LDG AT-100Pro User manual

AT-100Pro
Automatic Antenna Tuner
Manual Version 1.1
Fa
LDG Electronics
1445 Parran Road, PO Box 48
St. Leonard MD 20685-2903 USA
Phone: [phone]
Fax: [phone]
[email protected]
www.ldgelectronics.com

LDG AT-100Pro
Automatic Antenna Tuner
Table of Contents
Introduction 2
Jumpstart, or “Real hams don’t read manuals!” 3
Specifications 4
An important word about power levels 4
Getting To Know Your AT-100Pro 5
Installation 6
Operation 8
Basic operation 8
Tuning 12
Advanced Operation 14
Application Notes 15
Mobile operation 16
When to use Auto mode 16
When to use Peak mode 17
When to turn the LEDs off 17
RF Noise 17
MARS/CAP coverage 17
Error indications 17
Theory Of Operation 17
Some basic ideas about impedance 18
Transmitters, transmission lines, antennas and impedance 18
The LDG AT-100Pro 19
A word about tuning etiquette 21
Care and Maintenance 21
Technical Support 21
Warranty and Service 21
Firmware upgrades 22
Feedback 22
2

Introduction
Congratulations on selecting the LDG AT-100Pro tuner. The AT-100Pro provides fully
automatic, any mode antenna tuning across the entire HF range plus 6 meters for less than the
cost of some manual tuners. It will tune dipoles, verticals, Yagis or virtually any coax-fed
antenna. It will match an amazing range of antennas and impedances, far greater than some other
tuners you may have considered. Also, it consumes very little power making it suitable for
battery-powered operations.
While resembling earlier LDG tuners in overall layout and function, the AT-100Pro represents a
quantum leap in features and performance. Enhanced tuning algorithms provide much faster,
precise and consistent tuning. Automatic tuning is now available during transmission, even SSB,
and many user-settable options are accessible from the front panel. The two large LED meters
provide simultaneous readings of power and SWR, and readouts of internal states and settings.
LDG pioneered the automatic, wide-range switched-L tuner in 1995. From its laboratories near
the nation’s capitol, LDG continues to define the state of the art in this field with innovative
automatic tuners and related products for every amateur need.
Jumpstart, or “Real hams don’t read manuals!”
Ok, but at least read this one section before you transmit:
1. Connect the antenna jack on your transceiver to the "Tx" jack on your AT-100Pro
tuner using a 50 Ohm coaxial cable jumper of appropriate power handling capability.
2. Connect your 50 Ohm antenna coax lead to the "Ant 1" jack on the back of your AT-
100Pro.
3. Connect your AT-100Pro to a source of 11 - 16 volts DC @ 500mA via the 2.5 by 5.5
mm power jack on the back (center positive).
4. Power up your transceiver and select the desired operating frequency.
5. Begin transmitting, any mode1.
6. Wait for the tuning cycle to end.
7. You’re now ready to operate.
1If using SSB mode, simply speak into the microphone. You can tune while transmitting up to 125 watts if
your transceiver has a “roll-back circuit” to protect it from high SWR. If it does not have a roll-back circuit,
limit power when tuning to 25 watts to avoid damage to your transmitter or transceiver.
3

Specifications
• 1 to 125 watt power range (SSB and CW).
• 1.8 to 54.0 MHz coverage. Built-in frequency sensor
• For Dipoles, Verticals, Vees, Beams or and Coax Fed Antenna
• Tunes 6 to 1000 ohm loads (16 to150 on 6M), 6 to 4000 ohms with optional 4:1 Balun
• Over 4,000 memories for nearly instant retuning
• Tuning time: 0.2 to 2.5 seconds full tune, 0.2 second memory recall
• Supports two antennas, with separate memories for each
• Uses latching relays, retains tuned setting indefinitely even when DC power is removed
• Enhanced tuning algorithms for fast, consistent tuning even on SSB
• Easy to read LED bargraph display for RF power, SWR and status
• Two position antenna switch. Separate memories for each antenna
• Selectable user parameters: Auto/Semi, 125/12.5 watt scale, power save and more.
• Optional external Balun allows tuning of random length, long wire or ladder line fed
antennas
• Interfaces for Icom and Yaesu included.
• Power requirements: 11 to 16 volts DC at 500 mA Max, 7mA idle
• Size: 7.5” x 5.5” x 2.0”. Weight: 1.5 pounds
An important word about power levels
The AT-100Pro is rated at 125 watts continuous power input at most. Many ham transmitters and
transceivers, and virtually all amplifiers, output well over 125 watts. Power levels significantly
exceeding specifications will definitely damage or destroy your AT-100Pro. If your tuner fails
during overload, it could damage your transmitter or transceiver. Be sure to observe the specified
power limitations.
IMPORTANT SAFETY WARNING
Never install antennas over or near power lines. You can be
seriousl
y
in
j
ured or killed if an
y
part of the antenna or
transmission line touches the power line. Alwa
y
s follow this
antenna safet
y
rule: the distance to the nearest power line
should be at least twice the length of the lon
g
est antenna
dimension.
4

