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Technical Basics Part 3


Frequency

3.c Frequencies used in power, audio and radio systems.

3c.1 Recall the unit of frequency and understand the meaning of the abbreviations RF and AF

This section takes us away from the complexity that you have read up to now and into a new area.

If you were asked "How many times a week do you eat an apple" then your answer could be 2.

You could also be asked "How frequently do you eat an apple in a week" and you could still answer two.

Frequency, in amateur radio Frequency is the number of cyles that occur in a second.

When an occurrence is once per second that is said to be one cycle per second or put it in the terms of electronics 1 Hertz or 1Hz.

The Hertz is the unit of frequency and 1Hz is a repetitive occurrence 1 per second, note the large "H" and small "z".

The abbreviation RF stand for Radio Frequency and AF is for audio Frequency ...more about that later.

Identify the graphic representation of a sine wave and recall that sine waves are produced by oscillators.

SINE WAVE

The sine wave is the most basic of wave forms and is one that you have to be able to recognise.

A graphic animation of a sine wave showing how it changes with time

Place your mouse pointer on the graphic to see the sine wave motion.

To make the diagram show more cycles of the same signal you would join another drawing with the A to the B as many times as required.

One complete cycle exits between points A & B, and also between any two other similar points on the wave form.

One complete cycle of the sine wave is also represents one complete revolution of the generator or dynamo.

In a transceiver that is a section called an oscillator and this produces sine waves to generate the frequency of operation.

Recall the frequency of the mains supply 50Hz.

50Hz Mains voltage

a diagram showing the shape of a sine wave set in a circle.and a text block saying UK mains electricity 50Hz

We have spoken about mains voltages and that it is alternating current AC. The number of times this changes per second is 50 hence we say mains frequency is at 50Hz.

Faster Frequencies

Frequencies in amateur radio and electronics in general have many things happening much faster than mains frequency.

AF = Audio Frequency

You have to know that the abbreviation for Audio Frequency is AF

Recall the range of frequencies for normal hearing 100Hz - 15kHz

Audio frequencies are the range of frequencies that can be heard with normal hearing. The range of frequencies for normal hearing between 100Hz- 15kHz

a drawing of a head with only ears and the text above stating 100Hz to 15kHZ

Recall the range of frequencies for audio communication 300Hz - 3kHz.

However the frequency range is much less for normal audio communication between 300Hz-3kHz. Sometime this is thought of as a band width of 2700Hz of 2.7kHz. Note the use of the small "k" for the kilo which is then linked with the Hz for Hertz.

a rectangular box shape with the words RADIO COMMUNICATION RECEIVER and above that text of Audio frequency range 300Hz to 3kHz

RF = Radio Frequency

You have to know that the abbreviation for Radio Frequency is RF.

For communication the RADIO FREQUENCY ranges go much much higher ...

Recall the frequency bands for HF, VHF and UHF radio signals

You need to know the frequency bands used by amateurs in the HF bands, VHF bands and UHF bands for radio signals up to 1,000 MHz.

The progression of HF VHF UHF is actually a progression of increasing frequency of the radio bands.

The amateur bands come into the radio spectrum just above the Medium wave broadcast bands also called MF or medium frequency so including this below the frequency ranges are

These are :-

136kHz - 536kHz

is called the LF (Low Frequency)

536kHz - 1.8MHz

is called the MF (Medium Frequency)

in the range 1.8MHz to 30MHz there are several frequency bands using by amateurs

160m ------ 1.8 - 2MHz

80m ------ 3.5 - 4MHz

40m ------- 7 - 7.3MHz

30m ------ 10 - 10.1MHz

20m ----- 14 - 14.350MHz

17m --- 18.068 - 18.168MHz

15m ----- 21.0 - 21.450MHz

12m ----- 24.890 - 24.990MHz

10m -------- 28.0 - 29.70MHz

Amateurs can be heard on air saying that they have work several stations on 15 meters and this means that they have work several other amateurs radio operators on the frequencies between 21.0 - 21.450MHz.

The use of the 15m is a sort of short hand rather than having to say 21.0 - 21.450MHz

is called the HF (High Frequency) amateur band

in the range 30MHz to 300MHz there are three frequency bands using by amateurs

6m ----- 50 - 54MHz

4m ----- 70 - 70.5MHz

2m ------ 144 - 148MHz

is called the VHF (Very High Frequency) amateur band

in the range 300MHz to over 1000MHz there is one frequency band using by amateurs

70cm ------ 430 - 440MHz

is called the UHF (Ultra High Frequency) amateur band

NOTE:- 1kHz =1,000Hz 1MHz = 1,000,000Hz

EXAM TIP

In addition to the abbreviation AF, RF, HF, VHF, and UHF the abbreviations of LF and MF should be understood so that you have a full understanding of the radio spectrum. See the diagram above to help you recognise the various abbreviations.


The next section is an easier section as you are provided with documentation in the exam.

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The origin of some of the text on this page is from the RSGB with additions by the web master

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