Friday, June 7, 2013

Setting the output power in Yaesu FT-911

Recently I have been using my Yaesu FT-911 as signal generator to test a 10GHz transverter's local oscillator multiplier stage (1242 x 8 = 9936 MHz). Everything worked fine, but the readings I had were not consistent. The output power was lower than expected.

When supplied at 13.8 volts, high output power should be 1W, but I was measuring around 550mW. Clearly something was wrong. Many time ago, when I purchased the walkie, I did a quick readjustment using an attenuator and a spectrum analyzer. Spectrum analyzers are fine to see spurs or relationships between carrier and harmonics but they are not good as absolute power meters, specially if you are using them at the top of their dynamic range, this is with insufficient attenuation between.

The Yaesu FT-911's service manual say to adjust output power, you must supply the walkie at 13.8 volts, set the dial frequency to 1260 MHz, set high power mode, and adjust VR701 to get exactly 1W output. Then you must set the walkie to the low power setting and check if the output level is 0.15 +/- 0.05 W. Then you must switch it back to high output mode and check all along the band if the output power is within 0.2W.

These are the results I obtained:

Frequency Power Hi Power Low
1240 MHz 1.08 W 0.22 W
1260 MHz 1.00 W 0.19 W
1280 MHz 0.88 W 0.17 W
1300 MHz 0.86 W 0.15 W

Adjusted to get 1W output on 1260 MHz gives 0.19W on low power mode. Because 0.19W is inside the 0.15+/-0.05W range, it is ok. Checking the high power output along the band, the values are within 0.22W, not 0.2W but close enough ;-)

It is interesting to observe the walkie gives more power at lower frequencies. Observing the results you can think to adjust the walkie to give 1W output power around 1296 - 1298 MHz, maybe the segment most used. You can do that, but then at the bottom of the band (1240 MHz) output power is nearly 1.4 watts, too much for the little hybrid module because it gets quite hot. Maximum output power using VR701 is nearly 2W but definitively it is not a good idea.

Once adjusted the output power with 1W at 1260MHz with 13.8 volts I checked the output power with 7.5 volts, a typical battery voltage. The output was between 0.85 and 0.9 watts, not bad considering it is almost half the supply voltage.

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