ELECTRO-MAGNETIC INTERFERENCE Interference to radio receivers has been around a long time. Remember the pictures of Grandpa in the parlor with his ear glued to the morning-glory speaker? Lots of fun on a hot summer night with thunderstorms around! These days, there is much more than thunderstorms to contend with. For EMS, Fire, Police and other Public Service personnel, the radio is an indispensable tool in the performance of their work. Unfortunately, there are alot of other 'transmitters' around to affect their ability to communicate in an emergency situation. Some of these problems are more pronounced at the UHF and VHF channels. A recent issue of PC WEEK carried an article 'THE GOVERNMENT'S RADIO REGULATORS', in which the author cited many cases of poorly shielded microcomputers being sold around the country. Of particular interest to Public Service personnel is this quotation: "In one city, for example, police radio for several blocks was wiped out by radio frequency interference from an insuffiently shielded microprocessor in a store cash register, FCC officials said." EMS personnel could be victimized by this sort of occurrence if, for instance, they were called into a store to attend a heart-attack victim and could not use their radios. A little reflection on this and you could imagine other situations involving police and fire personnel carrying radios rendered useless by this type of interference. Microprocessor interference is characterized by buzzsaw-like tones, with different pitches. Or a soundless carrier, like a radio station broadcasting no sounds. These may completely block your receivers. You may transmit and be heard by receivers outside of the range of the interference, but you might not be heard by another radio within range of this EMI. There are many other types of interference which can affect your receiver. One, called Intermodulation, or 'intermod', occurs when two or more transmitters on different frequencies are activated at the same time; their frequencies combine in ways to produce 'sum' and 'difference' frequencies, one or more of which may be on your receiver freqency. In these case, you may hear the voices of those other stations on your channel. The causes of 'intermod' can be very complex and usually require the services of an expert to locate and correct the offending transmitter. The real point of this is - the time to locate these bad areas is BEFORE you are called into them to provide services. Interference of many types can be a violation of FCC regulations. It's worth your while to locate these trouble spots before your have to perform near them! Enforcement is possible if voluntary compliance is not forthcoming. How to find them? Well, just drive through, or walk through these areas with your receiver's squelch just barely on, while listening for the interference. Try to identify the source. Make a note of the locations where EMI occurs to alert personnel if they are called into the area. If, in a particular building, it is possible to locate 'quiet' spaces, these might be useful to mark on a layout of the building if the problem is severe and the likelihood of working there is great. By the way, those 'rubber-duckie' antennas are just great when it comes to preventing the skewering of your partner or your own kidneys but, for effectiveness against these kinds of interference, they are the worst. The reason is, they are extremely inefficient. If you have an option, plug in a quarter- wave antenna onto your 2-way portable. There are some telescoping antennas which fasten neatly onto your radio. When folded, they are about the size of a pocket tire guage. A couple of these among your equipment could come in handy. There are many other types of EMI and solutions, too. This little text wasn't intended to address them all, even if the writer knew them all, which he doesn't. The solutions proposed here do work. I have used them, and maybe they could help you out of a tight spot. Larry Bay 15 Sep 1986