Getting To Know Your AT-100Pro
Your AT-100Pro is a quality, precision instrument that will give you many years of outstanding
service; take a few minutes to get to know it.
• Your AT-100Pro can be used with any transceiver or transmitter with coax output operating
in the HF range at no more than 125 watts output. You can set the unit to tune automatically
whenever the SWR exceeds a set value, or you can set it to tune semi-automatically when you
start a tuning cycle by pressing the Tune button.
The front panel presents seven pushbutton controls, and two LED meters:
• Tune: Initiates either a memory or full tuning cycle, and also places the tuner in "bypass"
mode.
• Ant: Selects one of two antennas
• C Up: Manually increase capacitance
• C Dn: Manually decrease capacitance
• L Up: Manually increase inductance
• L Dn: Manually decrease inductance
• Func: Selects alternate functions for the other six buttons (see below)
• PWR: LED meter indicates forward power in 12.5 and 125 watt ranges
• SWR: LED meter indicates SWR while tuning or transmitting
There is no “Off” button; the AT-100Pro powers up as soon as you plug in an external power
supply. After tuning, it automatically enters a “deep sleep” state in which it draws only a few
milliamps of current, effectively off. The tuner will automatically "wake up" the next time you
start a tuning cycle, when RF is present, a button is pushed or when an automatic cycle is needed.
Your AT-100Pro employs latching relays which hold the tuned setting indefinitely, even when
DC power is completely removed by unplugging the power connector. Tuning memories are
stored indefinitely in EEPROM memory.
5

The AT-100Pro has over 2,000 memories for each of two antennas, each of which stores the
tuning settings for a specific frequency. When you transmit near a previously tuned frequency,
you can use “Memory Tune” to reset the tuner in only a fraction of a second. The process of
storing tuning data in memory is completely automatic; your AT-100Pro “learns” as you use it,
adapting itself to all of the bands and frequencies you use.
On the back panel, there are six connectors:
• RF input (marked “Tx”, standard SO-239 socket)
• Antenna connector 1 (marked "Ant 1", standard SO-239 socket)
• Antenna connector 2 (marked "Ant 2", standard SO-239 socket)
• DC power in (2.5 by 5.5 mm power jack marked "Power", center positive)
• Stereo 1/8” jack marked "Radio" for connecting a control cable to a compatible
transceiver
• Ground connector (wing nut)
6

Installation
Your AT-100Pro tuner is intended for indoor use only; it is not water resistant. If you use it
outdoors (Field Day, for example) you must protect it from rain. The AT-100Pro is designed for
use with coax-fed antennas. If you wish to use it with longwires or antennas fed with a balanced
transmission line (e.g., ladder line), an external balun is required; the LDG RBA4:1 or RBA-1:1 is
ideal.
Always turn your radio off before plugging or unplugging anything. Your radio may be damaged if
you connect or disconnect a cable with the unit powered up. Note that some radios do not power
down the tuner even when the radio is turned off. To be safe, unplug the power connector before
plugging or unplugging anything.
Connect the HF antenna jack on your transmitter or transceiver to the Tx jack on the back of your
AT-100Pro tuner using a coax jumper with standard PL-259 plugs (not provided). Properly
soldered connectors will be far more satisfactory and reliable than crimp-on or “solderless”
connectors. Attach your antenna lead-in coax to either the Ant 1 or Ant 2 jack on the back of your
AT-100Pro tuner. You will select the appropriate antenna from the front panel (see Operating
Instructions).
Your AT-100Pro can interface directly with several popular transceivers. For Icom radios, it will
enable their “Tune” button to start a tuning cycle and provide power to the tuner.
For Icom radios that are AH-3 or AH-4 compatible, connect the white molex connector into the
radio’s tuner port. The power and radio plugs then connect to the tuner. The tuning process can
start by either pressing the tune button on the tuner or the radio.
For Yaesu FT-897 and 857, use the Y-ACC cable and plug the red end marked “Radio” into the
radio’s ACC port. Connect the black end marked “Tuner” into the tuner’s interface jack. The radio
does not supply power to the tuner. The tune button on the tuner is used for starting the tune
process.
If you are not using an Icom interface for powering the tuner, connect your AT-100Pro to a source
of DC power capable of providing 11 – 16 volts DC at 500 mA, using the provided 2.5x5.5mm
coaxial cable (center positive). If your radio is powered by 12 VDC, you can use the same power
supply for the AT-100Pro, providing the power supply can source the extra 500 mA required by
the tuner during a tuning cycle.
Grounding your tuner will enhance its performance and safety. LDG recommends that you connect
your tuner to a suitable ground; a common ground rod connected to buried radials is preferred, but
a single ground rod, a cold water pipe or the screw that holds the cover on an AC outlet can
provide a serviceable ground. LDG strongly recommends that you use a properly installed, high
quality lightning arrestor on all antenna cables.
7

