| Q: |
Can you
explain antenna propagation and the proper grounding
planes for the different types of antennas available? |
| A: |
Ground
planes are required to properly impedance match quarter-wave or collinear antennas
launched directly from a conductive surface. The first element relies on image theory,
which is described as a virtual antenna of equivalent length and spacing below the ground
plane performing cooperatively with the physical antenna to provide impedance matching and
far field radiation. Additional half-wave or 5/8-wave radiators can be stacked vertically
to achieve higher gain. The proper ground plane dimensions are defined by a ground plane
which is much larger (in both directions) than the antenna length. |
|
|
| Q: |
Is the
antenna length detrimental to reception? |
| A: |
Antenna
length is critical to maximize performance and provide the proper radiation
characteristics. |
|
|
| Q: |
Explain the difference between unity gain, 3 dB gain,
and 5 dB gain. |
| A: |
Unity (0
dBd), 3 and 5 dBd designs differ by the number of elements incorporated to achieve
increased gain. This gain increase is achievable by stacking multiple elements in a
collinear manner to compress the vertical plane pattern and direct more energy along the
horizon, toward the cell site. |
|
|
| Q: |
How much loss is induced with each connector/fitting that is installed on the
cable? |
| A: |
Connector
insertion loss is negligible with respect to the overall cable loss. Some manufacturers
suggest adding 0.1 dB for every 2 connectors within a cable system for planning purposes. |
|
|
| Q: |
If you
cut a 3 dB whip to match a 1/4 wave antenna, are there any
concerns? |
| A: |
The only
concern is that the technician has the appropriate RF equipment to verify the VSWR or
impedance match of the cut antenna. Actual physical length of the antenna is dependent on
the mount type and radiator diameter, in order to optimally match the antenna. |
|
|
| Q: |
Will dual band antennas work as well as single band or do they
match the antennas in the middle of the frequency range? |
| A: |
Properly
designed dual band antennas will provide a good impedance match (i.e. 2:1 VSWR) over both
bands. Dual band designs should be designed for resonance and proper phasing in each band
and not centered between the bands. However, compromises must be made with respect to the
radiation efficiencies in each band. Single band antennas are optimized for single band
performance and typically perform better than dual band antennas, in their respective band
of operation. Reference "Dual-Band Vehicular Glass Mount Antennas", Antenna
Specialists bulletin SD-1301, April 1999. |
|
|
| Q: |
Explain the theory behind portable antenna design and their
length with respect to operational wavelength. |
| A: |
Traditional
portable antennas are generally less efficient, short radiators that are impedance matched
to the radio circuitry and case. At higher frequencies (shorter wavelengths) such as
800/1900 MHz, end fed designs can accommodate true ½-wave radiators that are independent
of the case and user, providing efficient dipole radiation characteristics. |
|
|
| Q: |
Can you
explain how a coupling box works? |
| A: |
The
coupling box is an RF circuit that acts as an impedance transformer and coupler to the
antenna whip through the glass. Optimally designed coupling boxes also provide a balanced
impedance transformation between the coaxial cable and the antenna. This balance
effectively eliminates unwanted RF currents on the outside of the coaxial cable and was
specifically developed and patented by Antenna Specialists for the original 800 MHz
"On-Glass" antenna. |
|
|
| Q: |
What's the difference between elevated feed and flat
surface roof mounts? |
| A: |
The
traditional roof mount antenna relies on the ground plane of installation to properly
impedance match the first element as a 1/4-wave radiator. Subsequent stacked and properly
phased radiators can be added as 1/2- or 5/8-wave sections for increased gain. Roof mount
antennas are known as ground plane dependent. The elevated feed antenna raises the feed point above the ground plane surface by
either center feeding or end feeding the first element as a dipole or
1/2-wave radiator. Radiators can then be serially stacked in a collinear fashion to
achieve higher gain. The elevated feed design is defined as ground plane independent. |
