MFJ-939 Plug&Play IntelliTuner Automatic Antenna Tuner
Instruction Manual
2006-2015 MFJ Enterprises, Inc.
24
MORSE CODE AND BEEPS
If insufficient power is applied for tuning (less than two watts), “QRO” (dah-dah-di-dah di-dah-dit dah-
dah-dah) will be sent on CW. Increasing the input power above two watts ends this message.
When input power is too high, the tuner enters a self-protection mode. The tuner will not allow any of its
relays to change to prevent damage to your tuner. If too much power is applied when tuning, the tuner
will cease the tuning routine and send “QRP” (dah-dah-di-dah di-dah-dit di-dah-dah-dit) on CW. This
occurs when the forward power exceeds 75 watts and the SWR is greater than 3.0, or when the forward
power exceeds 125 watts regardless of the SWR.
If more than 200 watts is applied to the tuner, the tuner will go into bypass mode and send “QRT” (dah-
dah-di-dah di-dah-dit dah) on CW.
If the tuning process is activated under these conditions, the tuner will not start the tuning. It will send the
appropriate code on CW.
The following table shows the various tuner beeps:
Function
Indicates By
One Beep
Indicates By
Two Beeps
By Pressing
Primary Memory
1
2
[ALT] < 1 second
Memory Bank
Bank A
(1 beep)
Bank B
(2 beeps)
Bank C
(3 beeps)
Bank D
(4 beeps)
Off
(5 beeps)
[ALT] > 1 second
Bypass Toggle
Bypass Mode
(L = 0 and C = 0)
Restore Last L/C Setting
[TUNE] < 0.5 sec.
Tuned SWR
SWR ≤1.5
(1 beep)
SWR ≤2.0
(2 beeps)
SWR ≤2.5
(3 beeps)
SWR ≤3.0
(4 beeps)
SWR >3.0
(“SWR”)
[TUNE] for
0.5 to 2 seconds
StickyTune
On
Off
[TUNE] > 2 seconds
Tuning Mode
Automatic
Semi-Automatic
[TUNE] + [ALT]
Table 8 Tuner Beeps.
TRANSCEIVER FOLDBACK
Modern transceivers with solid-state finals usually have a foldback circuit to protect the final transistors
from damage due to high SWR. A foldback circuit detects the SWR during transmit and reduces the
output power as the SWR rises above a preset threshold, usually 2:1. The higher the SWR the lower the
power is set to prevent damage.
If your transceiver has a foldback circuit, you can simply key down and tune at any power level from 2 to
100 watts. If your transceiver does not have a foldback circuit, you must manually set the power level to
20 watts or less for tuning. At higher power levels, the reflected power occurred during tuning can
damage your transceiver or amplifier and causes component arcing which can damage the relays in the
tuner.
Check your transceiver owner's manual to see if it has a foldback circuit. When an amplifier is used, the
amplifier must be bypassed during tuning.
Note:
MFJ recommends that a transceiver-specific interface cable be used between the MFJ-939
and your transceiver to ensure that tuning only occurs at low power.