Operation
Basic operation
All functions are controlled through the seven front panel buttons. Each function is invoked when
the key is released. Some commands are indicated by the length of time you hold a button before
releasing it. There are three lengths of press: short (less than .5 seconds), medium (.5 – 2.5
seconds) and long (more than 2.5 seconds). This sounds a lot more complicated than actually it is;
this interface will quickly become familiar and easy.
Setting Options
All settings are retained in EEPROM memory indefinitely, even if power is disconnected.
Separate settings are stored for each antenna, except for LED On/Off and Antenna selection,
which are global settings that apply to both antennas.
Startup Options
Your AT-100Pro has three startup commands, invoked by holding down one or more keys while
powering up the unit (usually by plugging in the coaxial power connector).
Version display: Press and hold Func while powering up. This will display the version number
of the firmware in your tuner on the LED meters. The first digit of the version number is
displayed on the PWR meter, the second on the SWR meter. The LEDs use a one-of-eight
display, indicating 1, 2, 3, etc from right to left. In the example below, version 1.2 is displayed
(this is just an example; your version number may be different).
Version Display
All Reset: Press and hold Func+Ant+Tune while powering up. This will reset the unit to
factory defaults, and will erase all memories.
LEDs On/Off: Press and hold Ant while powering up. This will toggle the unit between
displaying the power and swr LED meter displays during transmit. These displays will be active
during a tuning cycle or after key presses regardless of this startup setting. You may want to
disable the LED meters when operating from battery power.
8

Menu Options
Your AT-100Pro offers several options to adapt the tuner to your operations. Many functions are
controlled using the Func button. Press and release this button to place the tuner in Function
mode. You will see an "up arrow" display on the LED meters, indicating Function mode.
Function mode will time out after a few seconds, and a "down arrow" display will appear;
subsequent key presses must occur while function mode is still active. You can also cancel the
function mode by momentarily pressing the Func button a second time.
Function Mode On
Function Mode Off
The status check functions require you to press and hold the Func button. The following display
is shown during function hold:
Function Hold
9

Peak Mode On/Off: the LED wattmeter will read in average or peak watts. Average mode is most
suitable for 100% duty cycle modes like FM or RTTY. Peak mode is most suitable for modes in
which the amplitude varies, such as SSB or AM. The default is average mode.
To toggle between these two modes, press and release the Func button. Then press and release
the C Up button. You will see the highest power LED "float" behind the others, indicating peak
mode. Repeat this process to toggle back to Average mode, shown by the absence of the
"floating" LED.
Peak Mode
Wattmeter Scale: the LED wattmeter will indicate 12.5 or 125 watts maximum; the default is 125
watts. You will probably want to use the 12.5 watt scale when operating QRP to provide greater
resolution. To toggle between these two scales, press and release the Func button. Then press
and release the L Up button. The LED marked "10" or "100" will light momentarily to indicate
the 12.5 or 125 watt scale, respectively.
100-Watt Scale
12.5 Watt Scale
Automatic Tuning Mode: you can set your AT-100Pro to automatically begin a tuning cycle
whenever the SWR exceeds a value you set, or you can set it to semi-automatic, to tune only
when you begin a tuning cycle by pressing the Tune button. The default is Auto mode. To toggle
between Auto and Semi-auto modes, press and release the Func button. Then, press and release
the C Dn button. The LEDs will show the following patterns to indicate Auto and Semi mode:
Semi Mode
Auto Mode
Auto Tune Threshold: You can set the SWR at which an automatic tuning cycle will begin. press
and release the Func button. Then press and release the L Dn button. The SWR meter will show
10

