While I have time on my hands I thought I'd discuss antenna gain as
it's a much misunderstood subject.
The theory
----------
The first thing to make clear is whoever coined the word "gain" in
regard to antenna performance was an idiot. Antennas do not and
cannot add energy to a radio signal. This is an immutable law of
physics. However we're stuck with the damn word now.
An antenna converts the electrical signals produced by a radio
transmitter into a form of non ionising radiation which we commonly
call a radio wave. You are, of course, well familiar with radio
waves; every time you are unfortunate enough to hear Jon Faine on 774
Melbourne he is annoying you via radio waves. Likewise radio waves
are the basis for your GPS, mobile phone and television and when you
see lightning and hear the radio crackle simultaneously that is also
a radio wave generated by the lightning. Radio waves travel at the
speed of light.
Your UHF CB radio produces around five watts (5W) maximum power, this
is not very much. If the electric kettle in your kitchen used only 5W
it would take around 28 hours to boil one litre of water. When this
5W is transmitted through your radio and via the antenna it spreads
out in the shape of an invisible giant doughnut and any particular
direction sees only a tiny proportion of that 5W energy. This is one
reason why UHF CB has limited range.
The shape in which radio waves are produced by the antenna (our giant
doughnut) is called the antenna "pattern" and is critical to the real
world performance of antennas.
A UHF CB antenna which produces a giant doughnut is wasting a lot of
energy. Why? Well... it is very unlikely we will want to talk to
anyone directly above or below us so all the energy we are
transmitting in those directions is wasted and it would be much more
efficient if we could design an antenna which focused its energy into
a cylinder shape or a deep circle around us. In that way people to
the front, rear and sides would be able to hear us but aeroplanes and
coffins would not and that is, generally, what we want from a CB.
That is exactly what antenna gain is - an antenna which is able to
focus its energy in the desired direction. The ratio or amount by
which it is able to redirect radiated energy from one direction to
another is the antenna gain and is measured in decibels (dB).
Note: Decibels are used to measure ratios and have no native unit so
antenna gain is usually referenced to either an isotropic antenna
(dBi) or a dipole antenna (dBd). In general it will be dBi because
that produces a bigger number and manufacturers like that.
You can think of antenna gain exactly like a torch with a focusing
ability; the tighter you make the focus the further you will be able
to see BUT you will only see a smaller area because you have made the
beam more narrow. If you open the focus up you'll be able to see a
much wider area but you won't see as far into the distance. As always
with energy you cannot get something for nothing.
The real world
--------------
You will see advertisements for UHF CB antennas which give a gain
figure, 4dB, 6dB and 8dB are common and their prices usually follow
the same rising pattern.
Vehicle antennas are almost always vertical and single element which
means any gain they have must come by redirecting vertical energy to
horizontal energy. Such an antenna cannot redirect energy in the
horizontal plane (corrections welcome). In other words it will
produce a circular pattern and is therefore pretty limited in how
much gain it can generate and I doubt if many commercial UHF CB
antennas could actually meet their specification.
The next problem we have is that a vertical antenna is only half of
the antenna system (we'll ignore so called ground independent
antennas for the moment) with the vehicle comprising the other half
and this is where the real world takes over.
The antenna manufacturer has no control over how or where the antenna
is mounted and, especially at UHF frequencies, this make a huge
difference to antenna performance and can easily negate all that
extra gain you thought you were buying. Each installation will be
different but it may well be that 70% of your CB's energy is being
transmitted south and west and only 30% north and east - these
variations are called "antenna lobes" and there is no simple way to
determine your particular antenna installation's pattern.
And, after all that, even if the You Beaut antenna works really well
and is a 4dB improvement over your old one it will only make the
smallest of difference to the signal actually received by another CB
user. ie. It will make an unreadable signal slightly less unreadable
it will not turn it into a fully readable one.
My suggestions: Buy the most popular, and probably amongst the
cheapest, antenna from one of the major antenna manufacturers and
mount it in the middle of the roof - this will give excellent
performance. If you don't wish to cut a hole in the roof a magnetic
mount works well at UHF otherwise a wing mount is probably the next
best.
