SM 5 BSZ - Excessive power output from the IC706MKIIG in low power mode.
(March 24 2004)

Output power regulation

The IC706MKIIG allows different output power settings from the front panel. The way power regulation is implemented is perfectly ok when the rig is in mobile usage and the main purpose of setting the power low is to save battery, but when using this rig together with a power amplifier one has to be careful. Then one should always run the IC706 at its maximum power setting "H" pulses of nearly full power are always emitted, even when the power is set low and that can destroy a solid state power amplifier.

Peak power measurements

To shed some light on the pulse power problem I have made some measurements on an IC706MKIIG. The unit was operated from a supply that provided 12.9 V unloaded, dropping to 12.6 V at full power on CW. To ensure good regulation I added 180000 microfarads of electrolytic capacitors - but that did not affect the observed results. The unit was connected to a 100W 20dB attenuator followed by 46 dB of fixed attenuators into a spectrum analyzer. According to a 8712C network analyzer the attenuation was 65.7 from the IC706MKIIG antenna connector to the spectrum analyzer input.

Table 1 shows the readings from the spectrum analyzer which was set to 100 kHz bandwidth in peak hold mode. The analyzer was set to a fixed frequency, 14.160 MHz where the IC706MKIIG was tuned.

Setting      CW       Carrier      SSB        PTToff
            (dBm)      (dBm)      (dBm)        (dBm)
   H       -16.95     -18.80      -15.36      -14.91
   9       -16.97     -18.73      -15.69      -14.89
   8       -16.89     -18.69      -15.75      -15.03
   7       -17.18     -18.52      -15.89      -14.99
   6       -17.58     -19.12      -15.90      -15.00
   5       -18.33     -20.42      -16.78      -15.05
   4       -19.99     -22.01      -18.21      -15.20
   3       -21.16     -23.46      -19.55      -15.41
   2       -23.57     -25.94      -20.60      -15.82
   1       -25.79     -28.74      -25.16      -16.71
   L       -28.52     -31.62      -26.18      -19.10
Table 1. Peak power levels after 65.7 dB of attenuation.

Table 2 shows selected data from table 1 converted to power levels at the antenna connector of the IC706MKIIG.
Setting      CW       Carrier      SSB        PTToff
            (W)        (W)         (W)          (W)
   H         75         49         108          120
   5         55         33.8        78          116
   2         16.4       9.5         32.4         97
   1         10.0       5.0         11.3         79
   L          5.2       2.5          9.0         46
Table 2. Peak power levels at the antenna connector as well as continous power in CW mode.


The power regulation in the IC706MKIIG is not well designed. On CW the power regulation causes severe keying clicks, particularly at low CW speeds. We see it here as peak powers in CW that exceed the continous wave power by about 3 dB.

In SSB mode a big pulse is emitted each time the PTT button is released. The level of the pulses is high enough to destroy solid state power amplifiers at power settings below 5.

The much lower power output in CW mode may also be a problem when a power amplifier is used with the IC706MKIIG. A power amplifier that is tuned for maximum legal power on a string of Morse code dots will be heavily saturated in SSB mode. It will also be saturated each time the key is pressed after a longer pause, e.g. between words. Having an external amplifier saturated will generally widen the spectrum causing the keying clicks to become even worse.

If you want a power amplifier for your IC706MKIIG on 14 MHz, select one that will need the full output power or use an attenuator. Do not use the power regulation of the IC706MKIIG with power amplifiers!!

It may be advantageous to use a directional coupler to allow the IC706 to run at full power while sending a suitable fraction of the power into a power amplifier. The excess power can be dumped into a dummy load, sent into another antenna than the one used for the power amplifier output or it can be combined with the power output from the power amplifier by use of a second directional coupler. Look at Lumped components directional coupler here: The site of Pino, ZP4KFX The link shows how to design a directional coupler for feeding a 50 MHz power amplifier from a IC706.

Lowering the voltage from 13.8 V will not affect the power output during normal transmission as long as the voltage is above about 11.1 V where serious misbehaviour starts so it does not solve the problem. On the contrary - at lower supply voltages the pulse becomes even stronger.

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