On Thu, 2009-05-21 at 14:48 -0700, Arthur Trampler wrote:
> Isn't one of the key distinctions of the Ten Tec 238/B/C that these are
> switched L networks, rather than T-networks, as I believe the Palstar is?
> Some years back QST reviewed several external auto-tuners and with respect to
> efficiency, the Ten Tec 253 came out far ahead of most competitors, as did
> (surprising me) an MFJ tuner. They both handily were measured as being far
> more efficient than the Palstar especially on the lower bands matching low-Z
> There is a common factor between the MFJ and the TT-253 (and 238), and that
> is that it is also an L network. Most Palstars are T-matches, right?
> So forgetting the questions/comments about baluns, and whether the lowest SWR
> is also the best match, and whether there may be multiple low-SWR tuning
> points with a T-match...
> Is an L-Match inherently less lossy than a T-match under most circumstances
> (ceteris parabis)?
Yes. Because it has only one possible loaded Q for any given impedance
ratio, the circulating current in the circuit is minimized. In a T or PI
the loaded Q can be higher resulting in more circulating current. In any
tuned circuit, the circulating current is higher than the external
current by the factor loaded Q. Typically Q is 12 for an amplifier
output network because that gives adequate harmonic reduction. But on
the higher bands, Q rises because the output device capacitance limits
the minimum Q. And efficiency suffers because of tank circuit (mostly
coil) losses from the higher current.
73, Jerry, K0CQ
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