LDG YT-450 User manual

YT-450 OPERATIONS MANUAL MANUAL REV A
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LDG YT-450 100-Watt
Automatic Tuner for Yaesu
FT-450 and FT-950
Transceivers
LDG Electronics
1445 Parran Road
St. Leonard MD 20685-2903 USA
Phone: [phone]
Fax: [phone]
[e-mail]
www.ldgelectronics.com

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Table Of Contents
Introduction 3
Jumpstart, or “Real hams don’t read manuals!” 3
Specifications 4
An Important Word About Power Levels 5
Important Safety Warning 5
Getting to know your YT-450 6
Front Panel 6
Rear Panel 7
Installation 8
Compatible Transceivers 8
Interface Cable 8
FT-450 Installation: 9
FT-950 Installation 10
Finishing Up Installation 11
Operation 11
Power-up 11
Basic Tuning Operation 11
Toggle Bypass Mode: 12
Initiate a Memory Tune Cycle: 13
Force a Full Tune Cycle: 14
Status LED 15
Application Information 15
Mobile Operation 15
Tuning On The 60 Meter Band 16
MARS/CAP Coverage 16
Operation with a PC / CAT 16
Using Both Antenna Ports on the FT-950 16
Theory of Operation 17
The LDG YT-450 20
A Word About Tuning Etiquette 21
Care and Maintenance 21
Technical Support 21
Two-Year Transferrable Warranty 21
Out Of Warranty Service 22
Returning Your Product For Service 22
Product Feedback 22

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INTRODUCTION
LDG pioneered the automatic, wide-range switched-L tuner in 1995. From its laboratories in
St. Leonard, Maryland, LDG continues to define the state of the art in this field with innovative
automatic tuners and related products for every amateur need.
Congratulations on selecting the YT-450 100-watt automatic tuner for the Yaesu FT-450 and
FT-950 transceivers. The YT-450 provides semi-automatic antenna tuning across the entire HF
spectrum plus 6 meters, at power levels up to 125 watts. 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, including the built-in tuners on many
radios.
The YT-450 is similar to previous LDG tuners, but is specially engineered to integrate with
your Yaesu FT-450 or FT-950 HF radio. The YT-450 connects to the CAT (Computer
Automated Transceiver) port on the back of the radio. The CAT interface allows an external
device such as a PC or the YT-450 to control the FT-450/950 by sending it serial commands. The
YT-450 takes advantage of this interface to simplify the tuning process -- one button push is all
that is needed in order to switch the radio to AM mode, reduce output power, transmit a carrier,
measure the transmit frequency, and then restore the radio to its previous mode and power level!
Additionally, the YT-450 also draws power directly from the radio, so no extra cable is needed.
JUMPSTART, OR “REAL HAMS DON’T READ MANUALS!”
Ok, but at least read this one section before operating the YT-450:
Turn off power to your FT-450/950 radio.
Connect the antenna jack on the transceiver to the “TX” jack on the YT-450, using a 50 ohm
coax cable jumper.
Connect the 50 ohm coax antenna feedline to the “ANT” jack on the YT-450.
Connect the 8-pin mini-DIN plug on the supplied radio interface cable to the “TUNER” port on
the back of your FT-450/950. The YT-450 draws 12V power from the radio via this jack.
Connect the DB-9 plug on that same end of the radio interface cable to the “CAT” port on the
back of your FT-450/950.
Connect the DC coax plug on the other end of the radio interface cable to the “Power” jack on
the rear of the YT-450.
Connect the remaining DB-9 plug to the DB-9 jack marked “Radio” on the rear of the YT-450.
Using the radio’s menu system, set the CAT RATE to 38,400 baud, set the CAT TOT to 100,
and set CAT RTS to “Enable”†.
Select the desired operating frequency and mode.
Push and hold the TUNE button on the front of the YT-450 for one second (until the Tuning
LED comes on), then release. The transceiver automatically switches to AM mode, and keys up
† Be sure you have also set up the AM Carrier level to 25 watts, as per your FT-950 instruction manual, if using an FT-950 radio.

