MF
300 Khz to 3 Mhz which mainly includes the A.M. radio
band of about 530 Khz to 1650 Khz (varies between countries).
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OHMS LAW
There are four basic electrical units here. They are
(1) Power - in watts - [P], (2) Voltage
- in volts - [E], (3) Current - in amperes or
amps - [I] and (4) Resistance - in ohms - [R].
Now how basic can you get? Easy to remember!.
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OSCILLATOR
An oscillator is an electronic circuit where some of
the amplified output is fed back to the input to maintain
a flywheel effect or oscillations. Circuit design, components
and layout the frequency of oscillation. For extreme
accuracy we might use a crystal to maintain frequency.
At its most basic we could design one simple oscillator
circuit to operate at 7050 Khz. Although not recommended
we could apply and remove power at a morse code rate
and we would have a simple yet extremely crude C.W. transmitter operating in the 40m amateur radio
band as a Q.R.P. transmitter. Don't even think about it!.
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RECEIVER
A collection of circuits designed to receive signals
over one or more bands of interest and covering one
or more modes of operation.
At its most simplest it could be a crystal set designed
to receive the a.m. radio band. At its most complex
it could be a very sophisticated surveillance receiver
designed to cover anything and everything.
Typical receivers are a.m. / f.m. tuners, t.v. receivers
or s.w. radio.
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RTTY or rtty
This stands for Radio Teletype where amateurs, amongst
others, would transmit signals generated by a keyboard
device not unlike you sending email. Instead of an internet
connection you have a radio connection. I had only a
passing interest in this aspect of the hobby and that
was about 25 years ago. As to what is happening today
I don't know but I suspect it's a fair bet that computers
and packet radio (bit like the www) have overtaken it.
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QRP or qrp ( Low
Power Transmission )
This is a very much reduced power mode of operation
favoured by many amateur radio operators because of
the skill and challenges involved in making contacts.
Power is limited to 5 watts maximum on C.W. or 10 watts
on S.S.B. although of course often less is used. Because
of the low power output the equipment can be battery
operated and is quite suitable for portable operation.
Frequently Q.R.P. activities are conducted in conjunction
with other recreational pursuits such as fishing, camping
etc. Often the equipmentis home made (home brewed).
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S/N RATIO
Signal to Noise Ratio. Noise is the ultimate
limiting factor in the reception of radio signals. Noise
is generally classified as either natural (QRN) or man
made (QRM).
Natural noise emanates from galactic and atmospheric
noise picked up by the antenna as well as thermal noise
generated in the antenna itself. Similarly man made
noise such a fluorescent lights, motors and a host of
other appliances and tools is also picked up by the
antenna. These sources of noise are then amplified by
the various stages in a receiver. However these amplifying
devices create noise of their own also.
The noise problem varies with the frequency of reception.
Generally the noise figure of a receiver (i.e. allowing
for the noise generated by the receiver itself) is not
of great importance for frequencies somewhat below 30
Mhz because the external noise combined i.e. natural
and man made will always exceed the noise figure of
the receiver.
Visualise if you can, an arbitrary noise level of 10
uV - these figures are for comparative or illustrative
purposes only and bear no resemblance to reality. Now
that is 10 one millionths of a volt.
If we wished to receive a certain signal that was received
on our antenna and had a strength of say 1000 uV or
100 times the noise voltage you can see such a signal
would be copied quite readily. On the other hand if
a desired signal was only 1 uV (and many often are at
this level) then the noise level outweighs it by a ratio
of 10:1.
Now that is a pretty rough explanation but you should
get the general idea. Of importance to reception, the
narrower the bandwidth of the receiver, the bigger the
improvement to your prospects of recovering the desired
signal.
Once a desired signal drops in level in comparison
to noise in a particular location then all hope of recovery
is lost.
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SSB or ssb ( Single
Side Band )
Assuming you have 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.
If our carrier (initially) in the transmitter is say
9000 Khz or 9 Mhz and we modulate that signal with useful
voice frequencies of say 300 Hz to 2400 Hz (this spectrum
contains all useful information and indeed is roughly
the bandwidth of your telephone system) then dealing
with the highest frequency of 2400 Hz (2.4 Khz) we get
side bands (including carrier) of 000 Khz to 9002.4
Khz as well as 9000 Khz down to 8997.6 Khz.
Fifty per cent of our power is in the 9000 Khz carrier
and 25% in each of our side bands. At high power levels
this would be both wasteful and inefficient. In fact
at this point in our transmitter we are only dealing
with very low power levels.
