Microwave Engineering Europe - April 2008 - (Page 21)

MILITARY/AEROSPACE FOCUS 21 design, additional methods are needed, such as shielding and screening, commonly used to reduce susceptibility and “choke” radiated interference or signal leakage from the radio. The electrical design must often be modiﬁed to achieve compliance with the requirements; sometimes, this has even architectural implications. And, as usual, different types of ﬁlters are needed, and they cannot be digital. The antenna The antenna is a very important part of any radio system. The antenna, which is actually a transducer converting electromagnetic ﬁeld energy into electrical power, is the real contribution Guglielmo Marconi made at the end of 19 th century which led to the invention of radio communication. The antenna is a resonant device, with parameters that strongly depend on the operating frequency. To achieve resonance at HF or VHF, large antennas are used; although a short antenna can be made to resonate at low frequency, its efﬁciency will be very low. The basic trade off in antenna engineering is among size, gain and bandwidth. Higher gain requires higher size for the same frequency. Wider bandwidth requires higher size to keep the gain constant [1]. Even fractal antennas are subject to the same trade off. It is hard to overestimate the importance of antennas for communication systems: reducing the antenna gain by only 1.5 dB reduces the channel budget by 3 dB: this is equivalent to halving the output power, or losing most of the gain achievable by complex FEC schemes. ISWR is supposed to operate in any frequency band, so very wideband antennas are required. When building practical wideband antennas, performance is compromised, material costs rises, and the space needed to mount the antenna (or antenna array) increases. Thus, the antenna problem is most acute for manpack and handheld radios. For example, the JTRS handheld radio (Cluster 2, MBITR) needs 3 separate antennas to provide the same communication range as its narrowband counterparts over the 30 to 512 MHz frequency range. The problem is further compounded by mutual coupling among antennas in dense communication sites, where collocation issues become a real nightmare. Very sophisticated, extremely costly external devices (multicouplers, combiners, switches, etc., — none of them classiﬁable as digital), are then required to alleviate the problem. Wideband versus narrowband Many components in the RF path (antennas, ﬁlters, matching networks, RF transistors) are resonant devices and consequently band limited. RF components technology is also frequency dependent: for example, at the lower end of the radio spectrum lumped components (coils, capacitors, transformers) are used, whereas above 1 GHz distributed components are much more practical. To achieve wideband performance, a so called “ﬂexible RF” approach is proposed. This approach is based on using a sufﬁciently large number of RF circuits, each optimized and tuned for operation over a limited range of frequencies, that are switched into the signal path in accordance with the desired operating frequency. Unfortunately, the auxiliary circuits needed to implement this approach introduce stray coupling and additional losses, which are then compensated for by further increasing complexity and equipment volume. The inescapable conclusion is that for similar performance, a wideband device will be larger and more expensive than a narrowband implementation. Microwave Engineering ● April 2008 ● www.mwee.com 018-019-020-021-022_MWEE.indd 21 26/03/08 18:07:40 http://www.lpkf.com http://www.lpkf.com http://www.mwee.com

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Microwave Engineering Europe - April 2008 News Contents Comment Test and Measurement: Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation Military/Aerospace Focus: Hardware Needs Limit Software Radio Interview — Mitsubishi Electric Europe: GaAs Technologies Spanning High-End Space and Radar Through to Cost-Sensitive Handset and LNB Applications How Do You Test ZigBee Transmitters? Advanced Receiver Design Boosts Performance CMOS PAs Pave the Way for One-Chip Phones Products Calendar

Microwave Engineering Europe - April 2008

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