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Transcript
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]