Bredhurst Receiving and Transmitting Society

Technical Basics Part 3

Prefixes

Prefixes milli, kilo and Mega.

This is something that we left over from the previous page as it does sometimes cause some students confusion.

I would think that many of you have heard of kilo - it is used in shops for say 1 kilo grams of potatoes and that means 1000 grams.

AND kilo is used in just the same way in electronics thus 1000 ohms is the same as 1k ohms, note the "k" in kilo is small.

If we want to go a 1000 times bigger than 1k ohms we used the word Mega.

and so 1000000 ohms is the same as 1 Mega or 1M, note the use of the large "M".

So that leaves milli, poor little milli. Well that might help you to remember her as she is 1000 times smaller than 1 which if we write it in decimal is 0.001

and so when this is used with say amps and we want 0.001 amps we write it as 1mA, note the small "m" and big "A".

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 is how many times a thing occurs in a certain time.

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.

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

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

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.

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 for HF, VHF and UHF radio signals up to 1,000 MHz

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)
1.8MHz to 30MHz	is called the HF (High Frequency) amateur band
30MHz to 300MHz	is called the VHF (Very High Frequency) amateur band
300MHz to over 1000MHz	is called the UHF (Ultra High Frequency) amateur band
144MHz to 146MHz	is the VHF amateur band
430MHz to 440MHz	is the UHF amateur band
NOTE:- 1kHz =1,000Hz            1MHz = 1,000,000Hz

EXAM TIP

In some exam papers in addition to the abbreviation AF, RF, HF, VHF, and UHF  the abbreviations of LF and MF have been are used. 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.