Specifications

From January 2005 QST © ARRL
Figure 3—A lighted magnifying lamp was
an excellent addition to my kit building
tools. Here I was working on the KDSP2
circuit board.
KAT100 control cable in parallel with an
RS232 computer control cable, so both
the tuner and the computer interface can
be used at the same time.
Alignment and Test
After the construction is complete there
are a few tests and alignment steps before
you are ready to operate through the tuner.
I was pleased to see all the proper LEDs
light and the K2 main display acknowledge
the tuner when I first connected the tuner
and turned on the radio. The adjustments
are made by connecting a DMM between
ground and selected pins of the op-amp
used to measure forward and reflected
power for the SWR readout, as well as the
power calibration.
Circuit Overview
Eight series inductors and eight parallel
capacitors form an L network for the
KAT100 matching network. The induc-
tance range is about 0 to 20 µH in 256
steps, and the capacitance range is about
0 to 2400 pF in 256 steps. The components
are rated so that the tuner can handle
150 W with an output SWR as high as 10:1.
The parallel capacitance can be switched
between the transceiver and the antenna
side of the tuner using one of the relays.
A relay also selects the
ANTENNA 1 or
ANTENNA 2 connector.
Front-panel LEDs indicate whether
ANTENNA 1 or 2 is selected, and also
indicate whether the K2 is set to a power
level below 20 W (
LOW ) or above 20 W
(
HIGH ). A 10 segment LED multicolor
bargraph indicates SWR—green from 1:1
to 1.5:1—yellow to 2.5:1 and red up to 5:1.
See Figure 4 for an inside view of the
completed KAT100.
Lab Test Engineer Michael Tracy,
KC1SX, noted that unlike other automatic
antenna tuners we have tested in recent
years, the KAT100 always tunes for the
best possible match instead of stopping
when it finds a match of (typically) 1.5:1.
Operating with the KAT100
The KAT100 requires a 10 to 15 V dc
power supply. Typical current drain is about
200 to 300 mA, although it can be as high
as 700 mA if all relays and LEDs are
activated at the same time. This might
occur, for example at the beginning of a
tune cycle, if the initial SWR were over 5:1.
Connect one or two antennas to the
antenna jacks, use a short jumper between
the K2 and the tuner
RF INPUT, and
connect the serial control cable between
the two units. You should also connect
good ground wires to the ground screws
on the back panel of the tuner and the K2.
The tuner uses a transistor switch to
turn it on when the K2 is powered on.
The appropriate antenna indicator and
power level indicator will light up,
showing you that the tuner is ready to go.
The first time you select any band with
one of the antenna jacks, you will have
to press and hold the
TUNE button on the
front of the K2. This will start a tune
sequence. As soon as the sequence starts
you can release the
TUNE button. The
radio will reduce power to 20 W if you
had it set to a higher power level. If you
are using the tuner with the power set for
QRP operation, it will decrease power to
2 W. (The tuner will operate with the
transmitter power set as low as 0.2 W.)
The microcontroller will step through the
relay positions and measure the SWR to
seek the best match conditions. Within
about 1 to 5 seconds you should notice
the SWR bar graph indicators on the tuner
drop to between 1:1 and 1.5:1.
The next time you come back to a band,
the microcontroller will automatically set
the relays to the saved tuner and antenna
port settings for that band. As you tune up
or down the band, the SWR indicator LEDs
on the tuner will show you if the SWR
starts to increase. In that case you will have
to press and hold the
TUNE button again.
It is worth mentioning that this is a
bit different than what I first expected of
an automatic antenna tuner. At least with
some automatic tuners, if the tuner senses
an SWR higher than some predetermined
level, such as 2:1 or 3:1, the tuner will
go into a tune cycle automatically to
correct the mismatch. While the KAT100
senses SWR and displays it on the LED
bar graph, it does not automatically go
into a tune sequence.
You can also use the K2 to read the
various L and C settings. The ATU menu
selections of
Lxx.x and Cxx.x show the
present settings in µH and nF. You can
also select individual inductor and
capacitor settings directly from the menu.
These features would be useful if you
have a problem with your tuner and have
to troubleshoot the operation.
My first operation with the KAT100
was for Field Day 2004. My wife Jean,
WB3IOS, and I went camping at
Burlingame State Park in Charlestown,
Rhode Island. I used a 20 foot telescoping
fishing pole and about 35 feet of number
10 wire. Near the middle of the pole I
wound the wire into a large coil. Then I
continued with the wire straight to the
base of the pole. Three guy ropes and
stakes held my vertical antenna in the
field at the front of our campsite. I ran
six radial wires along the ground and
connected a piece of coaxial cable to the
antenna and ground system.
The KAT100 tuned that antenna on all
bands, 80 through10 meters, and we had a
great time. The tuner and radio operated
flawlessly, with the radio operating at 100
W all weekend. We powered everything
from a large gelled electrolyte battery, with
a small solar panel and the Micro M Plus
charge controller from The ARRL
Handbook.
3
This illustrates one great
feature of the K2 station. Even with the
KPA100 100 W amplifier installed and
using the KAT100 automatic antenna
tuner, this station is designed to operate
efficiently from battery power.
Firmware Upgrade and Keying
Waveform Modification
Elecraft is always looking for ways to
improve their products. As changes are
made to the circuitry and firmware for the
various microcontrollers, they publish the
information and make upgrade kits
available. One small circuit change came
out shortly after I completed building the
KPA100 amplifier.
There had been some discussion about
the CW keying bandwidth of the K2, and
many other modern transceivers, for that
matter. The sharp turn-off on key-up on
most radios can cause a slight click at the
Figure 4—A view inside the completed
KAT100 automatic antenna tuner. Note
the front-panel LEDs.
3
D. Reed, Editor, The ARRL Handbook for
Radio Communications , 2005 Edition,
p 17.41.