CalAmp GUARDIAN Manuel d'utilisateur télécharger pdf (Page 20)

2.4.2 SITE SURVEY

A Site Survey is an RF propagation study of the RF path between two points or between one point and multiple points.

UHF radio signals travel primarily by line of sight and obstructions between the sending and receiving stations will

affect system performance. Signal propagation is also affected by attenuation from obstructions such as terrain,

foliage, or buildings in the transmission path. A Site Survey is recommended for most projects to determine the optimal

RF paths for each link. This is especially true when more than one RF coverage area is required. A Site Survey will

determine the best unit location for the Relay Points.

2.5 SELECTING ANTENNA AND LIGHTNING ARRESTOR COMBINATIONS

RF engineers and installers have seen many types of radio installations over the years, and they know there are certain

details that must not be overlooked at any installation. Most radio installations contain some form of lightning

protection. However, the wrong combination of antenna and lightning arrestor can create high voltage transients on

the radio’s antenna port having devastating impacts on the life and reliability of modern day radio equipment.

2.5.1 LIGHTNING ARRESTOR OVERVIEW

Lightning arrestors can take many forms. But some of the most common lightning arrestors use gas discharge tubes

that turn on when the voltage across their terminals exceeds the specified threshold. Under normal conditions, these

devices have very high impedance and no current flows through the device. When the turn on voltage threshold is

exceeded, the gas discharge tube turns on instantaneously and becomes a short.

This functionality works well to limit the magnitude of a transient from a nearby lightning discharge. However, it can

have very negative consequences if a gas discharge lightning arrestor is used with the wrong antenna.

2.5.2 ANTENNA OVERVIEW

Antennas can come in just about any shape or size. However, there is one parameter, in particular, that the system

designer should not overlook, especially if the radio installation uses gas discharge tube lightning arrestors. The

parameter is the DC grounding of the active element in the antenna.

A DC grounded antenna will measure 0 ohms from the active element to ground when tested with an ohm-meter. One

way to test this is to connect the ohm-meter from the center conductor to ground of the RF cable that is attached

directly to the antenna. This will read as a short for a DC grounded antenna, and as an open for a non DC grounded

antenna. Note: Some antenna datasheets are misleading and will indicate the antenna is DC grounded. However, the

datasheet may be referring to the body of the antenna and not necessarily the active element. For this reason, it is best

to measure the antenna you plan to use to verify the active element is DC grounded.

2.5.3 THE WRONG COMBINATION

The combination of a DC open antenna and a DC blocked gas discharge tube lightning arrestor creates a situation

where static charge can build up slowly on the active element of the antenna. Static charge can be created by wind

Guardian™ Serial Modem or Analog Radio for Licensed Spectrum PN 001-5006-000 Rev. 3 | Page 12

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CalAmp GUARDIAN Manuel d'utilisateur Page 20