AGC (Automatic Gain Control )

Is the the automatic regulation (electronically) of the gain of a receiver in inverse proportion to the received signal strength. This allows, within certain limits, the audio output of a receiver to remain relatively constant over a range of fading signal conditions.

Where the "phase" of the current amplitude varies with time. One complete cycle occupies 360 degrees irrespective of amplitude (visualise a circle). The number of these cycles-per-second is the frequency of the signal.

For mathematical reasons this is referred to as a sine wave. A signal may commence at 0 degrees then go to its most positive value at 90 degrees then recede back to zero value at 180 degrees and continue to its most negative value at 270 degrees and then turn back to zero again at 360 degrees. This is then one complete cycle.

Perhaps the most common frequency around a home is our power mains. In Australia the frequency used for power mains is 50 cycles per second or now referred to as 50 Hz. The abbreviation is an acknowledgement to Heinrich Hertz. In the U.S.A. and other parts of the world the mains frequency is 60 Hz.

With a 50 Hz mains frequency one cycle occupies 1 / 50th of a second or 20 milli-seconds.

Therefore the signal is most positive after 5 milliseconds, back to zero after another 5 milliseconds, down to its most negative after the next 5 milliseconds and finally back to zero after a final 5 milliseconds. This whole cycle occupies 20 milliseconds or 20 mS and repeats 50 times a second.

With a 60 Hz mains frequency of course one cycle occupies 1 / 60th of a second or 16.67 milli-seconds.

A.C. at audio frequencies extends from 20 Hz to about 20,000 Hz or 20 Khz. Depending upon your age you will not actually hear it beyond 15 Khz and older people are unable to hear much beyond 10 Khz. Animals can hear much higher frequencies. The audio A.C. frequencies are referred to as A.F.

Signals beyond those above are referred to as radio frequencies ( RF ) and generally cover the spectrum:

L.F. - 30 Khz to 300 Khz although there are signals transmitted well below this region principally the OMEGA navigation network.

M.F. - 300 Khz to 3 Mhz which mainly includes the A.M. radio band of about 530 Khz to 1650 Khz (varies between countries).

H.F. - 3 Mhz to 30 Mhz and comprises amateur radio, short wave broadcasters among a host of others. Largely becoming superseded by satellite transmissions.

V.H.F. - 30 Mhz to 300 Mhz occupied by traditional T.V. stations, some amateur bands, commercial two way radio, maritime and aircraft bands as well as the F.M. radio band of 88 - 108 Mhz.

U.H.F. - 300 Mhz to 3 Ghz this band is occupied by U.H.F. T.V., some radar installations, mobile phones, two way radios and a heap of other stuff.

Beyond 3 Ghz is virtually satellite transmissions.

It is interesting to note by way of numerical comparison that firstly, each band is 10 times the previous band. Secondly the L.F. band spanning 30 to 300 Khz could be duplicated 10,000 times over in the space occupied by the U.H.F. band.

Also at the bottom end of 30 Khz the signal cycle repeats 30,000 times a second. At the of the U.H.F. band the signal cycle repeats 3,000,000,000 times a second (mind boggling?).

A very important attribute of A.C. (e.g. 50 / 60 Hz) is that it is generally easy to convert voltages with the aid of power transformers.

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Now we have learnt above about audio frequencies A.F. and also about radio frequencies R.F.

In the early days of what is now known as early radio transmissions, say about 100 years ago, signals were generated by various means but only up to the L.F. region.

Communication was by way of morse code much in the form that a short transmission denoted a dot (dit) and a longer transmission was a dash (dah). This was the only form of radio transmission until the 1920's and only of use to the military, commercial telegraph companies and amateur experimenters.

Then it was discovered that if the amplitude (voltage levels - plus and minus about zero) could be controlled or varied by a much lower frequency such as A.F. then real intelligence could be conveyed e.g. speech and music. This process could be easily reversed by simple means at the receiving end by using diode detectors. This is called modulation and obviously in this case amplitude modulation or A.M.

