1. License
2. Overview
3. Icom AH4 Auto-Tuner Protocol
4. Icom IC-7610 AH4 Tuner Interface
5. Fox Delta HFB3 HF Balanced Bridge
6. Fox Delta HFB3 HF Balanced Bridge Calibration
7. Arduino AH4 Protocol Emulator
8. Arduino AH4 Protocol Emulator Construction
9. Software Overview
10. SourceCode
11. Station Configuration
12. MFJ-994BRT Inverted-L Installation
13. Device Operation
14. Testing & Evaluation
15. Conclusions

BSD-3 Clause License

Copyright © 2022 Raymond B Montagne All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
3. Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

My station includes an Icom transceiver driving an Elecraft KPA-500 amplifier and an Elecraft KAT-500 auto-tuner. I recently installed an Inverted-L antenna that is located more than 200-feet from the radio installation. Using the KAT-500 auto-tuner to match the Inverted-L with over 200-feet of LMR-400 feed-line resulted in an inability to tune the antenna on a large number of frequencies.

Operationally, the main goal is to achieve a high-power Automatic Link Establishment, using ION2G, primarily supporting operation on NTIA channels that are outside of Part 97 frequency allocations.

My past experience with the Inverted-L demonstrated that when coupled with a remote auto-tuner, located at the base of the vertical segment of the antenna, there were no frequencies that the auto-tuner could not produce a proper impedance match. My prior experience was with an LDG RT-100 ATU, a 100-watt ATU that powers the ATU through the coaxial feed-line via a Bias-T. LDG had manufactured a higher power remote ATU, the RT-600 with a 600-watt rating. Unfortunately, the RT-600 ATU is no longer available.

The search for a remote ATU kept landing on MFJ. I have not been a big fan of MFJ products, but decided to give the MFJ-994BRT a go. Like the LDG RT-600, the MFJ-994BRT is rated for 600-watts power, and this provides some adequate headroom above the KPA-500 amplifier's 500-watt output.

No matter which remote ATU I chose, there are some significant differences between the Elecraft KAT-500 ATU and the available remote ATU offerings. Among these differences are the following critical features found in the Elecraft KAT-500 / KPA-500 configuration:

• The Elecraft KAT-500 implements the Icom AH4 Auto-Tuner control protocol, which fully automates the tuning process.
• When the AH4 protocol has the KAT-500 actively tuning, the key-line from the Icom transceiver (ie. SEND) is blocked from keying the KAT-500 amplifier, allowing the normal transceiver produced tuning power level to be applied during the tuning process.

Lacking the above features that are inherent with the Elecraft configuration, using a remote auto-tuner reverts to a largely manual process that requires the following steps:

1. Place the Elecraft KPA-500 into bypass mode.
2. Set the transceiver power output level to a level appropriate for the auto-tuning process.
3. Key the transceiver with a carrier or tone (I usually use the TUNE button in the FLDIGI application), and remain keyed while monitoring the transceiver VSWR until the VSWR stabilizes.
4. Un-key the transceiver.
5. Restore the transceiver power output setting to a level appropriate for driving the Elecraft KPA-500 amplifier.
6. Restore the operating mode (if changed to accomplish step 3 above).

Comparing this to the KPA-500 / KAT-500 configuration required a single push of the TUNE button on the KAT-500 auto-tuner to achieve proper operation after changing frequency.

The increased operating complexity with the remote auto-tuner is undesirable. If using digital communications that are frequency agile, such as Automatic Link Establishment, the increased operational complexity is unacceptable.

What is needed is an emulation of the Icom AH4 auto-tuner protocol, with the addition of blocking the key-line to the KPA-500 amplifier during a tuning sequence, to restore an automated one-touch operation.

Icom AH4 Normal Tuning

The Icom AH4 protocol is implemented with two signals, START and KEY. A normal AH4 auto-tuning sequence occurs as follows:

1. The START signal is driven from the transmitter and signals the auto-tuner that a request to tune is being made.
2. The auto-tuner responds to the START signal by asserting a KEY signal back to the transceiver, causing the transceiver to key up with a carrier in a low-power state that will not result in damage to the auto-tuner.
3. The transceiver responds to the KEY signal by negating the START signal and then waits for the KEY signal to be negated by the auto-tuner when the auto-tuning sequence has completed.
4. When the auto-tuning sequence completes (usually due to resolving the tuning problem and achieivng a stable low VSWR), the auto-tuner negates the KEY signal.
5. The transciever responds to the negated KEY signal by un-keying the transceiver before restoring the selected operating mode.