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with a minimal amount of power, and the YT-450 begins a tuning cycle. At the end of the tuning
cycle, the original mode and power level is restored.
Wait for the tuning cycle to end; you’re now ready to operate!
SPECIFICATIONS
•1 to 125 watts SSB and CW peak power, 100 watts on 6 meters, and 30 watts on PSK and
digital modes.
•Latching relays for ultra-low power operation.
•2,000 memories for instantaneous frequency and band changing.
•Power: 12V power supplied directly from the radio.
•Designed specifically for the Yaesu FT-450 and FT-950 HF transceivers.
•Pass-thru CAT port allows YT-450 to control the FT-xxx over the CAT bus while still
allowing a host PC to also control the radio.
•Pass-thru CAT port waits for idle CAT activity before controlling the transceiver.
•1.8 to 54.0 MHz coverage. Frequency for memory storage is read from the radio via CAT.
•Tunes 4 to 800 ohm loads (16 to 150 on 6M), 16 to 3200 ohms with optional 4:1 Balun.
•For Dipoles, Verticals, Vees, Beams or any Coax Fed Antenna.
•IC-450 Interface cable included.
•Optional external Balun allows tuning of random length, long wire or ladder line fed
antennas. See web site for details.
•Dimensions: 7.25”L x 7.75”W x 2.25”H.
•Weight: 1 pound 8 ounces

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AN IMPORTANT WORD ABOUT POWER LEVELS
The YT-450 is rated at 125 watts maximum power input at most. Many ham transmitters and
transceivers, and virtually all amplifiers, output well over 125 watts. Power levels that
significantly exceed specifications will definitely damage or destroy your YT-450. If your tuner
fails during overload, it could also damage your transmitter or transceiver. Be sure to observe the
specified power limitations.
IMPORTANT SAFETY WARNING
Never install antennas or transmission lines over or near power lines. You can be
seriously injured or killed if any part of the antenna, support or transmission line touches
a power line. Always follow this antenna safety rule: the distance to the nearest power
line should be at least twice the length of the longest antenna, transmission line or
support dimension.

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GETTING TO KNOW YOUR YT-450
Your YT-450 is a quality, precision instrument that will give you many years of outstanding
service; take a few minutes to get to know it.
The YT-450 is designed specifically for use with the FT-450 and FT-950 radios. Tuning is
performed when the Tune button is pushed on the front of the YT-450 and held for one second.
The tuner can be placed in bypass mode by pressing the Tune button momentarily.
The YT-450 is powered via the supplied DC coax plug. The YT-450 automatically powers
up at the start of a tuning cycle, and goes into an ultra-low-power sleep mode when tuning is
complete. The latching relays hold the tuned configuration indefinitely, even when DC power is
completely removed. Tuning memories are stored in FLASH memory.
The YT-450 has 2,000 frequency memories. When tuning on or near a previously tuned
frequency, the YT-450 uses “Memory Tune” to recall the previous tuning parameters in a
fraction of a second. If no memorized settings are available, the tuner runs a full tuning cycle,
storing the parameters for memory recall on subsequent tuning cycles on that frequency. In this
manner, the YT-450 “learns” as it is used, adapting to the bands and frequencies as it goes.
Front Panel
On the front panel there is one pushbutton and one LED indicator light.
Tune Button: Initiates either a memory tune or a full tune, and also toggles the tuner
between “active” and “bypass” modes.
Status LED: Lights to give feedback on button presses, lights during tuning; gives tune
status at the end of a tuning cycle.

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Rear Panel
The rear panel of the YT-450 features six connectors.
ANT connector: Connect a 50-ohm coax antenna feedline to this standard SO-239
connector.
GND connector (wing nut): Connect to antenna system ground.
TX connector: Connect a 50-ohm coax jumper cable from this standard SO-239 connector
to the ANT jack on the back of the transceiver.
PC connector: This 9-pin D-sub connector connects to a personal computer via a 9 pin
female-to-female straight-thru cable. Use of this port is optional; it is provided for those hams
who would like to control their radio via computer. This is a pass-thru port to the RADIO port,
and is switched under software control by the YT-450’s microprocessor. The firmware of the
YT-450 has been written such that this will seem transparent to the user -- if using CAT to
control your FT-450/950, just plug the PC’s CAT cable into this port instead of the CAT jack on
the back of the transceiver.
RADIO connector: This 9-pin D-sub connector is for connection to the FT-450/950’s
CAT jack. The YT-450 controls the PTT, power level, and operating mode via CAT commands
sent to the transceiver. The YT-450 also reads the operating frequency directly from the
transceiver so that it knows where to store tuning memory data.
POWER connector: (DC coax jack): Connect to 12VDC supply capable of supplying at
least 500 mA. Center pin is positive.

