RF Propagation Study FCC levels and db to Watts conversion


Here's where legal combinations of antenna gain and input power can get confusing. Starting from an antenna of 6dBi, we can put 30dBm of power into the antenna. For every 3dB over the 6dBi antenna, we have to subtract just 1dB of input power.

dBi dBm EIRP in dBm EIRP in watts
6 30 36 4
9 29 38 6.3
12 28 40 10
15 27 42 15.8
18 26 44 25.1
21 25 46 39.8
24 24 48 63.1
27 23 50 100
30 22 52 158.5
33 21 54 251.2
36 20 56 398.1
39 19 58 631
42 18 60 1000

in bold above: The perfect combo: 24dBi antenna with a senao 200mW / 23dBm output power

IEEE 802.11A CSMA/CA 2.4GHz-2.4835GHz (Industrial Scientific and Medical-ISM band)






A wider selection is available for the 2.4-GHz band since the FCC, as of April 10, 1997, allows higher ERP (effective radiated power) than the 36 dBm (4 watts) maximum stipulated in the original rules for point-to-point communications. This allows the use of very high-gain antennas, substantially increasing the reliability of point-to-point communications by increasing the link margin. For the 900-MHz band, the 36-dBm limit still applies.



Point-to-Point Communications

A wide variety of antennas are offered for S-Band point-to-point communications to accommodate various range requirements and take advantage of new FCC rules lifting ERP limits.

For short ranges in installations where a semi-parabolic antenna is not desired, the Model No. DA2.4-18 offers 18 dBi gain and can be used at ranges up to 10 km.

For longer ranges, Model No. DA2.4-24, a semi-parabolic antenna, offers 24 dBi gain and can accommodate 128 Kbps full-duplex operation at ranges up to 25 km. Link margin is 30 dB. This antenna is also recommended for shorter range where a semi-parabolic antenna is acceptable because it is less expensive than the DA-2.4-18.

Model No. DA2.4-27 is a 4 foot diameter grid parabolic antenna with 27 dBi gain which can be used at ranges up to 50 km with 30 dB link margin.

Model No. DA2.4-31 is a 6 foot diameter solid parabolic antenna with 31 dBi gain. It is recommended for difficult locations where there may be high interference levels or line-of-sight conditions may not be perfect.



Power and Sensitivity

Transmit power and receiver sensitivity are expressed relative to a reference level of 1 milli-Watt (mW) and abbreviated dBm. In the unlicensed ISM bands, the maximum power we are allowed to feed the antenna in the USA is 1 W or 30 dBm. In Europe, it is 250 mW or 24 dBm. The sensitivity of a good ISM band receiver ranges from -75 dBm to -90 dBm. (-90 dBm means the receiver can decode a signal at 1 nanoWatt !)

Transmit power is limited by the regulatory authority.

Receiver sensitivity is generally measured by reducing the input power until the error level exceeds a defined threshold. It is common to indicate the sensitivity as the level when the error rate has increased to 10E-6 (one bit error per 1 million bits of data). With a lower data rate, the connection will be more robust. Typically, the sensitivity decreases by 3dB when the data rate is doubled.

Antenna Systems

The ability of the antenna to shape the signal and focus it in a particular direction is called "antenna gain" and is expressed in terms of how much stronger the signal in the desired direction is, compared to the worst possible antenna, which distributes the signal evenly in all directions (an "isotropic radiator"). To express the relationship to the isotropic reference, this is abbreviated dBi. The typical omni-directional "stick" antenna is rated at 6-8 dBi, indicating that by redirecting the signal that would have gone straight up or down to the horizontal level, 4 times as much signal is available horizontally. A parabolic reflector design can easily achieve 24 dBi.

Under the antenna system, we also need to account for losses in the cables between the radio and the antenna. Count on 1 dB of loss for each connector and the following losses per 100 feet of feed cable (the figure in parenthesis is how many feet of cable it takes to lose 10 dB):

Cable Type


900 MHz



8.2dB (122 ft)

20dB (n/a)

n/a (n/a)


6 dB (160 ft)

11.1dB (75 ft)

19dB (n/a)

Belden 9913

2.3dB (434 ft)

4.2dB (238 ft)

8.0dB (125 ft)


2.2dB (450 ft)

3.9dB (250 ft)

6.8dB (147 ft)

Free-Space Loss

As the radio signal travels through space, it deteriorates for two reasons:

The free space loss can be calculated according to the formula

-L = C + 20 * log(D) + 20 * log(F)

where D is the distance, and F is the frequency in MHz. The constant C is 36.6 if D is measured in miles, and 32.5 if D is in kilometers. The following are some examples of free space losses:


Loss at F=

in miles

in km

900 MHz



1.6 mi

2.5 km

99 dB

108 dB

116 dB

3.1 mi

5 km

106 dB

114 dB

122 dB

5 mi

8 km

110 dB

118 dB

126 dB

6.2 mi

10 km

112 dB

120 dB

128 dB

10 mi

16 km

116 dB

124 dB

132 dB

These figures do not take into account deterioration due to weather. Typically, we recommend allowing 20 dB of margin to accommodate for weather related losses.

Putting it Together

Assume that you have a 2.4GHz multipoint radio system consisting of

The maximum allowable loss would be 123 dB. If we want a 15 dB link margin to protect against weather, then we are at 108 dB allowance for distance, which would be 1.6 miles.













Receiver sensitivity
link margin


-5 dB

+6 dBi

-108 dB

+24 dBi

-6 dB

= -80 dBm +15dB

If the radio system allows you to improve the sensitivity by dropping the link speed, you can typically gain 3dB of sensitivity by dropping to half speed. This would allow you to increase the distance to 2.3 miles.

If this system is used in point-to-point mode, the 6 dB omni antenna can be replaced with a 24dBi directional antenna, which would allow you to run 12.4 miles at full speed.

If your radio had a sensitivity of -90 dB instead of -80 dB, your multipoint system can serve an area out to 5 miles instead of 1.6 miles at full speed.

Designing for Maximum Range




Cable Attenuation is 10.8dBm for 100ft of LMR 400 with 2 AMPS
Antenna Gain is 16dBi

RF Propagation Study
Origin of Trig
Theory of Trig
Sine Cosine and Tangent
Gis Faq

Calculations Page for RF
Raw conversion page
Calculating Downtilt
Calculations HPBW Touchdown
Common Connectors

Antenna Gain There are many ways that antenna gain can be expressed: dBi Gain over an 'Isotropic Source' (a theoretical antenna with no dimensions). dBd Gain over a dipole (an antenna 1/2 wavelength long). dBq Gain over a quarterwave monopole. dBadv Meaningless random large numbers generated by advertizing departments. A dipole antenna has 0 dBd gain or 2.15 dBi gain or 3 dBq gain or (large number) dBadv gain. It's the same antenna only the numbers are different. Unless you really understand antenna theory, the numbers become meaningless. Unless you know that the gain numbers published by a particular manufacturer are accurate, don't believe any of them. The only accurate measure of antenna gain is size - the bigger the antenna, the more gain. There are a few exceptions to this; the most notable being the 5/8 wavelength antenna; it is really a 1/2 wavelength antenna fed with 1/8 wavelength of single-wire feedline. Remember: it is physically impossible to increase the gain of an antenna without increasing its size.