the new setting. Repeat this sequence to cycle through the available values (1.1 - 3.5). The default
value is 2.0. The example below shows the threshold set to 1.7.
Auto Threshold Display
High/Low Impedance Toggle: You can manually set your AT-100Pro for Hi-Z or Lo-Z antennas.
Press and release the Func button. While this function is rarely used, it is available for the more
advanced user to temporarily set the tuner to an LC or CL configuration. To toggle the Hi/Lo-Z
setting, press and release the Ant button. The LEDs will show the following patterns to indicate
low and high impedance. This setting is relevant only when you are manually adjusting the match
using the Cand Lbuttons. It is automatically set when you run any automatic tuning cycle.
High Impedance
Low Impedance
Antenna Selection: Press the Ant button to toggle between Ant 1 and Ant 2; the default is Ant 1.
The LED display points toward the selected antenna connector (as viewed from the front of the
unit). You cannot change antennas while transmitting; the antenna selection function is disabled
when RF is present. When you change antennas, the tuner references the last frequency used on
that antenna, and recalls tuning settings for that frequency, if any.
Antenna 2
Antenna 1
11

Tuning
Auto and Semi-Auto Modes
In Auto mode (see section on selection Auto or Semi mode) the tuner will begin a tuning cycle
whenever the SWR exceeds the value you set (see section on setting autotune threshold). In Semi
mode a tuning cycle begins only when you start one by pressing the Tune button, regardless of
SWR. You can also start a Semi tuning cycle while the tuner is in Auto mode. Your personal
operating practices will determine which mode is best for you. The default mode is Auto.
Auto mode tuning works well while you are transmitting in SSB, AM, CW or any digital mode.
Recent advances in LDG's tuning algorithms allow your AT-100Pro to tune effectively even with
a varying RF signal. This means that as you change frequencies, antennas or bands, you don't
have to make a separate tuning transmission; just start talking and your AT-100Pro will match up
in as little as 0.2 seconds.
Your AT-100Pro features over 2,000 memories for each antenna that store the tuning parameters
for each frequency as it tunes. When you transmit on or near that frequency again, the tuner
recalls and resets those parameters in a fraction of a second, much faster than a full tuning cycle.
When you tune, you can select between a Memory tune and a Full tune. If you select Memory
tune and there is no appropriate memory data, the tuner will automatically begin a full tuning
cycle. Tuning parameters are stored in EEPROM memory, and are retained indefinitely, even if
power is removed.
Full Tuning Cycle (Long press)
Set your transmitter or transceiver to SSB, AM, FM, CW or Packet mode, and a power level of no
more than 125 watts if your radio has a power rollback circuit. If your radio lacks a rollback
circuit (see your owner’s manual), set the power level to no more than 25 watts.
Press the PTT switch on the microphone (close key on CW) to transmit a carrier, or simply begin
talking on SSB or AM. While transmitting, press and hold the Tune button on the front of your
AT-100Pro for at least 2.5 seconds.
Long Press > 2.5 Sec for Manual Tune
Release the Tune button; an automatic tuning cycle will begin. You will hear the relays in your
AT-100Pro as they switch in and out seeking a match; they make a buzzing noise. The tuning
cycle will end in a few seconds with the SWR LED meter indicating the final match. At the end
of a full tuning cycle, the SWR LEDs will scan inward from both ends to indicate a successful
tune to an SWR of less than 1.1.
12

