A COMPACT HIGH POWER UHF COMBINER FOR MULTIPLE CHANNELS OVER A WIDE FREQUENCY SPAN Lewis Steer Radio Frequency Systems, Melbourne, Australia
Abstract Conventional UHF high power balanced combiners are large and only capable of handling multiple channels over less than a 20% frequency span due to the limitations of their waveguide couplers. Apart from restricting the channels which may be planned for an antenna, such systems do not allow for the flexibility of adding unplanned channels later if they are outside that span. The new Directional Waveguide Combiner configuration has a 46% frequency span limited only by the bandwidth of waveguide enabling US channels 14 to 60 to be transmitted at high power through one antenna system for the first time. Its small size and convenient daisychaining allows many channels to be combined in a confined space. This paper describes the useable frequency span of waveguide, explains why components of conventional waveguide combiners further restrict this span by more than half, and illustrates how the new Directional Waveguide Combiner regains the full waveguide span while becoming more compact. The innovations to achieve compact, full guide bandwidth power dividers and waveguide-to-coax transitions, vital parts of the new combiner system, are outlined. An excellent operational example of the new system exists on the 100th floor of the world s tallest office building, the Sears Tower in Chicago. As fourteen high power channels spanning 19 to 60 are combined for three antenna systems in a room 20x21 feet, the Sears installation is used throughout this paper to illustrate the discussion. Bandpass Filters I. INTRODUCTION TO BALANCED COMBINERS The purpose of a balanced combiner is to combine multiple transmitter signals into a single antenna system while keeping the transmitters isolated and properly matched. Fig.1 is a schematic of a balanced combiner module. A module consists of a balanced pair of filters, two 3dB couplers and a balancing load. The transmitter signal enters the narrowband input, the channels previously combined (or just another single channel) enter the wideband input, and the combined output is directed to the wide band input of the next module (for a cascaded multi-channel combiner) or finally to the antenna. In detail, the narrowband signal is split in two at 90 degrees phase difference by the input coupler, passes through the bandpass filters (tuned to pass this channel only) and is recombined in the output coupler to add at the wideband output port and cancel at the wideband input. It has the frequency response or selectivity of the bandpass filters. Any transmitter signal reflected by the filters goes into the balancing load port. This results in the transmitter working into an impedance matched system across a wide band, hence it is a balanced combiner. Similarly, the wideband signals are split by the output coupler, then reflect from the bandpass filters and recombine in the output coupler to add at the wideband output port combined with the narrowband signal. The critical point is that the frequency span of channels that may be combined is limited by the operable span of the wideband coupler. In high power UHF combiners, waveguide is used along the cascaded wideband path because of its higher power rating. The next section describes the inherent frequency limitations of rectangular waveguide. II. USEABLE BANDWIDTH IN RECTANGULAR WAVEGUIDE The width of a rectangular waveguide defines the useable frequency span where the desirable single mode, TE 10, can propagate. At and below the cutoff frequency Fc (equation 1), no propagation is possible. Deleted: Fig.1: Balanced Combiner Schematic
Fc = c/2a (1) Where: c = speed of light and a = width of waveguide. Due to the asymptotic dissipative attenuation in waveguide [1], the lower useable frequency is about 19% higher than the cutoff frequency for the short lengths within combiners, or about 24% higher for greater lengths. Undesirable TE 20 mode propagation is possible at frequencies higher than twice the cutoff frequency. To be sure of avoiding the complications TE 20 modes bring, the upper useable frequency is 95% higher than the cutoff frequency. For example, the WR1500 waveguide often used in UHF combiners is 15 inches wide and has a maximum useable bandwidth of 470 MHz to 754 MHz (US channels 14 to 60) in combiner systems. The next section explains why the conventional implementation of waveguide combiners gives only half of this useable span. III. LIMITATIONS OF CONVENTIONAL WAVEGUIDE COMBINERS Conventional high power UHF combiners are usually implemented in: Two physically separate waveguide 3dB couplers Two physically separate waveguide filters Fig.2 illustrates a typical example. Such combiners are very large as a result of their large components and their interconnections. The combinable span of channels is limited by the components along the wideband path such as elbows, waveguide-to-coax adaptors on the outputs, and the 3dB coupler configurations (Riblet Couplers, as illustrated, and Hybrid Tees known as magic tees ). Inside an individual coupler, the width of guide varies along the wideband path. Wider sections lower the useful upper frequency by lowering the TE 20 frequency threshold, and narrower sections raise the useful lower frequency by raising the cut off frequency. More significantly, the changes in width or height in waveguide couplers represent large impedance changes, which can only be matched over narrower bandwidths. If the maximum useable frequency span of waveguide is to be regained in a combiner, variations of width and impedance in the waveguide wideband path have to be eliminated. This does not appear possible using conventional components. The following section describes a new implementation that does achieve this. IV. IMPLEMENTATION AND ADVANTAGES OF DIRECTIONAL WAVEGUIDE COMBINERS Bandpass Filters Fig.2: Typical Conventional Combiner
