LDG Z-817H User manual

Z-817H OPERATIONS MANUAL MANUAL REV A
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LDG Z-817H 75-Watt
Automatic Tuner
for Yaesu FT-817 and
Other QRP Radios with
Amplifiers
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 Z-817H 6
Front Panel 6
Rear Panel 7
Installation 8
Battery Installation 8
Compatible Transceivers 9
Interface Cable 9
FT-817 Installation, No Amplifier 10
FT-817 Installation, With Amplifier 11
Finishing Up FT-817 Installation 12
Hookup For RadiosOther Than FT-817 12
Operation 12
Power-up 12
Basic Tuning Operation, Using With The FT-817 Radio 12
Toggle Bypass Mode 13
Initiate a Memory Tune Cycle 14
Force a Full Tune Cycle 15
Using The Z-817H With Other Radios 16
Status LED 16
Application Information 17
Mobile Operation 17
MARS/CAP Coverage 17
Operation with a PC / CAT 17
Theory of Operation 17
The LDG Z-817H 19
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 Z-817H 75-watt automatic tuner for the Yaesu FT-817
transceiver. The Z-817H provides semi-automatic antenna tuning across the entire HF spectrum
plus 6 meters, at power levels up to 75 watts. It will tune dipoles, verticals, Yagis, or virtually
any coax-fed antenna. It will match an amazing range of antennas and impedances (up to 10 to 1
SWR), far greater than some other tuners you may have considered, including the built-in tuners
on many radios.
The Z-817H is similar to previous LDG tuners, but is specially engineered to integrate with
your Yaesu FT-817 HF radio and external amplifier. The Z-817H connects to the ACC port or
sometimes called CAT (Computer Automated Transceiver) port on the back of the radio. The
CAT interface allows an external device such as a PC or the Z-817H to control the FT-817 by
sending it serial commands. The Z-817H 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 PKT mode, transmit
a carrier, read the transmit frequency, and then restore the radio to its previous mode and power
level! Additionally, the Z-817H is powered by internal batteries, so no extra power cable is
needed.
JUMPSTART, OR “REAL HAMS DON’T READ MANUALS!”
Ok, but at least read this one section before operating the Z-817H:
1. Turn off power to your FT-817 radio.
2. Open up the Z-817H and install 4 AA alkaline batteries, being careful to observe correct
polarity. Reassemble the tuner.
3. Connect the antenna jack on the transceiver to the “TX” or “INPUT” jack on the external
amplifier, using a 50 ohm coax cable jumper.
4. Connect the “ANTENNA” or “OUTPUT” jack of the amplifier to the “ANT” jack on the Z-
817H.
5. Connect a 50 ohm coax antenna feedline to the “ANT” jack on the Z-817H.
6. Connect one end of the 8-pin mini-DIN plug on the supplied radio interface cable to the
“ACC” port on the back of your FT-817.
7. Connect the other end of the radio interface cable to the “Radio” port on the back of your Z-
817H.
8. Using the radio’s menu system, set the CAT RATE to 38,400 baud.
9. Select the desired operating frequency and mode.
10. Bypass the external amplifier.

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11. Push and hold the TUNE button on the front of the Z-817H for one second (until the Status
LED comes on), then release. The transceiver automatically switches to PKT mode, and keys
up with a minimal amount of power, and the Z-817H begins a tuning cycle. At the end of the
tuning cycle, the original mode and power level is restored.
12. Wait for the tuning cycle to end, then un-bypass the amplifier if desired, and you’re now
ready to operate!
SPECIFICATIONS
•0.1 to 75 watts SSB and CW peak power, 50W on digital modes and 6 meters. 50W max
when used with non-FT-817 radio.
•Latching relays for ultra-low power operation.
•2,000+ memories for instantaneous frequency and band changing.
•Power: 6V power supplied from long-lasting internal AA alkaline batteries.
•Designed specifically for the Yaesu FT-817 transceiver.
•Will also work with non-FT-817 radios, but without automation of the radio.
•Pass-thru CAT port allows Z-817H to control the FT-817 over the CAT bus while still
allowing a host PC to also control the radio. Also permits seamless integration with Tokyo
Hi Power HL-45B amplifier.
•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.
•Optional external Balun allows tuning of random length, long wire or ladder line fed
antennas.
•Dimensions: 7.6”L x 5.6”W x 1.75”H.
•Weight: 1 lb., 6 oz.

