CalAmp GUARDIAN Manuel d'utilisateur télécharger pdf (Page 22)

2.5.4 GOOD DESIGN PRACTICES

There are two relatively easy ways to avoid creating large transients due to static buildup on an antenna and the

subsequent firing of the gas discharge tube in the lightning arrestor. Following either or both of the recommendations

below will eliminate this potential problem.

1. Use antennas with a DC grounded active element. Antennas can easily be tested, by using an ohm meter, to

measure the resistance from the center conductor to the ground of the RF cable that is directly attached to the

antenna. The ohm-meter should indicate a short. (Some antenna designs, such as folded dipole or folded dipole Yagi

antennas, inherently have a DC ground on the active element due to the nature of the antenna design.)

2. Use a lightning arrestor that does not have a gas discharge tube. PolyPhaser makes several DC blocked lightning

arrestors that have an inductor to ground instead of a gas tube. These lightning arrestors will not allow the static to

build up on the antenna, and there is no gas tube that can trigger causing a transient into the antenna port of the

radio. The following lightning arrestors, manufactured by PolyPhaser, have inductors to ground instead of gas tubes:

a. PolyPhaser Part Number: VHF50HN Frequency Range: 100MHz - 512MHz, 750W

b. PolyPhaser Part Number: TSX-NFF Frequency Range: 700MHz - 2.7GHz, 750W

Tip: Lightning arrestors that use gas tubes will normally specify a “Turn-On Voltage” in the data sheet. If you see this

specification in the datasheet, it is very likely that the lightning arrestor has a gas discharge tube. If you are still unsure,

contact the manufacturer.

2.6 SELECTING ANTENNA AND FEEDLINE

The Guardian can be used with a variety of antenna types. The exact style used depends on the physical size and layout

of a system. The Guardian device has been tested and approved with antennas having a maximum gain of 10 dBi.

2.6.1 ANTENNA GAIN

Antenna gain is usually measured in comparison to a dipole. A dipole acts much like the filament of a flashlight bulb: it

radiates energy in almost all directions. One bulb like this would provide very dim room lighting. Add a reflector

capable of concentrating all the energy into a narrow angle of radiation and you have a flashlight. Within that bright

spot on the wall, the light might be a thousand times greater than it would be without the reflector. The resulting bulb-

reflector combination has a gain of 1000, or 30 dB, compared to the bulb alone. Gain can be achieved by concentrating

the energy both vertically and horizontally, as in the case of the flashlight and Yagi antenna. Gain can be also be

achieved by reducing the vertical angle of radiation, leaving the horizontal alone. In this case, the antenna will radiate

equally in all horizontal directions, but will take energy that otherwise would have gone skywards and use it to increase

the horizontal radiation.

The required antenna impedance is 50 ohms. To reduce potential radio interference, the antenna type and its gain

should be chosen to ensure the effective isotropic radiated power (EIRP) is not more than required for successful

communication.

A number of FCC-approved antennas have been tested for use with the Guardian. Similar antenna types from other

manufacturers are equally acceptable. It is important to follow the manufacturer’s recommended installation

procedures and instructions when mounting any antenna.

Guardian™ Serial Modem or Analog Radio for Licensed Spectrum PN 001-5006-000 Rev. 3 | Page 14

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CalAmp GUARDIAN Manuel d'utilisateur Page 22