Narrowband EMI consists of a single frequency or a narrowband of interference frequencies. Narrowband EMI  usually  has  a  minor  effect  on  communications or electronic equipment. It can be tuned out or filtered out. Broadband EMI is not a discrete frequency. It occupies a relatively large part of the electromagnetic spectrum. It causes the majority of EMI problems in digital data equipment. It will be especially noticeable data on Link 11. CONTROL OF EMI. —EMI can be controlled or eliminated if some simple procedures are followed and good  installation  practices  adhered  to.  Let’s  look  at control  and  reduction  for  shipboard  and  shorebased installations. Many of the problems are the same for both installations. Shipboard EMI Control. —Shipboard EMI control is greatly simplified for the typical digital data installation. Because  of  the  ship’s  steel  hull  and construction, a great deal of shielding and isolation are provided the typical shipboard computer room or digital equipment  space. This blocks out the majority of broadband interference generated both internally and externally. Five major factors are considered in a shipboard computer and digital equipment installation. They  are  equipment  location,  equipment  shielding, system  and  equipment  grounds,  interconnection cabling, and power source. Equipment location —Computers  should  be located in spaces that are free of sources of EMI. They should not be located in spaces that contain radars, radio transmitters, generators, or other rotating machinery. Equipment   shielding  —Digital  computers should never be operated with drawers extended, cover plates removed, or doors open. Modem computers contain EMI-reducing gaskets and shields that enclose the equipment. Always reinstall cover plates with all the fasteners in place. If a cover plate or shield has to be removed in the course of corrective maintenance, ensure that the EMI reducing contacts or wire gaskets on the equipment opening are in good condition before the cover or shield is replaced. System  and  equipment  grounds —System and equipment grounds are extremely important in digital computer   installations. All  cabinets  should  be grounded together on a common system ground bus. Each  equipment  cabinet  is  connected  to  the  system ground by a heavy ground cable. The system ground is securely attached to the hull of the ship and provides a good ground reference for the system. Paint on ground straps or on the metal decks where the ground straps are mechanically attached will result in poor electrical connections. Interconnecting cables —All interconnecting cables used in a shipboard digital data system should be shielded cables. They should be assembled correctly according to installation drawings. The shield and connector shell should be electrically connected and properly secured at either end. The cables should never be run in the same cableways as cables carrying rf signals or high- power pulse cables. The shielding protects the data cables from EMI to a great extent. Power   source   —Power   lines   for   digital computers  can  provide  a  transmission  path  for  EMI from machinery spaces. The majority of input power passes through noise elimination filters as it enters computers. Unusual  random  problems  in  digital computers can sometimes be traced to defective line filters. Shorebased EMI Control. —Control of EMI at a shorebased installation requires the same consideration or the same factors as a shipboard system with two additions-site  location  and  soil  quality. Site   location   —Shorebased   digital   data equipment  sites  are  sometimes  built  where  the  need dictates or where a convenient building is available. They are not always ideal sites. Sites built near a large industrial complex such as a shipyard repair facility (SRF) or a naval depot (NADEP) may be subjected to EMI.  They  also  can  cause  power  line  fluctuations  if  the power source of the shore site and the SRF and NADEP are the same. Additional line filters and regulators for power lines may also be required to reduce EMI and provide line power within the limits prescribed by equipment manufacturers. Soil  quality —At a shore installation, a system ground bus is usually attached to a grounding rod driven into the soil. If the soil is dry, sandy, rocky soil as found in  the  Southwestern  United  States  and  some  places overseas, you will have a poor ground. A suspected system ground can be checked with an oscilloscope and 1:1 probe. Refer to  Electromagnetic   Compatibility, NAVELEX O967-LP-624-6010, for more information. DIRECTIVES.  —Many   directives   provide guidelines to follow for avoiding or reducing the effects   of   EMI.   The   EIMB   handbook   entitled Electromagnetic  Interference  Reduction,  NAVSHIPS 0967-LP-000-0150, includes topics of shipboard EMI 1-15