LAPLACE INSTRUMENTS RF200 User Manual - page 3
RF200/500 user manual
3
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.