pass
Filter (LPF). This has the advantage that it doesn't necessarily have
to be retuned if you change your frequency. Tune it for best match and minimum
insertion loss.
As you may have guessed, you can't use any connectors at VHF as the
connectors have to "match" the amplifier and feeder. Use BNC or
UHF series (PL259 plug and SO239 socket). UHF is preferable for
higher powers as you can get a wider cable into the plug. N type is also good
but more expensive.
So you've got your nice clean harmonic-free signal coming out of your
bandpass filter - we're on the home run. All that's left is to get the signal up
the aerial feeder to the aerial and we're away. However, as always, there's a
bit more to it than that. The aerial cable needs to match the
TXs output stage at one end and the aerial at the other end. That's right, the
aerial cable has a characteristic impedance as well, just like the TXs output,
the connectors and the aerial, and to match this, the impedance should be 50R.
As well as having the right impedance, it also got to be Low
Loss or your watts will be turned into heat. This is not the same as a
bad VSWR where you lose energy in the TX, a good VSWR does not necessarily mean
the cable is okay. The sort of cable we're talking about is called
coaxial cable or coax, which has an inner conductor totally
surrounded by a circular outer sheath. Decent coax cables for short runs are
UR76 and RG58U. For longer runs or higher powers use UR67. These cables are
available in the UK from Cirkit or
Maplin, amongst others.
At last, the aerial! You can run a pirate station knowing next to nothing
about transmitters, but if you know nothing of aerials you'll have few
listeners. I recommend you read a book on it. I used to recommend the "The Two
Metre Antenna Handbook" by FC Judd G2BCX, a paperback by Newnes Technical Books.
This is now out of print, but you may get it in a library. Some of the aerials
are not particularly useful for our purposes, but he covers subjects like
propagation, matching and VSWR in more detail. All the dimensions he gives are
for the 2m amateur band centred on 145MHz. Any other book covering the 2m radio
amateur band is worth a look.
To convert designs for the 2m band to other frequencies, all dimensions
(including diameter of aerial element, etc) should be multiplied by 145 and then
divided by your frequency in MHz. For example 978mm for 2m becomes 978 x 145/103
= 1377mm for 103MHz.
Another decision you will have to make is what Polarisation
to use. The main ones are horizontal and
vertical. To simplify you can say a horizontally placed aerial
produces horizontally polarised radio waves and a vertically placed one produces
vertically polarised radio waves. To receive a horizontally polarised signal you
need a horizontally polarised aerial, and for a vertical one a vertically
polarised aerial. Most domestic hi-fi FM receivers have horizontally polarised
aerials, but car aerials are vertically polarised. So what polarisation you go
for depends on the audience you expect. For example on Sunday afternoon you'd
expect people at home so use horizontal, whilst in rush hour you might favour
vertical. You can build an aerial which splits the power between both
polarisations, as used by legal stations, this is Mixed
polarisation. But the effect of radio waves bouncing off buildings etc tends to
twist the polarisation of your signal from horizontal to vertical and vice
versa, so your signal will still be picked up by the other sort of aerial.
Your transmitting site will affect your choice of aerial. In the middle of
the area you want to cover you'll need an Omnidirectional
aerial which transmits equally all ways, while outside your coverage area you
can beam the signal in, with a Directional aerial.
The simplest possible aerial for VHF is known as the Half-wave
Dipole and looks like this:
The elements can be bits of thin aluminium or copper tube. The lengths of
each dipole, L, you calculate from your transmitting frequency by the
formula
L = 71 metres
(f in MHz)
The impedance is about 72R, which is close enough to 50R to be fed from 50R
cable without too much power loss. A half-wave dipole used vertically is
omnidirectional, but when used horizontally it has a figure of eight coverage
like this:
which isn't very useful. Furthermore a dipole needs a Balanced
Feed as it is symmetrical, but a coaxial cable provides an
Unbalanced Feed. What's needed is a Balun
(BALance to UNbalance) transformer. These can
be easily made out of bits of coaxial cable. If you don't do this power will be
radiated from the feeder. An aerial with an impedance greatly different from 50R
needs an Impedance Transformer, which can also made out of bits
of coaxial cable. See an aerial book for details.
Before going on air get a low VSWR by adjusting the position of the aerial
and any of it's adjustable pieces. Aim for 2:1 or less. Use low power into the
aerial when tuning it up and making adjustments. If you were using 100s of watts
and a bit of the aerial came off in your hand the VSWR could be so bad as to
blow the final transistor. For the same reason check the DC continuity of the
aerial with an ohmmeter before plugging it in, to be sure it's what it's meant
to be, either a short circuit or an open one, depending on the aerial type. A
dipole should be an open circuit. How to make an
Weatherproof Vertically Polarised Omnidirectional Aerial
Siting is very important. Once again you can have got everything right so
far, but if you fall foul with your site you're facing no listeners syndrome.
The most important factor is Height. This is because VHF radio
waves travel in practically straight lines. If you can see somewhere, you can
transmit to it. So find somewhere with a nice view. The height of your aerial
will determine how far your signal will go, while the power of your TX will
determine how strong the signal is when you get there. If you run 100W into an
aerial in your front room you'll produce a super signal but only your neighbours
will be privileged to hear it. Conversely a 5W TX running into a good location
could give you a coverage of 5 miles (8 km) or more. Keep the aerial well anyway
from anything else and above any obstructions like trees and chimneys. VHF radio
waves do bend a bit so you'll get a bit more coverage than you can see, but its
hard to say how much. Go for It!
Here's a picture
of one I prepared earlier - a 100W Stereo PLL Controlled Tx Graphic
is 73KB.
Exciter Reviews
This page maintained by radio@irational.org Last updated 18
April 2001