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INSTALLATION
The YT-450 tuner is designed for indoor operation only; it is not water resistant. If you use it
outdoors (Field Day, for example), you must protect it from the rain. The YT-450 is designed for
use with coax-fed antennas. If use with longwires or ladder-line-fed antennas is desired, an
external balun is required. The LDG RBA-4:1 or RBA-1:1 is ideal, depending on the antenna
and transmission line used.
Always turn your radio off before plugging or unplugging anything. The radio may be
damaged if cables are connected or disconnected while the power is on.
Compatible Transceivers
The YT-450 is designed to be used ONLY with the following Yaesu 100 watt transceivers:
FT-450
FT-950
WARNING: Do not attempt to use the YT-450 with any other transceivers, even if the CAT
plug fits. At best, the YT-450 simply won’t work with these radios. At worst, it could cause
damage to the YT-450, the radio, or both.
It is possible that more Yaesu transceivers will be added to this list as Yaesu releases new
models. Be sure to check the LDG website for additional information.
Interface Cable
The YT-450 is supplied with a radio interface cable. Each end has two connectors. The end
with an 8-pin mini-DIN connector goes to the radio, and the other end goes to the tuner. Control
data is carried over the DB-9 connections; 12V power is tapped from the 8-pin mini-DIN, and is
delivered to the tuner via the round DC coax plug.

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FT-450 Installation:
Connect the antenna jack on the FT-450 to the TX jack on the back of the YT-450, using a
50 ohm coax cable rated 125 watts or greater.
Connect the 8-pin mini-DIN connector on the supplied radio interface cable to the jack
marked “TUNER” on the FT-450. Connect the adjacent DB-9 plug to the “CAT” jack on the FT-
450.
On the other end of the interface cable, connect the DC coax plug to the jack marked
“Power” on the rear of the YT-450 tuner. Connect the remaining DB-9 plug to the jack marked
“Radio” on the YT-450.
On the FT-450, press and hold the “F” button on the front panel for one second, to enter the
extended menu. Rotate the DSP/SEL knob until “EXT MENU” is showing. Momentarily push
in on the DSP/SEL button to select EXT MENU, and then turn the DSP/SEL knob clockwise
until “ON” is displayed. Push in on the DSP/SEL knob again.
Now rotate the DSP/SEL knob until “CAT RTS” is showing. Push in on the DSP/SEL knob,
and then turn the DSP/SEL knob until “ENABLE” is showing. Push in on the DSP/SEL knob
gain. Rotate one click clockwise until “CAT TOT” is showing. Push in on the DSP/SEL knob.
Rotate until “100” is showing. Push in on the DSP/SEL knob again.

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Rotate the DSP/SEL knob one click clockwise until “CATRATE” is showing. Push in on the
DSP/SEL knob. Rotate the DSP/SEL knob until “38400” is showing. Push in on the DSP/SEL
knob one final time. Now push and hold the “F” button to return to normal operation.
FT-950 Installation
Connect the ANT 1 jack on the FT-950 to the TX jack on the back of the YT-450, using a 50
ohm coax cable rated 125 watts or greater.
Connect the 8-pin mini-DIN connector on the supplied radio interface cable to the jack
marked “TUNER” on the FT-950. Connect the adjacent DB-9 plug to the “CAT” jack on the FT-
950.
On the other end of the interface cable, connect the DC coax plug to the jack marked
“Power” on the rear of the YT-450 tuner. Connect the remaining DB-9 plug to the jack marked
“Radio” on the YT-450.
On the FT-950, press the MENU button on the front panel to bring up the menu system. Push
in on the SELECT knob once or twice so that the non-numbered menu items are showing.

