And using a bridge such as one of the MFJ's or Autec's will tell more about
the bandwidth and Q of the network and serve to keep QRM off of the air and
make transistors in the PA last a lot longer.
I much prefer the L type network over the Pi or T for the network. The L
will have typically only one place that the match is good and that place it
the lowest loss configuration.
Then too the old Johnson Matchbox is the daddy of matching devices for
balanced feed systems.
73
Bob, K4TAX
>
>> 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
> coil.
>
> 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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