IW3HEV embedded antenna analyser

The
IW3HEV antenna analyser is a relatively
simple antenna analyser. It has a 1-60MHz DDS signal generator and uses a
homebrew directional coupler plus the Analog
Devices AD8302 IF Gain Phase detector. Controlled through a PC's parallel
port it can measure SWR, Return-loss, Phase and Z. It cannot determine the real
and imaginary parts of Z however.

I read about this design in the
december 2004 issue of Funkamateur in
an article written by IW3HEV and IW3IJZ. Together with two other radio-amateurs
we started collecting parts and producing the PCB. It turned out to be easy to
reproduce the design. Once I started using the analyser however, I wondered if I
could make an embedded version so that I could do without the PC. Yes, I could
carry a laptop up on the roof to do antenna measurements, but a small
self-contained box would be much easier and I alway wanted to have a 'digital
MFJ-259' anyway.
An embedded version has to emulate the essential functions
of the PC:

- sweep the IW3HEV analyser accross a frequency range
-
readout the ADC's and compute SWR etc.
- display the result on a small
graphics display

I had a few 128x64 pixel graphics displays with a
Toshiba T6963 controller. Bascom has routines for controlling such a display.
The display is quite small however, so I had to limit the embedded version in
several ways:

- the frequency is swept accross 115 points as opposed to
500 point in the PC program
- the frequency settings are controlled using six
keys
- only the swr graph is displayed, Return-loss and Z is displayed only
for the centre-frequency

I use an Atmel ATMega32 microcontroller in the 40-pin
DIP version. This controller has enough I/O to control the graphics display, the
IW3HEV analyser and read the keys. This first version has some I/O pins left for
future enhancements. The controllers speed is app. 13.5MHz, resulting in app.
two sweeps per second. Not too fast, but considering the amount of arithmetic
the ATMega32 has to perform, entirely satisfactory.
The first version of the
program (v0.23) uses approximately half of the ATMega32 program memory, so there
is plenty of room for extras.

Frequency settings are controlled in a
spectrum-analyser fashion. Two keys each are used to step the centre-frequency,
frequency-span and frequency-step up or down. Frequency-step can be set to 10,
20, 50, 100, 200, 500kHz, 1, 2, 5, 10MHz.
Frequency span can be set to 0, 20,
50, 100, 200, 500kHz, 1, 2, 5, 10, 20, 50MHz.
'Zero-span' means that a
constant frequency is generated accross the sweep. The analyser could be used as
a (primitive) signal generator this way.

As the size of the built-in font
of the T6963 display was too large, I made a smaller 5x5 pixel character set.
This makes reading the display a little harder, but also enables a larger graph
area.
There is a seventh key on the box. This key can be used in future
versions to call up a menu.

The schematic:

is
quite simple, most of the complexity is in the software! The IW3HEV analyser is
built as standard, no changes are needed for this embedded version. You can
build the analyser, test it with a PC and then connect it unchanged to the
controller. The Int pins and RS232 pins on PortD are not used in this version,
they are available for future enhancements. Also, pin 7 of PortA is free. This
pin could be used for monitoring battery voltage.

The Bascom
program is indeed a little more complex than the schematic. The program
starts by clearing the graphics display and initialising the DDS. It then cycles
continually through the sweep mode until a key is pressed. A key press may
result in a change in the frequency setting after which the graph is updated and
sweeping is resumed.
If the sweep is at the centre frequency all parameters
are calculated, for the other points only SWR is calculated to speed up the
sweep. The ATMega32 is quite fast but it is not a floating-point number
cruncher!

After experimenting on a breadboard
I built everything into an enclosure
I used is made of plastic and contains
the IW3HEV analyser, the graphics display, the controller board (in dead bug
style), a small board with seven keys, two switches: on/off and lcd backlight,
two led's (currently unused), a small board with a power supply, a 12V power
supply entry and a 9-pin and a 25-pin DB connector (both unused).
The power
supply uses a National Semiconductor
LM2825 one-chip integrated DC-DC converter. This chip has a wide input voltage
range and can easily supply 5V/300mA (backlight on). I am working on a 2x2-cell
Li-Ion battery charger circuit.

A few pictures of the graphics
display:

The
'welcome' screen

A
small magnetic-loop antenna.
The top line displays frequency-step value, SWR,
Return-loss and Z.
Left axis show SWR ticks at 1, 1.2, 1.5, 1.7, 2 and
"High".
Bottom line shows start-, centre- and stop-frequency.

Same
as above, but with 'zero-span'.

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