Hi again,
Covering comments since I last posted - yes chokes may provide sufficient
attenuation for the purpose - but to confirm this all the possible issues need
to be considered.
With a waveguide beyond cut off, filling the hole with a dielectric
effectively increases the frequency that it will pass. 3mm at 40GHz gave
100dB. Fitted with PVC coated fibre the attenuation dropped to 60dB - we ended
up with 1.5mm fibre through the waveguide.
In the EU - the EMC Directive now explicitly places requirements on "Fixed
Installations". Now I know I will get some disagreement, but in my opinion, we
all work in "fixed installations" therefore an EMC facility is either a fixed
installation in its own right OR it is part of a bigger fixed installation.
The radiated immunity signal is - by definition - a transmission. If this is
done in a screened room, then the radiation is suppressed radiation.
Strangely, there is a UK Statutory Instrument regarding Suppressed Radiation
which dictates the limits of radiation detectable outside the suppression. If
these limits are met, and operation is contained within the frequency range,
then no license is required. It would therefore seem that, if the limits are
exceeded or the frequencies in use extend beyond the frequency range in the
regulations - then a licence IS required.
The table for the UK is given below.
I have no idea what other Member States require in the way of limits and
frequency ranges -or indeed whether they have any regulations of the nature of
the UK regulations. However, returning to the EMC Directive, there is a
requirement for the fixed installation to meet the protection requirements of
the Directive AND for record to be kept.
My Company - the Company I work for - lies underneath the flight path for
London Luton airport. ALL engineering (not just the EMC) is carried out inside
screened rooms - about 3,000 sq.m. of them. The EMC Support equipment, the
high power amplifiers and the project support equipment is all contained
within their own screened rooms - that is the emc facility is a suit of four
screened rooms. The screening is maintained so that is does not fall below a
level that maintains the emissions at levels lower than those given in the
table.
You will smile when you look up SELEX Galileo - Sensors and Airborne Systems -
as you will see why we go to so much trouble to stop signal leaking out.
An alternative view, would be to treat the fixed installation (the EMC
Facility) and apply the limits of a well known EN for radiated emissions - at
10m from the screened room. If you have 10V/m in the screened room at 3m from
the antenna, that is 140dBuV/m at 3m, then it is easy to calculate the level
at, say, 16 or 20m from the antenna. The minimum screening that this approach
would give would be, say 140 - 20log(20m/3m)-37dBuV (for frequencies above
230MHz) = 140 - 16.5 -37 = 86.5dB. Below 230MHz, on the same basis, 93.5dB
attenuation would be required.
If this can be obtained via through connections, filters, the grade of
screening on the coax, etc. then there should be no problems being under the
flight path of an airport.
Food for thought.
Regards
Tim
Statutory Instrument 1989 No. 1842
SCHEDULE
Regulation 5
TERMS, PROVISIONS AND LIMITATIONS OF EXEMPTION
1. Use of relevant stations and relevant apparatus for emission
shall
be limited to use -
(a) on a frequency within a frequency band specified in table
1 below; and
(b) where the maximum field strength of the emission does not
exceed the
limit specified in relation to that frequency band when measured at the
distance specified in relation thereto.
TABLE 1
Frequency Band (MHz) Limit for maximum field strength (dB.013 V/m)
Distance
at which measurement taken (m)
0.150 - 0.2835 34 100
0.5265 - 1.605 34 100
1.605 - 2.1735 48 100
2.1905 - 3.950 48 100
22.00 - 29.999 34 100
30.00 - 70.50 30 30
71.50 - 74.60 30 30
75.40 - 80.00 30 30
84.00 - 108.00 30 30
137.00 - 143.00 30 30
144.00 - 146.00 30 30
148.00 - 153.00 30 30
156.8375 - 225.00 30 30
400.00 - 405.50 30 30
406.50 - 450.00 30 30
453.00 - 464.00 30 30
467.00 - 960.00 30 30
2. Relevant stations and relevant apparatus shall not radiate,
in any
frequency band, spurious emissions of a maximum field strength in excess of
the limit, when measured at the distance specified in table 2 below in
relation to each frequency band -
TABLE 2
Frequency Band (MHz) Limit for maximum field strength (dBV/m)
Distance at
which measurement taken (m)
below 30 23 100
30 and above 23 30
3. In paragraphs 1 and 2 above -
"distance" means the distance from the relevant station or relevant
apparatus; and
"measured" means ascertained by making measurements of all components
of the
emission from such number of directions in -
(a) in a horizontal plane; or
(b) where the configuration of the site is such that it is not
reasonably
practicable to make measurements in an exactly horizontal plane, in a plane
which is as near to horizontal as reasonably practicable, including at least
four -
(i) orthogonal directions; or
(ii) where the configuration of the site is such that
it is not reasonably
practicable to make measurements from exactly orthogonal directions,
directions which are as near to orthogonal as reasonably practicable,
as is reasonably necessary to determine the maximum field
strength of the
emission at the distance specified, and the highest value obtained shall be
taken to be the maximum field strength.
SELEX Sensors and Airborne Systems Limited
Registered Office: Sigma House, Christopher Martin Road, Basildon, Essex SS14
3EL
A company registered in England & Wales. Company no. 02426132
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