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AT-AUTO Auto Tuner

AT-AUTO Tuner image

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The AT-AUTO is a fully autonomous, fully automatic impedance matching network (an antenna tuner) that employs an embedded microprocessor to position a conventional roller inductor and a split-stator variable capacitor. It provides continuous coverage tuning from 1.8 - 54 MHz (including MARS, DoD, and commercial frequency bands, etc.) and is rated for 1500 W continuous carrier operation. The AT-AUTO is quiet and reliable, and with its industrial-rated stepper motors, provides a superior impedance match to that provided by the loud clanking boxes of relays provided by the competitors.

The AT-AUTO continuously measures forward and reflected RF power. As long as the SWR is below a user-selectable threshold, the AT-AUTO provides a "Good Match" cue in the Liquid Crystal Display (LCD) and takes no further action. Whenever the SWR exceeds the "Tune-Start" SWR, the AT-AUTO then counts the RF carrier frequency, looks up previously stored Inductor (L) and Capacitor (C) settings for the frequency of operation and then repositions L & C to the recalled settings. If the SWR is still excessive, the AT-AUTO automatically begins tuning, and provides user prompts during the entire process via the LCD. Once a good match is found, the new match settings are stored in memory for subsequent recall. The default behavior is highly customizable to suit individual needs.

The AT-AUTO is also able to obtain frequency information via the radio's serial data port (for these compatible radios so equipped). By doing so, the AT-AUTO instantly follows frequency changes (VFO adjustment, etc.) and immediately recalls and sets the L & C for the new frequency - without the need to generate an RF carrier! The recalled match should provide an excellent SWR so that when RF is finally applied, subsequent tuning will not be necessary. While the serial port is not necessary for automatic tuning, this "Smart Mode" of operation greatly speeds the entire process of changing frequencies.

The AT-AUTO is also capable of automatically selecting up to 8 coaxial outputs (via the companion CX-AUTO coaxial switch). Additionally, the AT-AUTO may also be shared with up to 8 different radios when utilizing a CX-AUTO on the "input-side" of the AT-AUTO. Please see the CX-AUTO link for additional information.

Note: The AT-AUTO is currently not in production and not available for sale. It is undergoing a redesign for release soon. The pictures and descriptions shown here are of the existing version(s) of the AT-AUTO tuner prior to its production cessation.

A new web page for the AT-AUTO II is coming in the near future.

Update Your Tuner's Firmware

Kessler Engineering provides the latest versions of the tuner's firmware available for download. Works on Windows PC's only. Support for older firmware versions is available here, too. Download the bootloader Zip file below to install firmware.

Bootloader zip file

AT-Auto Info


Kessler Engineering, LLC is able to offer repair and upgrade service for legacy AT-AUTOs originally manufactured by Palstar.

Please contact us directly, should your legacy AT-AUTO require service or upgrade.

Please do not send any products to Kessler Engineering until you have been issued a valid Return Merchandise Authorization (RMA).

AT-Auto tuner internal component view, currently out of production. Click to view larger image.

The AT-AUTO employs an embedded microprocessor to precisely adjust the tuning network comprised of a split-stator capacitor and a roller inductor, arranged in a classic "T-Match" configuration. Additionally, the microprocessor is able to receive serial frequency data directly from a radio's serial data port, and the AT-AUTO is also capable of being remotely controlled via its serial data port.

  • Power Rating: 1800 W PEP SSB, 1500 W CW key-down
  • Frequency Range 1.8-54 MHz Continuous
  • Impedance Range 15-1500 ohms
  • Balanced Output 4:1 Current Balun
  • Tuning Time approx. 1-30 Seconds
  • Tuning Method: Microprocessor - controlled, stepper motor-driven differential T impedance matching network
  • Dimensions: 12.5" Wide, 6.25" High, 17" Deep
  • Weight: 17 Lbs
  • Materials: 0.090" aluminum, gold iridited, powder coated
  • Power Requirements: 12VDC, 3.5 A while tuning, 100mA idle
  • Data I/O: RS-232 or 5 V Open-Collector NRZ (CI-V)

Optional interface cables available for Elecraft, ICOM, Kenwood, TenTec, Yaesu.


All products manufactured by Kessler Engineering, LLC are either personally built my myself (Don Kessler) or by a select member of the Kessler Engineering staff. All products are tested exclusively by me and I can assure you of the high level of component and build quality as well as strict attention to detail.

Kessler Engineering products are therefore warranted to be free from defects in material and workmanship for a period of one year from the delivery date to the customer. However, when services or upgrades are provided to legacy AT-AUTOs, this warranty shall only cover those specific items, products or services installed or upgraded by Kessler Engineering.

During this one year warranty period, Kessler Engineering, LLC shall either issue credit, repair, or replace any Kessler Engineering-manufactured product proved to be defective.

Should the original purchaser notify Kessler Engineering of the suspected product defect, Kessler Engineering will supply instructions for returning the product. If the product is indeed determined by Kessler Engineering staff to be defective, the product will be repaired, replaced or a refund provided, at the discretion of Kessler Engineering.

No products will be accepted for return unless accompanied by a Kessler Engineering-issued Return Merchandise Authorization (RMA).