Comments and corrections most welcome.
MH
it's a much misunderstood subject.
The theory
----------
The first thing to make clear is whoever coined the word "gain" in
regard to antenna performance was an idiot. Antennas do not and
cannot add energy to a radio signal. This is an immutable law of
physics. However we're stuck with the damn word now.
An antenna converts the electrical signals produced by a radio
transmitter into a form of non ionising radiation which we commonly
call a radio wave. You are, of course, well familiar with radio
waves; every time you are unfortunate enough to hear Jon Faine on 774
Melbourne he is annoying you via radio waves. Likewise radio waves
are the basis for your GPS, mobile phone and television and when you
see lightning and hear the radio crackle simultaneously that is also
a radio wave generated by the lightning. Radio waves travel at the
speed of light.
Your UHF CB radio produces around five watts (5W) maximum power, this
is not very much. If the electric kettle in your kitchen used only 5W
it would take around 28 hours to boil one litre of water. When this
5W is transmitted through your radio and via the antenna it spreads
out in the shape of an invisible giant doughnut and any particular
direction sees only a tiny proportion of that 5W energy. This is one
reason why UHF CB has limited range.
The shape in which radio waves are produced by the antenna (our giant
doughnut) is called the antenna "pattern" and is critical to the real
world performance of antennas.
A UHF CB antenna which produces a giant doughnut is wasting a lot of
energy. Why? Well... it is very unlikely we will want to talk to
anyone directly above or below us so all the energy we are
transmitting in those directions is wasted and it would be much more
efficient if we could design an antenna which focused its energy into
a cylinder shape or a deep circle around us. In that way people to
the front, rear and sides would be able to hear us but aeroplanes and
coffins would not and that is, generally, what we want from a CB.
That is exactly what antenna gain is - an antenna which is able to
focus its energy in the desired direction. The ratio or amount by
which it is able to redirect radiated energy from one direction to
another is the antenna gain and is measured in decibels (dB).
Note: Decibels are used to measure ratios and have no native unit so
antenna gain is usually referenced to either an isotropic antenna
(dBi) or a dipole antenna (dBd). In general it will be dBi because
that produces a bigger number and manufacturers like that.
You can think of antenna gain exactly like a torch with a focusing
ability; the tighter you make the focus the further you will be able
to see BUT you will only see a smaller area because you have made the
beam more narrow. If you open the focus up you'll be able to see a
much wider area but you won't see as far into the distance. As always
with energy you cannot get something for nothing.
The real world
--------------
You will see advertisements for UHF CB antennas which give a gain
figure, 4dB, 6dB and 8dB are common and their prices usually follow
the same rising pattern.
Vehicle antennas are almost always vertical and single element which
means any gain they have must come by redirecting vertical energy to
horizontal energy. Such an antenna cannot redirect energy in the
horizontal plane (corrections welcome). In other words it will
produce a circular pattern and is therefore pretty limited in how
much gain it can generate and I doubt if many commercial UHF CB
antennas could actually meet their specification.
The next problem we have is that a vertical antenna is only half of
the antenna system (we'll ignore so called ground independent
antennas for the moment) with the vehicle comprising the other half
and this is where the real world takes over.
The antenna manufacturer has no control over how or where the antenna
is mounted and, especially at UHF frequencies, this make a huge
difference to antenna performance and can easily negate all that
extra gain you thought you were buying. Each installation will be
different but it may well be that 70% of your CB's energy is being
transmitted south and west and only 30% north and east - these
variations are called "antenna lobes" and there is no simple way to
determine your particular antenna installation's pattern.
And, after all that, even if the You Beaut antenna works really well
and is a 4dB improvement over your old one it will only make the
smallest of difference to the signal actually received by another CB
user. ie. It will make an unreadable signal slightly less unreadable
it will not turn it into a fully readable one.
My suggestions: Buy the most popular, and probably amongst the
cheapest, antenna from one of the major antenna manufacturers and
mount it in the middle of the roof - this will give excellent
performance. If you don't wish to cut a hole in the roof a magnetic
mount works well at UHF otherwise a wing mount is probably the next
best.
Comments and corrections most welcome.
MH
