7a.1 Understand
that all electronic equipment is capable of radiating and absorbing
radio frequency energy. Recall that the basic principle of
electromagnetic compatibility is that apparatus should be able to
function satisfactorily in its electromagnetic environment and without
causing intolerable electromagnetic disturbance to other apparatus in
that environment. Introduction
All electronic equipment is capable of radiating radio frequencies. All equipment can also absorb radio waves.
Electro magnetic compatibility (EMC)is all about building
electronic equipment that reduces the pick up of electromagnetic
radiation and reduces its own output of electromagnetic radiation.
7a.2 Understand
that transmitters in domestic environments may give rise to RF fields
stronger than the agreed limits. Understand that transmitters in
domestic environments are not ‘normal’ situations and special measures
may have to be taken.
An amateur transmitter in a
domestic environment may produce RF fields stronger than the agreed
limits. This will cause breakthrough even though the apparatus is
designed to meet the regulations
A radio transmitter in a
domestic environment is not a normal situation, and if breakthrough
occurs special measures will have to be taken to reduce or eradicate
the problem.
7a.3
Understand that new electronic equipment should meet the European EMC
immunity requirements but that existing equipment and poorly installed
equipment may not. New electronic equipment
should be designed and built to meet European EMC immunity
requirements. Older equipment and equipment that is not properly
installed may not meet the European standards.
Good radio housekeeping
7b.1 Recall how to interconnect the transmitter, microphone, power
supply, SWR meter and band or low pass filters, using appropriate
cables, to minimise EMC problems.
The first step in reducing EMC is to make sure that radio equipment is installed properly.
Microphones should have shielded cables that are earthed to prevent RF
entering the audio stages and causing feedback. Ferrite beads could
also be placed over the wires from the microphone
Power cables should have good connections.
Interconnecting cables to SWR meters, linears, external low pass
filters and or bandpass filters should be made with good quality 50ohm
coax and connectors which maintain a continuous shield along the system.
Filters, power supplies and SWR meters should be in metal enclosures to prevent RF signals from being radiated.
7b.2 Understand that filters can be fitted in the leads from the power
supply to the transmitter to help minimise RF energy entering the mains
wiring.
RF filters can be fitted into
the leads taking power from the power supply to the transmitter. This
will reduce the risk of RF entering the mains. The image opposite shows a common type of filter. It clamps onto the mains cable. The
ferrite material increases the resistance to RF current and reduces RF
on the wire.
Mains wire can also be threaded through a ferrite ring.
Both these techniques to involve dismantling the cable.
This filter could be plugged in line with the mains or built into the
equipment. The capacitors remove HF signals on the mains line. The
ferrite ring reduces common mode interference.
7b.3 Recall what constitutes a good RF earth, its purpose and use.
The earthing system on a radio has 2 purposes
1. to be a good earth for mains equipment to reduce the chance of users getting electric shocks
2. to be a good earth at RF particularly for end fed wires and verticals.
In this section we deal with RF earthing.
If you run a dipole, or any balanced antenna system you do not need to
bother with RF earths. But, if you run long wire antennas you
do.
Ideally your shack is at ground level. You connect one end of your ATU
to the antenna and the other end to an earth wire. This earth wire (or
better still copper tube) goes down to an earth rod which is connected
to as much buried uninsulated wire as possible. Some people use rolls
of wire netting (unrolled of course!) to increase the surface are of
the earth and thus lower its resistance.
In reality a lot of radios are located above ground level on the first floor or in an atic.. If you use an end fed
wire from here what do you connect the other end to? Well you could try
running a length of thick copper wire down the side of the house to an
earth system as explained above. What happens is that the ATU sees this
run of earth cable as part of the antenna and it will radiate causing
interference to nearby electronic equipment. So, not a good idea. In
addition the transmitter will see the mains earth as part of the
antenna and RF will run to earth via the mains earthing system causing
further interference as most electronic appliances are connected to this.
A better solution is to use a counterpoise. This consists of about a
quarter wavelength of insulated wire, insulated at the ends and running
around the skirting boards or better still over the roof tiles. This
will provide a lower impedance than the other methods and so the RF will tend to travel down this wire.
7b.4 Recall how to use a suitable general coverage receiver to check for spurious and harmonic emissions from the station.
We mentioned in
lesson 15 that transmitters may produce harmonic and spurious emissions
which could cause interference. A simple check for this is to use a
general coverage receive. Set the transmitter to low power and connect
a dummy load to the receiver.
Find the frequency of
transmission on the receiver. Slowly increase the transmitter
power until the S meter is reading maximum.
Then, leaving the transmitter
on the same frequency, slowly tune the general coverage receiver to see
if there are any harmonics or spurious signals. Make a note of the
strength of these. You can identify harmonics because they will
be multiples of the fundamental frequency and will appear above the
fundamental frequency. Spurious emissions will not be multiples of the
fundamental frequency and could occur above and below the fundamental
frequency.
7b.5 Understand that siting a transmitting antenna close to mains
wiring, TV or radio aerials and downleads is a potential problem
exacerbated by the use of a loft or indoor transmitting antenna. The siting of a transmitter antenna is an important factor in reducing interference. Ideally transmitting antennas should be: 1. outdoors - indoor antennas will increase the chance of interference. 2. well away from
power lines as the power line could pick up the RF and carry it to all
electronic equipment connected to the mains. 3. Well away from TV and domestic radio antennas which will pick up RF and carry it to the TV or radio.
Not only will
these decrease the likelihood of causing interference from the
transmitter, but also decrease the chances of picking up interference.
On the diagram opposite position 3 is the best place for an antenna.
