On Tue, 2010-02-23 at 16:35 -0600, Stuart Rohre wrote:
> If you read "Reflections" by Walter Maxwell; the function of the tuner
> at the rig is to adjust the matching to the line to account for the
> situation on the antenna end as well; well enough to minimize losses to
> the practical extent. The whole of the tuner, transmission line, and
> antenna acts as a tuned system where the matching box adjusts such that
> the transmitter sees on 50 ohms non reactive, if the matching box has
> the proper range of adjustments for the band in use. It is true that
> SWR extremes do not introduce as high a loss in parallel line as coax,
> for coax is a high current line, and parallel line is higher impedance,
> lower current. Thus, I squared R losses are less than coax I squared I
> losses.
>
But in the extreme cases of feeding a dipole over a wide range where the
feed impedance may be 50 ohms at half wave resonance not too high above
the ground up to 1000 ohms at full wave resonance, the SWR and the
currents on the 300 to 600 ohm line will be more reasonable than the
will be at the extremes with 50 ohm coax. And tuning will be easier with
the high impedance line most of the time. The worst cases with the high
impedance line are with the antenna impedance down at 50 ohms and the
line with 600 ohm impedance being a quarter wave long. Then that 50 ohms
transforms to 7200 ohms, high but tunable with reasonable components.
But the voltage at the tuner will be high. 850 volts for 100 watts. When
the antenna has the 1000 ohm feed impedance the impedance at the tuner
if a quarter wave (or 3 quarter waves or 5 or any odd number of quarter
waves) is decent at 360 ohms. 300 ohms would be present a nicer range of
impedances to the tuner, max of 1800 ohms, min of 90 ohms.
The worst case with the dipole and a coax feed is when the feed is a
quarter wave long and the antenna is full wave resonant with a 1000 ohm
feed point impedance. Then the input to the coax looks like 2.5 ohms
which demands large currents and large capacitors (especially on the
lower bands). 100 watts at 2.5 ohms means over 6 amps coax current and
likely more in the tuner if the tuner has a loaded Q greater than 1. A
tuning situation not unlike feeding a base loaded 8 foot whip on 160
meters. And some tuners, even the automatic tuners, will make that work.
Some will fry trying.
> -Stuart
> K5KVH
>
>
73, Jerry, K0CQ
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