By Michael F. Young
Former President and CTO (YDI Wireless)
Radio acts like light
For people who have no RF experience, it may be difficult to visualize how radio waves travel, or propagate, through
the air. Even for those with RF experience, this concept is sometimes difficult to understand. An easy way to
think about microwave signals (generally those frequencies above 1000 MHz or 1 GHz) is to use light as an analogy.
Light is an electromagnetic signal as are radio frequency waves.
Light bulb analogy
For purposes of this light-radio analogy, we will create a hypothetical example that should help the thought process
of understanding radio frequency. Imagine a dark, overcast night sky with no moon or stars shining through
the high clouds, away from any city lights, where the area is totally pitch black, but closer to the ground visibility
is clear. If one were to disassemble the light mechanism and remove the reflective mirror from behind the bulb
of a standard flashlight with two D-cell batteries, and then set it up so that the light bulb is hanging in
free space, the bulb lights up the room, but there is not even enough light to read by.
The power output of this bulb is only about 2 watts. In the license free 2.4 GHz radio band, the most power that
the FCC allows for powering an omni-directional antenna is 1 watt.
If half of the light's power is removed by removing one of the two batteries, the intensity of the bulb drops
considerably. The light's output decreases because the power output is proportional to the square of the voltage,
meaning that, if the voltage is cut in half, the power decreases to 1/4, i.e. it goes down by 75%.
The next part of our analogy is to imagine installing this 1 watt light on a tall radio tower, mountaintop, or
tall building. The amount of light output represents roughly the radiation power that is present with an amplifier
feeding a 6 dBi (decibel) gain omni-directional antenna.
Transmit range tests
At a distance of approximately ½ mile from this hypothetical tower, one should be able to see the light
with the naked eye, but just barely. This arrangement using the naked eye would be analogous to a low gain dipole
antenna.
At a distance of a mile or two away, one will likely not be able to see the bulb anymore. Using an X10 (times
ten) telescope and aiming it at the bulb on the tower, the light bulb is now visible. This layout would be analogous
to using a 10 dBi gain unidirectional antenna, such as a flat panel or Yagi antenna. A 10 dBi gain antenna has
about ten times the focusing gain over a simple whip or dipole antenna.
From a distance of five or six miles out, the light is so weak that even the X10 scope cannot see it. Using an
X100 scope, the light comes in clearly, but the viewing area of the telescope is much smaller, which makes properly
aiming the telescope (analogous to an antenna) even more critical. This setup would be comparable to using a 20
dBi dish antenna. A 20 dBi gain unidirectional antenna has 100 times the focusing power over a dipole antenna.
From a distance of ten miles or more, presuming that the bulb is mounted high enough up so that there is clear
line of sight back to it, even the X100 scope does not see the bulb. If one were to use a X100 night scope, like
the ones that military and law enforcement use, the bulb is now clearly visible, but so is everything around the
bulb and the background ("background noise" as infrared light). This configuration is analogous to using
a radio amplifier at the client site, which, in this example, would be where the high-powered night scope is.
In order for the bulb to be brighter, the brightness control (gain) on the night scope can be increased. As the
bulb gets brighter through the night scope, so does all the background light. If the brightness control is turned
up full, the light from the bulb is overcome by all the background noise created by the light amplification circuitry
and the light itself gets lost in this background light.
If the gain on the night scope is turned down to the point at which the bulb is as bright as possible without
an intolerable increase in the background light to your eye, this point represents the optimum "signal to
noise" ratio for this particular configuration. Turning up the brightness (gain) did not improve the visibility
of the bulb, but instead stressed the viewer's eyes and made the viewer's iris close up to compensate for the increase
in the overall light level caused by the increased brightness (gain).
The lesson from this situation is: use only as much receiver gain as is necessary because too much gain is bad.
A delicate balance exists within the ratio of the signal to the noise when working with radio frequency.
