EMC - Introduction Flashcards
EMC (Electromagnetic Compatability)
Consumer Appliances and Radios together in perfect harmony
EMC has two aspects
- Avoidance of generating inteference
- Immunting of own kit from being intefered with
- The more power you run the greater the likelhood of causing inteference
- The type of mode in use has a great affect on the inteference be it to TV Broadcast Radio - items such as Hifi
Generally speaking amplitude based modes cause more inteference
AM - Any diode junction in equipment may detect and demodulate it.
SSB - Similar but often worse than AM
CW - Can cause clicks/glitches (even with a key-click filter)
Consumer electronics equipment can fairly easily demodualte strong rf amplitude changed to give disorted audio,clicks etc
Some items such as Hi-Fi speaks leads can inadvetanbvtly act as a dipole antenna and thus make the unit more vulnerable to strong RF
In contrast FM is a friendly mode from an EMC point of view but very strong FM signals can still cause problems
Domestic EMC issues
Domestic Broadcast radio recievers, music centres Hi-Fi are particularly vulnerable to AM/SSB inteference
Users may hear distorted voices like sounds from SSB but it may be more intelligble if itis from AM
Overhead telephone lines and domestic housing wiring are also potential reception mechanisms that can give EMC issues
Another domestic issue is wideband masthead preamps used for recieveing weak TV. They can suffer blocking by strong nearby amateru signals
Good practice by amateurs can help minimize the likeihood of a problem but it may not be the full answer
Supress problems at their source
Select and site antennas carefully keep them balanced and well mainted
Ensure you test from time to time as antenna/radio faults can contribute to EMC issues - Log the tests
Ensure key click filters for Morse Keys are fitted
A wide variety of filters and other aids are availiable
If in doubt seekadvice from experience amateurs
Ofcom can assist as they are a last resort and can charge money
Use a Balun if coax feed
Use a good RF earth and dont use Mains Earths or join mains and RF earths
Filter
LPF for HF
BPF for VHF/UHF - VSWR Meter
Morse key use key filter
Microphone restrict audio bandwith (DO NOT OVERMODULATE TRANSMITTER)
Filter mains and screen units well escpecially the transmitter.
RF Earth Connection
Use a good RF earth connection in a HF transmitted station
Provides a path to ground to minimise RF currents enttering the mains Earth System
Minimize the possiblity of inteference to other electronic equipment
Should consist of the shortest possible length of thick copper braid to an earth stake in the ground
EMC correctantenna system
Antenna as high and far as possible aeway from TV antenna
Good quality Coax run underground where possible
Coax earthed at point of entry to House
Balun used
Balanced antenna system
Long Wire /end fed system
Unbalanced antenna system
End-Fed wire
Strong RF fields near the house
Poor earth system
Near TV antenna
Most likely to cause inteference
Best EMC - Balanced system
Excellent EMC antenna system
Balanced Antenna
Well located and well earthed
Antenna position
Least likely to cause inteference
Digital TVis affected dirfference quite differenced to old analogue sets
There is no clear evidence of what is really happening other