This warranty shall be void if the product has been tampered with, abused, or used in an inappropriate manner, fashion, or application. Kessler Engineering, LLC is not responsible for loss or damage to any other equipment and does not make any claim or warranty of product merchantability or fitness. No one is authorized to assume any obligation or responsibility on behalf of Kessler Engineering LLC.

Additional AT-AUTO Downloads

AT-AUTO firmware update instructions (PDF)

AT-AUTO Tuner Manual (PDF) (Updated Jan. 2018)

AT-Auto Tuner Cables

The AT-AUTO sends and receives serial data to/from the associated radio via the radio's serial port. This is commonly referred to as Computer Aided Tuning or CAT. In order for the radio and the AT-AUTO to be able to communicate serially, an appropriate CAT cable must be used.

Because there is no defacto standard for CAT, there are variations in connectors and pin assignments from manufacturer to manufacturer. Therefore, there are unique AT-AUTO CAT cables for Elecraft, Kenwood, TenTec, Yaesu, and others. The CAT cables themselves are nearly identical, with minor variations in the specific connector and pin assignments.

ICOM has their own CAT implementation known as CI-V which uses entirely different protocols. ICOM radios, therefore, do not use the CAT cable, but instead use a CI-V cable.

Cable Sets consist of a Firmware Update Cable, A CAT/CI-V Cable, and in the case of some Kenwood radios, a Tuner-Handshake Cable. Schematic diagrams of the various cables are provided in the AT-AUTO user manual.

Purchase Cables for the AT-Auto Tuner

Universal Serial Interface

It comes supplied with three identical serial cables and the interface board. The universal interface eliminates the need for different radio-specific cables and only uses typical 9-pin serial cables. The interface has three switches that permit quick selection of mode as well as radio-specific settings. This is the best option for your situation and it less expensive than buying multiple sets of cables.

A PDF manual is available here.

Price: $58.00

ICOM Interface Cable Set

The Icom Tuner-Handshake is much simpler to implement with no special circuit boards required. This feature is incorporated into the ICOM CI-V cable; 6 feet long.

Price: $40.00

Firmware Update Cable

The AT-AUTO employs a microprocessor and the programming (firmware) may be updated by the user as new features and improvements are made. This requires a standard 9-pin male-to-female serial cable, which we refer to as the Firmware Update Cable; 6 feet long.

Price: $7.50

Elecraft Serial Data Interface

Elecraft Serial Data Interface Cable Set, 6 feet long.

Price: $30.00

TenTec Serial Data Interface

TenTec Serial Data Interface Cable Set, 6 feet long.

Price: $30.00

Kenwood/Yaesu Serial Data Interface

Kenwood/Yaesu Serial Data Interface Cable Set, 6 feet long.

Price: $30.00

AT-AUTO Power Cable

AT-AUTO Power Cable

Price: $28.00

AT-Auto Tuner Upgrades

Although currently out of production, we are dedicated to providing outstanding product support for the legacy AT-AUTO tuner. Until now, that product support has consisted primarily of repair and firmware updates. However, in to ensure AT-AUTO operational longevity and to prevent product obsolescence, we are in the process of developing a series of hardware improvements for the legacy AT-AUTO.

As those hardware improvements occur, they will be made available to AT-AUTO owners.

Replacement Meter Kit

One of the most common complaint areas concerns the AT-AUTO's stock meter. Meter problem areas have been "sticking" needle movements due to static electricity accumulation, illumination "hot spots" and difficulty obtaining replacement incandescent meter lamps. Other complaints involve meter face clutter and lack of resolution.

In order to solve the meter issues, we developed a replacement meter using better plastics that prevents the static electricity build-up and is back-lit with three LEDs, providing nearly uniform illumination and eliminating the incandescent lamp burnout/replacement issues.

QRO Keyline Upgrade Kit

The AT-AUTO first went into production in 2006, however, the QRO Keyline hardware was not developed until 2008. The QRO Keyline was created as a method to prevent damage to an HF amplifier or to the AT-AUTO when inadvertently driving mis-tuned loads at QRO power levels.

The QRO Keyline feature was then included in all 2009 and subsequent models of the AT-AUTO. AT-AUTOs so equipped with the QRO Keyline are able to automatically permit/inhibit keying of an HF amplifier. Use of the QRO Keyline hardware is entirely optional and completely independent of the automatic tuning process and does not otherwise change or alter the operation of the AT-AUTO.

With the QRO Keyline, the user may configure the AT-AUTO to automatically prevent an HF linear amplifier from being keyed if the SWR exceeds a user-selectable level, to immediately unkey an HF amplifier should the SWR become excessive, or while the AT-AUTO is tuning. This may be particularly advantageous when using the AT-AUTO with HF amplifiers that feature SWR protection circuitry. If the user neglected to unkey the amplifier while the AT-AUTO is tuning, the amplifier's self-protection circuitry would likely disable the amplifier, necessitating a reset of the amplifier itself.

Purchase AT-Auto Tuner Upgrades

AT-AUTO Replacement Meter Kit

The manual is available as a PDF here: AT-AUTO Meter Upgrade Kit Manual.

Price: $53.00

QRO Keyline Upgrade Kit

The manual is available as a PDF here: AT-AUTO QRO Keyline Upgrade Kit Manual.

Price: $35.00