Unkey your transmitter or transceiver and reset to your desired power if you changed for tuning −
you’re ready to operate. The new tuning parameters are stored for the present operating
frequency, replacing any parameters that were previously stored for that frequency.
Memory Tuning Cycle (Medium press)
If you are tuning near a frequency at which you have already completed a tuning cycle, you can
reset the tuner very quickly by using a Memory Tuning Cycle. Key your transmitter as described
above, and press and hold the Tune button for .5 to 2.5 seconds, then release. The tuner will
automatically check for a saved tuning setting, and if present will restore that setting in a small
fraction of a second. If no tuned setting is saved near the present frequency, the tuner will begin a
full tuning cycle, saving the result when finished for future operation on that frequency. In this
way, your AT-100Pro “learns”; the longer you use it, the more closely it adapts itself to the bands
and frequencies you use. You will probably use Memory Tuning most of the time; it takes
advantage of any stored tuning settings, but automatically defaults to a Full Tuning cycle if no
stored data is available.
Memory frequency step size varies with band. Steps are very small on the 75 meter band to
accommodate the expected higher Q of most antennas, whereas the step size is somewhat larger
on 10 meters where antenna Q is usually lower.
Medium Press > .5 Sec for Auto Tune
Bypass mode (Short press)
To place your AT-100Pro in bypass mode, press the Tune button less than a half-second. The
tuner will switch to bypass. All LEDs will flash once to confirm bypass mode. In bypass mode
RF from your transmitter goes directly to the antenna with no matching. A second short press
toggles the tuner back to its last tuned setting; the LEDs show which C and L relays are
energized. Bypass followed by Func+Tune will clear the present memory. Bypass mode is not
saved when you power down. On the next power up, the tuner will reset the last tuned settings.
You can if you wish press Func-Tune to save the bypass setting, erasing the memory for that
frequency.
Power and SWR Indication
During a tuning cycle the Power and SWR LED meter indicate forward power and the present
SWR, respectively. If you are using a transmitter or transceiver with SWR rollback protection,
you will see the forward power vary up and down quite a bit during a tuning cycle. During
transmit, both forward power and SWR are displayed if this option is selected in the startup
options.
When no LEDs are lit on the SWR meter when RF is present, it means that the SWR is less than
1.1. When the LED segment labeled 1.1 is lit, it means that the SWR is between 1.1 and 1.3. The
1.3 LED indicates an SWR between 1.3 and 1.5, and so on. Read each LED segment to mean
"equal to or greater than the marked SWR, but less than the next higher marked SWR".
13

Off Mode
When any tuning cycle ends, the tuner automatically enters a “deep sleep” state in which it draws
just a few milliamps; it is effectively off. The tuner will automatically “wake up” the next time
you start a tuning cycle, when RF is present, a button is pressed or when an automatic tuning
cycle is required, if this option is selected. The latching relays will hold the tuned configuration
indefinitely, even when you unplug the power cable or turn off the transceiver powering the
tuner.
Advanced Operation
Manual adjustments
In rare cases, it may be desirable to manually adjust the match after a tuning cycle. This will
happen most often with antennas that are far from resonance at the operating frequency. The C
Up, C Dn, L Up and L Dn buttons increase and decrease capacitance and inductance,
respectively. Momentarily press any of these buttons to see the present setting; the value is
indicated on the display (C on the upper display, L on the lower), left justified (that is, the lowest
order relay is shown on the left). To changes these values, press repeatedly or hold any of these
buttons; you will see the value change. Then, key your transmitter to observe the achieved SWR
on the SWR meter. You can also manually change C and L as you transmit, observing the SWR
on the meter; however, the C and L values are not displayed in this case.
After manually adjusting the match, you can manually store the tuning parameters for the present
frequency; press the Func button, then the Tune button. The parameters are stored for later
recall, replacing any previously saved parameters for that frequency.
Frankly, you won't use the manual adjustments very much; your AT-100Pro is very, very good at
finding a match. These functions are included only to provide you with the maximum utility and
flexibility.
Status Query
You can query the status of most settings by pressing and holding the Func button while
pressing the relevant setting button. For example, to check the power scale of the PWR meter,
press and hold the Func key while pressing the L Up button. The PWR meter will indicate the
present scale setting without changing it. When done, release all buttons. Other status queries
include:
• Ant: Presently selected antenna
• C Up: Present PWR meter mode
• C Dn: Present tune mode (Auto or Semi)
• L Up: Present PWR meter scale
• L Dn: Present Auto Tune threshold
14

Button
Primary Function Secondary Function
(Func + Button)
Status Check (Func
Hold + Button)
Power-Up
Function
Tune
Manual Tune (Long)
Auto Tune (Medium)
Bypass (Short)
Manually Store Tuning
Parameters Present Relay Settings NA
Ant
Switch between Ant 1
and Ant 2 connectors
on back panel
Toggle Hi-Lo Impedance Present Antenna
[Ant 1]
LEDs
On/Off
C Up Manually increase
capacitance
Toggle Power meter
Peak/Average
Present PWR Meter Mode
[Avg] NA
C Dn Manually decrease
capacitance Set Auto/Manual Mode
Present Mode (Auto or
Semi)
[Auto]
NA
L Up Manually increase
inductance Set PWR Meter Scale Present PWR Meter Scale
[125 Watts] NA
L Dn Manually decrease
inductance
Set Auto Tuning
Threshold SWR
Present Auto Tune
Threshold
[2.0]
NA
Func
Select Alternative
Functions For Other
Buttons
NA NA Version
Number
Func+
Ant+
Tune
NA NA NA All
Reset
AT-100Pro Command Reference
[
Default settin
g
s in brackets
]
Peak/Avg
Mtr Mode
NA
Hi/Lo Imp
Sel Ant
LEDs On/Off
Auto/Man
Tune Mode
NA
Man Store
Rly Settings
NA
12/125 Watts
Mtr Scale
NA
Auto Thresh.
Pres. Thresh.
NA
Func C Up L Up
Ant C Dn L Dn
Tune
Func + Key
Func Hold + Key
Hold on Power Up
Hold on power-up
for All Reset
15