To emulate the AH4 protocol, a microcontroller is needed to model the state transitions that occur with the AH4 protocol signals, and an ability to determine when the VSWR has stabilized so that the KEY request to the transceiver can be terminated. An Arduino Pro Mini is used to implement the AH4 protocol state machine, and a Fox Delta FD-HFB3-1213 Balanced Bridge is interfaced to the Arudino to provide VSWR sensing.

NOTE: This project is emulating only the normal AH4 tuning protocol. Not supported are the AH4 protocols for placing the auto-tuner into BYPASS mode, or the AH4 protocol for indicating an auto-tuning error (ie. when no tuning solution was determined). There is no method of determing an auto-tuning error from the MFJ-994BRT remote auto-tuner, other than possibly an unacceptable VSWR. The non-remote version of the MFJ-994 does support a BYPASS mode, but with the control board buttons inside the case of the MFJ-994BRT remote tuner, placing the auto-tuner into BYPASS mode is only possible by removing the DC power to the remote tuner (which is best done with the power switch on the BIAS-T device).

Icom AH4 Normal Tuning - Failed Tuning

Icom AH4 Bypass Tuning

The Icom IC-7610 AH4 Tuner Interface has a weak pull-down termination (R7025) on the START signal. The START signal has a strong pull-up that is located in the auto-tuner. If no auto-tuner is present, the weak pull-down on the START signal will result in a low being presented to the TCON signal and the IC-7610 will then determine that no external auto-tuner is attached during power-on boot. If the START signal is read as high during the power-up boot of the IC-7610, via the TCON signal, the IC-7610 determines that an external auto-tuner is attached.

The IC-7610 provides a strong pull-up termination (R7022) of the KEY signal, an input to the IC-7610. The auto-tuner should provide a weak pull-down of the KEY signal, and should have the ability to read the state of the KEY signal. If the KEY signal is read by the auto-tuner, and a low is detected while the auto-tuner is not driving the KEY signal low, the auto-tuner should determine that the IC-7610 is powered off, and should wait for the KEY signal and the START signal to go high prior to responding to any low assertion of the START signal.

The 1KΩ series resistors (R7024 & R7027) on each of the AH-4 Signals limit the current through the clamping diodes (D7022 & D7021) that prevent the I/O signals from exceeding the signal levels specified by the I/O logic devices.

The auto-tuner/emulator should provide a strong pull-up termination of the START signal. This is necessary to overcome the weak pull-down in the IC-7610 for both signal termination and so that the IC-7610 properly detects the presence of the tuner during power-on boot.

The auto-tuner/emulator should provide a weak pull-down termination of the KEY signal. This is necessary so that the KEY SENSE signal properly indicates when the IC-7610 is not powered on, and is used to gate accepting an assertion of the START signal to only occur when the IC-7610 is powewred-on.

The Fox Delta FD-HFB3-1213 Balanced Bridge is available in kit form. The HFB-3 provides two adjustable outputs that represent a forward voltage and a reflected voltage. These outputs are adjustable and can be scaled to remain within the 0 to full-scale range of an analog to digital converter (ADC).

The following schematic depicts the Fox Delta HFB3 circuit. Note that this schematic corrects two errors on the schematic provided by Fox Delta, where the Forward and Reflected voltages were depicted as being routed off the top of the trim resistors. The corrected circuit has the Forward and Reflected voltages taken from the variable tap on the trim resistors. This error only appears on the schematic. The printed circuit board is implemented correctly.

Calibration of the Fox Delta HFB3-1213 HF Balanced Bridge should be performed before connecting the HFB3 to the Arduino Pro Mini. Note that the HFB3 is limited to transmitters operating at 100-watts or less.

