PRODUCT NOTE 7106
GRE A
MERICA
, Inc. 425 Harbor Blvd. Belmont, California 94002 phone (650) 591-1400 fax (650) 591-2001
2 of 5
ANTENNA THEORY
To get the carrier signal from “here” to “there” involves an antenna system. The
height, type, and efficiency of this system determines the distance between transmitting and receiving stations.
As mentioned, UHF wireless and light are alike is many ways, especially in propagation. They each tend to
travel in a straight path called “line of sight” but because the wireless signal is at a lower frequency, it bends
some and will go a bit farther than light. This increase at 900 MHz is approximately 1.18 times the line of sight
distance. Because we live on a rounded world, the line of sight distance will vary with height Above Ground
Level or AGL. At 6 feet AGL, line of sight is about 3 miles. At 10 feet AGL, it is about 3.9 miles. At 25 feet
AGL, the line of sight is about 6 miles but the UHF wireless distance is around 7 miles. The formula for
calculating antenna height or distance is: H=(DxD)/2, or : D= sq rt.(2H). H=feet, D=miles.
6 FRESNEL ZONE
Another consideration with wireless is keeping the lower .6 Fresnel Zone in the clear
to prevent echoes or multipath from reducing the received signal. Multipath is to wireless as “ghosting” is to
TV. The lower part of the .6 Fresnel Zone is like a “sag” or widening of the radio beam at the middle of the
path. Its width (900 MHz) is about 44 feet at 2.5 miles for a 5 mile link and about 72 feet at 7 miles for a 14
mile link. The lower .6 Fresnel Zone, as well as the radio center line between the antennas, must clear all
obstacles for best results. The antenna AGL is about 97 feet at each end of a 14 mile unobstructed path over
“flat” ground. The 900 MHz formula for calculating the lower 1st Fresnel Zone is: F1=72.1 x sq. rt. (dxd / 2fd)
where: F1=1
st
Fresnel Zone radius in feet, d=midway distance in miles, and f= frequency in GHz. Then
.6F=(F1x.6).
SPACE ATTENUATION
Wireless signals get weaker as the distance increases. In the 900 MHz region,
the attenuation is -96 dBm for the first mile and increases by -6 dBm each time the distance doubles. 2 miles
would equal -102 dBm, 4 miles would equal -108 dBm, 8 miles would equal -114 dBm, and 16 miles would
equal -120 dBm. These numbers are important in determining how strong the received signal will be and if a
proposed link is practical.
DECIBELS
The term dB is a logarithmic ratio that compares (in this case) two power levels. An easy way
to appreciate the magnitude of the ratio is to remember that a loss of 1 dB equals a loss of .2 (x.8 of the
whole); a loss of 3 dB equals .5 (x.5 of the whole); a loss of 6 dB equals .75 (x .25 of the whole); and a loss of
10 dB equals .9 (x .1 of the whole). An increase is the reciprocal. 1 dB equals times 1.25; 3 dB equals times 2;
6 dB equals times 4; and 10 dB equals times 10. The term dBm is the same ratio but related to .001 watt. An
example: the 1 watt transmitter output power of GINA is the same as +30 dBm. [By definition .001 watt is “0”
dBm, then .01 watt (x10) is +10 dBm, .1 watt (x10 again) is + 20 dBm and I watt (x10 again) is +30 dBm. To
multiply using logarithms, one must add.]
ANTENNA TYPES
The next step is to design the antenna system. There are many options in choosing a
proper antenna and a very common external array is a 50 ohm, Yagi-Uda or simply Yagi. The Yagi is a
directional antenna that has a relatively wide transmitting and receiving angle. It can be mounted for either
vertical or horizontal polarity and is offered with several gain figures. The higher the gain, the narrower the
angle. Another popular antenna is a panel type used mostly with Cellular systems. Then, there are times when
it is necessary to install a non-directional or omnidirectional antenna. These also come in various gains, with
the 6 dB a good choice. A gain limit is given by the FCC to reduce range and interference. This limit is termed
Effective Isotropic Radiated Power or EIRP and the value will be +36 dBm or about 4 watts. It means the total
power level in dBm of the radio (+30 dBm) plus the antenna gain (+6 dBm) must not 36 dBm, so the
antenna gain figure will be important in meeting the EIRP technical requirement.