It does get confusing when neither end is properly terminated to the characteristic impedance of the transmission line.
You have reflections from both ends, and reflections of those reflection, until everything dies out.
And considering this, I suppose my analysis of the SWR in the transmission line when using an antenna tuner
may be wrong, that 100/25=4:1 SWR calculation assumes the transmitter has a 100 ohm source impedance
to kill any reflections there, matching the 100 ohms of your transmission line.
However,
No, you can't just add transmission line length and get any purely resistive load looking into the transmission line that you might wish.
You get a couple purely resistive choices, one high and one low, all other impedances seen by the transmitter will
have significant inductive or capacitive reactance added in. On the Smith chart, we are drawing a big circle
centered on that perfect match in the middle of the chart as we add length to the transmission line.
I am convinced that at all times, a lossless transmission line can be considered to have the same SWR
at any point along it's length, assuming steady state conditions. All voltage waves travelling out to the antenna
including all those reflections have the same frequency, so they will sum to a single sine wave. Likewise for
any reflections coming back from the antenna. Those outgoing and incoming voltage waves will interfere
to create a single standing wave pattern.
Jerry, KE7ER
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On Wed, Aug 19, 2020 at 09:26 AM, Miro, N9LR wrote:
'm not sure I completely agree :) Fortunately (or unfortunately) the
transmission line acts as a impedance transformer. Given the length of the
transmission line, impedance "shown" on to the TX might be the same as
antenna's impedance, or quite different (depends on transmission line length
and it's impedance).
Like wise, what TX "presents" to the t-line is not what will be presented to
antenna (the same principle where t-line transforms impedance).
I do NOT known if this transformation done by the t-line is reciprocal: let's
take a simple example. Take antenna tuner out of picture, so you have TX(50),
t-line(100), and antenna (25). By carefully selecting t-line's length, you can
transform antenna's 25 to match TX's 50. I think (!?!) that at the same time,
on the antenna side, using the same t-line magic, TX's 50 will be now
"presented" to antenna as 25!
If this is true (is it?), you can expend to any length of the t-line by adding
antenna tuner - whatever t-line presents to TX side (let's call that Zx) will
be matched to 50, and like wise, TX side will appear to t-line as Zx, and
t-line will transform that to 25 on the antenna side.
If all this holds water, if TX sees SWR as 1:1, that means that SWR on the
antenna side is the same!
Two (hand picked) examples of reciprocity:
1) lambda/4 t-line transforms short (0 ohm) to open (>>0). Wise versa is the
case as well: >>0 on one side transforms to 0 on the other
2) lambda /12 t-line does magic Z1*Z2=sqrt(Zt) (multiplied impedances from
each side are square root of t-line's impedance
If, in generic case (any t-line length/impedance) the following holds true
Z1=k*Z2 and Z2=1/k*Z1 ("k" is impedance transformation "factor" that t-line
does), it appears that good SWR on one side is good SWR on the other as well,
right?
Now, you make an interesting point, if all this magic is happening by
"bouncing" waves back and forth, cost of doing it with real t-line with losses
might be significant - wave bouncing back and forth will lose some of it's
energy in the t-line, and seeing SWR meter showing 1:1 on the either side will
NOT indicate that such losses are present. Antenna tuner will make
t-line/antenna appear "friendly" to TX, but will not deliver all the power to
ether