What if we introduce a highly specialized and highly
accurate filter that will only accept frequencies in
the range of 9000.3 Khz to 9002.4 Khz and reject all
others. Presto we have a signal which occupies a bandwidth
of only 2.1 Khz wide, therefore more channels then can
be accommodated in the same band. Further the power
amplification formerly available to us can now be devoted
exclusively to our narrow band signal in a linear amplifier.
Unfortunately at the receiving end things get a lot
more complicated and expensive. Firstly our I.F. Amplifier
must accept signals no wider than the 2.1 Khz. In practice
you use a similar crystal filter or if the receiver
is part of a transceiver then you use the same filter
as was used in the transmit section. Secondly because
no carrier is transmitted with the S.S.B. signal we
must provide one locally in the receiver. This is called
a B.F.O. or Beat Frequency Oscillator and 9000 Khz is
typical but not the only frequency or method.
When mixed with the received signal the B.F.O. and
Detector (sometimes called Product Mixer) will put out
our original audio of 300 Hz to 2400 Hz. All other frequencies
are filtered out.
In the transmit section the 9000.3 to 9002.4 signal
is mixed with a local oscillator signal or frequency
to produce a signal at our final frequency of transmission.
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SWR or swr
This is called Standing Wave Ratio (or more correctly
V.S.W.R.) and is much beloved by many who like to become
paranoid over something. For some unknown reason C.B.'ers
seem to excel at this.
Contrary to popular belief it is not the holy grail.
Whilst everyone should strive for technical excellence
as well as efficiency there is absolutely no reason
to slash your wrists because you can't get an ideal
S.W.R.
A transmitter requires a load to deliver power to.
This is called an Antenna.
If some of the transmitted power is reflected back
along the transmission line toward the transmitter then
we have a situation where voltage standing wave patterns
exist.
"The ratio of the maximum voltage on the line to the
minimum value (provided the line is longer than a quarter
wavelength) is defined as the voltage standing wave
ratio or vwsr"
It is often mistakenly assumed that power reflected
from a load is power lost. If there is proper matching
at the input end of the line this is only true if there
is considerable loss in the line itself.
Be technically efficient not paranoid.
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S METER
You need to first understand both I.F. amplifiers and automatic gain control (A.G.C.)
An S-meter is simply a meter set up to measure the
current through the agc control line so that on strong
signals it shows say S9 + 20 dB while on weak signals
it might be at the bottom end of the scale on say S1
or S2.
Be aware, S-meters are notoriously inaccurate, can
not be compared between various receivers and are only
useful for relative measurements applicable to your
receiver, using your current antenna, at your present
location and on one particular band. They are widely
misunderstood and all too often given a level of importance
they don't rightly deserve.
S-meters are as accurate as a group of people standing
by the road and individually estimating the speed of
passing vehicles.
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TRANSCEIVER
A unit which contains both the transmitter and receiver.
It has the advantage that common electronic circuits
are shared rather than duplicated if you operated the
two units separately.
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TRANSISTOR
Firstly we had valves and then two bright sparks in
Bell Labs. back in 1948 invented the transistor. Almost
similar to a valve (triode) the transistor has revolutionized
the world.
Not only are there types which handle very high voltages
there are types capable of very large amounts of power.
Because of transistors much equipment has shrunk to
a fraction of its former size while extending capabilities
almost beyond imagination.
By the 1960's transistors began to become integrated
together in single packages to perform all sorts of
logic blocks. The digital explosion had begun.
Today literally millions and millions of transistors
are formed on the one thin wafer to produce devices
like the Pentium IV processor at previously unheard
of speeds and power.
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TRANSMITTER
see Oscillator
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UHF or uhf
300 Mhz to 3 Ghz (that's 3,000 Mhz or 3,000,000,000
cycles per second) - this band is occupied by U.H.F.
T.V., some radar installations, mobile phones, two way
radios and a heap of other exciting stuff.
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USB or usb ( Upper
Side Band )
Identical but the direct opposite to L.S.B. (Lower Side band) - see also A.M. and S.S.B.
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VHF or vhf
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.
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WATT
The fundamental unit of power consumed see Ohms Law
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WAVELENGTH
Originally in radio, the frequency of signals was not
mentioned. The custom was to refer to the 'wave length'.
This is easily computed from:
In reality this indicates, from a purely technical
standpoint, that a wavelength is determined by dividing
the speed of light by the signal frequency in cycles
per second. The underlying reason here is that radio
waves do travel at the speed of light. This is approximated
as 300 million metres-per-second (and no correspondence
will be entered into on that point).
As frequencies increased by about the 1930's the wavelengths
diminished in physical size to the point the term 'short-wave'
came into vogue.
To understand radio waves etc. visualise a pebble dropped
into a pond. At the point where the pebble hits the
water the transmitting antenna is situated. Waves then
radiate outward from that antenna.