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Rotate the SELECT knob until the menu “CAT BPS” is showing. You may have to rotate
for a while; the FT-950 has a LOT of menu options.
With “CAT BPS” showing on the screen, rotate the CLAR/VFO-B knob fully clockwise
until “384H” is showing. Rotate the SELECT knob one click clockwise until “CAT TOT” is
showing. Now rotate the CLAR/VFO-B dial until “100” is showing.
Rotate the SELECT knob one more click clockwise, until “CAT RTS” is displayed. Now
rotate the CLAR/VFO-B knob until “On” is displayed. Finally, press and hold the MENU
button for 2 seconds so that these new settings take effect.
Also, be sure to set your FT-950’s AM CAR menu per the FT-950 instruction manual, to
select 25 watts as the AM carrier level.
Finishing Up Installation
Connect a 50-ohm coax feedline to the jack marked ANT on the rear of the YT-450, and
connect this to the antenna system.
If it is desired to operate the FT-450 / 950 via computer control, connect a straight-thru 9-pin
female-to-female cable (not supplied) to the jack on the YT-450 marked “PC”. The YT-450 will
automatically feed through any CAT commands coming in over this port to the transceiver and
vice versa. Note that any software used to control the transceiver will have to be set for 38,400
baud while connected through the YT-450.
Grounding the YT-450 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 the use of a properly
installed, high quality lightning arrestor on all antenna cables.
OPERATION
Power-up
The YT-450 is powered by the transceiver. When the YT-450 is first powered on, the Status
LED will blink once to indicate it is functioning properly. The first time the TUNE button is
pressed, the YT-450 checks the connection to the transceiver. If this check fails, the Status LED
will blink continuously.
Possible causes for failure to detect the radio properly are an improperly seated CAT cable, a
damaged CAT cable, incorrectly selected CAT baud rate, or plugging the CAT cable into a radio
other than an FT-450 or FT-950. If checking all of these things does not correct the situation, try
turning the radio and tuner off and back on again.
Basic Tuning Operation
The YT-450 is operated from the front panel TUNE button on the YT-450. Two types of
tuning cycles are available; a memory tuning cycle and a full tuning cycle.
The memory tuning cycle attempts to tune quickly based on having previously tuned on the
present frequency selection. If the tuner previously was successful in tuning on the currently
selected frequency, the settings for that match will be loaded into the tuner relays, and checked to
see that an acceptable SWR match is found.

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A full tuning cycle “starts from scratch” and begins a fixed tuning sequence where the YT-
450 rapidly tries varying combinations of inductance and capacitance values, and then zeroes-in
on the best match possible. When the tuning cycle is complete, if an acceptable match was
found, the inductance and capacitance settings are saved in a memory associated with the
selected frequency, so that they may be recalled quickly in the future via a memory tuning cycle.
In this manner, the YT-450 “learns”; the longer you use it, the more closely it adapts itself to
the bands and frequencies used. Most users will probably use memory tuning most of the time; it
takes advantage of any saved tuning settings, but automatically defaults to a full tuning cycle if
no stored data is available.
In both cases, at the end of the tuning cycle, the carrier is held for 1.5 seconds after tuning is
complete, so that the final SWR may be read on the transceiver’s internal SWR meter or another
inline SWR meter, and the front panel LED will indicate the status of the tuning cycle.
The tuner may also be placed in “bypass” mode where it is electrically removed from the
antenna system.
Toggle Bypass Mode:
To toggle between bypassed and active mode, press the front panel Tune button on the YT-
450 momentarily. The Status LED will flash three times to indicate that the tuner is in bypass
mode. Press the front panel Tune button momentarily again to recall the previous tuner settings.
The Status LED will flash once to indicate that the tuner is no longer bypassed. This function

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may be useful if you wish to compare antenna performance with and without the benefit of the
tuner’s matching network.
Initiate a Memory Tune Cycle:
To initiate a memory tuning cycle, press and hold the Tune button on the front of the YT-450
until the Status LED lights up. A memory tuning cycle will begin. The YT-450 will force the
transceiver into AM mode, change the power level to one that is appropriate for tuning, and key
the radio. When tuning is complete, the transceiver will return to the operating mode and power
level previously set.
You will notice that the radio toggles between VFO A and VFO B before and after a tuning
cycle. This is normal; the YT-450 is determining if the radio is operating in split mode, and
acting accordingly. The YT-450 will tune under all sorts of odd-split conditions, storing the
tuning memory information associated with the transmit frequency.
Be aware that the YT-450 tunes in AM mode. The microphone input is live during tuning, so
anything you say will be heard on the air while tuning!