The following steps can be used to calibrate the HFB3 to produce Forward output voltage that is compatible with the 5-volt Arduino Pro Mini:

1. Connect transmit antenna port to RF-Input port on the HFB3.
2. Connect a Dummy Load to the RF-Output port on the HFB3.
3. Adjust the transmitter for 100-Watts output.
4. Place the transmitter in CW mode.
5. Key the transmitter.
6. Adjust P1 so that the FWD signal on pin 3 of the D-SUB 9-pin connector is at 5.0000 volts.
7. Un-key the transmitter.

The following steps can be used to calibrate the HFB3 to produce Reflected output voltage that is compatible with the 5-volt Arduino Pro Mini:

1. Connect transmit antenna port to RF-Output port on the HFB3.
2. Connect a Dummy Load to the RF-Input port on the HFB3.
3. Adjust the transmitter for 100-Watts output.
4. Place the transmitter in CW mode.
5. Key the transmitter.
6. Adjust P2 so that the REF signal on pin 4 of the D-SUB 9-pin connector is at 5.0000 volts.
7. Un-key the transmitter.

Although not used in this project, the following steps can be used to calibrate the Frequency Sense output voltage:

1. Connect transmit antenna port to RF-Input port on the HFB3.
2. Connect a Dummy Load to the RF-Output port on the HFB3.
3. Adjust the transmitter for the minimum output power level that the transmitter may be operated at.
4. Place the transmitter in CW mode.
5. Adjust P3 to its minimum value.
6. Key the transmitter.
7. Adjust P3 so that the Frequency Sense signal on pin 9 of the D-SUB 9-pin connector produces a square wave as observed on an oscillocope.
8. Un-key the transmitter.

The AH4 Protocol Emulator includes the following features:

• A MOSFET interface used to translate between the Icom transceiver AH4 interface levels and the Arduino microcontroller.
• A TUNE button interface.
• A SPDT relay that is used to block keying of the Elecraft KPA-500 amplifier. The normally open contacts are used so that, in the event of a relay driver failure, the amplifier is prevented from keying so that the MFJ-994BRT is protected from possible tuning scenarios occurring at a high transmit power level.
• An Analog to Digital Converter (ADC) interface to the external Fox Delta HFB3 HF Balanced Bridge for VSWR measurements.
• A Mode switch to support dynamic configuration as an AH4 protocol emulator or an AH4 repeater.

The Mode switch is used in a configuration that has another auto-tuner, such as an Elecraft KAT500, providing RF routing through antenna switches. In such a configuration, one antenna port/switch will route RF to the MFJ-994BRT Remote Tuner and the remaining ports/switches will route RF to other antennas. When the antenna port assigned to the MFJ-BRT994 Remote Tuner is selected, the KAT500 must be placed into BYPASS mode and the AH4 ATU Protocol Emulator MODE switch must be placed into MODE A / EMULATION. When another antenna port is selected, that does not route RF to the MFJ-994BRT, the AH4 ATU Protocol Emulator MODE switch must be placed into MODE B.

When in MODE B, the AH4 ATU Protocol Emulator will enable the amplifier keying and act as a repeater for the AH4 protocol (ie. AH4 START from the transceiver is forwarded to the KAT500, and AH4 KEY from the KAT500 is forwarded to the transceiver). The AH4 ATU Protocol Emulator TUNE button is disabled in MODE B.

The source code is written in the C programming language and can be compiled with the Arduino IDE. Source code tab-stops are based on a 4-character per tab-stop setting.

The source code implements a state machine that provides the AH4 protocol emulation. State machine execution is as follows:

1. WAIT RADIO POWERUP

   This is the initial state. Should the KEY SENSE signal be sensed low when the KEY signal is not being driven by the AH4 Emulator, execution will return to this state under the assumption that the transceiver has been powered down.

   When the KEY SENSE signal indicates that the transceiver has powered up, the state machine will advance to the IDLE state.

2. IDLE

   The IDLE state detects an assertion of the START signal to start the tuning process. If START is detected, this state will block the keying of the KPA-500 amplifier. A timer is then primed to produce the delay between START assertion and KEY assertion before advancing to the POST START DELAY state.

