1. Electromagnetic Compatibility (EMC)
Electromagnetic compatibilty is the avoidance of interference between two pieces of electronic equipment.
Interference and Immunity
Interference from an amateur transmitter falls into two categories:
Interference from the legitimate amateur signal to some susceptible piece of equipment – breakthrough; Electromagnetic Compatabilty (EMC) Standards endeavour to set standards for immunity.
Interference due to unwanted (spurious) emissions from the amateur station.

2. Overload
TV and radio receiver input stages and mast-head pre-amplifier stages have a wide bandwidth to prevent the necessity of retuning or “peaking” as frequency is changed.
Amplifier stages become overloaded and non-linear resulting in interference to the TV picture or radio signal

3. Cross Modulation and Blocking
When a strong signal overloads an amplifier it can cause the gain of the amplifier to vary in time with its modulation, imposing its modulation on a wanted AM signal, e.g., TV video, as light and dark horizontal lines; lesser impact on FM signals.
SSB is worst for this; interference from FM transmissions might go unnoticed.
A strong signal may affect the receiver’s AGC circuits, or overload an amplifier stage, turning down gain and causing the wanted signal to become weak or / and noisy; this is known as desensing or blocking.
It is more common from FM or data modes.

4. Intermodulation Audio Circuits etc.
Intermodulation distortion (IMD) is where two signals mix together due to non-linearity and produce spurious signals.
The TV or radio receiver may have poor dynamic range and the spurious signal may appear in, or be generated in, the i.f. passband resulting in interference.
Audio Circuits etc.
Audio stages of receivers / hi-fi may experience interference due to rectification in a diode or semiconductor junction in the circuit, or non-linearity.
Interference may be caused to security systems, telephones, IR detectors through a similar mechanism.

5. Transmitter Field Strength
An amateur station should only use as much power as is really necessary to make the contact.
Field strength falls off over the first few metres from the antenna
A field strength of 3 V/m is near the highest level that electronic equipment might be expected to cope with.
Antennas should be kept well away from other antennas and wires.

6. Spurious Radiation
Passive Intermodulation Products (PIPs) the so-called “rusty bolt” effect can be caused by high field strength, causing re-radiation at harmonic frequencies.
Transmitter harmonics themselves can cause difficulties and good final amplifier design and operation coupled with low pass or band pass filtering is important.
Spurious mixer products should be minimised by good design.

7. Spurious Radiation
Parasitic oscillations, self-oscillation in the transmitter due to poor design or mismatch can lead to interfering signals not related to the band in use.
Very poor oscillator stability can result in the Tx drifting outside the allocated amateur band and interfering with other services.
Overdriving, excessive microphone/audio gain setting can lead to overloading, non-linearity, excessive bandwidth and harmonic generation in transmitters.

8. Spurious Radiation
When a carrier is interrupted, as in CW, a sharp interruption will cause sidebands which manifest as Key Clicks. These can cause interference over a long distance and wide band of frequencies.
Additionally, depending on Tx design, there may be a small spark as the key or keying relay operates which can cause localised interference.
The rise time should be conditioned with a key click filter

9. Routes Taken
Interfering signals can enter TV and radio receivers via the antenna input or downlead or poor quality cable TV systems
Mains borne interference, either through coupling or the Tx power supply
Coupling into loudspeaker, hi-fi interconnect or telephone leads
Direct radiation from a poorly-shielded Tx

10. Prevention Appropriate transmit power.
Mode of transmission – FM is the most benign, ssb the worst offender.
Choice of antenna type, location, feeder – balanced antennas and feeders minimise feeder radiation; balanced feed should not run near ground or metal objects as this will unbalance it; use of a balun if feeding a dipole, with good quality co-ax.
Appropriate filters.

11. Prevention
Suitable filter at the output of the transmitter and/or at the electronic device being interfered with.
Ensuring that the complete transmitted signal is inside the allocated amateur band.

12. Types of Filters
Low pass filter passes low frequencies, stops high frequencies.
High pass filter passes high frequencies, stops low frequencies.
Band Pass passes a range of frequencies and stops (rejects) frequencies outside the passband.
Band stop rejects a range of frequencies and passes all others; if sharp called a notch

13. Filters
Band Pass (a,b,c,d); shape is not ideal as circuit is only an approximation; in (c,d) filters are cascaded to give better response.
Band stop (e,f)
LC tuned circuits can be used for either

14. Filters Low pass (a,b) High pass (c,d)
RC at audio frequencies.  LC at RF
(b) and (d) are known as Pi filters due to similarity of configuration to Greek letter π

15. Filters
RC twin T-notch filter can provide a sharp notch at audio frequencies.
LCR T-notch filter (bridged-T) can provide notch at higher frequencies. L adjusts frequency, R depth of notch.

16. Filtering
Use of a low pass filter (l.p.f.) with a cut-off at 30Mhz on HF; bandpass filter on VHF.
Ideally filter is installed after SWR meter as diodes in this may cause difficulties.
Use of an inline mains filter which filters out high frequencies

17. Filtering
Common mode currents which run on the braid of shielded cables both in the station (microphone, computer, etc.) and on TV/radio downleads, speaker leads and interconnects can be treated with ferrite chokes, also called braid breakers; they are also effective on mains cables.
The cable is wound around a ferrite ring (toroid) or passes through a ferrite bead close to where it enters the device to be treated

18. Filtering
Use of a high pass filter inserted in the TV antenna downlead to filter out HF signals.
Use of an LC notch filter for specific bands at HF or VHF.  Use of an open-circuit electrical λ/4 stub which will act as a notch (short circuit) at its design frequency.

19. Decoupling
Adequate decoupling by appropriately sized capacitors in homebrew equipment is essential.
Leads should be decoupled where they enter the enclosure (power supply, microphone, control lines, etc.).
Decoupling will often eliminate interference to telephones and other devices; however a difficulty is that it involves modifying the device.

20. Shielding and Earthing
Equipment should be adequately shielded, generally in an earthed metal enclosure.
This prevents radiation leaving or entering the device
A separate RF earth will prevent signals flowing on the mains (safety) earth; it will also reduce noise pick-up by the amateur station.
This separate earth should consist of several earth rods, connected by thick wire or braid.