New Antenna Designs for DTV Implementation

Transcription

1 New Antenna Designs for DTV Implementation JOHN L. SCHADLER and KERRY COZAD Dielectric Communications Raymond, Maine ABSTRACT WIDE BAND CAVITY ANTENNA (TFU-WB) Over the past few years the implementation of DTV transmission facilities has gone through many phases. One of the major obstacles to maximization remains the capacity of the tower to handle the components necessary for both analog and digital transmission. To meet the challenging needs of the broadcasters in smaller markets, improved antenna system designs are now available. This paper will review the latest designs for multi-channel antenna systems that can be used for analog and digital transmission as well as provide back-up antenna capacity for existing analog installations. INTRODUCTION Smaller market economics in conjunction with many existing towers being currently stressed out has forced today s broadcasters toward broadband antenna solutions with lighter weight, lower windload, and lower cost without sacrificing reliability. This paper describes two of the latest technological innovations in antenna design driven by these criteria. The most common broadband UHF broadcast antenna solution has historically been the panel. Panel antenna solutions are ideal for multiplexing high power stations that require pattern versatility, but panel antennas can also be synonymous with high price and high installation costs, as well as high windload. It is also well known that a weak link of any RF system can be an electrical connection or a joint which are typically numerous in broadband antennas due to their need for a branch feed system. With this in mind, a design philosophy of keep it simple is instilled in the following new antennas. 8 Bay TFU-WB Antenna Section The TFU-WB (Wideband) antenna is a medium power, directional, broadband UHF broadcast antenna capable of transmitting two or more channels within a 30 channel span. The new design offers the broadcaster an economical alternative to a panel antenna with the loading characteristics of a pylon antenna. The TFU-WB is suited for combined analog and digital transmission as well as a multi-channel backup for existing installations TFU-WB Antenna: Standard Azimuth Pattern

2 The basic building block of the new design is a patent-pending slot cavity radiator that provides true wideband performance. The slot cavity radiator can be viewed as a broadband coax to waveguide transition. The outer conductor of the coaxial line is terminated at the waveguide wall while the inner conductor simply extends into the cavity parallel to the guide s electric field lines and forms a probe antenna which radiates from the waveguide. This inherently simple and rugged probe style feed is free of connections and solder joints and allows for excellent power handling. 8 Bay Section with Shell Removed Slot Slot Cavity Radiator The individual cavity radiators are paired up into doublets and fed with a reinforced rigid 1-5/8 coax power divider, making the doublet unit manageable, uncomplicated and sturdy. Full 8 Bay Section Cutouts are placed into the rear of the main shell for easy access to all connections leaving the entire feed system free from entrapment within the antenna structure but at the same time completely protected from the environment. Doublet Slot Cavity Radiator Four sets of doublet radiator units are then clamped into a compact cylindrical outer shell and fed by 1-5/8 flexible feedlines from a main power divider located at the center of the 8 layer section. The end result is a simplified branch fed system which maintains pattern stability over the operating bandwidth. Access Cutouts in Main Shell

3 The 3-1/8 main power divider is supported and protected by a solid cylindrical shroud, which is flange mounted to the outer shell. This unique packaging of the antenna aperture makes it lightweight with low windload as well as completely serviceable on the tower. The basic array design of the TFU-WB antenna is modeled after the popular sectionalized side mounted slotted coaxial antenna. The 8 layer sections are each mounted separately to the tower and fed from a main power divider, forming 8, 16, 24 or 32 layer arrays. This concept offers flexibility in side mounting as well as easily serviceable components with a minimum number of external connections Degrees Below Horizontal 16 Layer Elevation Pattern Typically broadband radiators are grounded by a stub located somewhere near the feed point. This technique adds at least two more small connections or solder joints at every radiator in the antenna. In following in the footsteps of the keep it simple concept, the TFU-WB antenna design eliminates all of these types of possible failure points by leaving the radiators themselves ungrounded and providing the DC ground through the use of a matched double short stub. The double short stub is located at the tower top near the TFU-WB antenna s input and is incorporated into the transmission line run. 16 Layer Array 2 Sections Matched Double Stub Short

