IC-746 FAQ
A possible problem we haven't already mentioned may stem from the possible similarity
between the IC-746 and the IC-756. The IC-756 users have been warned not to use either
their "Auto Tune" or "PTT Tune" features while on 6m. (even the manuals mention this,
although in passing). Several have damaged their internal ATUs this way, causing them to
get stuck in 6m operation after damaging the relays.
It has been speculated that some of the reported mishaps with the IC-746 internal ATU with
6m may be related. It's not hard to imagine that the 746/756 share a common ATU design.
We'd appreciate someone (with access to both service manuals) doing a detailed circuit
comparison to confirm/refute this.
Finally, the internal ATU is only matching at the internal PA. You'll still have high SWR out
on your coax, as expected.
3.2.9
How do I use a "beverage" antenna?
3.2.10 How do I use different antennas for receive vs. transmit?
As previously stated, nearly ANY antenna can be used with the IC-746, provided it can be
matched to the 746's 50 ohm, unbalanced antenna terminals. However, there is more to using
a "beverage" antenna than just impedance matching.
Beverage antennas tend to sacrifice gain in order to improve their signal to noise ratio. So,
while they are good for receiving, they are poor for transmitting. Therefore, you would
typically use a separate antenna for transmitting.
Split VFO Method
While not convenient, the IC-746 can be configured to use one of its antenna ports for
transmitting and the other for receiving. Run the rig in split mode with both VFOs on the
same frequency, but have VFO A use antenna 1 and VFO B use antenna 2.
External Antenna Switcher Method
A more traditional approach (albeit, more work) is to take the HF transmit signal from one of
the accessory ports activate a coaxial relay (via a driver transistor) to switch between
antennas. This is the same basic setup you would use to control an external amplifier.
(refer to articles and external amplifier questions for more information)
External RF Sense Antenna Preamp Method
Some manufacturers make antenna preamps that sense the RF on the line to switch between
antennas as well as switch in/out the preamp. The Ameco PT3 preamp is one example.
3.2.11 How do I measure SWR while in SSB?
Usually, you don't. Measuring SWR requires a signal. The only way to get a signal on SSB
is to modulate; i.e. you talk.
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2003-02-08