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Simple Wireless Internet Range Extending Systems For many years, the sensitivity or noise figure of wireless receivers has achieved such low values that sensitivity is now within a few decibels of the ambient background noise temperature or equivalent noise power as determined by the Rayleigh-Jeans approximation:- Vn = Equivalent Voltage at antenna due to thermal noise k = Boltzmann's constant T = Temperature in deg. Kelvin B = Bandwidth (Hz) R= Resistance (ohms) The maximum Effective Isotropic Radiated Power (EIRP) of wireless transmitters is a measure used by regulatory authorities to limit emissions. Prior to this innovation, licence exempt and low cost point to point wireless (such as Bluetooth and Wireless Local Area Networks WLANs which operate world-wide under the IEEE 802.11a specification, at very fast broadband 11 Mbits/sec data rates) are generally believed to only legally operable within a respective 30 and 300 metre approximate radius since the Effective Isotropic Radiated Power (EIRP) in the latter is limited to less than 100 milli-Watts. At UHF and microwave frequencies, domestic sized parabolic dish and other high performance antennas might be used which improve the operating range by an order or two of magnitude (e.g. x 30). However should an improved antenna of this kind be used for licence exempt products this causes the sending appliance to exceed the conformal EIRP allowed for unlicensed operation which in some countries including the United Kingdom, constitutes a criminal offence. 2 STATE OF THE ART On close examination any two-way wireless communications point to point link actually consists of four discrete system elements consisting of two transmitter and two receiver systems as shown in fig.1 Standard Two Way Radio Link (fig.1) Any improvement to a single system element within a one-way "leg" of a working two-way point-to-point link will not be useable systemwise unless complemented on the other half of the system. Common Antenna: (fig.2) Equipment is generally provided with a single antenna connection for both transmitting and receiving. In a typical two-way link the respective transmitters and receivers are connected by an internal combining or switching circuit to a (generally assumed to be) common antenna as shown below as fig.2 Fig.2 Common Antenna short range It is generally not possible for a user to alter the internal equipment circuitry without compromising "type approval" requirements within the national regulatory type approval frameworks… 3 THE EIRP REGULATOR In this scheme a wireless tranceiver uses a single "high gain antenna". The receive and transmit antenna paths are differentiated so that the increased transmitter output signal radiation (as a function of actual antenna gain) is selectively attenuated so as to never exceed the EIRP maximum laid down by national government regulatory agencies. Passive Circuit The passive automatic wireless emissions management circuit is in effect a single path unidirectional circuit which passes signals in the forward (receive) path with little or no degradation but reliably attenuates signals in the return or backward (transmit) path as shown in fig.5. Fig.5 Passive Automatic Wireless Emissions Management In its simplest form suitable for milliwatt power transmitters the backward-attenuator is:1. A biased switching diode circuit. 2. An unswitched low-noise pre-amplifier with adjustable backward attenuation. 3. A phase balanced (and unbalanced) network. 4. A "circulator". Or 5. An "isolator". The use of items 4 and 5 (but for completely different purposes) is well known. The automatic wireless emissions management circuit (“ACA”) is employed by simply inserting it in the antenna feed cable or built into the transmitter stages of a newly designed appliance. 4 Fig.7 Overall System (EIRP Compliant) long range Summary: With a wireless emissions management circuit described above high gain antennas of many kinds such as omnidirectional, sector, horn, yagi or parabolic may now be freely employed to increase the apparent receiver gain and therefore effective range without exceeding the transmitter power required for conformance. Unlike any existing system, this increase in system range is equal to that afforded by the high gain antenna at the end of the link path with the least gain and not the more usual algebraic sum of both. At least one high gain antenna is therefore absolutely essential AT EACH END of a long-range link…. Whilst offering no great scientific merit, this is a practical innovation which offers a means for ordinary consumers to purchase "range enhancing antenna" for use with low power wireless systems and use them safely without fear of infringing the EIRP levels as set by their local regulatory authorities. Henry O'Tani March 2001 Bath Tel: 01225 789144 Email: [email protected]