   Alternatively, a manual tuning sequence can be initiated by depressing the TUNE button. Like a START initiated tuning sequence, , this state will block the keying of the KPA-500 amplifier. A timer is then primed to produce the delay between START assertion and KEY assertion before advancing to the POST START DELAY state.

3. POST START DELAY

   This state runs the delay timer to terminal count before advancing to the ASSERT KEY state.

4. ASSERT KEY

   If the START signal remains asserted, or manual tuning was started via pressing the TUNE button, this state will assert the KEY signal. The timer is then primed to support a delay to allow the transceiver transmitter to key before checking for a tuning solution in the next state. The state is then advanced to the Wait For Transmit RF state.

   If the START signal is not asserted and the tuning was not started manually, the state advances to the Wait For Tune Button Release state.

5. WAIT FOR TRANSMIT RF

   This state runs the timer down to terminal count while sampling RF to determine if the transmitter has keyed. If the transmitter RF is detected before the timer reaches terminal count, the state is advanced to the Wait For Stable VSWR state.

   If no transmitter RF is detected and the timer reaches terminal count, the state advances to the Negate Key state.

• WAIT FOR STABLE VSWR

  This state accumulates transmitter RF detection samples and waits for a stable VSWR state. If a stable VSWR is detected before the timer reaches terminal count, the state is advanced to the Negate Key state to terminate the tuning sequence.

  A stable VSWR is defined as meeting the following metrics:

  • The maximum VSWR that is encountered over eight consecutive measurments must be no higher than 2.0:1.
  • The average VSWR over eight consecutive measurements must be equal to or less than the mid-point between the maximum and minimum VSWR encountered over the same eight measurements if the minimum VSWR is not equal to the maximum VSWR.
  • The range of VSWR measurements over an eight sample span must be less than or equal to 0.250.
  • The minimum VSWR over an eight sample span must be equal to or greater than 92% of the maximum VSWR encountered over the same eight sample span.

  Once transmit RF is detected, VSWR samples are aquired with a minimum interval of 50 milliseconds between samples until either a tuning sollution is found or the tuning timer reaches terminal count. The timer will reach terminal count after 15-seconds.

• NEGATE KEY

  This state negates the KEY signal. If a stable and viable VSWR was detected, the state will advance to the Negate Amp Inhibit state. If the VSWR was not stable, or not viable, the amp keying will remain inhibiting by skipping over the Negate Amp Inhibit state, in order to protect the amplifier, and go directly to the Wait For Tune Button Release state.

• NEGATE AMP INHIBIT

  This state enables the keying of the KPA-500 amplifier by the Icom transceiver and then advances to the Wait For Tune Button Release.

• WAIT FOR TUNE BUTTON RELEASE

  This state waits for the TUNE button to be released and then advances to the Idle state.

Supporting functions include:

• float getVswrData ( void )

  This function reads the two Analog to Digital Converters to obtain the voltage that represents the Forward and Reflected power, and then calculates the VSWR using the VSWR = ( Forward + Reflected ) ÷ ( Forward - Reflected ) formula. The VSWR is only represented to a tenth value, with the value rounded to the nearest tenth.

• float analogReadPinAndScale ( int pin )

  This function reads the integer value of the specified Analog Digital Converter (ie. Forward or Reflected) and converts the integer value into a float value that represents the voltage that was read.

• void pushVswrData ( float vswrData )

  A small buffer is maintained to store VSWR measurement samples. The contents of the buffer can be evaluated, external to this function, to determine when the VSWR has stabilized. This function is used to enter a VSWR measurement sample into the buffer.

• uint8_t getNumberOfVswrMeasurements ( void )

  This function returns the number of VSWR measurement samples contained in the buffer.

• boolean checkIfVswrIsStable ( void )

  This function evaluates all of the VSWR measurement samples contained in the buffer to determine if the VSWR has stabilized. This function returns true if the VSWR has stabilized.

• void setAh4TunerKeyOutState ( boolean ah4KeyState )

  Passing TRUE to this function will assert the KEY signal to the transceiver AH4 interface. Passing FALSE to this function will negate the KEY signal to the transceiver AH4 interface.

