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 2.4GHz2.4835GHz (Industrial Scientific and MedicalISM band)
A wider selection is available for the 2.4GHz 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 pointtopoint communications. This allows the use of very highgain antennas, substantially increasing the reliability of pointtopoint communications by increasing the link margin. For the 900MHz band, the 36dBm limit still applies.
A
wide variety of antennas are offered for SBand pointtopoint communications
to accommodate various range requirements and take advantage of new FCC rules
lifting ERP limits.
For short ranges in installations where a semiparabolic antenna is not desired, the Model No. DA2.418 offers 18 dBi gain and can be used at ranges up to 10 km.
For longer ranges, Model No. DA2.424, a semiparabolic antenna, offers 24 dBi gain and can accommodate 128 Kbps fullduplex operation at ranges up to 25 km. Link margin is 30 dB. This antenna is also recommended for shorter range where a semiparabolic antenna is acceptable because it is less expensive than the DA2.418.
Model No. DA2.427 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. DA2.431 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 lineofsight conditions may not be perfect.
Transmit
power and receiver sensitivity are expressed relative to a reference level of 1
milliWatt (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 10E6 (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.
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 omnidirectional
"stick" antenna is rated at 68 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 
325MHz 
900 MHz 
2.4GHz 
RG58/U 
8.2dB (122 ft) 
20dB (n/a) 
n/a (n/a) 
LMR195 
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) 
LMR400 
2.2dB (450 ft) 
3.9dB (250 ft) 
6.8dB (147 ft) 
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:
Distance 
Loss at F= 

in miles 
in km 
900 MHz 
2.4GHz 
5.8GHz 
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.
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.
Transmit 
Transmit 
Omni 
Path 
Directional 
Receive 
Receiver sensitivity 







+24dBm 
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 pointtopoint 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.
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 singlewire feedline.
Remember: it is physically impossible to increase the gain of an antenna without increasing its size.