Application Notes
Mobile operation
The AT-100Pro is perfectly suited to mobile operation. You can install it under the dash along
with your transceiver, or remotely in the trunk. The only requirements are that the tuner remains
dry, and that you provide a fused source of DC power in accordance with the specifications. The
AT-100Pro does not include an internal fuse; you must provide one in the DC line. LDG
recommends a 2 Amp "fast blow" fuse. Make sure the tuner is well grounded to the car body.
To install the unit under the dash, you can "homebrew" a bracket. You can also use Velcro tape,
although the adhesive tends to soften in hot weather. In the trunk, Velcro works well to secure the
unit to the trunk floor. Simply run coax from your transceiver to the tuner, run a fused DC line to
the tuner and connect your mobile antenna. Simply set the tuner to Automatic mode, and you're
ready to go; the tuner will adjust the match as needed whenever you transmit on a new band or
frequency. You can observe the progress of the match on your rig's SWR meter, or on the power
output meter (not while driving; keep your eyes on the road!). When the reading stops changing,
the tuner has finished tuning. Most of the time this will happen very quickly, as the AT-100Pro
tunes from memory in less than a second.
Tuner
Xcvr
DC
2A
When to use Auto mode
Auto mode is most useful when you are often changing frequencies and bands (a contest, for
instance). With memory operation, Auto mode will retune almost instantly whenever you transmit
on a new frequency. On the other hand, if your antenna SWR is relatively flat across the band of
frequencies you're using, retuning might not be necessary, and semi-automatic mode would be
preferable.
16

When to use Peak mode
Any averaging wattmeter will read correctly only when excited by a steady, unmodulated carrier.
Any modulation will cause the reading to vary. Peak mode measures the peak power as the name
indicates, and is much steadier in the presence of modulated highs and lows. You will generally
want to use Peak mode when using SSB or AM, and average mode when using FM, RTTY or any
other 100% duty cycle mode that provides a steady carrier to the wattmeter circuitry.
When to turn the LEDs off
You can set the LEDs to remain off when transmitting. This will help conserve power when
operating from batteries (Field Day, for instance), or when you don't want the distraction of the
flashing LEDs.
RF Noise
The processor in your AT-100Pro creates a certain about of RF noise, which you may notice in
your receiver while tuning (only if you are using a separate receiver and not a transceiver) or
when information is being displayed in the LED display. This will be most noticeable when
operating CW with break-in, while using the Peak power display. This should present no
problem, as the Peak mode is not very useful on CW. The extremely short tuning cycle
minimizes the impact of the noise; the tuner creates no RF noise while not tuning.
MARS/CAP coverage
Your AT-100Pro provides tuning continuously over its specified range, not just in the ham bands.
This makes it useful for MARS or CAP operation, or any other legal HF operation.
Error indications
There are four error states that are indicated on the Power LED display by all LEDs flashing
twice:
1. No RF is present when a tune command is executed. So, if you press and hold Tune for a full
tuning cycle, but no RF is present, the LEDs will flash.
2. RF removed during a tune. If you stop transmitting during a tuning cycle, the LEDs will
flash.
3. Full tune below Auto tune threshold. If you start a full tuning cycle when the SWR is already
below the Auto tune threshold, the LEDs will flash.
4. Reading over-range.
To correct error conditions 1 or 2, simply provide RF for the duration of the tuning cycle. Error
condition 3 indicates that a tune cycle is not needed. Error condition 4 indicates power or SWR
beyond the range of the meters.
17