• boolean ah4TuneStartInIsAsserted ( void )

  Returns TRUE if the transceiver AH4 START signal has been asserted by the transceiver.

• boolean ah4TunerKeyInIsAsserted ( void )

  Returns TRUE if the second auto-tuner, used in MODE B operations where the AH4 Emulator acts like an AH4 protocol repeater, has asserted the KEY signal on the AH4 interface between the second auto-tuner and the AH4 Emulator.

• boolean ah4TunerKeySenseIsAsserted ( void )

  Returns TRUE if the KEY IN SENSE is in a low state. This signal should only be observed if the KEY OUT signal has not been driven low (asserted) by the AH4 Emulator.

• void setAmplifierEnableState ( boolean ampEnableState )

  This function controls the relay that is used to enable the keying of the amplifier or to place the amplifier into bypass mode. When passing a TRUE argument, the amplifier keying is enabled. When passing a FALSE argument, the amplifier is bypassed.

• boolean tuneButtonIsAsserted ( void )

  This function returns FALSE if the TUNE button is released. This function returns TRUE if the TUNE button is pressed.

• boolean isInModeA ( void )

  This function is used to determine if the operator has placed the AH4 emulator into emulation mode or into repeater mode. When in emulation mode (ie. controlling the MFJ-994BRT), a value of TRUE is returned. When in repeater mode (ie. controlling another AH4 compatible auto-tuner), a value of FALSE is returned.

• void setAh4TunerStartOutState ( boolean ah4StartOuttate )

  This function is used only when the AH4 emulator is in repeater mode, and is used to control the state of the START signal from the AH4 emulator to the second auto-tuner that is AH4 protocol compatible.

• boolean xmitRfDetected ( void )

  This function reads the Forward and Reflected voltages from the Fox Delta HFB3 HF Balanced Bridge to determine if the transmitter is active. If the transmitter is active, this function returns TRUE. If the transmitter is inactive, this function returns FALSE.

• void stateMachine ( void )

  This function is executed at intervals that are no shorter than 1 millisecond, and both monitors and controls the state of the AH4 protocol interface signals in order to implement, or repeat, the AH4 auto-tuner protocol.

• void repeatAh4Protocol ( void )

  This function is executed with the AH4 emulator is in repeat mode. This function then repeats the START signal from the transceiver to the second auto-tuner, and repeats the KEY signal from the second auto-tuner back to the transceiver.

• void setup ( void )

  This function is executed once at AH4 emulator power-up time and performs all AH4 hardware and software initialization tasks.

• void loop ( void )

  This function is executed repeatedly. This function monitors the current time and then enables further execution only when at least 1 millisecond has transpired since the last time that further execution occurred.

  When in AH4 emulation mode, further execution passes control to the stateMachine () function.

  When in AH4 repeater mode, further execution passes control to the repeatAh4Protocol () function.

  Further execution also includes ensuring an orderly transition between emulation and repeater modes. A transition from emulation mode to repeater mode, or from repeater mode to emulation mode, will be delayed until any pending AH4 protocol sequences have self terminated.

The following block diagram shows haw the station is configured where only AH4 Emulation is required (ie. no repeat operation). Note that the Fox Delta HFB3 HF Balanced Bridge is placed between the transmitter and the amplifier, where the power level will not exceed 100-watts. This placement of the Fox Delta HFB3 is between the transceiver and the amplifier. The amplifier is bypassed (inhibited) during the tuning process by blocking the routing of the transceiver SEND singal to the amplifier PTT signal. This results in the transmission of a low power carrier, resulting from the AH4 KEY line assertion to the transceiver, during the tuning cycle and prevents damage to the auto-tuner by avoiding the auto-tune process at power levels that exceed the limits of the auto-tuner during the tuning sequence.

Under normal operation, a transmitter output level of 23-watts produces 500-watts output from the Elecraft KPA-500 amplifier. When tuning, in response to assertion of the AH4 KEY signal, the transmitter output level is approximately 10-watts.

