Description:

1. A monopole antenna is commonly used m conjunciion with frequency hopping radio transceivers with an optimal iength of ½ the moduiaion frequency wavelength. Maximum efficiency requires discrete antenna lengths and for each modulation frequency the iength is equal to; c

^ ¹ 4[{fmin + (n_hop - 1)A op]

fmax-fmin

Ahop - ·

n-l

Where c is the speed of light, fmin is the minimum hop frequency, fmax is the maximum hop frequency, and n_hop is the hop frequency number with 1 representing the lowest frequency and n representing the highest frequency.

The antenna width is determined as a function of capacitance per unit length so as to match the antenna characteristic impedance to the output impedance of the transceiver. The Programmable Antenna Controlled Impedance yosfet is used as a digitally controlled variable Iength antenna with a geometry that produces the required characteristic impedance.

Semiconductors utilize siiicon doping to provide both low conductance and high conduciance regions to control current. Doped silicon Is commonly denoted as either n-type or p-type. A low conductance p-type semi-conductor is created by doping the silicon crystal with an impurity that accepts electrons. A high conductance n-type semi-conductor is created by doping the Silicon crystal with art impurity that contributes electrons.

An n-channel metal-oxide semi-conductor field effect transistor (Mosfet) is a three terminal device that can be used either as a switch or amplifier element providing voltage control of the Drain to Source current via the Gate to Source voltage. The Drain and Source are directly tied to conductive n-type semiconductors while the Gate is indirectly tied to a non-conductive p-type semiconductor through a Gate insulator (Fig. 1).

The Drain to Source conductivity is increased by applying a positive voltage between the Gate and Source terminals. The Gate to Source voltage produces an electric field which enhances a channel with the avaiiable p-type semiconductor electrons drawn towards the Gate insulator lowering the Drain to Source impedance. The Drain to Source conductivity is increased as the Gate to Source voltage is increased to its threshold voltage. A programmable switch is possible using the n-channel mosfet with a digitally compatibie threshold voltage (Fig.. 2). The conductive enhancement of the p-type semi-conductor material In the n- channel mosfet Is traditionally accomplished with a s ngle controlling Gate input for controlling the drain to source current(Flg. 2). Further control of the enhancement region can be gained by adding another Gate input (Fig, 3). I! the two controlling Gates are of equal dimension the length of the enhancement region can be halved by shorting the second Gate to the Source (Fig. 4),

The characteristic impedance for a uniform transmission line is given by;

Where L is the inductance per unit length and G is the capacitance per unit length. This same relationship can be applied to the monopole antenna which can be designed onto a Printed Circuit Board (PCB) using a copper trace with the length set to ¾ modulation frequency wave length and the width set to provide the necessary capacitance to match the antenna characteristic impedance to the transceiver output impedance.

The Programmable Antenna Controlled impedance Mosfet consists of an n- channel mosfet configured with multiple gate inputs for digitally controiiing the enhancement channel length (Fig. 5). The enhancement channel characteristic impedance is set by the Gate capacitance and the enhancement channel inductance per unit length. The maximum enhanced channel length is produced with all gates digitally driven over the Gate to Source threshold voltage. The gate function for a Bluetooth Low Energy application is as follows; n G(Hex)

1 FFFFFFFF FF

2 FFf FFFfF FE

3 FFFFFFFF F8

4 FFFFFFFF FO

40 8000000000

The Gate capacitance is given by;

CoCrA

C

d

Where€o is the permittivity of free space,€r is the relative permittivity of the Gate Insulator, A is the Gate area, and d is the Gate Insulator thickness.

The Programmable Antenna Controlled Impedance Mosfet enhancement channel characteristic impedance is set with maximum channel length equal to ½ the minimum modulation frequency wavelength and by adjusting device geometry and Gate Insulator relative permittivity.

The device is configured as a programmable antenna by driving the Source terminal, leaving the Drain terminal open, and digitally controlling n Gat inputs where n is the number of hopping frequencies. The Gate inputs are individually driven using low impedance drivers so that th Gate capacitance and the enhanced channel antenna length set the characteristic impedance.