Posted by
Ullrich von Bassewitz on
Sep 14, 2004; 7:22pm
URL: http://elecraft.85.s1.nabble.com/K2-ATU-question-tp370291p370293.html
On Tue, Sep 14, 2004 at 11:23:18AM -0300, Michael Harris wrote:
> Can't see the problem. I have a log periodic for 18-30MHz on ant-1 and a
> vertical for lower bands on ant-2. It does not matter if the atu stays in
> circuit and "tunes" the matched resonant antenna. Just the same as
> matching to the dummy load. My vertical is a good 50 ohm match on 10MHz
> but I also very occasionally press it into service on 7MHz where the ATU
> keeps the PA happy.
Unfortunately, it's not that easy. Matching a combined feeder/antenna system
with a tuner so that the PA sees 50 ohms doesn't mean that the antenna is
actually working. What you say is true, provided that the antenna is resonant,
and has 50 ohms with no imaginary part. But if you don't know that for sure,
for example if you're experimenting, or because the antenna has a very low
bandwidth (see below), just matching it with the ATU won't help.
A mag loop is special, because it has a very narrow bandwidth (often 10khz or
less), so changing frequency means retuning the antenna. But to tune the
antenna, one needs to know the real SWR. And this is the point where the ATU
needs to be switched off, because it will just correct the SWR as far as the
PA is concerned, but it will not make the antenna resonant.
Regards
Uz
--
Ullrich von Bassewitz
[hidden email]
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