The following block diagram shows haw the station is configured where both AH4 Emulation and AH4 repeat operation are required. Note that the Fox Delta HFB3 HF In this configuration, the station configuration would have the MFJ-994BRT on one KAT500 antenna port and that antenna port must be operated in the BYPASS mode. Note that it is not possible to force a specified antenna port to always be in BYPASS mode with the KAT500 Utility, so this must be managed by the radio operator.

MFJ-994BRT Remote Auto-Tuner installed on tower stand-off with lightning arrestor and 4:1 balun. The total lenght of the stand-off arm is 4-feet, and is mounted approximately 6-feet above the ground to keep the installation above the normal snow depth. A ground rod is installed at the base of the tower.

An identical stand-off is mounted at the 56-foot level of the 60-foot tower.

Arial view of Inverted-L antenna layout. The end of the Inverted-L that is opposite to the feed point is tied off to a tree that is 124-feet from the tower. There are no trees inside of the "L". The vertical segment of wire is 50-feet and the sloping portion is 130-feet long, for a total wire length of 180-feet.

Tower, tower-stand-offs and Inverted-L antenna.

The board supports two operating modes. In the first operating mode, MODE A / EMULATION, the AH4 ATU Emulator will provide handshanking with the AH4 protocol interface with the transceiver, using the Fox Delta HFB3-1213 HF Balance Bridge for VSWR measurement in support of the MFJ-994BRT Remote Tuner. In the second operating mode, MODE B / REPEAT, the AH4 ATU Emulator will repeat the AHR protocol signals between the transceiver and a secondary tuner that is attached to the AH4 ATU Emulator to enable the secondary tuner to provide antenna selection and antenna matching for antennas that are directly attached to the secondary tuner and are not connected through the MFJ-994BRT Remote Tuner.

• MODE A - EMULATION - Auto-Tuning Initiated Manually

  Select the antenna port on the KAT500 auto-tuner that has the MFJ-994BRT connected to it, and place the KAT500 auto-tuner into BYPASS mode.

  The Icom IC-7610 MENU ▸ SET ▸ FUNCTION ▸ TUNER ▸ PTT START should be set to OFF.

  In manually invoked tuning, the TUNE button on the AH4 Protocol Emulator is pressed to cause the AH4 Protocol Emulator to issue a request to the Icom transceiver to initiate an auto-tuning sequence. This request is made by asserting the AH4 KEY signal to the transceiver. Keying of an amplifier will be inhibited during the auto-tuning sequence. Once the auto-tune sequence has completed, keying of the amplifier will be enabled if a valid and stable VSWR resulted from the auto-tuning sequence. If a value and stable VSWR was not achieved, the amplifier will remain bypassed and subsequent transmissions will not key the amplifier.

• MODE A - EMULATION - Auto-Tuning Initiated from Transceiver

  Select the antenna port on the KAT500 auto-tuner that has the MFJ-994BRT connected to it, and place the KAT500 auto-tuner into BYPASS mode.

  The Icom IC-7610 MENU ▸ SET ▸ FUNCTION ▸ TUNER ▸ PTT START should be set to ON.

  Tuning from the transceiver is best achieved by setting up the transceiver to automatically iniitate an auto-tuning sequence on the first key-down event after the transceiver frequency has been changed.

• MODE B - REPEAT - Auto-Tuning Initiated Manually

  Select the antenna port on the KAT500 auto-tuner that has an antenna other than the MFJ-994BRT connected to it, and place the KAT500 auto-tuner into MANUAL or AUTO mode.

  The Icom IC-7610 MENU ▸ SET ▸ FUNCTION ▸ TUNER ▸ PTT START should be set to OFF.

  In mode B, the AH4 Protocol Emulator will pass the amplifier key line through so that it is controlled by another auto-tuner, such as the an Elecracft KAT500 auto-tuner. The AH4 Protocol Emulator will simply repeat the START signal from the transceiver to the KAT500, and repeat the KEY signal from the KAT500 to the transceiver. Manual tuning is initiated by pressing the TUNE button on the KAT500.

• MODE B - REPEAT - Auto-Tuning Initiated from Transceiver

  Select the antenna port on the KAT500 auto-tuner that has an antenna other than the MFJ-994BRT connected to it, and place the KAT500 auto-tuner into MANUAL or AUTO mode.

