KY-28 Speech Security Unit

Speech Security Unit KY-28

The UHF communication system is capable of providing either conventional or crypto voice transmission and reception. Both UHF and intercom systems can be used in combination with the KY-28 Speech Security Unit, if so desired in a tactical or hostile environment. The KY-28 can either cipher outgoing voice transmissions or decipher received voice transmissions; however, these functions cannot be performed simultaneously. The crew can choose between plain or cipher mode of operation. In the cipher mode operation the KY-28 converts voice inputs through the microphone into crypto transmissions and deciphers crypto replies that are being received to the crews headsets. In the plain mode operation the UHF functions as a conventional voice transceiver.

KY-28 Controls and Indicators


The KY-28 is controlled by the KY-28 control panel and respectively the conventional UHF or Intercom controls. Both front and rear cockpit instrument panels display mode lights for either mode. If the mode light P is illuminated (1) the KY-28 and UHF/Intercom is operated in plain mode. If the mode light C is illuminated (2) the KY-28 and UHF/intercom is operated in the cipher mode.

KY-28 Control Panel


The KY-28 control panel is in the rear cockpit. It has a power knob, a mode switch and a code zeroize button. The power knob (3) provides power to the KY-28 and can be set to either OFF, ON or RLY (relay). The KY-28 is not powered in the OFF position. It is powered in the ON and RLY positions. If a direct communication between KY units is desired, the ON position is to be selected.

💡 RLY would provide the ability to use the KY-28 as a retransmission facility, which is not modeled in DCS.

The mode switch (2) can be set to either P (plain) or C (cipher) operating modes. To change modes the switch has to be pulled outwards. If the KY-28 is powered and the mode switch is set to the C position, the UHF transmitter is automatically selected for front cockpit transmissions. Pressing the zeroize button neutralizes the preset code in the KY-28. The zeroize button (1) is guarded and should only be used in emergencies or after aircraft shutdown if required. As the code can only be set on the ground, pressing the zeroize button during flight makes the cipher mode inoperable for the crew for the remainder of the flight.

💡 To permit conventional UHF communications the mode must be set to P (plain) even if the KY-28 power knob is in the OFF position. If the mode is set to C (cipher), conventional UHF communication is inhibited in both the front and rear cockpit. An exception to this is transmission and reception on guard or aux receivers, as these are not affected by the cipher mode.

KY-28 Modes of Operation

All transmitted and received signals are routed through the KY-28 unit. When the KY-28 mode is set to P (plain), the UHF transceiver operates in the conventional voice manner, whether the KY-28 is powered or not.

To operate the UHF transceiver in the KY-28s C (cipher) mode, the KY-28 has to be powered. When operating in the cipher mode, the KY-28 unit functions in either standby, receive, or transmit.

All KY-28 units remain in the STANDBY condition until either the microphone button is actuated or a sync preamble is received. Once transmission or reception of the crypto message has been completed the KY-28 automatically reverts to the standby condition. While the KY-28 is in the standby condition the radio equipment also functions as a traditional receiver as all incoming non-crypto transmissions are passed directly to the headsets.

The unit switches to the cipher RECEIVE condition whenever a sync preamble is received. The sync preamble is generated by other KY-8, KY-28 or KY-38 units when the transmitting stations operator actuates the microphone switch. The sync preamble ensures that all units in the network are in the cipher receive condition. The enciphered message is received immediately after the sync preamble signal and the message is decoded by the KY-28 and passed to the aircrew headsets in the form of plain language.

When the UHF microphone switch is set to the UHF position, the KY-28 switches to the TRANSMIT condition. The sync preamble signal is transmitted to all receiving KY stations to switch to the cipher receive condition. After actuating the microphone switch, a brief tone is heard in the aircrew headsets. The tone signal indicates that the sync preamble signal has been completed and transmitted, after which the KY-28 is ready to transmit the crews voice input. When the mic button is released again, the KY-28 reverts to the standby condition. The WSO must select UHF communications with the radio selector switch to transmit ciphered messages. The UHF transmitter is automatically selected for front cockpit transmissions if the KY-28 is operated in mode C.

💡 With the C (cipher) mode selected, the transmit condition overrides the cipher receive condition. Hence during cipher receive operation the UHF microphone switch should not be actuated until the incoming message has been completed.

For operations procedure see Chapter KY-28 Operations.