The RF Spectrum On-Line

The Latest in Broadcast Technology - WINTER, 1997/98

Combiners and Everything Else Under the Tower

Combining FMs requires an entirely new outlook on tower and antenna design, and some new hardware as well. RF Specialties and Shively Labs have been promoting dual antenna systems on existing structures in order to extend the usability of frequency tuned section towers, for example. "If it comes down to the tower being so resonant at one frequency and not the other, sometimes it makes a master antenna system not doable and you have to go to separate antennas," said Bob Surette, manager of RF engineering for Shively Labs, who suggests the Shively 6814 antenna for combining frequencies across 6MHz. For existing FM towers adding a tenant or two to one master antenna, Shively, among others, also offers branched combiners that combine two or three station frequencies through a tee junction.

When new steel is needed for a large combined antenna system, a good option is Electronics Research's quad-sided tower/panel antenna structure. The four legs of the tower act as reflective screens, and the antenna elements are mounted in the center. The structure cuts down on windload because the master antenna is built-in and it offers exceptional omnidirectional propagation with efficient use of tower facing. "You basically get more than one tower top. The structural elements are also the electrical elements of the antenna," said Dave Davies, an RF design engineer with Electronics Research, which ships the structure pre-assembled, panel antenna and all.

When it comes to the balanced combiner for the large multi-station system, there are many choices to consider, depending on the need. Isolation a concern? Shively's 2540 regularly achieves isolation of 50dB or better, even for frequencies 0.8 MHz apart. Or, is it group delay that's a primary concern? Dielectric added a fifth stage to its balanced combiner to compensate for group delay. Electronics Research, Jampro, Shively and Dielectric offer balanced combiner designs uniquely suited to differing applications.

There are new options in transmission line, as well. Cablewave recently came out with a 7/8 inch Teflon (tm) dielectric cable that has the same power handling capabilities as a 1-5/8 inch cable for interconnecting bays of large systems and Shively recently came out with its affordable rigid B-Line 3-, 4-, and 6-inch coaxial. RF Specialties offers rigid and Heliax® line from Andrew, Cablewave, Shively and Myat. Coaxial switches, RF panels, surge arrestors, guy cables, and just about everything RF under the tower is available through RF Specialties.