|
Antennas
The choice and positioning of transmitter and receiver
antennas is of the utmost importance and is the single most
significant factor in determining system range. The following
notes are intended to assist the user in choosing the most
effective antenna type for any given application.
Integral antennas
These are relatively inefficient compared to the larger
externally-mounted types and hence tend to be effective
only over limited ranges. They do however result in physically
compact equipment and for this reason are often preferred
for portable applications. Particular care is required with
this type of antenna to achieve optimum results and the
following should be taken into account:
1. Nearby conducting objects such as a PCB or battery can
cause detuning or screening of the antenna which severely
reduces efficiency. Ideally the antenna should stick out
from the top of the product and be entirely in the clear,
however this is often not desirable for practical/ergonomic
reasons and a compromise may need to be reached. If an internal
antenna must be used try to keep it away from other metal
components and pay particular attention to the "hot"
end (i.e. the far end) as this is generally the most susceptible
to detuning. The space around the antenna is as important
as the antenna itself.
2. Microprocessors and microcontrollers tend to radiate
significant amounts of radio frequency hash which can cause
desensitisation of the receiver if its antenna is in close
proximity. The problem becomes worse as logic speeds increase,
because fast logic edges generate harmonics across the VHF
range which are then radiated effectively by the PCB tracking.
In extreme cases system range may be reduced by a factor
of 5 or more. To minimise any adverse effects situate antenna
and module as far as possible from any such circuitry and
keep PCB track lengths to the minimum possible. A ground
plane can be highly effective in cutting radiated interference
and its use is strongly recommended.
The following types of integral antenna are in common
use:
Quarter-wave whip: This consists simply of a piece
of wire or rod connected to the module at one end. At 151MHz
the total length should be 471mm from module pin to antenna
tip including any interconnecting wire or tracking. Because
of the length of this antenna it is almost always external
to the product casing.
Helical: This is a more compact but slightly less
effective antenna formed from a coil of wire. It is very
efficient for its size, but because of its high Q it suffers
badly from detuning caused by proximity to nearby conductive
objects and needs to be carefully trimmed for best performance
in a given situation. The size shown is about the maximum
commonly used at 151MHz and appropriate scaling of length,
diameter and number of turns can make individual designs
much smaller.
Loop: A loop of PCB track having an inside area
as large as possible (minimum about 5cm2), tuned and matched
with 2 capacitors. Loops are relatively inefficient but
have good immunity to proximity detuning, so may be preferred
in shorter range applications where high component packing
density is necessary.
Integral antenna summary:
|
whip
|
helical
|
loop
|
| Ultimate performance |
***
|
**
|
*
|
| Ease of design set-up
|
***
|
**
|
*
|
| Size |
*
|
***
|
**
|
| Immunity to proximity
effects |
**
|
*
|
***
|
|