This discovery spawned whole new industries and revolutionized the world of communications. Industries grew up manufacturing radio parts, receiver manufacturers, radio stations, news agencies, recording industries etc.

There are three distinct disadvantages to A.M. radio however.

Firstly because of the modulation process we generate at least two copies of the intelligence plus the carrier. For example consider a local radio station transmitting on say 900 Khz. This frequency will be very stable and held to a tight tolerance. To suit our discussion and keep it as simple as possible we will have the transmission modulated by a 1000 Hz or 1Khz tone.

At the receiving end 3 frequencies will be available. 900 Khz, 901 Khz and 899 Khz i.e. the original 900 Khz (the carrier) plus and minus the modulating frequency which are called side bands. For very simple receivers such as a cheap transistor radio we only require the original plus either one of the side bands. The other one is a total waste. For sophisticated receivers one side band can be eliminated.

The net effect is A.M. radio stations are spaced 10 Khz apart (9 kHz in Australia) e.g. 530 Khz...540 Khz...550 Khz. This spacing could be reduced and nearly twice as many stations accommodated by deleting one side band. Unfortunately the increased cost of receiver complexity forbids this but it certainly is feasible - see Single Side Band.

The second disadvantage is half the transmitted power is in the carrier (900 Khz in our example) and 25% is in each side band of which we only need one. For a commercial radio station transmitting at say 20 Kw of power, about 15 Kw is wasted but for them this is no great burden because availability of cheap and simple receivers for the listener is of far greater importance.

The third disadvantage is that whilst the signal is amplitude modulated, common forms of radio interference are also amplitude in nature. Examples of such interference to radio reception are natural phenomena such as electrical storms etc. (QRN) as well as man made noise (QRM) which can emanate from nearby electrical appliances, lights, electric drills or even the humble electronic calculator and most probably your computer.

To get away from this amplitude affect by noise F.M. Radio was devised.

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Ever since radio transmissions first began there were experimenters and tinkerers'. Indeed even today a great many of the advances in radio science continue to come from this band of people.

Most are now called "Amateur Radio Operators" and to prevent total chaos each would-be operator sits for modest examinations set by the laws of his or her region to gain a licence.

You will find amateurs are courteous, helpful, constructive (in more ways than one) and always have a warm welcome for newcomers. If you need help just politely ask.

Amateur Operators have assigned bands and modes of operation. They also observe certain standards of etiquette and ethics. Amateurs play a significant role in providing communication links when needed, particularly in times of natural disaster. It is a wonderful fraternity which over-rides the boundaries of nationality, politics and religion.

Virtually every country in the world has an umbrella amateur radio organization. In the U.S.A. it is the American Amateur Radio League (A.R.R.L.), Great Britain has the Radio Society of Great Britain (R.S.G.B.) and Australia has the Wireless Institute of Australia (W.I.A.).

If you are a newcomer to radio and are keen to pursue it as a hobby then contact the amateur radio organization for your region. Help is available everywhere.

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This is the unit of electrical current flow. The rate at which current flows past a given point.

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An attenuator is a passive network comprising usually, but not always, resistors that reduce the power or voltage level of a signal without introducing significant distortion.

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See amplitude modulation.

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A generalized expression meaning morse code transmission.

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This is a relative power unit. At audio frequencies a change of one decibel (abbreviated dB) is just detectable as a change in loudness under ideal conditions.

For a given power ratio the decibel change is calculated as:

If we used voltage or current ratios instead then our formula becomes:

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Direct Current is where at all times the voltage polarity remains constant. Unlike a.c. there is no varying cycle.

D.C. may however, particularly where it is rectified from mains a.c., contain residual a.c. superimposed or part of the voltage. This is often referred to as mains hum.

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A means or circuit designed to convert amplified R.F. energy into recovered audio which contains the desired intelligence.

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A dipole is an antenna. It is a fundamental form of antenna consisting of a single wire whose length is approximately equal to half the transmitting wavelength.