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AN IMPORTANT WORD ABOUT POWER LEVELS
The Z-817H is rated at 75 watts maximum power input at most, when connected to an FT-
817 via the supplied CAT radio interface cable, and 50 watts maximum if not using the supplied
CAT cable. Many ham transmitters and transceivers, and virtually all amplifiers, are capable of
transmitting well over 75 watts. Power levels that significantly exceed specifications will
definitely damage or destroy your Z-817H. 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 Z-817H
Your Z-817H is a quality, precision instrument that will give you many years of outstanding
service; take a few minutes to get to know it.
The Z-817H is designed specifically for use with the Yaesu FT-817 radio. Tuning is
performed when the Tune button is pushed on the front of the Z-817H and held for one second.
The tuner can be placed in bypass mode by pressing the Tune button momentarily. The Z-817H
may also be used with other radios; see the section on Using The Z-817H With Other Radios.
The Z-817H is powered via four internal AA alkaline batteries. The Z-817H powers up at the
start of a tuning cycle, and powers-down completely when tuning is complete. The latching
relays hold the tuned configuration indefinitely, even when DC power is completely removed.
Tuning memories are stored in non-volatile FLASH memory.
The Z-817H has 2,000 frequency memories. When tuning on or near a previously tuned
frequency, the Z-817H 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 Z-817H “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 two LED indicator lights.
•Tune Button: Initiates either a memory tune or a full tune, and also toggles the tuner
between “active” and “bypass” modes.
•Green SWR LED: Blinks tune status at the end of a tuning cycle.
•Red Status LED: Lights to give feedback on button presses, lights during tuning.

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Rear Panel
The rear panel of the Z-817H features five 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 (OUTPUT) jack on the back of the amplifier. If running barefoot (no amplifier),
connect to the FT-817’s ANT jack.
•PC connector: This 8-pin mini-DIN connector connects to a personal computer via CT-62
compatible PC interface cable (not included). 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 Z-817H’s microprocessor. The
firmware of the Z-817H has been written such that this will seem transparent to the user -- if
using CAT to control your FT-817, just plug the PC’s CAT cable into this port instead of the
CAT jack on the back of the transceiver.
•RADIO connector: This 8-pin mini-DIN connector is for connection to the FT-817’s ACC
jack. The Z-817H controls the PTT, power level, and operating mode via CAT commands sent
to the transceiver. The Z-817H also reads the operating frequency directly from the transceiver
so that it knows where to store tuning memory data.

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INSTALLATION
The Z-817H 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 Z-817H 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.
Battery Installation
The Z-817H is powered from 4 internal AA alkaline batteries. To install the batteries, first
remove the four screws that hold on the lid, using a Philips screwdriver.
Next, remove the lid by gently lifting it upward:

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Now, install 4 AA alkaline batteries, being careful to observe correct polarity. Reinstall the
lid, and reinstall the four screws to secure the lid. Never operate the Z-817H with the lid off, as
hazardous and potentially lethal RF energy is present while transmitting.
Compatible Transceivers
Although the Z-817H is designed to work with any radio / amplifier combination, the CAT1
interface allows additional automation of the tuning process with compatible transceivers. The Z-
817H CAT radio interface cable is designed to be used ONLY with the following Yaesu
transceivers:
•FT-817
•FT-817ND
WARNING: Do not attempt to use the Z-817H’s radio interface cable with any other
transceivers, even if the CAT plug fits. At best, the Z-817H simply won’t work with these radios.
At worst, it could cause damage to the Z-817H, the radio, or both. If using the Z-817H with
another radio, do not hook up the included CAT cable.
Interface Cable
The Z-817H is supplied with a radio interface cable. Each end has an 8-pin mini-DIN
connector. The end with a round 8-pin mini-DIN connector goes to the radio, and the other end
goes to the tuner.
1 CAT -ComputerAssisted Transceiver, a serial communicationprotocolallowingexternaldevices to control certain Yaesuradios.

