LAPLACE INSTRUMENTS RF200 User Manual

RF200

Antenna

User Manual

Version 3.0

January 2004

LAPLACE INSTRUMENTS LTD

3B, Middlebrook Way

CROMER

Norfolk NR27 9JR

UK

Tel: [phone]
Fax: [phone]

RF200/500 user manual

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Index

1.0 Introduction Page 3

Background

2.0 RF200 broadband antenna Page 4

Assembly Page 4

In use Page 6

Antenna Factor Page 7

3.0 Ground Plane Page 8

4.0 Test site calibration Page 9

RF200/500 user manual

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1.0 Introduction

1.1 Antenna Background

For measurement of field strength (far field emission level) an antenna is required
which will act as a transducer, converting field strength (mV/m) to mV signals output
down a coax cable.
Antennas to cover the wide frequency ranges required by the legislation are not simple
devices! The standards call for the use of a ‘tuned dipole’. Whilst this is simple to
manufacture and produces a easily definable output, it will only work at one
frequency, the tuned frequency. Dipoles are tuned by adjusting the length of their
elements. For serious emissions measurement work, the constant retuning of the
antenna for each peak of interest is time consuming, hence the introduction of ‘broad
band’ antennas that cover a wide spectrum without the need for any retuning. These
include log periodic, bi-conical, bi-log and other specialist types. All suffer from
variation of sensitivity with frequency and need a correction chart so that the
appropriate adjustment can be made to the spectrum. This correction chart is called
the antenna factor.
The Laplace RF200 broadband antenna has a relatively ripple free antenna factor
characteristic, close to the optimum.
If the antenna is used with the Laplace EMC analysers and the EMCEngineer
software, selection of the RF200 item in the input menu automatically applies the
RF200 antenna factor correction to the spectrum.

1.2 Dipole or broadband antenna, which to use?

EN50022 specifies that a tuned dipole be used as the antenna for radiated emissions
testing. The dipole is a basic standard that, at its tuned frequency, has an easily
definable output vs field strength characteristic. Dipoles are tuned by adjusting the
length of each element to be ¼wavelength long. If measuring the emissions from a
product over a wide frequency range, this is tedious, time consuming and is a source
of error. Broadband antennas have a known response over a wide range of
frequencies and need no adjustment. The response is not flat, and all broadband
antennas should be supplied with an ‘antenna factor’ curve. This is a plot of
sensitivity vs frequency over the full working range of the antenna. The RF200 has a
working range of 30MHz to 1GHz and thus matches the requirements of the EN
standards.
The RF200 may be used with any analyser or receiver but the SA1020 pre-amplifier
should be used to ensure that the characteristics of the antenna match the published
data.
Basically, you need the Broadband antenna if it is necessary to measure absolute field
strength with a reasonable level of confidence and have an effective test site, free of
reflections.
Note that the antenna factor for the RF200 is included in the SA1000 software.
This antenna means that you can cover the whole radiated emissions frequency range
in one sweep. No need to adjust dipoles to each frequency of interest, no need to
switch between log periodic and biconical types half way through the testing.

RF200/500 user manual

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2.0 RF200 Broadband antenna

The broadband antenna will allow the user to detect and measure radiation over the
frequency range 30MHz to 1GHz.
This is shipped in a ‘knocked-down’ form to ease packaging and to minimise the
potential for damage in transit. Assembly is straightforward but must be done with
care.
The basic design of the antenna consists of a central main beam, itself comprising two
parallel aluminium sections spaced apart by insulators. Equal length pairs of stainless
steel rods form the antenna elements, these mounted on the main beam in order of
length, the shortest at the end from which the output lead is attached. An insulating
mounting block provides attachment for the stand with facilities for horizontal and
vertical mounting. The non-metallic stand allows adjustment of antenna height and
direction.

2.1 Antenna assembly (see fig 1)

1. The aluminium alloy elements are secured to the central beam using the M4 bolts
and washers provided. An M4 hex driver is also included to facilitate assembly. These
elements are mounted in equal length pairs with the shortest at the end of the central
beam where the output cable is attached. There are two copper crinkle washers with
each bolt. Ensure that one washer is under the bolt head and the other is under the
antenna element. Tighten the bolts until the crinkle washer is flat. Do not overtighten
as this may distort the beam. Alternate the element direction as shown in the diagram
so that for each side of the central beam, the elements alternate up, down, up,
down...etc. until element 9 which is out-of-sequence and is mounted same side as
element 8. Element 10 is alternate to element 9 as shown in Fig 1.

3. The number of elements (10 pairs) should match the number of hole pairs along the
central beam.

4. A pre-drilled plastic block is supplied to form a central mounting block and pre­amp support. This is screwed to both beams using the supplied nylon screws.