The length of a half wave in space is approximately:

The actual physical length in practice is slightly different from this owing to other factors.

A most popular dipole known to almost everyone is the folded dipole which forms part of most V.H.F. T.V. Antennas.

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D.X. is a short hand way of saying 'long distance'. In the early days of radio a lot of short hand was devised to minimize morse code transmissions.

A dx'er is rather like an angler who has gone fishing. The angler seeks a catch of the biggest fish whilst the avid dx'er seeks the elusive 'long distance' contact.

Whether he/she be an amateur radio operator, short wave listener (s.w.l.'er) or even an a.m.b.c.b. dx'er (a.m. radio broadcast band dx'er}. Absolutely fascinating!.

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Frequency modulation was devised to overcome the problem that A.M. reception was susceptible to noise interference.

With F.M.  instead of the carrier having its amplitude modulated the signal frequency is varied or controlled by the modulating (audio) frequency.

In the receiver the signal undergoes a great deal of amplification where the s and bottoms are chopped of the signal - this is called 'limiting'. By limiting the amplitude of the signal all a.m. components (including noise) are thereby removed. This is why F.M. is preferred for quality music transmission. On the downside it tends to occupy greater bandwidth although narrow band F.M. does exist for two-way communication.

Commercial F.M. broadcasts occupy 200 Khz channels throughout the 88 - 108 Mhz band. This compares with the 10 Khz (or 9 Khz) channel spacing in the a.m. radio band or short wave broadcasting.

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3 Mhz to 30 Mhz and comprises amateur radio, short wave broadcasters among a host of others. Largely becoming superseded by satellite transmissions.

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Tuned circuits in radios have one severe limitation - bandwidth. Without going into a complex explanation let us assume that the best response can be about 2% of the signal frequency. In the early days of a.m. radio, circuits simply tuned straight across the frequency band of interest.

Applying our 2% rule we find at say 540 Khz, the bandwidth is 10.8 Khz. We would be able to receive this signal without a great deal or little interference from adjacent channels. On the downside if we wanted to receive a signal at say 1550 Khz our bandwidth becomes 31 Khz or spanning 3 channels. We would have little hope of satisfactorily receiving a signal because our bandwidth also now includes both adjacent channels.

A method of receiving called the 'superhetrodyne' principle evolved.

Here as part of our receiver we have a 'local oscillator' or mini transmitter where the incoming received signal is mixed with the local oscillator. As a result 4 frequencies become available.

Firstly the original signal, (2) then the original local oscillator signal, (3) then the original signal plus the local oscillator signal and then finally (4) the original signal minus the local oscillator signal.

Confused?. Consider this practical example of your little transistor a.m. radio. It is designed to receive about 540 - 1650 Khz. The local oscillator will always tune in tandem with the input section to produce another signal at 995 - 2105 Khz.

At all times the difference frequency is a constant 455 Khz or what is called the intermediate frequency or I.F. All other frequencies arising from this process are then filtered out.

When you tune your radio you are actually tuning the local oscillator which is more correctly called the 'V.F.O.' or variable frequency oscillator.

Because we always have a constant difference frequency of 455 Khz it is relatively easy to design and construct narrow band circuits to suit our requirements. It is in these circuits (I.F. Amplifier) that the greatest amplification occurs.

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30 Khz to 300 Khz, although there are signals transmitted well below this region, principally the OMEGA naval navigation network.

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Assuming you had read the section on A.M. you would be aware that two of the disadvantages of a.m. transmission are the twice the bandwidth to convey the same information and only 25% of the power is used in each side band. The remaining 50% of power is expended in the carrier.

It makes more sense in terms of economy of bandwidth as well as economy of power to simply transmit only one side band. This is called S.S.B. or Single Side band.

Depending upon which side band is chosen to transmit one is called upper side band and the other is called lower side band. In amateur radio, conventions exist as to which side band is transmitted in a particular amateur band.

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