End Aperture Bandpass Filters Once again, combiner modules may be cascaded to form a multiple channel combiner system. But in these Directional Waveguide Combiners, the wideband path forms a straight length of rectangular waveguide known as the spine. A 6 channel system operating on the 100 th floor of the Sears Tower in Chicago was installed in 2000 and is shown by Fig.4. The combined span was Ch19 to Ch60. Note the straight spine. Space was at a premium but the system was so compact that it was able to fit in that floor s removed restroom. Ouput End Aperture Ch 19 For Future Channel s Ch 60 Fig.3: Directional Waveguide Fig.3 illustrates a Directional Waveguide Combiner module. Schematically, it is equivalent to a conventional balanced combiner. The narrowband signal in rectangular guide reaches the end aperture where it is simultaneously split into two orthogonal waves and coupled into the circular waveguide filter at the required filter input coupling. The end aperture is designed with a particular offset and shape. The resulting orthogonal signals in the circular waveguide are equal in magnitude, have a phase difference of 90 degrees, and their E fields are physically at right angles to each other. In effect, there are two filters in the same waveguide with a 90 degrees phase difference, schematically equivalent to conventional combiners. 16 Feet Straight Spine Fig.4: Sears Tower 6 Channel Combiner The orthogonal signals reach the end aperture at the output where they simultaneously couple out of the filter, add together in the wideband output and cancel in the wideband input. Meanwhile, the wideband input signal reaches the output end aperture where it is immediately reflected by the filters, and continues along the rectangular guide combined with the narrowband signal almost as if the guide had been continued with no aperture.
The combinable span of channels is equal to the maximum useable frequency span of the spine waveguide because there are: no width (or height) changes along the spine, only small impedance changes at the end apertures, made easily manageable by the short spine length, compared to the longer and more complex wideband path of many conventional combiners. This also makes adding/removing future channels more practical. Directional Waveguide Combiners are more compact than conventional waveguide combiners because: both the 3dB coupler and filter input / output coupling functions are implemented in the small end apertures, both filters are physically in the one series of cavities. An added benefit of this is that the filters are physically matched and therefore intrinsically have better matched electrical characteristics. As combiner temperatures vary with power and ambient temperature, the filters drift together and remain well matched. This is beneficial to N+1 adjacent channel combiners which require the most stable of configurations. The physically separate filters of conventional combiners are subject to unequal cooling drafts and blowers, so their filters may drift unequally and become unmatched. V. REALISATION OF UHF DIRECTIONAL WAVEGUIDE COMBINERS Directional Waveguide Combiners are not new. They have been used extensively in MMDS systems. MMDS channels have a much smaller bandwidth to frequency ratio compared with UHF television channels, so their filters are much narrower, meaning their end aperture coupling requirements are much smaller. Also, the power levels passing through MMDS combiners are relatively small compared to UHF combiners which can have outputs in the hundreds of kilowatts. The realisation of UHF TV Directional Waveguide Combiners and keeping them compact was not merely a matter of physically scaling MMDS combiners. A number of innovations were required for this development and are the subject of patent applications. Reduction of Rectangular Waveguide Height The height of the waveguide was reduced from standard WR1500 (which has a width-to-height ratio of 2:1) for two reasons: 1. To enable sufficient filter coupling. 2. To make components more compact without compromising the frequency span. The impedance of standard 2:1 waveguide is much higher than 50Ω coax, so a long impedance transformer is required in waveguide to coax transitions (on outputs and inputs). If the height is halved, then the impedance is halved, enabling much shorter transitions. E plane sweeps and mitres are also made more compact. A reduced height guide of about 4:1 width-toheight ratio is ideal because it enables the coupling and size requirements to be met, while not compromising the practical requirements of: tolerance to unintended distortions average power rating (half height WR1500 can handle about 240kW average) peak power rating Higher Peak Power Ratings, More Channels and Adjacent Channel Combiners As DTV is phased in, the number of high power channels being combined and the implementation of adjacent channel combiners may be limited in conventional waveguide combiner systems by the peak power ratings of the filter output couplings. These couplings are often comprised of narrow slots with extreme electric field intensities resulting in lower peak power ratings above which arcing will occur. A number of conventional adjacent channel combiners, particularly N-1 (where the analog channel is higher than the digital and hence the high power of the Vision Carrier is close to the digital channel s frequency), have been known to burn out or have their power ratings greatly reduced. However, the end apertures of Directional Waveguide Combiners have been specially designed so the electric field intensities are significantly reduced, greatly increasing the peak power ratings, and increasing the number of channels capable of being combined and the ratings of adjacent channel combiners.
