There exist many situations where it is desirable to have a variable length radio frequency (RF) connection. For instance, many RF antennas, such as for news vans or other mobile antenna platforms, have a variable-length (e.g., height) RF antenna, where one end of an RF cable is attached to the variable-length RF antenna, and another end is attached to a stationary opposite end.
To date, to allow the RF cable to accommodate the adjustable-length RF antenna, a variety of techniques have been used. In one technique, the maximum amount of needed RF cable according to the maximum length of the antenna is connected, and when the length of the antenna is anything other than the maximum length, the excess cable is manually coiled on a vertical surface in the shape of a side-ways figure eight held up by a pair of lobes or hooks. Excess cable can also be stored horizontally by coiling it inside a protective enclosure (often referred to as a “cable coffin”), as neatly as the operator is capable of accomplishing. In another technique, a cable reel may be used to spool and unspool the RF cable in response to the adjusted length of the RF antenna, however, due to the rotation of the cable reel, an operator has to manually disconnect and reconnect the RF cable end whenever the antenna height is adjusted. Both of these two manual techniques pose great inconveniences to the operators, and result in increased set-up (emplacement) and tear-down times of the antennas.
In another known technique, the RF cable may be wound around the adjustable-length RF antenna in a generally helical (spiraling/coiling) manner, much like a television news truck. As the RF antenna is extended (e.g., raised), the RF coil expands like a spring around the antenna (e.g., a mast on which the antenna rests). As the RF antenna is shortened (e.g., lowered), the RF coil compresses. While this arrangement may be suitable for certain situations, such as low power and/or low frequency transmissions, as will be appreciated by those skilled in the art, RF cable length is a great contributor to line loss, typically expressed in decibels (dB). In addition to cable length, line loss in a given cable is a function of the dielectric material between the center and outer conductor, the diameter of the center conductor, the diameter of the outer conductor, and also the frequency. Lower frequency communications such as VHF (below 300 MHz frequency) have less line loss for a given length than higher, microwave frequencies (e.g., C-Band or X-Band). For example, using a premium, high-performance, large diameter cable, approximate loss may be characterized for high frequency transmissions as 1 dB of signal loss for every 20 feet of RF cable. Due to the spiraling nature of this particular alternative, the length of the RF cable is roughly eight to ten times the maximum length (e.g., height) in order to allow the proper expansion of the RF cable spiral to match the length of the variable-length antenna. In addition to other signal losses, such as from weather, interference from jamming, line of site, curvature of the earth, etc., the signal loss due to the extra cable length may be significant and unsatisfactory (e.g., particularly to maintain a high baud rate over the RF transmission).
Additionally, slip-rings may be used for certain classes of cable reels, such as multi-conductor power, discrete controls, and low frequency communications, often found on harbor and boom cranes. For this a slip-ring is typically enclosed in an environmental housing, sized for the number of channels (conductors) and current carrying capability required by the load. Typical slip-ring housings for a cable reel (e.g., a 10-inch wide cable reel supporting 1-inch diameter cable), however, are approximately 6-8 inches in diameter and 8-10 inches high. This extra size and weight may pose particular difficulty to mobile applications. Slip-ring frequency capability is limited by the geometry of the ring and brush assembly, as may be appreciated by those skilled in the art, has a cut-off limit of approximately 100 kHz. For this reason slip rings are typically not used for RF communications.
For higher frequency RF communications an RF rotary coupling is typically used. These devices are designed specifically for higher frequencies and use contacting (single channel) or wave-guide (multi-channel) architectures. As with slip rings, implementation of an RF rotary coupling with a cable reel requires a suitable environmental enclosure. The weight and volume requirements for a 1-inch diameter high-performance coaxial cable are approximately the same as a slip-ring for a power cable of the same size. The additional volume and weight required poses great difficulty to mobile applications.