> The feed impedance of that short antenna is much too low. You
> are going to
> melt down your tuner. Any tuner will get hot and consume a
> large percentage
> of your RF power when attempting to match such a low Z.
> Steve N4LQ
No, not necessarily true. Normally a blanket statement like this wouldn't
bother me at all, but to say that "Any tuner will get hot and consume a large
percentage of your RF power" demands a little bit more engineering and
mathematical rigor in its defense.
We can, for example, take a link-coupled tuner with large components of
sufficiently high Q (air variables and large diameter, large gauge coil) that
would have very little loss, simply because there are predominantly reactances
in the circuit, which dissipate no heat. Many such tuners are in use all over
the place, happily matching short antennas at low frequencies with very little
power loss.
Of course, for the vast majority of tuners out there, including and especially
the automatic tuners that are especially in vogue (re: November QST Short Takes
product review) nowadays this statement is, unfortunately, absolutely correct.
Eliminate the baluns and toroidal coils and small capacitors and insufficient
inductors, and the problem of matching a very low impedance goes away
completely.
The efficiency of the antenna itself (as compared to, say, a half-wave dipole)
and ground losses are other topics that we can deal with separately.
To the original poster of the question: The SWR drift you are seeing is a
result of the components in your tuner heating up. You need a different tuner
architecture to handle the low impedance presented by your antenna. Discontinue
use of any tuners with toroidal baluns immediately.
Al W6LX
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