4 The measured VSWR of an 8 bay section is excellent over a 224 MHz bandwidth, with most of a 36 channel spread under a 1.04:1. Since the radiator is essentially waveguide driven, the useful range of operating bandwidth cannot be increased and is governed by the cavity dimensions. The 30+ channel sweet spot is easily shifted up or down in frequency by adjusting the length, width and depth of the cavity opening. THE BOW TIE SLOT TURNSTILE ANTENNA CH1 RFL SWR 100 m REF 1.1 PRm 1_: MHz Cor MARKER MHz 2_: START STOP Measured VSWR of 8 Bay TFU-WB Antenna SUMMARY OF THE WIDE BAND CAVITY ANTENNA (TFU-WB) The TFU-WB antenna is a basic, uncomplicated, economical alternative to a panel antenna. The low windload and wideband performance characteristics make the design well suited for combined analog and digital transmission as well as a multi-channel backup for existing installations. Bow Tie Slot Turnstile Antenna The bow tie slot turnstile is a low power economical broadband UHF omnidirectional antenna that features a simple feed system and durable construction Azimuth Patterns of Bow Tie Turnstile Antenna 500 MHz / 600 MHz / 700 MHz / 800 MHz

5 Before going into detail on how the bow tie slot turnstile antenna is constructed and operates, it is important to understand basic characteristics of the common batwing Superturnstile. The Superturnstile antenna is the most widely used VHF television broadcast antenna in the United States. Bow Tie Radiator The two individual radiators are then oriented orthogonally creating the patented cross bow tie slot configuration. VHF Superturnstile Each layer employs four batwing type elements mounted symmetrically around a pole. The Superturnstile uses a branching type feed system in which each radiator is fed by a separate feedline. One pair of radiators (North-South) is fed 90 degrees out of phase with respect to the other pair (East- West). This is usually accomplished through the use of a hybrid. By alternating the feed straps + to the result is radiators fed in 0, 90, 180, and 270 degree phase rotation, creating an azimuth pattern which is essentially omnidirectional. Crossed Bow Ties In this configuration, the antenna is effectively four batwing shaped radiator openings. One can think of this as the conjugate to the North-South-East-West radiators of the Superturnstile antenna. The bow tie slot turnstile antenna utilizes bow tie shaped slots which are cut into two thin flat conductive sheets with each fed by a coaxial feedline across the centerline of the slot.

6 Full Two Layer Bow Tie Turnstile Bow Tie Slot Turnstile Feed System Schematic In keeping true to the keep it simple design philosophy, it is easily seen that the cross bow tie configuration provides light weight with structural integrity without the need for a mast or the extra mounting-clamping hardware associated with the mast and separate radiators. Since each bow tie represents two batwings in the turnstile configuration, the design eliminates one half the feedlines (and possible failure points) needed to feed the radiators. CH1 RFL SWR 100 m PRm Cor REF 1.1 REFERENCE LINE POSITION 1.1 Div 4 : MHz CH1 Marker 1 : MHz 2 : MHz 3 : MHz An individual bow tie slot is a naturally broadband radiator but unfortunately a flat 1.1:1 VSWR performance cannot be achieved across the entire UHF band. To compensate, the turnstile feed design allows for creative impedance cancellation to ensure wideband operation. There is first order 90 degree cancellation occurring at each power divider head and another second order cancellation through the 90 degree ports of the hybrid. This technique maintains true wideband performance from 470 MHz to 860 MHz. 1 2 START STOP Measured VSWR of Bow Tie Turnstile Antenna Since the antenna is packaged with an internal hybrid, the design allows for dual inputs. The dual input design has been developed for single channel or for the simultaneous broadcast of two channels without the need for a separate combiner. 3 4

7 Note Dual Inputs at Bottom of Antenna SUMMARY OF BOW TIE SLOT TURNSTILE ANTENNA In essence this antenna provides the capability to achieve broadband turnstile performance with half the number of feedlines and feedline clamping hardware, half the number of radiators, as well as eliminates the need for a support mast. CONCLUSION Two types of broadcast antennas and the new technologies which allow for the novel design solutions have been discussed. These designs provide economical, broadband multi-channel operation, as well as low windload without sacrificing reliability. The features and flexibility of the antenna systems presented meet the need for DTV and NTSC multi-channel transmission as well as provide for a permanent or a temporary back-up that can be moved from site to site. ACKNOWLEDGEMENTS The authors would like to thank the Development Team at Dielectric for their dedicated support, encouragement and creative influence. INTELLECTUAL PROPERTY RIGHTS These products are subject to patents in the United States and corresponding patents in other countries.