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RE: [AV Media Matters] Magnetic fields

At 02:45 PM 06/08/2001 -0700, Linstead, Cheryl ISTA:EX wrote:
>[cross-posted] {reply, too}
>I have a query regarding magnetic fields:
>"What is the effect of ship radar systems on magnetic media? Information
>on protection from such damage is requested."
>Can anyone offer what it is that radar systems (from ships or other
>sources) produce and how much?

I think the concerns go beyond the magnetic fields to the electrical fields
as well. I would be concerned about how any
recording/reproducing/duplicating systems would work in the presence of
these fields. This reminds me of a drive in Pennsylvania I used to take 25
years ago where I could clearly hear a large radar chirping in my auto
radio (pre-tape player).

I do not have direct experience with this,  but a quick Google search
turned up a very interesting British document from their health and safety


There is a paper of historical interest (can't let this one go by) that
applies to 50-year-old technology but supplies some background of radar
sets in parts 8 and 9 of the paper

Related information on health issues and power levels from Norway can be
found at

http://www.kelvinhughes.co.uk/k_hughes/50years.htm shows us that after
1984, the International Maritime Organization required ships over 10,000
gross registered tons to have ARPA radars installed. Searching on ARPA
radars led to one of the major manufacturers of this equipment, Furuno
http://www.furuno.com/commercl/radarpdf/far2805.pdf is a brochure relating
to their high-end ship-board radars that provide power levels. These units
are available in up to 60kW (of course this is pulsed) and in  a brief
discussion with Chuck at Furuno in the Seattle area just now, he confirmed
what they call "sidelobe attenuation" is probably what we call antenna
gain. This is around 30dB.

Chuck suggested that a measurement be taken and this is the normal case.
The frequencies involved are 3.05GHz and 9.41GHz, by the way.

>What are the limits from which magnetic media must be shielded?

There is an interesting paper at the Magnetic Reference Labs site:

http://www.flash.net/~mrltapes/field-strength-for-partial-erasure.pdf which
is in keeping with the information from a magnetic consultant I worked with
on a recent project. They are a manufacturer, but also offer consulting
services and provide useful information on the Web at:
If you want to contact the consultant there, I'd suggest it. He's very
knowledgeable and can perform on-site surveys. He is "Larry Maltin"

>Does a building afford sufficient protection, or should an Archives building
>install additional measures?

The magnetic shielding of a building is quite variable depending on
construction techniques and materials. The wavelengths that you are
attempting to shield against determine the effectiveness to a great extent.
For example, at my office which is a steel frame building with metal and
glass spandrel panels, it is very difficult to receive AM radio stations,
while cell phones work throughout.

>Here at the BC Archives in Victoria, we are in immediate proximity of
>cargo ships, cruise/passenger ships, and Navy ships/vessels.

Remember, the intensity falls off as the square of the distance (inverse
square law), so while "immediate proximity" is important, does that mean 50
feet or 500 feet? Although the distance is 10x in this example, the power
is reduced to 1%.

I've provided a fair amount of raw material, but don't have the time to
process it all. I would suggest that you ask Larry Maltin to do it--he is
far more facile at this than I! He's not on either of these lists, but I've
copied him on this.



>Direct responses and/or references to publications are most appreciated.
>Thanks alot,
>Cheryl Linstead
>Audio-Visual Records Curator, BC Archives
>Ministry of Management Services
>Phone: (250) 387-2959  /  Fax: (250) 387-2072

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