     
© Radiometrix Ltd
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Using Radiometrix modules operating
on 418 or 433.92MHz
in North America
under FCC Regulation Part 15, Section 231,
Paragraph (e)
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Radiometrix modules operate on licence exempt frequencies of 418MHz
or 433.92MHz. Their Effective Radiated Power (ERP) outputs
and spurious emissions meet the UK and European Type approval requirements.
However, Electric Field Strength limit imposed by FCC is very
much lower than European power limit as shown below.
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Figure 1: Electric Field Strength limits on signal
& spurious emission
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The following Table shows the conversion from dBm to Electric
Field Strength in mV/m
for common power and spurious emission level
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Power
|
P (dBm)
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E (mV/m) @3m
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E (dBmV/m) @3m
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Comments |
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10mW
|
10
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182,574
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105.2
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European limit for 433.92MHz |
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1mW
|
0
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57,735
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95.2
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Typical 418MHz modules |
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0.25mW
|
-6
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28,868
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89.2
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UK limit for 418MHz
modules |
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5.8m W
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-22
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4,399
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72.9
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FCC limit for 433.92MHz |
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5.1m W
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-23
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4,133
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72.3
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FCC limit for 418MHz |
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75nW
|
-41
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500
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54
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FCC Spurious Emission
limit for >470MHz |
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1m W
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-30
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1,826
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65.2
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European Spurious Emission
limit for
> 1 to 4GHz |
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0.25m W
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-36
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915
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59.2
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European Spurious Emission
limit for
25-47, 74-87.5, 118-174, 230-470, 862-1000 MHz |
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4nW
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-54
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115
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41.2
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European Spurious Emission
limit for
47-74, 87.5-118, 174-230, 470-862 MHz |
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The above limitations on Electric Field Strength would require,
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25dB (max) power attenuation on 418MHz modules
35dB (max) power attenuation on 433.92MHz module.
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The above values are only required if used with a high gain
antennas like dipole or yagi antenna.
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When using antennas like 1/4 wavelength whip, helical or loop antenna,
ERP will be less than the Power Output at the RF pin of the Transmitter.
i.e. required attenuation value will be few dBs less than the calculated
values.
Note: An imperial 'rule of thumb' for in-building
operation requires a 15dB change in path loss capability to change
the range by a factor of two. This is a very cruel law when compared
to free space propagation where only a 6dB change is required to
double / halve the range.
To reduce the Effective Radiated Power (ERP) either one of the following
can be used:
- Passive attenuation network
- Very short monopole antenn
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Figure 2: Methods to reduce ERP
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The following Table gives Pi-Network Resistor values for Attenuators
(50W )
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Attenuation (dB)
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R1 (W )
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R2 (W )
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5
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179
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30
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10
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96
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71
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15
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72
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136
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20
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61
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248
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25
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56
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443
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30
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53
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790
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35
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52
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1405
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It is recommended that the above Pi-Network is implemented with a
good ground plane using Surface Mount Resistors.
Power or Electric Field Strength Equation
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Equation (1) |
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|
| where |
Pr received power density [W/m2] |
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Pt transmitted power [W] |
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G numerical gain of the transmitting antenna relative
to an isotropic source |
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r distance of the measuring point from the electrical
centre of the antenna in metres |
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E electric field strength [V/m] |
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Zo characteristic impedance of free space
[W ] |
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FCC specifies limit in terms of the Electric Field Strength in m
V/m at a distance of 3m.
Using Equation (1),
When G=1, r=3m, Zo@ 120p
W
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Equation (2) |
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