  The Icom IC-7610 MENU ▸ SET ▸ FUNCTION ▸ TUNER ▸ PTT START should be set to ON.

  Operation is identical to MODE A Auto-Tuning Initiated from Transceiver, but because the AH4 Protocol Emulator is simply repeating the AH4 protocol signals between the transceiver and the KAT500 auto-tuner, the KAT500 will perform the handshaking with the transceiver instead of the AH4 Protocol Emulator.

NOTE: Switching from MODE A / EMULATION to MODE B / REPEAT will remove any amplifier lock-out. Switching back from MODE B / REPEAT to MODE A / EMULATION will not restore the amplifier lock-out until an auto-tune sequence is started.

This section provides an evaluation of the performance of the AH4 Protocol Emulator. An evaluation of the MFJ-994BRT Remote Auto-Tuner is not provided. In all cases, a tuning solution has previously been resolved and the tuning parameters have been stored in memory within the MFJ-994BRT Remote Auto-Tuner. This means that all applications of a tuning solution with the AH4 Protocol Emulator will draw tuning coefficients from memory, resulting in a very rapid tuning solution.

• MODE A / EMULATION - Auto-Tuning Initiated from Transceiver

  The transceiver has been set to initiate an auto-tuning sequence upon first transmission after moving frequency (ie. the Tuner section of the Function page has the "PTT Start" parameter set to "ON").

  • With the transceiver powered down, the AH4 Emulator waits until the transceiver powers up before responding to any signal or tune switch input.
  • After powering up the transceiver, and if the transceiver has not moved from the last used frequency where transmission occurs, the transceiver does not initiate an auto-tune sequence. This is as expected.
  • After moving frequency, of a magnitude that the transceiver deams appropriate for an auto-tune sequence to be initiated, the auto-tune sequence initiates upon key-down. The AH4 Emulator is properly handshaking the AH4 interface signals. The longest observed tuning sequence, when migrating across {160m, 80m, 75m, 40m and 30m}, was just under 4.3 seconds and resulted in an average VSWR of 1.7:1, with a range of 0.0 when the AH4 protocol terminated with the negation of the KEY signal to the transceiver. The shortest auto-tuning sequence was observed to consume 675 milliseconds.
    
    Further, it was observed that the Elecraft KPA500 amplifier is in bypass mode and is not being keyed during the auto-tune sequence. Amplifier keying does occur after the auto-tuning sequence has successfully completed.
  • Note that with the antenna configuration, there is some variation in VSWR measurement at the VSWR sample rate while in the auto-tuning sequence. This is where the longest tuning sequences occur, but the measurement seems to always settle to within the specifications set by the software (ie. VSWR MIN is greater than or equal to 92% of VSWR MAX, the average measured VSWR across eight samples is less than or equal to ( MAX VSWR + MIN VSWR ) / 2, and the MAX VSWR is <= 2.0:1).
  • Powering down the transceiver does place the AH4 Emulator into a state where it waits for the transcieiver to be powered-on.
  • The AH4 Protocol Emulator functions properly with remote operation using SDR-Control, by Marcus Roskosch, and the AH4 Emulator performs flawlessly in this environment.

  Operation of the AH4 Emulator in EMULATION mode appears to be functioning correctly.

• MODE A / EMULATION - Auto-Tuning Initiated from AH4 Protocol Emulator

  The ability of the transceiver to initiate an auto-tuning sequence upon first transmission after moving frequency has been disabled for this test (ie. the Tuner section of the Function page has the "PTT Start" parameter set to "OFF"). Auto-tuning sequences were initiated manually, after a frequency change, by pressing the TUNE button on the AH4 Protocol Emulator.

  • With the transceiver powered down, the AH4 Emulator waits until the transceiver powers up before responding to any signal or tune switch input.
  • After powering up the transceiver, depressing the TUNE button does initiate an auto-tune sequence.
  • The AH4 Emulator is properly handshaking the AH4 interface signals. The longest observed tuning sequence, without moving bands, was just over 5.5 seconds and resulted in an average VSWR of 1.7:1, with a range of 0.0 when the AH4 protocol terminated with the negation of the KEY signal to the transceiver. The shortest auto-tuning sequence was observed to consume 790 milliseconds.
    
