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Appendix H. Distance vs. Antenna Gain, Terrain, and Other Factors
H-4
Antenna Gain
Antenna gain is specified either in dBi (decibels of gain relative to an isotropic
radiator) or in dBd (decibels of gain relative to a dipole). The relationship is:
dBi = dBd + 2.15
Some antennas that are FCC approved for use with the RF400 series are:
Mfg. Antenna Type Band Model CSI Item # dBd
Gain
dBi
Gain
Size
Astron Omni (1/2 wave) 900 MHz
AXH900 RP SMA R
14204 0 2.15 6.75”
Antenex Collinear 900 MHz FG9023 14221 3 5.15 24”
MaxRad Yagi 900 MHz BMOY8905 14201 9 11.15 21.4”
LINX Omni (1/2 wave) 2.4 GHz ANT-2.4-CW-RCT-RP 16005 0 2.15 4.5”
MaxRad Enclosed Yagi 2.4 GHz WISP24015PTNF 16755 13 15.1 17”
Receiver Sensitivity
Receiver sensitivity is usually specified in dBm for a specific bit error rate
(BER). The transceiver module used in the RF400 (either 900 MHz radio) is
specified at –110 dBm at ~10
-4
raw BER.
If the received signal strength is greater than the receiver sensitivity, a link can
be established. Any excess signal strength above the receiver sensitivity is
“link margin”, and is a very good thing; a minimum of 6 dB of link margin
should be sought.
Path Loss
We have combined in this section the normal “free space” path loss (only seen
in mountaintop to mountaintop scenarios) with loss due to ground reflections,
diffraction, leaf/forest absorption, etc. It is all loss!
A starting point is the “free space” path loss. Here are two equations for this:
Lp (dB) = 32.4 + 20 x log( f ) + 20 x log ( d ) dB (f in MHz, d in km)
Lp (dB) = 36.6 + 20 x log( f ) + 20 x log ( d ) dB (f in MHz, d in miles)
Here is a table showing the free space path loss (in dB). Note the effect of
frequency.
Frequency Distance
1 mi. 2 mi. 4 mi. 8 mi. 10 mi. 16 mi. 22 mi. 26 mi. 30 mi.
400 MHz 89 95 101 107 109 113 115 117 118
915 MHz 96 102 108 114 116 120 123 124 125
2.4 GHz 104 110 116 122 124 128 131 133 134
Notice the relationship between path loss and distance: each time you double
the distance, you lose 6 dB of signal under free space conditions. Or, put
another way, if you add 6 dB of gain (for example with 6 dB of additional
antenna gain, or 6 dB less cable loss), you can double the distance for free
space conditions.