Receive range tests
The above example explained the analysis of what the client side of the RF link would see. Below, we will look
in the other direction of the signal: what the unit on the tower would see. A wireless LAN requires two-way
communication, and it does no good if the client can see the tower's signal, but the tower cannot see the client's
signal.
Continuing with the light analogy, if the voltage to the bulb is decreased so that, instead of radiating 1 watt,
it puts out fifty-thousandths of a watt (50 mW), barely lighting the filament. Fifty milliwatts is equivalent to
the transmitter power emitted by typical wireless LAN cards and access points. At this level of power the bulb
cannot be seen past several hundred feet away. Using the same X100 night scope mentioned earlier, the bulb is visible.
The night scope will be the viewing mechanism, representing an amplifier, for the remainder of this example. At
a distance of half mile, the bare bulb can be seen with a properly adjusted night scope.
At a mile or two away, the bare bulb is not visible because the light's intensity is too weak. If the bulb is
setup behind an X10 telescope eyepiece, so that the X10 eyepiece is aimed back up at the tower, this setup would
be equivalent to feeding the radio signal into a directional high-gain antenna. With the X10 telescope aimed towards
the tower the bulb is visible from the tower's X100 night scope.
At five miles out, the X100 telescope is necessary in order for the client on the tower to see the bulb. The light
is not strong, but it is visible. At ten miles, the bulb is not visible at all, so the voltage feeding the bulb
is increased. The bulb is now radiating 250 milliwatts, which represents the maximum the FCC allows into a 24 dBi
gain dish antenna. But the bare bulb is still not visible from the tower. With the X10 scope in front of the bulb,
the light is visible, but it is not strong. With the X100 telescope, the light is quite bright.
The lesson here is that high gain directional antennas are needed at the client end of a wireless link.
Obstacles
If there were any obstacles in the way, the bulb would not be visible. This situation is one area in which the
light bulb analogy begins to break down. A 2.4 GHz radio signal will go through walls and floors. Light will
not. How many walls and floors the radio signal will go through depends on the type and the thickness of the
material of the walls. RF signals easily travel through sheet rock walls, such as those found in offices and
homes, but are seriously attenuated (weakened) through steel reinforced concrete walls and floors.
At long distances, the analogy holds up. A large building in the way will definitely block the radio signal. At
close range (a mile or less), reflection and/or refraction of the radio signal will possibly allow connectivity,
but that connectivity is both unpredictable and unreliable.
Increasing power at the tower
Wireless LAN users frequently want high-power amplifiers (for use at a tower) that exceed FCC Part 15 regulations.
When asked why, they reply, "Because we want a strong signal to reach our clients." When asked if they
intend to put amplifiers at their clients' sites, they invariably say "no." The next step for the wireless
LAN expert is to point out that it makes no sense to put several watts of transmit power at the tower site while
their clients only have perhaps 30 milliwatts of transmit power. Since a wireless LAN is a two-way system, if
the base cannot hear the weak client, it does not matter how strong the signal from the base is. There must be
amplification at both ends for a balanced system.
Reflections
In cases where the client is located close to the base station, it is possible to get a non line-of-site connection
off of a reflection from a nearby building. Once again, if everything is very close together (less than 1000
feet) the weak reflection from the building may have enough "illumination" to be captured by a high-gain
antenna aimed at the reflecting point.
Conclusions
In conclusion, below are several points to remember when implementing wireless links.
- Antennas, like telescopes, focus the signal and offer the same gain for both transmitted and received signals.
- The tower should always use an amplifier.
- Clients (except those in close proximity to the tower) need to use high-gain directional antennas when possible.
- Clients at a distance from the tower may need amplifiers.
- Clear, unobstructed line-of-sight is required, except perhaps for clients in close proximity to the tower.
- Fresnel Zone encroachments will reduce the strength of the radio signal.
- It is illegal, per FCC Part 15 regulations, to implement a wireless LAN that is not certified. Violation of
FCC regulations can result in fines, imprisonment, and confiscation of the wireless link that violates the
regulations.
- Reflected signals may be strong enough if the distances are short.
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