Use of a KVM extension to reduce electromagnetic exposure

Some offices need quite large computers and such computers may be big sources of electromagnetic emissions, especially if they have one or more external communication devices like ADSL modems.

In such case a possible solution to reduce the exposure of people to this electomagnetic radiation is to move the computer away leaving just the user devices, keyboard, mouse and monitor, on the desk.

Sometimes this is needed also for small computers if they have some particular kind of devices attached to them. Moving the computer in another room allows also to not to hear the computer noise anymore. Finally, having the computer box in a separate room may be very useful also to protect it from damages and unallowed access.

However our aim is to reduce the dangerous electromagnetic radiation in the workplace as more as possible so we are going to examine the possibility of moving the computer in another room for this purpose.

Extension cords

The first solution to have the computer in another room is to use extension cords for mouse, keyboard and monitor. Such extension cables are available in many shops, but this solution has several problems.

First of all the video signal looses quality if the cable is too long, so we need a high quality cable for this, and anyway the maximum length is quite limited, the maximum length we have found for VGA cables without intermediate devices is 7.5 meters.

Second, there would be multiple cables side by side, one for the monitor, one for the keyboard, one for the mouse, and possibly also one for the headphones/microphone ( two cables if the microphone is not embedded in the headphones ). All these cables would be laying on the floor, with the risk that someone steps on them, stumbles, and breaks something.

Third, the multiple cables create more electromagnetic emissions than a single cable, and they capture also electromagnetic fields from the floor and the near sources, bringing them to the desktop.

In conclusion the common extension cords are not a good solution.

KVM extenders over network cables

The industries have designed and developed some devices for today's computer farms, where dozens or hundreds of typical computer boxes are installed and maintained. These devices are called KVM extenders, where the acronym KVM stands for Keyboard, Video and Mouse.

These devices use a standard network cable like CAT5, to transport the signals from the computer to the console and back. The first products of this kind a few years ago where quite poor because they allowed a very limited screen resolution, but recently these products have been improved at a point that makes them really useful, at last.

A typical KVM over CAT5 extender is composed by two devices, one called the Transmitter, and one called the Receiver, connected by a typical network cable of type CAT5 or superior. The Transmitter is attached to the computer's ports, while the Receiver is attached to the keyboard, mouse and monitor on a desk. Of course the system transmits also signals from the keyboard and mouse to the computer so the names Transmitter and Receiver are just conventional.

There are lots of different models and designs of KVM extenders, some have additional channels to connect multiple computers to the same console, or to connect other types of computer peripherals. However a typical office would need just the basic KVM channels, with possibly the audio output and input too. The most advanced products of this kind have also audio channels, but KVM extenders with audio support are currently much more expensive than basic KVM systems.

The network cable between the Transmitter and the Receiver can be at least 20 meters long, while the enterprise grade products support cables of several hundreds meters.

There are also products that use an optical cable instead of a common CAT5 cable, and they allow even kilometric distances. For the reduction of electromagnetic radiation, optical cables may be interesting because they create virtually zero electromagnetic field around the cable. However the biggest electromagnetic emissions of a KVM extender are not caused by the cable but by the communication devices at its ends. Even optical cables need complex integrated circuits that perform the conversion of the signals, and these circuits are the biggest sources of electromagnetic radiation, so the optical cables are generally not worth their cost. A good network cable like CAT7 is good enough for most cases.


The cable of a KVM extender is better in terms of electromagnetic environment than a set of extension cords, because the video, mouse and keyboard signals are all carried by a single network cable with very little electromagnetic emissions. However the overall result of using a KVM extender is not as good as it could be, because of the receiver device.


The biggest problem of the KVM extenders over network cable, for the improvement of the electromagnetic environment in the workplace, is the receiver device. The receiver has three cables that go to the mouse, keyboard and monitor, but these cables are very short, and so the receiver has to be on the desk or very near to it, the farthest point where it can be is hanging from the edge of the table.

The receiver emits some electromagnetic radiation of high frequency because of its internal circuits, and in most cases there is no shielding at all since the enclosure is made of plastic.

The receiver also needs a small power supply and the power supply provided by the manufacturers is of an extremely cheap design and creates a lot of electromagnetic radiation. It is like many other small power adapters, which work on the principle of the switching circuit to convert the mains AC voltage into an output voltage which is constant on average. The problem is that the output voltage has an extremely high amount of electromagnetic noise, that is spread over a very large frequency band. This EMF is almost impossible to filter or to shield. The effects of such small power supply is so big that it cancels out completely the benefits of having the KVM extension.

The electromagnetic emissions created by this power supply not only radiate from the small power supply box, but are carried by the wire to the KVM receiver box, from which they are radiated much more. Part of this electromagnetic radiation goes also to the mouse, keyboard and monitor connectors which become additional emitters.

In practice, with the original power supply the KVM extender cannot be used, it creates a mess of electromagnetic radiation at the receiver side, that vanishes completely the advantages of having a KVM extension.

Perhaps with a low EMF power supply, and a metal box for the receiver, the results would be much better.

Related pages

ADSL wires
Network cables
USB cables