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FT-817 Installation, No Amplifier
If using the FT-817 radio “barefoot” (no external amplifier), connect the ANT output of the
FT-817 radio to the TX input on the rear of the Z-817H. Then connect the antenna feedline to the
ANT jack on the Z-817H. Finally, connect the supplied CAT radio interface cable from the 8-pin
mini-DIN jack marked “Radio” on the rear of the Z-817H to the jack marked “ACC” on the rear
of the FT-817.
Grounding the Z-817H 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, such as the LDG SP-200, on all antenna cables.
Once the cables are all hooked up, turn on the FT-817 radio, then press and hold the “F”
button on the front panel for one second, to enter the extended menu. Rotate the SEL knob until
menu # 14, “CAT RATE” is showing. Rotate the tuning dial until “38400” is displayed. Now
press and hold the “F” button again to resume normal operation.

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FT-817 Installation, With Amplifier
Connect the antenna jack on the FT-817 to the TX or INPUT jack on the back of the external
amplifier, using a 50 ohm coax jumper cable.
Connect the ANT or OUTPUT jack on the amplifier to the TX jack on the rear of the Z-
817H with a coax jumper.
Connect the antenna system feedline to the ANT jack on the back of the Z-817H.
Connect the round end of the supplied radio interface cable to the jack marked “ACC” on the
FT-817. Connect the opposite end to the jack marked “Radio” on the rear of the Z-817H.
If your amplifier interfaces directly with the FT-817’s ACC port, you may plug the
amplifier’s interface cable into the PC port on the rear of the Z-817H. Amplifiers such as the
Tokyo Hi-Power HL-45B use the ACC port to determine the radio’s band and also to send back
ALC data to the radio. The Z-817H is designed to allow such amplifiers to still work correctly
with the FT817.
Once the cables are all hooked up, turn on the FT-817 radio, then press and hold the “F”
button on the front panel for one second, to enter the extended menu. Rotate the SEL knob until
menu # 14, “CAT RATE” is showing. Rotate the tuning dial until “38400” is displayed. Now
press and hold the “F” button again to resume normal operation.

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Finishing Up FT-817 Installation
If it is desired to operate the FT-817 via computer control, connect a Yaesu CT-62 cable (not
supplied) to the jack on the Z-817H marked “PC”. The Z-817H 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 Z-817H.
Hookup For Radios Other Than FT-817
If using the Z-817H tuner with a radio other than a Yaesu FT-817, the hookup is the same as
for use with the FT-817 (with or without amplifier), but the CAT radio interface cable is not
used. Tuning operation with a non-FT-817 radio is slightly different; see the section on Using
The Z-817H With Other Radios.
OPERATION
Power-up
The Z-817H is powered by internal batteries. Each time the TUNE button is pressed, the Z-
817H first checks the connection to the transceiver. If an FT-817 radio is detected, the SWR
LED will flash briefly before continuing on with normal operation. If an FT-817 radio is hooked
up, but the SWR LED does not flash briefly each time the TUNE button is pressed, then the Z-
817H has not properly detected the attached radio.
Possible causes for failure to detect the radio properly are: an improperly seated CAT cable,
a damaged CAT cable, incorrectly selected CAT baud rate, FT-817 not turned on, or plugging
the CAT cable into a radio other than an FT-817. If checking all of these things does not correct
the situation, try turning the radio off and back on again.
Basic Tuning Operation, Using With The FT-817 Radio
The Z-817H is operated from the front panel TUNE button on the Z-817H. 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.
A full tuning cycle “starts from scratch” and begins a fixed tuning sequence where the Z-
817H 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 Z-817H “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.

