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RE: [TenTec] Hi power Ten Tec ant tuner swr drift?

To: <tentec@contesting.com>
Subject: RE: [TenTec] Hi power Ten Tec ant tuner swr drift?
From: <al_lorona@agilent.com>
Reply-to: tentec@contesting.com
Date: Sun, 14 Nov 2004 16:30:01 -0700
List-post: <mailto:tentec@contesting.com>
> 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 

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.


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