Old Tip of the Week 9
CB BEAM
ANTENNAS
HELPFUL TIPS
EFFECT OF OTHER ANTENNAS
When two antennas are mounted near each other (even
if used for different frequencies), a coupling effect usually results which
in some way alters their operation. This coupling is even more pronounced
when the antennas are mounted less than one wavelength (36 ft.) apart and
may change your antenna's performance in some way.
EFFECT OF METAL STRUCTURES
Not only antennas, but water towers, power lines,
buildings, or any material of a conductive nature has the ability to misdirect
transmission. Sometimes these obstacles may act as directors or as
reflectors---causing the signal to increase or decrease in the intended
direction. Complaints of poor front- to- back ratio or lower than
expected gain can be traced to this above circumstance--especially in beam-type
operation.
SIGNAL INTENSITY
The signal strength of a transmitting station can
never be assumed to be of the same strength as in previous transmissions.
Signals of incoming stations should be recalibrated to the antennas being
compared. For this reason, you cannot take down one antenna, put
up another one week later, and expect to make accurate measurements.
If the stations being used are using beam type antennas, a slight change
in the beams' direction can also be critical. Contacts with mobiles
are even less valid. A movement of five feet sometimes makes measurable
differences in mobile communications.
S METER CALIBRATION
Depending upon the CB set, an S meter is calibrated
so that one S unit is equal to 6db. Therefore, an antenna responsible
for 1 S unit gain over another has also about 6 db over that antenna.
Some S meters, however, are calibrated at only 3 db per S unit and others
at 3 or 4 at low end, and 6 or 7 at the top of the scale.
Another problem encountered with S meters is the inability
to measure high signal strength inputs. Some bounce back as a powerful
signal and appear erratic in operation even reading lower on the scale
with an increase signal.
COAX AND CONNECTORS
The quality of a coax and connectors and especially
the soldering of the coax to the connector can effect SWR and gain.
An unsuspecting CB'er may buy a lower grade coax and lose 2 or 3 db after
paying good money for a expensive transceiver and antenna. A quick
check for good coax and connectors can be run by substituting a dummy load
on the antenna end of the coax. If all is right, the SWR with the
dummy load should be a 1 to 1 match. SWR can also be affected by
the length and conductivity characteristics of the coaxial cable.
If using solid dielectric coax i.e.. polyethylene cut to multiples of 12
ft. If using a coax of a foam dielectric cut in multiples of 14 ft.
ANTENNA HEIGHT
Whenever antennas are being compared, they should
be installed at the proper distance above the ground and preferably in
an open field. Ideally antennas should be one wavelength (36 feet
at CB frequency) or more above the ground. For comparison testing
always install test antennas at the same height.
TIME VARIATION
Any test of antennas should be performed with a time
variation of about 15 minutes or less or variations due to tropospheric
shifts and other changes will affect performance.
GUY WIRES AND SUPPORTING
STRUCTURES
Guy wires should be of the non-metallic type using
ski tow rope or other plastic or plastic coated lines. If metallic,
they should be broken up at uneven intervals along their length to avoid
interference and possible high SWR, in many cases the mast or tower being
used will act as a radiating element.
