From May 2006 High Frequency Electronics

High Frequency Design

Copyright © 2006 Summit Technical Media

THROUGH-WALL COUPLER

Microwave Coupler Feeds

Outdoor Antenna

Through Walls

By G. Knapp

Consultant

M

icrowave  wall

Over the years FDTD became a main

FDTD modeling was used to

couplers  were

choice for electromagnetic simulations due to

develop this coupler,

originally devel-

its good accuracy, aided by the rapid increase

which can be used to send

oped for room-to-room

of computational speed and RAM capacity of

signals from an outdoor

LAN connections, where

PCs. With the simulations that follow, about 5

WLAN antenna through a

inexpensive  2.4  GHz

to 12 hours are needed for one computer run

wall to indoor equipment

access points, operating

with 6 cuts through the object's dimension

as bridges, are connected

using a 1.4 GHz computer with a 512 MB

on each side of a transmission blocking wall by

RAM. The program really does not care in

microwave wall couplers (Fig. 1).

what area you want your simulation done. It is

Now, with citywide WiFi becoming avail-

important to include enough cells to cover

able in many cities and towns, a new applica-

plenty of free space around the objects for

tion for the couplers can simplify and improve

fields you may need later. For the couplers a

wireless installation for many houses. Figure

block of cells, called grid or mesh, with x = 200,

2 shows a passive antenna feed, with a panel

y = 200 and height z = 300 cells, a total of 12

antenna directly connected to a coupler. Inside

million cells are needed for computation and

the house an identical coupler receives the

high-resolution display without need for a

WiFi signal through the wall (or roof), which is

finer subgrid, which is optional for fine tuning

then retransmitted by a wireless access point

of the geometry at a later time.

In real dimensions, the grid is 400 × 400 ×

to computers and wireless devices inside.

In order to keep coupling losses as low as

600 mm in size. The 2 mm cell size gives good

possible, a good design for the coupler had to

resolution of the electromagnetic field for all

be found. Since magnetic and acoustic tech-

data. The accuracy, however, is dependent

niques do not provide sufficient bandwidth

upon a number of time steps of the input

and reach, a coupling microwave patch anten-

source. The shortest calculation time is

na system was designed using common design

reached with transient and sinusoidal excita-

equation [1] and refined by Finite Difference

tion, when near-zone field points data do not

Time Domain (FDTD) simulation [2].

change significantly with an increase in the

Figure 1 · Through-wall couplers eliminate the need for repeaters or wired connections.

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High Frequency Electronics