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Antenna technology basics PDF
 

Antenna technology basics


This application note is not intended as a replacement for the books and
literature about antennas and RF radiation but it should help developers
who are coming from other business fields other than RF technology to
select the correct and best antenna system for their wireless application.
Most of the examples we explain are with GSM/GPRS modules because
these modules always come without a built-in antenna, but the general
principles are still valid for other wireless modules from Round Solutions,
like Bluetooth, ISM, WIFI or GPS modules.


1 Dipole, Monopole and Ground plane
The basic antenna is a half-wave dipole. A half-wave dipole has two rods.
External antennas for broadcast radios are often half-wave dipoles.
Half wave dipole for broadcast radio
Diagram of a half-wave dipole and quarter-wave dipole
In the wireless communication business, we are looking for small
antennas. Even the smallest antenna is the best for some applications.
Such small antennas are usually quarter-wave antennas but they still
need the second rod of the half-wave dipole as well.
Question: So where is the other half of the antenna?
The answer:
The other half of the antenna is the ground plane. Using the roof antenna
of a car as an example it is easy to explain. The roof of a car is a big
metal plate. The quarter-wave rod and the roof of the car are the two
parts of the complete antenna system. Such an ideal quarter-wave
antenna gives you a gain of 0 dBd = 2,15 dBi. The recommended
minimum ground plane is:
Wavelenght/4 x Wavelength/4 = Lambda/4 x Lambda/4
Lambda (wavelength) = C/F
where C is the speed of light
and F is the frequency of the signal
Using GSM 850 the lowest frequency is 824 MHz.
299,793,077 m/s
Lambda = ---------------------- = 0.363 m = 36,3 cm = 363 mm
824,000,000 1/s
Recommended ground plane = Lambda/4 x Lambda/4
363 mm 363 mm
Recommended ground plane = ----------- x ---------- = 91 mm x 91 mm
4 4
The smaller the ground plane the higher will be the loss of gain. Think
about a loss of 3 dB as halving the output power or halving the received
signal. The lower the gain or losses in the antenna system the faster the
application will disconnect from the base station.
As you can see the quarter-wave dipole with a small ground plane is
always a compromise between performance and size of the product.
You and we all own mobile phones have probably seen a situation where
one person in a room might have no problem placing a call but another
person in the same room might have problems placing a call. The chipset
and amplifiers in different mobiles are quite often the same. Even when
you compare mobiles with the same brand name, you will find differences
in performance.
Lets go back to an application based on GSM/GPRS module. The physics
and the mathematical rules for the radiation of microwaves are valid for
GSM modules as well.

2 Examples for special situations:

In the wireless communication business, we are looking for small
antennas. Even the smallest antenna is the best for some applications.
Such small antennas are usually quarter-wave antennas but they still
need the second rod of the half-wave dipole as well.
Question: So where is the other half of the antenna?
The answer:
The other half of the antenna is the ground plane. Using the roof antenna
of a car as an example it is easy to explain. The roof of a car is a big
metal plate. The quarter-wave rod and the roof of the car are the two
parts of the complete antenna system. Such an ideal quarter-wave
antenna gives you a gain of 0 dBd = 2,15 dBi. The recommended
minimum ground plane is:
Wavelenght/4 x Wavelength/4 = Lambda/4 x Lambda/4
Lambda (wavelength) = C/F
where C is the speed of light
and F is the frequency of the signal
Using GSM 850 the lowest frequency is 824 MHz.
299,793,077 m/s
Lambda = ---------------------- = 0.363 m = 36,3 cm = 363 mm
824,000,000 1/s
Recommended ground plane = Lambda/4 x Lambda/4
363 mm 363 mm
Recommended ground plane = ----------- x ---------- = 91 mm x 91 mm
4 4
The smaller the ground plane the higher will be the loss of gain. Think
about a loss of 3 dB as halving the output power or halving the received
signal. The lower the gain or losses in the antenna system the faster the
application will disconnect from the base station.
As you can see the quarter-wave dipole with a small ground plane is
always a compromise between performance and size of the product.
You and we all own mobile phones have probably seen a situation where
one person in a room might have no problem placing a call but another
person in the same room might have problems placing a call. The chipset
and amplifiers in different mobiles are quite often the same. Even when
you compare mobiles with the same brand name, you will find differences
in performance.
Lets go back to an application based on GSM/GPRS module. The physics
and the mathematical rules for the radiation of microwaves are valid for
GSM modules as well.

2 Examples for special situations:


2.1 ANT-GXR103 or ANTGNR100 mounted on non-metal roof
A caravan, a camping car or a truck with a bedroom for
the driver on the top of the truck are made with plastic.
The recommendation of Round Solutions is to stick some
aluminium foil on the plastic roof. With such a piece of
foil you will easily get your simple ground plane. Please
take care that the metal RF connector has electrical
contact to foil.


2.2 Quarter-wave dipole (ANT-GXH205) on plastic enclosure
These simple tricks will help—
• Aluminium or copper foil—
• Spraying metal paint on the inside of the plastic
box
Please take care, that the metal RF connector has
electrical contact to the foil or metal paint inside the
enclosure.


2.3 Magnetic mount antenna
A magnetic mounted antenna like the ANT-GXM102 or
ANT-GXM602 is also a quarter-wave dipole. They are
made for mounting on the metal roof of a car. By using it
without metal under the magnet you will get a quarterwave
dipole without a ground plane. A piece of steel of
size Lamda/4 x Lambda/4 will help to build that ground
plane.


2.4 Slim embedded antennas on PCB
Embedded antennas like the ANT-GXE475 do need a
specific ground plane as well. Please note that tests must
made on test PCB and measured in free field conditions.
A real application might not have enough ground plane
and will always have other components close to the
antenna. Optimisation can be obtained by measurement
on your prototype PCB and customisation of the antenna.
 

 

2.5 Embedded PCB antennas
Embedded antennas like the ANT-GXE479 do need a
specific ground plane as well. Please note that tests must
made on test PCB and measured in free field conditions.
A real application might not have enough ground plane
and will always have other components close to the
antenna. Optimisation can obtained by measurement on
your prototype PCB and customisation of the antenna.


2.6 PCB antennas with their own ground plane
PCB antennas like the ANT-GXP113 or ANT-GXP401 are
ground plane independent because they come with their
own ground plane. Best performance will be achieved if
they are 40 mm or more away from metal objects. Do
you remember Lamda/4? The 4 cm is not magic or
random number but just a quarter wavelength. If you are
close to big metal objects like batteries then you will lose
signal strength.


2.7 Other ground plane independent antennas
Fixed mounted antennas like the ANT-GXS106 or ANTGXS506
are ground independent as well.
Other examples are the flat antenna ANT-GXR531 or
the ANT-GNM530. These antennas come with a big
metal ground plane inside the antenna.

 
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Embedded GSM antenna ANT-GXE474
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