Premium RG-58 type cables, such as Belden 7806R, offer loss of about 4.7 dB at 200 MHz and 8.9 dB at 700 MHz. Still, at 700 MHz, only 68 feet (20 m) of this cable will cut range in half. Larger RG-8 style cables have considerably lower losses; about 3 dB per 100 ft at VHF and 6 dB at 700 MHz. For best results when long coax cable runs are necessary, a premium foam-dielectric cable, such as Belden 9913, should be used. This cable has only 1.8 dB of loss per 100 feet at 200 MHz, and 3.6 dB at 700 MHz.

Generally, it is preferable to run audio cables out to remote receivers, thus keeping RF cables short. This is particularly true when cable runs will be more than longer than 75 feet (23 m) or so. If remotely locating wireless receivers is not feasible in a given application, using a high-quality, low-loss cable and keeping the loss to 6 dB or less is recommended.

Remote Whips. Connecting whip antennas to the wireless receiver via coax cable is a very common cause of range problems. Whip antennas need a “ground plane” to operate properly; when they are connected directly to the receiver, the receiver chassis or circuit board provides the necessary antenna ground plane. The same is not necessarily true when connecting whips remotely with coax cable.

Don’t misunderstand: whip antennas may be used remotely if the necessary ground plane is provided. If the outside shell of the cable or antenna connector is electrically connected to a good-sized metal object, such as a pipe, cabinet, microphone stand or the like, good performance is possible. Remember, however, that the whip must extend away from the object being used for the ground plane.

For best results, a metal plate should be used. For UHF systems, a plate approximately 12 inches (30 cm) square is optimum. For VHF, plate size should be at least 30 inches (75 cm). A hole is drilled in the middle of the plate, and a “feed thru” RF connector installed. The whip is then connected to the top of the feed thru, with the cable connected to the bottom. If the whip is a right angle design, it must be changed to the straight configuration.

Antenna Splitters. When remote antennas are used, it often becomes convenient to use antenna splitters. A common, and serious, mistake is to use “T” connectors to make splits. Because of the resulting impedance mismatch, signals are rarely divided properly, and it is entirely possible for a particular receiver to get essentially no signal at all. In addition, much of the received RF energy may simply be reflected back towards the antenna.