On Wed, 2007-08-08 at 20:33 -0400, Larry DiGioia wrote:
> Now you've done it. Don't let Kurt hear you say that.
> Interestingly, he just addressed a similar issue in his last column.
> I am a user of random length dipoles and truly balanced tuners. I am 8
> countries away from 5BDXCC with that (OK, I have a Hy-Tower too,) and
> 100W. Anyway, I used to fight tuning the wires, lengthening and
> shortening them, because I believed that the more turns of inductance
> and the more meshed the capacitors were, the more I was "losing in the
> Not true. See the last issue of "Worldradio" for an explanation.
> CATFISHTWO@aol.com wrote:
> > but when you use a
> > random dipole and a tuner, you are leaving some of your power in the tuner,
The power you loose in the tuner can depend on the impedance ratio
between the feed line and the desired Z for the transmitter, the tuner
circuit, and the losses in the tuner components. When the load is highly
reactive and the impedance a long ways away from 50 ohms, the T, PI, or
L match sometimes requires running at a high loaded Q and that makes the
circulating currents higher. And its those high circulating currents
that lead to greater losses in the unbalanced tuner. Tuning through a
balun is often fraught with great difficulty, more times than I care to
have tried I've found a tuning position that presented a match to the
transmitter, but that match didn't change with the antenna disconnected.
All the power was going into the ferrite cored balun and the coil in the
tuner. The DX-40 would tune and load up to full power with no antenna on
its Pi network. I suppose all the RF power was being used to heat the
For a balanced load, its hard to beat a link coupled parallel or series
tuned tuner. Series tuned for low impedance, parallel tuned for high.
And its highly practical to tap the feed line down from the ends of the
parallel tuned link coupled tuner. Its been around for 3/4 century and
still works fine.
As for tuner sizes, a compact tuner is the enemy of efficiency. Small
coils are always more lossy than large coils. Air spaced capacitors are
really pretty good unless made of steel and not silver plated. Silver
plated copper are best and losses can be reduced a bit more for the old
capacitor by adding a braid or strap connection for the rotor, getting
away from friction and a dirty sliding contact.
If there are two or more tuner settings that will match the antenna, the
one with the greater bandwidth should be the one with lower loss because
greater bandwidth says lower loaded Q and so smaller circulating
currents in the tuner circuits.
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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