Using Radiometrix modules operating on 418 or 433.92MHz in North America under FCC Regulation Part 15, Section 231, Paragraph (e) 
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.
Figure 1: Electric Field Strength limits on signal & spurious emission
The following Table shows the conversion from dBm to Electric Field Strength in mV/m for common power and spurious emission level
Power 
P (dBm) 
E (mV/m) @3m 
E (dBmV/m) @3m 
Comments 
10mW 
10 
182,574 
105.2 
European limit for 433.92MHz 
1mW 
0 
57,735 
95.2 
Typical 418MHz modules 
0.25mW 
6 
28,868 
89.2 
UK limit for 418MHz modules 
5.8m W 
22 
4,399 
72.9 
FCC limit for 433.92MHz 
5.1m W 
23 
4,133 
72.3 
FCC limit for 418MHz 
75nW 
41 
500 
54 
FCC Spurious Emission limit for >470MHz 
1m W 
30 
1,826 
65.2 
European Spurious Emission limit for
> 1 to 4GHz 
0.25m W 
36 
915 
59.2 
European Spurious Emission limit for
2547, 7487.5, 118174, 230470, 8621000 MHz 
4nW 
54 
115 
41.2 
European Spurious Emission limit for
4774, 87.5118, 174230, 470862 MHz 
The above limitations on Electric Field Strength would require,
25dB (max) power attenuation on 418MHz modules
35dB (max) power attenuation on 433.92MHz module.
The above values are only required if used with a high gain antennas like dipole or yagi antenna.
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 inbuilding 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
Figure 2: Methods to reduce ERP

The following Table gives PiNetwork Resistor values for Attenuators (50W )
Attenuation (dB) 
R1 (W ) 
R2 (W ) 
5 
179 
30 
10 
96 
71 
15 
72 
136 
20 
61 
248 
25 
56 
443 
30 
53 
790 
35 
52 
1405 

It is recommended that the above PiNetwork is implemented with a good ground plane using Surface Mount Resistors.
Power or Electric Field Strength Equation

Equation (1) 

where 
Pr received power density [W/m2] 

Pt transmitted power [W] 

G numerical gain of the transmitting antenna relative to an isotropic source 

r distance of the measuring point from the electrical centre of the antenna in metres 

E electric field strength [V/m] 

Zo characteristic impedance of free space [W ] 



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

Equation (2) 