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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 Z-
817H 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
may be useful if you wish to compare antenna performance with and without the benefit of the
tuner’s matching network.

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Initiate a Memory Tune Cycle
To initiate a memory tuning cycle, first, bypass the amplifier. Next, press and hold the Tune
button on the front of the Z-817H until the Status LED lights up. A memory tuning cycle will
begin. The Z-817H will force the transceiver into PKT mode, and key the radio. When tuning is
complete, the transceiver will return to the operating mode and power level previously set. Place
the amplifier back in “active” mode and begin transmitting.
You will notice that the radio toggles between VFO A and VFO B before and after a tuning
cycle. This is normal; the Z-817H is determining if the radio is operating in split mode, and
acting accordingly. The Z-817H will tune under all sorts of odd-split conditions, storing the
tuning memory information associated with the transmit frequency.

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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 Z-817H to seek a better match than the
match already stored in memory for this frequency.
To force a full tuning cycle, first bypass the amplifier. Then, press and hold the Tune
button on the front panel of the Z-817H 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. Place the amplifier back in active mode, and begin
transmitting.

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Using The Z-817H With Other Radios
Although the Z-817H is designed to integrate seamlessly with the Yaesu FT-817 radio, it can
also be used to tune other QRP radios with external amplifiers, so long as the maximum power
output is kept under 50 watts.
Additionally, because there is no frequency counter on the Z-817H, memory recall tuning
works slightly differently. The button push sequence is the same as for tuning with an FT-817,
but memories are stored sequentially rather than stored in a memory associated with the
operating frequency. Over 200 sequential memories are available, and the Z-817H searches the
memory from most recently used frequencies to least frequently used, so memory recall in most
cases is still nearly instantaneous.
Because the Z-817H’s CAT interface only works with FT-817 radios, the Z-817H will not be
able to automatically key the radio. Thus, you must key the radio during tuning.
Tuning operation is the same as for tuning with an FT-817, except that you must key the
radio manually during the entire tuning process. CW or AM modes work best for tuning, but it is
possible to tune in other modes as long as a carrier is present. Be sure to bypass the external
amplifier first before tuning, then re-enable it once tuning is complete.
StatusLED
The Status LED is used to indicate both operating modes, 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 blinks once. Tuner in “Active” mode.
Status LED blinks three times. Tuner is bypassed.
Status LED goes out, then SWR LED on
steady for 1 second. Tuning cycle is complete, tuning match is less than 1.5:1 SWR.
Status LED goes out, then SWR LED
blinks five times. Tuning cycle is complete, tuning match is between 1.5 and 3.0:1
Status LED goes out, no SWR LED. Tuning cycle failed, match greater than
3.0:1.
Status LED blinks 4 times. RF lost during tune.
Status LED blinks 5 times. No RF detected.

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APPLICATION INFORMATION
Mobile Operation
The Z-817H 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.
MARS/CAP Coverage
The Z-817H 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 Z-817H uses the transceiver’s CAT port for tuning control, the Z-817H 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 connect a CT-62-compatible cable from the PC
to the 8-pin mini DIN jack marked “Computer” on the rear of the Z-817H.
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 Z-817H for controlling the radio.
The Z-817H 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 Z-817H 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 Z-817H 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.
THEORY OF OPERATION
Somebasic 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
beyond the scope of this manual to present a tutorial on this subject2, but a little background will
help in understanding what the Z-817H 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.
2 For a very completetreatment of this subject,seeany editionofthe ARRLHandbookfor RadioCommunications(previouslythe HandbookFor Radio Amateurs).

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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,transmissionlines, 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.
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.

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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 Z-817H 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.
THE LDG Z-817H
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
SWR=1+RF
1−R
F

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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 Z-817H to handle loads that are either greater than or less than 50 ohms. All relays are
sized to carry 75 watts.
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 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, 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 get the SWR as low as possible (not just to 1.5); it takes about
half a second to run.