    Further, it was observed that the Elecraft KPA500 amplifier is in bypass mode and is not being keyed during the auto-tune sequence. Amplifier keying does occur after the auto-tuning sequence has successfully completed.
  • Powering down the transceiver does place the AH4 Emulator into a state where it waits for the transcieiver to be powered-on.

• MODE A / EMULATION - Auto-Tuning Initiated from ION2G as a prefix operation to an ALE Call

  The ability of the transceiver to initiate an auto-tuning sequence upon first transmission after moving frequency has been disabled for this test (ie. the Tuner section of the Function page has the "PTT Start" parameter set to "OFF"). Auto-tuning sequences were initiated by a short tuning tone that is transmitted by the ION2G software as a prefix operation to an ALE Call.

  It should be noted that the ION2G software sends the prefix tuning tone at the same amplitude as an ALE Call. This means that the KPA-500 amplifier will have a short transmission at full-power, with the incorrect tuning parameters applied at the MFJ-994BRT Remote Auto-Tuner. This is followed by a short pause with no transmission, where the MFJ-994BRT will then apply the tuning parameters from memory, and then a resumption of transmission at full-power to initiate the ALE Call.

  The ION2G prefix tuning tone duration was set to 700 milliseconds, with a non-keyed gap following the tuning tone with a 100 millisecond duration.

  • A calling sequence was issued with ION2G and the AH4 Emulator was observed to perform an auto-tuning sequence on each channel as the calling sequence progressed across the scan group.
  • Longer auto-tuning sequences (ie. longer than expected when the auto-tuning parameters are pulled from the auto-tuner memory) were observed on some of the calling channels. During some of the longer auto-tuning sequences, the measured average VSWR across eight samples was slightly higher than the calculated mid-point between the MAX VSWR and the MIN VSWR, and it is this measurement that is holding off the completion of the auto-tuning sequence. This appears to occur as a one-time event on each channel and was likely due to a tuning solution not being stored in the tuner memory for the channel. Once the long tuning sequence completed, returning to the channel did initiate an auto-tuning sequence, but the duration of the tuning cycle was much shorter.
  
  

• MODE B / REPEAT - Invoking Manual Tune on the Elecraft KAT500

  • User interaction points toward REPEAT mode not satisfying requiriments, and in part, this is due to the Elecraft KAT-500 not remembering and applying the BYPASS mode when switching back to antenna 1 (ie. the Inverted-L / MFJ-994BRT). When switching to the Yagi, the user switches to repeat mode, changes the IC-7610 to an appropriate frequency, and keys-down. The Elecraft KAT-500 will detect the frequency and apply the appropriate antenna selection that is based on the profile stored in the KAT-500 and take the KAT-500 out of bypass mode. Auto-tuning then occurs and completes normally. The radio is now ready to use on the Yagi, but this is a one-way trip. When returning to a frequency that is appropriate for antenna 1 (ie. the MFJ-994BRT) and keying down initiates an auto-tuning sequence. The KAT-500 will select antenna 1, but will not place the KAT-500 auto-tuner into bypass mode. Because the KAT-500 is not in bypass mode, neither auto-tuner is guaranteed to solve the tuning problem.
    
    The best option here is to force the KAT-500 auto-tuner to memorize the tuning solutions for the planned frequencies of operation, and then leave the AH4 Emulator in emulation mode. Then manually manage the bypass mode on the KAT-500 auto-tuner.
    
    This is less than ideal.
  
  

The configuration where this project succeeds is as follows:

The above configuration may simply omit the Emulate / Repeat switch.

This project is considered to be incomplete. A future modification will attempt to address the current issues by interfacing a serial port between the AH4 Emulator and the KAT500 to apply command control to (1) place the KAT500 in bypass when entering emulation mode, and (2) place the KAT500 in manual tuning mode when entering repeat mode. A breif low power transmission will be needed immediately after the transission of the mode switch and the sending of the command to allow the KAT500 to apply the frequency dependent antenna selection, followed by a full tuning cycle. Stay tuned...

73, Ray Montagne (W7CIA)