Icom IC-746, Часто задаваемые вопросы

Страница: 29 / 113

IC-746 FAQ
3.2.2 What range can the automatic antenna tuning unit (ATU) handle?
Officially, the automatic ATU in the IC-746 can handle SWR mismatches of up to 3:1 on HF
and up to 2.5:1 on 6 meters. In practice, many people get even wider ranges, a few as high as
7:1, but be wary of pushing the ATU that hard.
The ATU does not function on 2 meters.
Some have pointed out that, even though the internal tuner may tune a particular antenna
configuration, if there is a high SWR, you could end up burning out the tuner.
We have at least one case (reported by KG6GLN) where the detector diodes in an IC-746
were fried. According to the ICOM tech, "... it could happen if you tried to use an external
antenna tuner in conjunction with the internal tuner.
"
3.2.3 Will the internal ATU handle "random" wire antennas?
3.2.4 Will the internal ATU handle "long wire" antennas?
No. You'll need an external ATU.
The internal ATU can only handle an SWR mismatch of about 3:1 on HF and 2.5:1 on 6
meters. Such antennas will exceed those limits somewhere, over stressing the ATU and
possibly blowing it out. Matching will be hit or miss (mostly miss).
Of course, there's always an exception. N4EUK reports successfully tuning his 100 foot wire
antenna using the internal ATU. Even he was surprised.
3.2.5 Are there any "tips" to using the ATU?
You mean, besides the tips already provided on page 58 of the Instruction Manual? Yes.
The ATU appears to pay attention to the reflected power, not the SWR. [Note: we'd love
someone to analyze the schematics to confirm or deny this.] This is evidenced by two effects.
When starting from a close match at QRP power levels, the auto ATU doesn't always home
in on the best match. Increasing the power level and tuning again often improves the match,
which the ATU then retains when returning to QRP power. In fact, the manual mentions at
least 8w on HF and 15w on 6m.
The converse situation can also cause trouble. When starting from a poor match at full
power, the auto ATU sometimes can't find a match at all. In this case, tuning at lower power
allows the ATU to get a match, which the ATU then retains when returning to high power.
If one end of a band consistently gives you trouble while the other end tunes easily, then you
may still be able to coax the ATU into matching the antenna. Start in the portion of the band
where the ATU works and tune there. Then, move your way across the band in increments
(maybe 25Khz at a time), tuning as you go until you reach the desired portion.
3.2.6 Can I use the internal ATU along with an external ATU?
Maybe. Theory says yes, but Icom says no. The results are mixed. Use at your own risk.
Page 29 of 113
2003-02-08