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Force a Full Tune Cycle:
Sometimes, if you are transmitting on a previously tuned frequency, but something has
slightly changed in your antenna system (maybe the antenna was re-oriented, for example),
performing a memory recall tune will find a stored match that is acceptable, but is not as optimal
as could be. In this case, forcing a full tune will cause the YT-450 to seek a better match than the
match already stored in memory for this frequency.
To force a full tuning cycle, press and hold the Tune button on the front panel of the YT-
450 until the Status LED lights up, and keep holding until the Status LED goes out again.
Release the Tune button once the Status LED goes out. A full tuning cycle will begin. When
tuning is complete, the transceiver will be restored to its previous operating mode and power
level.
You will notice that the radio toggles between VFO A and VFO B before and after a tuning
cycle. This is normal. Be aware that the YT-450 tunes in AM mode. The microphone input is
live during tuning, so anything you say will be heard while tuning!

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Status LED
The Status LED is used to indicate both modes of operation, tuning status, and error codes.
The following table lists the LED status codes and their meaning.
LED Indication Meaning
Status LED on. Tuner is tuning.
Status LED goes out, then blinks once. Tuner has completed a tuning cycle; a
good SWR match was found.
Status LED goes out, then blinks twice. Tuning cycle is complete, tuning match is
between 1.5:1 and 3.0:1 SWR.
Status LED goes out, then blinks three
times. Tuning cycle is complete, tuning match is greater than 3.0:1 SWR.
Status LED blinks 4 times. Tuning cycle failed, RF was lost in the
middle of the tune.
Status LED blinks 5 times. Tuning cycle failed, no RF was detected.
Status LED blinks continuously Communication with radio failed.
APPLICATION INFORMATION
Mobile Operation
The YT-450 is perfectly suited to mobile operation. It can be installed under the dashboard
along with the transceiver, or mounted remotely. The only requirement is that the tuner remains
dry.
The supplied radio interface cable is 14 inches long. If it is desired that the YT-450 is
positioned farther from the transceiver than this cable length allows, a custom cable will need to
be constructed. This can be accomplished in two ways: Cut the supplied cable and solder a
jumper wire between all the connections, or purchase new connectors and cable to construct a
custom-length interface cable from scratch.
The 9-pin CAT interface connector is Kobiconn part number 156-1309T-E, available from
http://www.mouser.com/ as Mouser part number 156-1309T-E. Pinning for this cable is one-to-
one, straight through; all nine signals are used. Be sure to use a D-sub hood for the connector as
well; this protects against shorts and other damage to the connector.

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Tuning On The 60 Meter Band
The YT-450 tunes in AM mode, but, in the United States, the FCC requires that only SSB
transmissions can be made in the 60 meter band. To that end, the FT-450 and FT-950 radios will
not switch to AM mode during tuning, but will remain in SSB mode. If no audio signal is present
during SSB transmission on 60 meters, then no RF will be generated, and the YT-450 will blink
a “No RF Was Detected” error.
In order to tune on the 60 meter band, then, speak into the microphone while tuning. It will
suffice to say “Ahh” for a few seconds while tuning. Be sure to identify your communications, as
is required by FCC rules.
MARS/CAP Coverage
The YT-450 provides continuous tuning coverage 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.
Operation with a PC / CAT
Although the YT-450 uses the transceiver’s CAT port for tuning control, the YT-450 is
designed to allow the user to continue to use the CAT interface with the transceiver for PC
control, also.
If PC control of the radio is desired, simply hook a 9 pin straight thru female-to-female cable
to the PC jack on the rear of the YT-450, and connect to the PC’s serial port.
Any rig control software on the computer must be set to use the 38,400 baud rate, as this is
the communication rate used by the YT-450 for controlling the radio.
The YT-450 monitors the Computer port for activity before beginning any tuning cycle.
Only when the CAT line is idle for a period of time will the YT-450 take over control of the
CAT line in order to perform a tuning cycle. When the tuning cycle is complete, control of the
CAT interface is returned to the PC.
This procedure is completely automatic, and is transparent to the user. Simply hook up a PC,
and use the rig control software as normal. Press the TUNE button on the YT-450 when tuning is
desired. Some rig control software will detect that the radio is no longer communicating with the
PC during tuning. This is normal, and communications with the PC will resume once the tuning
cycle is complete.
Note: The YT-450 must be powered-on in order for its pass-through CAT port to function.
Using Both Antenna Ports on the FT-950
The FT-950 radio comes with two antenna ports, selectable via the ANT 1-2 button in the
upper left corner of the FT-950’s front panel. It is also possible to set up an “odd split” on the
radio where transmission is done on one antenna, and reception on the other. Simply choose
different antennas for VFO-A and VFO-B, and then enable split mode.
With a pair of YT-450 tuners, it is possible to use both antenna ports, and have one-button
pushbutton tuning for each antenna. This is possible via “daisy-chaining” the CAT ports on the
two YT-450 tuners. Just be sure to have the correct antenna selected on the radio when pushing
the TUNE button on the YT-450 connected to that antenna port.

