Specifications

FCS1362 Page 46 of 74 © Nov 2010
The tests are as follows:
• Centre conductor continuity check
o low resistance between the centre pin of the coaxial connector and the antenna radiating
element (less than 0.3 ohm)
• Earth continuity check
o low resistance between the body of the co-axial connector and vehicle earth/ground plane
(less than 0.2 ohm). This test is applicable to standard body mounted antennas.
• Short circuit check
o In the case of a simple panel-mount quarter-wavelength aerial there should be no connection
between the centre pin and body of the co-axial connector.
Note that some types of antennas present a DC short across the coaxial cable, for example some glass
mount, centre tapped, end-fed half wave etc. In this case and where possible the cable should be
disconnected from the antenna and checks for continuity and no short circuit carried out before
reconnection to the antenna.
Active GPS antennas can, dependent on make of unit, present a resistance (usually 300 to 400 ohm)
between centre conductor and connector body. Check with manufacturer for specification. For ‘GPS
Mouse’ (combined receiver/antenna unit) type only a functional test needs to be carried out, see section
4.8.3.
For an antenna with ’phantom’ power feed, do not connect it to the equipment if there appears to be a
short circuit fault as this may cause damage to the equipment concerned.
Consult the manufacturer’s installation instructions for guidance where applicable.
4.8.2. VSWR measurements and analysis for transmitting antennas
A VSWR test is either recommended or mandatory depending on type of installation (see matrix table in
section 4.8). This is due to the variation on measurement and adjustment that can be undertaken for
different equipment using different radio frequency bands. For example a GSM mobile phone antenna
usually does not provide any scope for measurement or adjustment being already tuned by the antenna
manufacturer.
In the case of a more complex installation whereby a tuneable antenna or a multiple antenna system is
employed, VSWR must be carried out.
The VSWR measurement should be taken using a calibrated tester suitable for the frequency band and
RF technology and power of the radio installation.
A suitable antenna tester may be either:
• a VSWR meter capable of producing a VSWR result at expected RF power level of the installation
• a through-line wattmeter capable of measuring both the forward and reverse RF power levels of the
installation and a chart to determine VSWR from the obtained results
For digital radio systems which use a pulsed RF carrier, for example GSM, 3G and TETRA, it may be
more beneficial to use an antenna analyser that works independently of the installed transceiver. This
device is self contained so does not require the radio equipment to transmit, as would be the case with a
more traditional power meter. The analyser connects directly to the antenna under test and generates a
low level RF carrier at the required frequency in order to measure the VSWR.
Analysers are suitable for analogue and digital systems whereas the traditional power meter is only
suitable for analogue systems.
4.8.2.1. Undertaking VSWR test
With all vehicle doors, bonnet, boot, etc closed check the match of the antenna using the tester.
The VSWR should be 1.5 to 1 or better (i.e. less than 1.5:1) when measured in the mobile transmit
(uplink) band.
For an internal window mounting or covert type antenna for cellular GSM/3G this limit may be increased
to 2.0:1.