Hi folks, Having had a good deal of experience in setting up small community stations in less than ideal circumstances, one very common source of noise is due to bad electrical wiring. In Mexico, we had a lot of problems with bad grounds and neutral lines that were not quite neutral. It is best not to use the ground provided by the wiring in the structure. Instead, drive a 1 1/2 or so meter long metal rod, preferably copper (have to watch for metal thieves) into the ground outside of the structure. Lay a sheet of metal on the table being used for the audio equipment, connect that sheet of metal to the ground pipe with a very thick insulated copper wire, 6 gauge or so. Use a terminal lug soldered to one end of the cable and bolted to the metal sheet. Clamp the other end (after stripping off several inches of insulation) securely to the metal rod. Then, connect a 12 gauge wire from the chassis of each piece of equipment to the metal sheet. If you are unable to provide a metal sheet, a heavy metal pipe running the length behind the equipment can work but not as well. Only use the hot and neutral lines from the structure's wiring. Test the neutral line with a volt meter connected between neutral and ground. If you see any amount of voltage present, you may have hot and neutral reversed.
Another source of noise can arise from the RF signal entering the audio chain. 3 meter length cables make great antennas since that is the wavelength of 100 MHz, also fractional lengths of 1/4 and 1/2 as well. Place the transmitter as far away from the audio gear as possible. Do not run high impedance audio cables more than 6 meters. If you need to place the transmitter at a further distance, use balanced, low impedance cables (i.e. three pin XLR microphone type cables) to connect the audio output from the final audio device to the transmitter. You may need to use matching transformer to convert from high impedance unbalanced (RCA type connections) to low impedance and back again. Audio baluns are available that have two RCA audio connectors on one side and a standard RJ45 connector for CAT5 type cable. This allows one to use cheap CAT5 cable for runs of 50 or meters. It is best to place the transmitter as close as possible to the antenna, this minimizes the length of the coaxial cable and hence the power loss in the cable. When feeding unbalanced antennas such as dipoles, make a 10-12 centimeter or so 4-5 turn coil of the coaxial cable right before the connection at the antenna. This help to prevent RF from migrating down the shield of the coax cable. A ferrite RF suppression clamp will do the same job as well, plastic piece that clamps around the cable with a good size piece of ferrite in the plastic shell. Seal the coax connection at the antenna very well to prevent moisture from getting in. We use a special rubbery kind of tape called coax seal, electrical tape will do but is not ideal. When you fabricate your coaxial cable, use heat shrink tubing over the spot where the cable enters the connector. A little silicone grease on the connector threads helps too. Consumer grade audio gear was never designed for use in an RF environment. A skilled tech can modify the input connections a bit to filter out RF. Everywhere there is an audio input jack, solder a .001uf /1nf capacitor to ground. If possible, break the connection and bridge it with a ferrite bead multicore inductor - gray piece of ferrite with 3 or 4 holes in it and small gauge bus wire wrapped in and out. And, bypass caps on either side of the ferrite. You might want employ sort of common mode filtering on the AC feed. Get a large ferrite toroid, about 3-5 cm diameter and two lengths of 10-12 gauge stranded insulated wire. wrap one wire about 6-10 turns on one half of the toroid and then wrap the other wire on the other half of the toroid winding it in the opposite direction - i.e. one clockwise and the other counter clockwise. Tape them securely. That is your common mode filter, insert between the AC feed from the structure and the line going to the equipment. Use a metal box with appropriate connector for this. Ground the box to the above ground system. Finally, in the same box, place some MOVs (surge protectors) from both AC lines to ground on the equipment side. This will help prevent voltage spikes from clobbering the equipment. Anyway, hope this helps. I will be happy to answer any questions you may have. Best regards, Stephen Dunifer _______________________________________________ cr-india mailing list [email protected] https://mail.sarai.net/mailman/listinfo/cr-india
