REDUCTION OF STRAY NOISE PICKUP
                 

One of the problems encountered after installation of an active antenna might be excessive noise pickup, greater than was expected. Even if the mounting location was first checked out with a portable receiver, disappointing results may be found after the active antenna is installed in the chosen location. When the antenna is disconnected from the cable, the receiver inside the house goes dead quiet. This rules out noise pickup due to the noise penetrating the cable. Usually what really is happening is that the noise from inside the house or other receiver location is being brought out on the shield of the coaxial cable, where it is picked up by the active antenna, as it appears in series with the effective antenna to ground impedance of the antenna. The cure is to isolate the cable shield from the antenna and provide a noise free ground for the active antenna.

However, since the active antenna often derives power from the same cable that is carrying the received signal back to the receiver, this may seem to not be possible. Using a separate battery supply for the active antenna and a separate RF coupling transformer is one solution, but this defeats the convienience of a single feedline for both received RF and DC power. It also adds complexity.

One solution which generally works well is to use a 1:1 choke balun that places a high impedance in series with the shield of the cable at radio frequencies but will still allow the cable to pass DC and RF. This balun is easily made from miniature coaxial cable such as RG-174/U and a small (1 inch dia) toroid made of a very high permeability ( u= 2000 to 8000 ) ferrite. The cable is wound on the toroid, with 20 to 30 turns and brought out to connectors on each end. The assembly is mounted in a plastic box to electrically isolate the input and output connectors. Since the ferrite does not appear in the field inside the cable, there is no effect on RF signals. However, the outside of the cable sees a very high inductance (10-50 millihenries depending on material and number of turns). This effectively disconnects the cable shield from the active antenna as far as noise on the external shield is concerned. Next, an earth ground is connected to the cable shield on the side of the balun connected to the active antenna. An 8 foot ground rod with a short (1 meter or less) ground lead usually is adequate, as the active antenna is a high impedance device. DO NOT GROUND anything else to this earth ground. Now there is no path for the cable shield noise on the receiver side to couple to the antenna except through the high impedance of the balun choke, and any noise that does is routed to ground on the antenna side of the choke. This effectively disconnects the cable from the antenna as far as conducted shield noise is concerned. The better the ground the better the results. It also helps to raise the active antenna 2 meters or more above ground.

Most noise situations are worse at AM and lower frequencies, and this technique works very well in reducing noise from flourescents and dimmer switches, and switching power supplies. This noise is generally worse at low frequencies (<3 MHz) and is mainly conducted noise. This technique is not as effective on radiated noise pickup. The only cure for radiated noise is locating the antenna a good distance from noise sources

 
TECHNICAL INDEX
ACTIVE ANTENNA
NORTH COUNTRY RADIO
PO Box 200, Hartford NY 12838-0200
EMAIL: support@northcountryradio.com
Tel 518-854-9280
Internet http://www.northcountryradio.com