Theory Of Operation
Some basic ideas about impedance
The theory underlying antennas and transmission lines is fairly complex, and in fact employs a
mathematical notation called “complex numbers” that have “real” and “imaginary” parts2. It is
beyond the scope of this manual to present a tutorial on this subject, but a little background will
help you understand what your AT-100Pro is doing, and how it does it.
In simple DC circuits, the wire resists the current flow, converting some of it into heat. The
relationship between voltage, current and resistance is described by the elegant and well-known
“Ohm’s Law”, named for Sir George Simon Ohm of England, who first discovered it in 1826. In
RF circuits, an analogous but far more complicated relationship exists.
RF circuits also resist the flow of electricity. However, the presence of capacitive and inductive
elements causes the voltage in the circuit to lead or lag the current, respectively. In RF circuits
this resistance to the flow of electricity is called “impedance”, and can include all three elements:
resistive, capacitive, and inductive.
Capacitive
Reactance
Inductive
Reactance
The output circuit of your transmitter consists of inductors and capacitors, usually in a
series/parallel configuration called a “pi network”. The transmission line can be thought of as a
long string of capacitors and inductors in series/parallel, and the antenna is a kind of resonant
circuit. At any given RF frequency, each of these can exhibit resistance, and impedance in the
form of capacitive or inductive “reactance”.
Transmitters, transmission lines, antennas and impedance
The output circuit of your transmitter, the transmission line, and the antenna all have a
characteristic impedance. For reasons too complicated to go into here, the standard impedance is
about 50 ohms resistive, with zero capacitive and inductive components. When all three parts of
the system have the same impedance, the system is said to be “matched”, and maximum transfer
of power from the transmitter to the antenna occurs. While the transmitter output circuit and
transmission line are of fixed, carefully designed impedance, the antenna presents a 50-ohm, non-
reactive load only at its natural resonant frequencies. At other frequencies, it will exhibit
capacitive or inductive reactance, causing it to have an impedance different from 50 ohms.
When the impedance of the antenna is different from that of the transmitter and transmission line,
a “mismatch” is said to exist. In this case, some of the RF energy from the transmitter is reflected
from the antenna back down the transmission line, and into the transmitter. If this reflected
energy is strong enough it can damage the transmitter’s output circuits.
The ratio of transmitted to reflected energy is called the “standing wave ratio”, or SWR. An SWR
of 1 (sometimes written 1:1) indicates a perfect match. As more energy is reflected, the SWR
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2For a very complete treatment of this subject, see any edition of the ARRL Handbook for Radio
Communications (previously the Handbook For Radio Amateurs)

rises to 2, 3 or higher. As a general rule, modern solid-state transmitters must operate with an
SWR of 2 or less. Tube exciters are somewhat more tolerant of high SWR. If your 50-ohm
antenna is resonant at your operating frequency, it will show an SWR close to 1. However, this is
usually not the case; operators often need to transmit at frequencies other than resonance,
resulting in a reactive antenna and a higher SWR.
FR
FR
SWR /1
/1
−
+
=
where F = Forward power (watts), R = Reflected power (watts)
SWR is measured using a device called an “SWR bridge”, inserted in the transmission line
between the transmitter and antenna. This circuit measures forward and reverse power from
which SWR may be calculated (some meters calculate SWR for you). More advanced units can
measure forward and reverse power simultaneously, and show these values and SWR at the same
time.
An antenna tuner is a device used to cancel out the effects of antenna reactance. Tuners add
capacitance to cancel out inductive reactance in the antenna, and vice versa. Simple tuners use
variable capacitors and inductors; the operator adjusts them by hand while observing reflected
power on the SWR meter until a minimum SWR is reached. Your LDG AT-100Pro automates
this process.
No tuner will fix a bad antenna. If your antenna is far from resonance, the inefficiencies inherent
in such operation are inescapable; it’s simple physics. Much of your transmitted power may be
dissipated in the tuner as heat, never reaching the antenna at all. A tuner simply “fools” your
transmitter into behaving as though the antenna were resonant, avoiding any damage that might
otherwise be caused by high-reflected power. Your antenna should always be as close to
resonance as practical.
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Forward Power (Watts)
20 30 40 50 60 70 80 90 100
2 1.92 1.70 1.58 1.50 1.45 1.41 1.38 1.35 1.33
4 2.62 2.15 1.92 1.79 1.70 1.63 1.58 1.53 1.50
6 3.42 2.62 2.26 2.06 1.92 1.83 1.75 1.70 1.65
8 4.44 3.14 2.62 2.33 2.15 2.02 1.92 1.85 1.79
10 5.83 3.73 3.00 2.62 2.38 2.22 2.09 2.00 1.92
12 7.87 4.44 3.42 2.92 2.62 2.41 2.26 2.15 2.06
14 11.24 5.31 3.90 3.25 2.87 2.62 2.44 2.30 2.20
16 17.94 6.42 4.44 3.60 3.14 2.83 2.62 2.46 2.33
18 37.97 7.87 5.08 4.00 3.42 3.06 2.80 2.62 2.47
20 - 9.90 5.83 4.44 3.73 3.30 3.00 2.78 2.62
22 - 12.92 6.74 4.94 4.07 3.55 3.21 2.96 2.77
24 - 17.94 7.87 5.51 4.44 3.83 3.42 3.14 2.92
26 - 27.96 9.32 6.17 4.85 4.12 3.65 3.32 3.08
28 - 57.98 11.24 6.95 5.31 4.44 3.90 3.52 3.25
30 - - 13.93 7.87 5.83 4.79 4.16 3.73 3.42
32 - - 17.94 9.00 6.42 5.18 4.44 3.95 3.60
34 - - 24.63 10.40 7.09 5.60 4.75 4.19 3.80
36 - - 37.97 12.20 7.87 6.07 5.08 4.44 4.00
38 - - 77.99 14.60 8.80 6.60 5.44 4.71 4.21
40 - - - 17.94 9.90 7.19 5.83 5.00 4.44
42 - - - 22.96 11.24 7.87 6.26 5.31 4.68
44 - - - 31.30 12.92 8.65 6.74 5.65 4.94
46 - - - 47.98 15.08 9.56 7.27 6.02 5.22
48 - - - 97.99 17.94 10.63 7.87 6.42 5.51
50 - - - - 21.95 11.92 8.55 6.85 5.83
Reflected Power (Watts)
SWR Lookup Table
Find SWR at intersection of
forward power column and
reflected power row.

