Page 16 May June 2014 TCA

ANTENNAS & TRANSMISSION LINES

USING SIMSMITH FOR ANTENNA SYSTEM DESIGN
AN INVERTED DELTA LOOP EXAMPLE

cable of negative length in the software. As I 2) A design example using a vertical inverted delta
write this column in January, the temperature in loop antenna (20 metre band) fed at the bottom.
my backyard is – 24° C and I am not inclined to In particular, it is shown how to match the antenna
put my antenna analyzer outside and hook it up for the 30 metre band which is quite far from the
to the antenna terminals to accomplish the resonant frequency of 14 MHz.
same task.
BACKGROUND
David Conn, VE3KL One use of SimSmith is simply that of a learning
24 Terrace Drive tool for those building their skills in the The idea behind the Smith Chart and SimSmith is
Ottawa, ON K2H 9N2 application of Smith Charts. This is useful in itself the refection of waves from some point (usually
E: davidrconn@rogers.com
since these charts are used by many authors but not always) at the load. In most cases, we
and appear in the ARRL antenna books. This want the refection of a forward wave at the load
INTRODUCTION learning tool is made possible by the use of to be small so that most of the power gets
drag and drop lumped element circuits (R, L, C, transmitted to the antenna or other load. The
This column provides an Transmission Lines and other components) to commonly used term Standing Wave Ratio (SWR) is
introduction to the use of see their interpretation as viewed on the Smith directly related to these refections. Below is the
a software program, Chart. No mathematics is used directly since basic equation used internally in SimSmith.
SimSmith, developed by SimSmith does it all for us. The catch is that we
AE6TY (see TCA hotlink 1), still have to understand the basics of refection
that adds one more tool coeffcient and its simple relationship to
for developing antenna standing waves.
and transmission line Where:
systems. As the name For most Amateurs who do not design their
suggests, the program is own antennas or antenna tuners, perhaps the Z is the load on a transmission line with
L
based on a graphical use most important use of SimSmith is that of characteristic impedance Z .
0
of Smith Charts. evaluating their system. For example, if a system
consists of a high grade transceiver and The Smith chart is simply a plot of the refection
More importantly, the amplifer in conjunction with an antenna tuner, coeffcient on a polar graph with various overlays
program links to antenna transmission line and a non-resonant antenna, superimposed on the basic graph. This is
simulators – such as EZNEC then SimSmith can easily evaluate the impact of analogous to the idea of layers used in
(see TCA hotlink 2) – and placing the antenna tuner at the transceiver photographic software editing packages but the
collects data from our instead of at the antenna. The total loss from basic graph (bottom layer) is always the refection
measurements made with transmitter to the load is also given. Previously coeffcient.
network and impedance this was a diffcult calculation to perform and
analyzers. It also uses was seldom done. SimSmith does the calculations for you and
transmission line data from displays the results using graphics. It is a great
TLDetails (see TCA hotlink 3). This column does not provide a tutorial on how learning tool because you can try many examples
This represents a big step to use SimSmith as these are well covered in the quickly using the sweep (change) function to gain
forward for Radio Amateurs SimSmith website. The purpose here is simply to an understanding of the process that is involved.
who want to design, show its use through examples and how For example, you can set Z to 50 Ohms and Z to
L
0
measure and evaluate their beginners can get started with SimSmith as a 100 Ohms and SimSmith immediately shows you
own antenna systems. learning tool. Please note that SimSmith is the value of the refection coeffcient while Z is
L
constantly being upgraded. I am currently using swept over a range of values. The SWR is directly
It makes it very easy to version 7.8. By the time you read this column, I am derived from the refection coeffcient as follows:
measure the impedance of sure that a new version will be in use. The main
an antenna at the far end impact is that some tutorials are based on older
of a cable (the radio shack) versions that have different graphics and
and then simply fnd the functions so the tutorials have to be used as a
impedance at the actual guide only. To get started, download SimSmith SWR is a good example of an overlay on the Smith
antenna by inserting a and read the introduction tutorial that gives a Chart. Often, circles of constant SWR are used as
tour of the main page and shows how to design one layer. I often plot an SWR circle of 1.5 on top
Note: This article uses TCA some simple circuits. of the basic plot.
hotlinks to provide access
to enriched media from the This column includes: The process used by SimSmith is shown in Figure 1
RAC website. For more on the next page. The main calculator is shown
information, please go to 1) Viewing simple circuits on SimSmith that surrounded by four blocks in this example.
http://www.rac.ca/tca. include resistors, inductors and transmission
lines.

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