>"So my question is: does the driver care (or even know) the difference
between a flat 50 ohm line to an amplifier or one that is running at a 2:1
SWR, but whose length is such that the driver "sees" an impedance of 50+j0
Ohms.
I say no."
That's correct Vic. The answer is no. The transmitter is terminated with
the same complex load that an impedance analyzer measures. If the complex
impedance at the input end of the line is 50+j0 from varying its line
length, then it does not matter that the line SWR is 10:1 or even 20:1 when
using very low loss lines.
I think what many folks have difficulty understanding is that a line with
very high SWR can in any way show 1:1 on a transmitter's SWR bridge. For
example, it's difficult to conceive at first that a 600-ohm line with a 20:1
SWR that's connected right to the back of the transmitter can still show 1:1
on the transmitter's SWR bridge. It can, and does under the right
conditions and line length. This is one of the mental blocks that lead
folks into thinking only "resonant" antennas can work efficiently. It's a
shame as some ops who have spent many decades in the hobby still believe
this.
The SWR meter was one of this hobby's greatest inventions -- and one of the
worst. When my OM guided me though my novice years, we didn't even own an
SWR meter. With low-loss lines on the HF bands, and the ability to dip and
load a tank circuit on a vacuum tube Pi network, it just didn't matter
provided that the radiator is close to a half-wave in length for dipoles
(1/4 wave for verticals) and reasonably short/low loss line is used.
Have a look at the history of the SWR meter, and we don't see it in the
amateur literature until the April, 1947 issue of QST. Through a series of
articles by Lew McCoy, It took another decade for the device to become
commonplace in every ham shack. Before the SWR meter, most ops owned a
grid-dip meter and adjusted their feed line lengths to achieve resonance on
the operating frequency and sometimes the harmonics of the operating
frequency. Achieving line resonance did NOT mean low line SWR. It only
meant cancellation of system reactance -- and that made it easy to transfer
power using link-coupled, push-pull output networks that were common in the
1930s, '40s, and through the mid-'50s.
Paul, W9AC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|