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System Design Concepts By Gus Roumeliotis 1 System Design Concepts Antenna Combining Systems • Allow multiple transmitters and receivers to share an antenna system • Provide adequate isolation to permit simultaneous operation of all frequencies • May consist of several antenna systems (multiple Tx & Rx antennas) Terminology • Multicoupling – Refers to the combining of multiple transmitters and receivers onto a single antenna. • Combiner – Transmit multicoupler with ferrite isolators 2 System Design Concepts Advantages • Maximize tower utilization – Reduces tower loading by minimizing the number of antennas & feedlines – Expandable systems avoid need for tower climb – Premium antenna location can be utilized by many users • Minimize interference risk – Proper multicoupler system is designed to avoid all transmitter noise, receiver desense, and intermod problems – Bandpass filters commonly used in multicoupler systems to provide carrier suppression against other transmitters and noise suppression to avoid causing problems for receivers. – Bandpass filters also inherently notch the second harmonic of the pass frequency 3 System Design Concepts • VHF systems – Virtually no band plan – Potential for extremely close Tx-Rx spacings • Most Tx combiner and Rx Multicoupler systems are custom designs – Lower frequency means separation must be greater between antennas • 30-40 feet tip-to-bottom to gives approx. 55 dB – Greater cavity efficiency in this frequency band. • 7” bandpass cavity set at 1.0 dB insertion loss will provide 20 dB isolation 1.0 MHz from f0 • UHF Systems – Paired services mean standard spacings – Watch for close minimum spacings • E.g. 461 Tx and 459 Rx – Higher frequency means separation can be less between antennas • 20 feet tip-to-bottom to get 55 dB – Lower cavity efficiency at this higher frequency band. • 7” bandpass cavity set at 1.0 dB insertion loss will provide 12 dB isolation 1.0 MHz from f0 4 System Design Concepts • 800 & 900 MHz systems – Paired services mean standard spacings – Watch for cell sites and paging transmitters • 894 MHz is 2MHz from 900 MHz SMR Rx band – Higher frequency means separation can be less between antennas • 10 feet tip-to-bottom to get 55 dB – Use of ¾ wavelength cavities required • 7” ¾ wave bandpass cavity set at 1.0 dB insertion loss will provide 11 dB isolation 1.0 MHz from f0 5 System Design Concepts The Antenna Combining System Design Objectives • Provide sufficient isolation between transmitters (Tx’s) and Receivers (Rx’s) to prevent RF interference in each receiver – Tx/Rx (Transmitter noise) – Rx/Tx (Receiver desensitization or Tx carrier) – Tx/Tx (Transmitter to transmitter isolation) – Rx/Rx (Receiver to receiver isolation) • Maintain proper impedance match with radios (Z0=50 OHMS) • Minimize insertion losses 6 System Design Concepts The Antenna Combining System Design Constraints • Number of Transmitters (Tx’s) and Receivers (Rx’s) • Frequency of Tx’s and Rx’s (MHz) • Tx power levels (Watts) • Radio manufacturers (Motorola, GE, Kenwood) • Number of antennas (Gain of antennas in dBd) • Horizontal or Vertical antenna separation (Feet) • Future expansion needs • Packaging • Other operating frequencies on site • Combine or mate with existing equipment • Desired Price and availability 7 System Design Concepts The Antenna Combining System Design Process • Obtain design constraint information – Tx’s & Rx’s frequencies, power levels, antennas, etc. • Perform intermodulation study • Examine frequency separations – Tx/Rx, Rx/Tx, Tx/Tx, Rx/Rx • Determine isolation requirements – Duplex operation curves (TNRD Curves) • Select type(s) of components and/or combiners to be utilized • Sketch design configuration using Visio – (Electrical and Mechanical) • Verify all design constraints incorporated and objectives met • Price system up, and check system availability • Finalize system diagram/quotation 8 System Design Concepts Sinclair Capabilities • Custom combining and multicoupling systems available • Complete line of components for embedding into OEM systems – Cavity filters – Duplexers – Preselectors – Isolators – Preamplifiers – Splitters 9 System Design Concepts Custom & Special Requirements 1. Simplex, half-duplex, full duplex radio combining 2. High power transmitters 3. High isolation/low loss 4. Small packaging 5. Cabinet systems 6. Various power supply configurations 7. Large channel capacity combiners 8. Intermod analysis & prevention 9. Isolation analysis & design 10. Retuning & expansion capabilities 11. Expansion/modification of existing antenna system 12. Environmental protection 13. Customized products (NPR’s, R&D, etc.) 10
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