User`s manual

CHAPTER 6
ANTENNAS
6-2
EST offers different types of antennas ranging from 1/4
wave to 5/8 wave in physical size. The user choice is
dependent on the application.
Communications in the VHF and UHF bands are
normally over
"Line of Sight (LOS)"
. Looking from the
antenna of one wireless modem you must be able to see
the antenna of the wireless modem you wish to
communicate with. If a large object obstructs the line of
sight view it is unlikely that satisfactory communications
will result. This means you must relocate the antennas or
use the REPEATER FEATURE and a second modem to
go over or around the object.
The United States Federal Communications Commission
(FCC) specifies that the antenna type used on the
ESTeem Model 95 be limited to one-half wave vertically
polarized omni-direction in design. This limits the
overall gain of the antenna to a maximum of
approximately 3 dB.
The Model 96 products are allowed by the FCC to use
high gain directional antennas.
It is noted that a 1/4 wave antenna that does not have
ground plane radials requires a ground plane to operate
at maximum efficiency. This can simply be a conducting
surface under the antenna that is a 1/4 wave length in
diameter. For the Model 95 this is approximately 42
inches. A conducting surface can be anything from the
roof top of an automobile to a file cabinet.
COAXIAL CABLES
To minimize signal loss, the overall length of the coaxial
cable should be as short as possible. To avoid corrosion
select coaxial cable manufacturers with tinned copper
braid, where possible. Listed below are representative
cable losses in db/100ft at the VHF and UHF frequencies:
Frequency (MHz) RG-58u RG-8 ½” Heliax
---------------------------------------------------------------------
72-79 - 3.8 - 1.5 - 0.6
402-420 -10.0 - 4.2 - 1.5
450-470 -10.8 - 4.4 - 1.5
In a severe noise environment it my be desirable to use a
double shield type of coax cable such as RG-214/U.
Notes:
1. Pre-made coax cables can be purchased from the
factory. A -3 dB loss means you have lost 1/2 of
your signal. A +3 dB gain means you have doubled
(x2) your signal.
2. Keep the antenna feedline as short as possible to
minimize losses.
3. Extreme care must be taken when attaching coax
connectors to the antenna feedlines. If there is any
error in making this connection the output of the
transmitter will be greatly reduced.
WEATHER PROOFING COAX
CONNECTIONS
1. Coat the threads of the connectors with silicone
lubricant prior to assembly (See Note 1) and hand
tighten. Care should be taken
not
to get any
lubricant on the center conductor.
2. Wrap the connector assembly with a vapor barrier
patch for weather proofing (See Note 2), ensuring to
overlap onto the coax cable approximately 1 1/2
inches.
3. Apply a electrical coating (sealing agent) over the
vapor barrier patch for added protection (See Note
3).
Notes:
1. Dow Corning RTV-3140 or equivalent.
2. Suggested vendors:
VAPOR-WRAP
Decibel Products
3184 Quebec St.
Dallas, TX 75356
214-631-0310
VYNIL-MASTIC, P/N 2200
3-M Company
Customer Service
512-984-1800
3. SCOTCHKOTE, 3-M Company, or equivalent.