EMC Concepts 6A2   38    Understand that the immunity of a device is affected by the nature of its installation and that poor installation of an otherwise good item of equipment can compromise its safe and compliant operation. Firstly what is immunity ? EMC immunity is the ability of an electronic device to withstand different types of electromagnetic radiation and continue to operate as if it were not in the presence of the electromagnetic radiation in amateur radio terms RF. Thus poor installation of an otherwise good item can compromise its safe and compliant operation. The possible outcomes of problems could be caused by incorrect choice of choice of aerial and its installation position which could result in excessive field strength and or spurious emission from the transmitter.
6A4   39   Recall that radio amateurs are not required to demonstrate compliance with EMC standards for equipment they put into service but remain responsible for complying with licence requirements regarding interference. There is a difference between compliance with EMC standards and interference. Operating a radio transmitter can cause interference whether or not the transmitter complies with the EMC standard as it is producing the Radio waves (RF) that can cause the interference.Thus even if the radio amateur has built or put into service equipment that DOES comply with EMC standard it does not necessarily prevent interference from occurring to other equipment. If you are building equipment you must comply with Section 2 sub section 7 of the licence condition ( which in May 2021 here is the link http://rsgb.org/main/files/2019/09/EX309-Sept-2019.pdf ) which indicates several things: stability and free of unwanted emissions as is possible taking into account technical developments bandwidth of the emissions  must not allow be greater than 1% of the mean power to fall outside the nominal carrier bandwidth.  Please read through the licence condition part for a fuller explanation and do not ignore the notes section (a).
Sources of interference and their effects 6B1   40  Recall that some imported or home constructed electronic equipment may not meet relevant EMC standards. Whilst here you just have to be able to remember (recall)  that some imported or home constructed electronic equipment may not meet relevant EMC standards you need to remember that commercial equipment should have the CE mark to indicate that they have been built to certain standards. The CE marking is an administrative marking with which the manufacturer or importer confirms the equipment conforms with European standards for products sold within the European Economic Area (EEA) but note it is not a quality indicator or a certification mark. However the CE mark is sometimes applied to equipment from outside EU which does not actually comply with the standards. As an aside post Brexit (2020) a new UKCA (UK Conformity Assessed) marking exists for a new UK product that is used for goods being placed on the market in Great Britain (England, Wales and Scotland). It covers most goods which previously required the CE marking. ------------------------------------------------------------------------------------------------ 6B1   40 continued  Recall that items containing radio communication facilities such as cordless and mobile telephones and information technology communication equipment may produce sufficiently strong signals to cause  short range interference but are otherwise generally satisfactory. The question here is what is short range ?? If a cordless phone is set right beside a radio receiver then it is likely that the receiver's front end could  overloaded and thereby interference caused by the phone to the receiver but move it say 3 meters away it would be much less likely to cause such interference. ------------------------------------------------------------------------------------------------ 6B1   40 continued  Recall that imported devices and toys may not be compliant with relevant regulations. Here you just have to be able to remember (recall)  that imported devices and toys may not be compliant with relevant regulations. --------------------------------------------------------------------------------------------------- 6B2   40  Understand that blocking (or desensitisation) is an effect in a radio receiver where a strong, consistent level interfering signal e.g. FM either swamps the wanted signal or drives the affected circuits out of their normal operating range such that the received audio or data is severely attenuated or muted. You need to know that where a strong, consistent level interfering signal e.g. FM ( from a baby monitor ) either swamps the wanted signal or drives the affected circuits out of their normal operating range such that the received audio or data is severely attenuated or muted it is called  blocking (or desensitisation or overloading) the front end of the receiver. ------------------------------------------------------------------------------------------------------------------- 6B2 40 continued  Understand that Cross-modulation is an effect in a radio receiver where the interfering signal is varying in strength e.g. AM or SSB such that the modulation on the interfering signal is added to the modulation on the wanted signal such that both may be heard with varying clarity. The syllabus, as above,  is quite informative if you read it carefully but to explain it another way Cross Modulation is an unwanted effect which can occur when two or more audio signals are present in addition to the wanted audio signal, additional outputs can occur which are the sum and difference between the signals. Check out page 88 at the bottom of first column. This section also refers to cross modulation causing problems with digital TV and DAB radios. ------------------------------------------------------------------------------------------------------------------- 6B3  40  Recall that passive intermodulation products can be caused by corrosion in any metallic junctions in metalwork, including transmitting and receiving antennas, supports and guttering. So what are passive intermodulation products ?  The passive intermodulation products are generated when two or more signals pass through a passive device with non-linear properties. Anywhere there are dissimilar or even similar metals touching each other and especially when corrosion occurs,  this is called a junction point and  is where passive intermodulation products can be created even in parts of transmitting and receiving antennas and / or their supporting metal structures or rainwater guttering and is often called the "Rusty Bolt Effect".