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The advantage of this configuration is that two separate antennas can be used, without
requiring retuning when switching between them. This is especially useful if a TX/RX split is set
up, where TX is on one antenna and RX is on the other. Just be sure to tune both antennas prior
to working the split and the FT-950 will automatically switch between TX and RX antennas
when you key the radio!
Note that, when daisy-chaining a second YT-450, there is only one TUNER port on the rear
of the FT-950, so there is no place to plug in the second YT-450’s 8-pin mini-DIN connector.
This connector provides power to the tuner. Instead, the second YT-450 will require an extra
LDG power cable, available from your favorite LDG dealer.
Two-antenna operation of FT-950 with two YT-450s:
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” parts. It is

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beyond the scope of this manual to present a tutorial on this subject
1
, but a little background will
help in understanding what the YT-450 is doing, and how it does it.
In simple DC circuits, the wire resists 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 Georg Simon Ohm of Germany, who first discovered the principle in
1826. In RF circuits, an analogous but more complicated relationship exists.
RF circuits also resist the flow of electricity. However, the presence of capacitive and
inductive elements causes the voltage 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.
The output circuit of a 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 a transmitter, the transmission line, and the antenna, all have a
characteristic impedance. For reasons beyond the scope of this document, the standard
impedance is nominally 50 ohms resistive, with zero capacitive and zero 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 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 other than 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.
1 For a very complete treatment of this subject, see any edition of the ARRL Handbook for Radio Communications (previously the Handbook For Radio Amateurs).

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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 increases 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 a 50 ohm
antenna is resonant at the 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.
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 the
antenna. This circuit measures
forward and reflected power from
which SWR may be calculated
(some meters calculate SWR for
you). More advanced units can
measure forward and reflected
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. The LDG
Electronics YT-450 automates this
process.
No tuner will fix a bad antenna. If the antenna is far from resonance, the inefficiencies
inherent in such operation are inescapable; it’s simple physics. Much of the transmitted power
may be dissipated in the tuner as heat, never reaching the antenna at all. A tuner simply “fools”
the transmitter into behaving as though the antenna were resonant, avoiding any damage that
might otherwise be caused by high reflected power. For best performance, the antenna used
should always be as close to resonance as is practical.
SWR =1+RF
1−R
F

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THE LDG YT-450
In 1995, LDG Electronics 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. An additional relay switches between high and low impedance ranges. 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, and, in
fact, virtually any coax-fed antenna.
The series inductors are switched in and out of the circuit, and the parallel capacitors are
switched to ground under microprocessor control. The high/low impedance relay switches the
capacitor bank either to the transmitter side of the inductor bank, or to the antenna side. This
allows the YT-450 to handle loads that are either greater than or less than 50 ohms. All relays are
sized to carry 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 that provide signals
proportional to the forward and reflected 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. These two voltages are read by the ADCs in the
microprocessor, and are used to compute SWR in real time.
The relays are powered by the 12VDC input provided by CAT interface cable. The relays
are a latching type, and so they consume no current when not actively switching.
Although the microprocessor’s oscillator runs at 32 MHz, which allows the main tuning
routine to execute in only a few milliseconds, the relays require several milliseconds of settling
time for every combination of inductors and capacitors. Thus, it may take several seconds before
all relay combinations are exhausted, in the case of a difficult tune.
The tuning routine 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 inductors and capacitors. The program checks LC combinations to see if a 1.5:1 or
lower SWR can be obtained and stops when it finds a good match.
The microprocessor runs a fine tune routine just after the tuner finds a match of 1.5:1 or less.
This fine tune routine now tries to tune the SWR as low as possible (not just to 1.5); it takes
about half a second to run.