The LDG AT-100Pro
In 1995 LDG pioneered a new type of automatic antenna tuner. The LDG design uses banks of
fixed capacitors and inductors, switched in and out of the circuit by relays under microprocessor
control. A built-in SWR sensor provides feedback; the microprocessor searches the capacitor and
inductor banks, seeking the lowest possible SWR. The tuner is a “Switched L” network consisting
of series inductors and parallel capacitors. LDG chose the L network for its minimum number of
parts and its ability to tune unbalanced loads, such as coax-fed dipoles, verticals, Yagis; in fact,
virtually any coax-fed antenna. The inductors are switched in and out of the circuit by relays
controlled by the microprocessor. An additional relay switches between high and low impedance
ranges.
The capacitors are connected to ground with the seven inductor relays. Another relay switches the
entire capacitor bank to the input or output side of the inductor. This switching allows the AT-
100Pro to automatically handle loads that are greater than 50 ohms (high setting) and less than 50
(low setting). All of the relays are DPDT types sized to handle up to 125 watts continuously.
The SWR sensor is a variation of the Bruene circuit. This SWR measuring technique is used in
most dual-meter and direct-reading SWR meters. Slight modifications were made to the circuit to
provide voltages (instead of currents) for the analog-to-digital converters (ADCs) that provide
signals proportional to the forward and reverse power levels. The single-lead primary through the
center of the sensor transformer provides RF current sampling. Diodes rectify the sample and
provide a dc voltage proportional to RF power. Variable resistors calibrate the FORWARD and
REVERSE power levels. Once adjusted, the forward and reverse power sensors produce a
calibrated DC voltage proportional to the forward and reverse RF power levels. These two
voltages are read by the ADCs in the microprocessor. Once in a digital format, they are used to
calculate SWR in real time.
The relays operate from DC supplied by via the power input jack. The total current drawn by the
AT-100Pro depends primarily on the number of energized relays, with the maximum current
drain being approximately 500 mA, but only during the few seconds a tuning cycle is running. At
all other times, the tuner is in a “deep sleep” mode drawing only a few milliamps. The latching
relays retain the tuned setting indefinitely, even when power is removed. The last tuned setting
will still be set on the next power-up.
Although the microprocessor’s oscillator runs at 20 MHz. The main tuning routine takes about 75
cycles to make a tuner adjustment and take a new SWR measurement, or 7 milliseconds per tuner
adjustment. If running at maximum speed, the microprocessor can try all inductor-capacitor
combinations in fewer than 3 seconds. Unfortunately, the mechanical relays can’t react as quickly
as the microprocessor, and the tuning speed must be slowed down to compensate for relay settling
time.
The tuning routine, written in assembly language, uses an algorithm to minimize the number of
tuner adjustments. The routine first de-energizes the high/low impedance relay if necessary, and
then individually steps through the inductors to find a coarse match. With the best inductor
selected, the tuner then steps through the individual capacitors to find the best coarse match. If no
match is found, the routine repeats the coarse tuning with the high/low impedance relay
energized. The routine then fine-tunes the capacitors and inductors. The program checks LC
combination to see if a 1.5 or lower SWR can be obtained, and stops when it finds a good match.
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