Compact Full Frequency Span Power Dividers Where multiple coaxial feeders are required from the combiner to the antenna to handle the power, the combiner s output must be split by a power divider also capable of handling the full waveguide bandwidth. s H-Plane Sweeps Full Span Coax Adaptors E-Plane Splitter Impedance Transformer 6 8 output must be equal. Two physical realisations are; 1. The Y shown in Fig.5, with its H-plane sweeps, and 2. The Stepped Arrangement used in the four-way power divider (240kW) shown in Fig.6, with its E-plane elbows. The stepped configuration is used in a very high power combiner in the Sears Tower because it is extremely compact compared to a cascade of three adaptors and three Y dividers. Full frequency span waveguide to coax adaptors. Stepped Coax Access From the Sears Tower 6 Channel Combiner Fig.5: Sears Tower 6 Channel Power Divider Many conventional combiners use Hybrid Tees or Riblet Couplers as power dividers, but as explained earlier, these limit the span of channels to less than half of the useable waveguide span. One full span solution is depicted by the two way power divider in Fig.5. It is comprised of: An E-plane splitter: The electric field is simply sliced into two waveguides of equal width (an E-plane split). The relative power split is a function of ratio of the heights of the output guides, but are typically made equal. Because the sum of their impedances equals that of the input waveguide, and they have the same width, the output waveguides are perfectly matched and inherit the full useable frequency span of waveguide. An impedance transformer (or waveguide height adaptor): To avoid reducing the output waveguide height to below the practical half height guide, a full bandwidth impedance transformer (or waveguide height adaptor) increases the height, to standard WR1500 in this example, prior to the E Plane Splitter. Structure enabling in-phase coaxial transition access: After the splitter, the electrical length of waveguide to the coax output and the direction of the E field with respect to the coax Full Span Coax Adaptors Impedance Transformer from Sears Tower Main High Power Combiner Fig.6: Sears Tower 4-Way Power Divider The compact E-Plane sweep connecting the output of the combiner chain s spine to the power divider keeps the total system s footprint small. Compact Adjacent Channels, DTV and NTSC Masks As with conventional combiners, high isolations between transmitters three or more channels apart are achievable with 3-cavity filters and semi-adjacent with 4-cavities. Adjacent channel combiners and DTV and NTSC masks have adequate isolations / selectivities using 8-cavity filters. But a Directional Waveguide Combiner with 8 cavities at the lower UHF channels would be about 12 feet tall, too tall for some rooms.
A solution that meets the selectivity requirements is a 6-cavity combiner with Cross Coupling, shown by Fig.7. Heights are reduced to a practical 9 feet or less, suitable for most rooms. It can be shown that the 6 pole, cross-coupled filter frequency response is equivalent to an 8 pole design in the critical regions adjacent to the passband and is superior in terms of lower loss and group delay within the passband. During the transition period from analog to digital, broadcasters may wish to plan and build NTSC mask/combiners convertible to DTV mask/combiners of the same channels. Directional Waveguide Combiner chains may be built to incorporate this. A DTV mask/combiner is fitted with an all-pass group delay pre-corrector to meet the NTSC requirements. The group delay pre-corrector is removed when NTSC transmission is replaced by DTV. Cross Coupling Fig.7: Adjacent Channel Combiner / DTV Mask VI. CONCLUSIONS UHF Directional Waveguide Combiners are a new and operationally proven alternative to conventional waveguide combiners and have the following advantages: Much wider span of combinable channels (eg US Ch14 to Ch60) More compact, lower cost and very easy to chain High peak power ratings crucial to DTV High number of channels per chain possible More practical in terms of adding/removing channels in the future High power and flexible adjacent channel combining and option of integrated DTV masking / NTSC cleanup filtering. ACKNOWLEDGEMENT It was the inspiration of Graham Broad, the author s mentor, to develop the Directional Waveguide Combiner as the vehicle for achieving compact, wide bandwidth, high power UHF combiners to complement existing wideband UHF antennas. The author wishes to thank Graham for his help and guidance in overcoming the technical hurdles presented by this development. Broadcasters can save space and cost by making the combiner also perform the DTV or NTSC masking functions, thus avoiding separate transmitter mask and clean-up systems. REFERENCES [1] P. A. Rizzi, "Microwave Engineering Passive Circuits" Prentice Hall., section 5-4, pp. 201-216, 1988.