Objective:
The purpose of this experiment was to examine the effects of antenna design and
the antennas angle to the earth on satellite transmission. From our research into
antenna’s we know that according to the IEEE definition an antenna is “That part of a
transmitting or receiving system that is designed to radiate or receive electromagnetic
waves”.
Theory:
The best place to start to learn antenna theory is with radio waves. Radio waves
are waves of energy that resemble closely to light waves and in fact they travel through
air at the speed of light. A radio wave is usually visualized to be a sine wave and the
period of this wave or cycle is known as its wavelength. (See figure below)
Figure 1
To better follow our experiment one be would be prudent to understand how an
antenna radiates electromagnetic waves or radio waves. This radiation occurs due to the
time varying properties of the current, or an acceleration or deceleration of charge.
Radiation however will still take place in a bent wire with a uniform velocity of its
charges. The intensity of the magnetic field can be calculated by H  (2I)  R . The
radiation that is produced can be shown using Figure 2.
This magnetic field is the signal and when it strikes another antenna such as the
satellite it induces a current on the antenna surface and the current produced is then

converted into data.
Figure 2
The type of antenna used on our test is a ¼ wave antenna, which has an omni
directional pattern meaning it radiates equally well in all directions. This works well
with the design of the buoy, as our bearing to the satellite is unknown and randomly
changing. It does however have the least amount of gain, which makes it poor for
antenna reciprocity, which is a feature that allows an antenna to both transmit and receive
signals.
According to wikipedia “Antenna gain is the ratio of the power density of an
antenna’s radiation pattern in the direction of strongest radiation to that of a reference
antenna.” It is measure in decibels (dB) and it measures the increase of field strength of
the antenna when compared to a reference antenna. The reference antenna is usually
either an isotropic antenna (dBi) or a dipole antenna (dBd).
The quarter wave antenna does however need a good ground plane to work
properly. A ground plane is a flat surface usually made of metal that limits the
downward radiation of the antenna and extends a minimum of one wavelength in each
direction from the antenna.
If a quarter wave antenna is used without a ground plane or one that is not
functioning properly then two things can happen. One is that the radiated power will tend
to go out in every direction up / down as well as towards the horizon. The other problem
is the antenna can become mismatched and a high Standing Wave Ratio will be seen.
Which causes the power that the transmitter generates to “bounce” back into the
transmitter and be lost as heat. This in turns reduces the signal that our satellite will see
causing more problems.
Standing Wave Ratio is a complex subject needing you to comprehend a few
other properties involved in antennas. On such factor is impedance when we think of
impedance we think of something that impedes or acts as a force. Impedance in antennas
refers to the ratio of voltage to current at any place on the antenna. This ratio is different
throughout the antenna, which means that its impedance also differs along the antenna.
This talk of impedance has to refer to our 50-ohm coaxial cable used to connect
our radio to its antenna. To achieve maximum power transfer so well taught to us by Dr.
Kumar we need match our impedances, which is known as tuning your antenna.
So what happens with mismatched impedances? Well the coax connection on the
antenna causes some of our wave to be reflected back depending on how out of tune your
antenna is. This combination of our original wave toward the antenna and the reflected
wave is known as Standing Wave Ratio. This ratio is generally tried to keep low,
preferably less then 2:1.
The last fundamental theory part to tackle is the theory of polarization. As
mentioned above a radio wave is composed of two fields an electric field and a magnetic
field. These fields are perpendicular to each other and there sum is called the electromagnetic field. The fields transfer energy back and forth to each other to what is called
oscillations.
Our primary field of interest is the electric field. The position and direction in
relation to the ground determines its wave polarization. So if the antenna is vertical then
the polarization is vertical and the same goes for horizontal. This can effect radiation
pattern and will be part of what we are testing.