You need to consider
The higher the gain the better the performance.
In order to get a higher gain the antenna needs to be longer, there is no such thing as a short high gain UHF antenna |
The longer the antenna the better the performance.
Long antennas have high gain on transmit
as well as receive. The highest gain on a moving vehicle should not be greater than 7.5dB as antenna becomes too long and the angle of radiation too low! |
The Higher the antenna the better the performance.
Where you put your antenna is
most important, It should be high as possible and as far away as possible
from any other antenna or obstacle. |
UHF antennas do not need to be tuned.
UHF Antennas are very broadband, 10 to 15MHz in some cases, this means they do not need tuning. |
When
deciding on an antenna
You need to ask
these questions
Where do I plan to put the antenna?
Bullbar, Roof, or Front Guard?
Best is usually the center of the roof, roof Gutter, front Guard then bullbar - the higher the better. If you put any antenna on a bullbar you should consider a spring to absorb
the vibration, this will lengthen the life of the antenna.
|
What is the longest antenna that I can cope with?
Length matters , but if you require an antenna that
is short but has a reasonably high gain
then I suggest the "RF470 Range" these are approximately one metre long and have 6dB transmit gain. |
What do I plan to do? Highway or Bush bashing?
This
will determine weather the antenna should be S/Steel or fibreglass |
There are two main types of UHF antennas
Monopole
& Ground
Plane Independent
Monopole
Antennas
"A single antenna that uses a metal area under the antenna
for the ground plane"
Monopole
antennas require to be mounted onto a Base and need
at least one square metre of metal as a ground plane
to work effectively, this ground plane can be a Bull
bar, roof, Balcony railing, any metal surface area. Monopole
antennas are low profile, easy to install, they are
available in various colours may either PVC (plastic) or polyolefin (rubber) covering.
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|
Some Do's & Don't's
with Monopole Antennas
Do - |
Test
for shorts or open circuit if possible with an multimeter
or check SWR with a SWR meter
Testing the antenna ensures that
all is in working order including the transceiver |
Do - |
Mount
the antenna high as possible - Gutter
mount or roof is best
Centre of the roof is not always
possible but it still is the best place, next best is on
a "BMS" or "BMB" mount |
Don't
- |
Use
a spring even if mounted on a bullbar - It
may damage your radio
The spring is important to absorb the
vibration from the engine but will make the antenna out of
phase greatly reducing the performance |
Don't
- |
Use
a wire strap to earth the base - It
may damage your radio
You must earth the antenna at the base,
supplying a sufficient ground plane area |
Don't
- |
Mount
the antenna along side any metal object - As
far away as possible
Any metal object will absorb the radiation
from the antenna, greatly reducing its performance, you need
at least 700mm from a wall or from another antenna for 477MHz
antennas, if this is not possible then mount the antenna
as far away from any other antenna or metal object as possible. |
Ground Plane Independent
Antennas
"A single antenna that has its own in built simulated Ground plane"
Monopole
antennas require to be mounted onto a Base and need
at least one square metre of metal as a ground plane
to work effectively, this ground plane can be a Bull
bar, roof, Balcony railing, any metal surface area. Monopole
antennas are low profile, easy to install, they are
available in various colours may either PVC (plastic) or polyolefin (rubber) covering.
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|
Ground Independent Antennas
There are three types of ground independent antennas
DC Grounded, RF Choke & Raised Feed
DC Grounded
Most high gain - high performance
antennas are DC Grounded and do not need tuning
| DC Grounded means the antenna has
what is known as a LC Network, Tuning stub or similar method for
matching the antenna in order for it to work on the frequency required,
what this means is that if you were to measure the connection at
the base of the antenna with a ohm meter it would read a DC dead
short (DC Grounded) - This is normal and is a common way of designing
antennas. DC Grounded antennas do not need a ground plane or earth as in the case
of monopole antennas. |
Some Do's & Don't's
with DC Grounded Antennas
| Do - |
Test
for shorts or open circuit if possible with an multimeter or
check SWR with a SWR meter
NOTE- When the antenna is tested with
a ohm meter the reading is 0 Ohms at the base
Testing the antenna ensures
that all is in working order including the transceiver |
| Do - |
Mount
the antenna high as possible - Bullbar
is often the best place for this style of antenna |
| Do
- |
Use
a spring if mounted on a bullbar. The
spring is important to absorb the vibration from the engine, without the spring the antenna eventually may
fall apart! |
| Do
& Don't |
Earthing
the antenna at its base, doesn't matter - It
makes no real difference to performance |
| Don't - |
Mount
the antenna along side any metal object - As
far away as possible
Any metal object will absorb
the radiation from the antenna, greatly reducing
its performance, you need at least 700mm from a
wall or from another antenna for 477MHz antennas,
if this is not possible then mount the antenna
as far away from any other antenna or metal object
as possible. |
RF Choke
| Mobile One make a few antennas
that use the RF Choke method of matching, this is achieved by
coiling the coax in a way to produce the correct matching for
the frequency, they do not need a ground plane nor do they need
to be earthed, These antennas are low profile and low gain but
have a large bandwidth that makes them ideal for use where height
and lack of ground plane is an issue. |
Some Do's & Don't's
with RF Choke Style Antennas
| Do - |
Mount
the antenna high as possible - Roof
or Roof gutter is best for this antenna |
| Do & Don't |
Earthing
the antenna at its base, doesn't matter - It
makes no real difference to performance |
| Don't
- |
Mount
the antenna along side any metal object - As
far away as possible
Any metal object will absorb the
radiation from the antenna, greatly reducing its performance,
you need at least 700mm from a wall or from another antenna
for 477MHz antennas, if this is not possible then mount
the antenna as far away from any other antenna or metal
object as possible. |
Raised Feed
Raised feed antennas can be used in
any and all applications including base station
What we call raised feed are really a simple dipole made up of a monopole antenna and a ground plane
in the form of a 1/4 wave long tube, this tube draws the radiation pattern toward itself producing
a lower angle of radiation that hugs the horizon more effectively
producing extra gain.
Some Do's & Don't's
with Raised Feed Antennas
| Do - |
Use
a spring if mounted on a bullbar - Some
bull bars are ok but some can severely vibrate. The
spring is important to absorb the vibration from the
engine otherwise the antenna will eventually fail. It
extremely important that you use the correct
spring in the right position |
| Do - |
Mount
the antenna high as possible - Roof
or Roof Gutter
is the best place for this style of antenna, next
best place is on the front guard |
| Do
- |
Use
a spring if mounted on a bullbar. The
spring is important to absorb the vibration from
the engine, without the spring the antenna eventually may
fall apart! |
| Do & Don't
- |
Earthing
the antenna at its base, does not matter - It
makes no difference to performance |
| Don't
- |
Mount
the antenna along side any metal object - As
far away as possible
Any metal object will absorb the
radiation from the antenna, greatly reducing its performance,
you need at least 700mm from a wall or from another antenna
for 477MHz antennas, if this is not possible then mount
the antenna as far away from any other antenna or metal
object as possible. |
| DON'T- |
Use
longer antennas other than supplied - You
WILL damage your transceiver
Some people like to put longer antennas on raised
feeds eg: a "M476 on a
RF470" shown below
The
only antennas that work with raised feeds are S474
style, M474 Style, and FDW's
any other antenna top longer than 24"
or 620mm WILL damage your radio
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by Pride Industries |
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