Radiometrix Ltd.,
BiM1 transceiver, TX1H high power transmitter data sheet
Page 11
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.
A simple test for interference is to monitor the receiver RSSI output voltage, which should be the same
regardless of whether the microcontroller or other logic circuitry is running or in reset.
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
**
*
***
Helical antenna
RF
Loop antenna
RF GND
RF
C
tune
C
match
capacitors may be variable or fixed
(values depend on loop dimensions)
track width = 1mm
min. area 500mm
2
Whip antenna
471mm @ 151MHz
RF
wire, rod, PCB track
or a combination of these
length(mm) = 71250 / freq(MHz)
trim wire length or expand coil
for best results
35-40 turns wire spring
length 120mm, dia 10mm
Figure 8: integral antenna configurations
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