I'll assume you have 50 ohm coax from transmitter to antenna tuner.
From antenna tuner to 25 ohm antenna is 100 ohm coax.
The antenna tuner is adjusted so the transmitter sees a 1:1 SWR in the coax between transmitter and tuner.
The impedance change from 100 to 25 ohms between the transmission line and the antenna will create
a reflection, some of the outgoing power heads back to the antenna tuner. This creates standing waves
as the reflection interferes with the outgoing power. The job of the antenna tuner is to reflect all of that power
coming back from the antenna, sending it back out to the antenna in phase with the power from
the transmitter. So assuming the 100 ohm coax is lossless, none of the power in those reflections is lost.
But there will be an SWR of 100/25 = 4:1 in the transmission line from antenna tuner to antenna.
Jerry, KE7ER
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On Wed, Aug 19, 2020 at 07:54 AM, Miro, N9LR wrote:
Might appear to be slightly off topic, but I hope it's not :)
Lets assume we have ideal/lossless "everything" from end to end. Source (TX)
is pure 50 Ohm, the same with coax 100 Ohm, antena 25 Ohm. Over simplified,
but will get us going. Reflections everywhere (TX to coax, coax to antenna).
Now, I put antenna tuner between TX and coax (or coax and antenna), make
transition from 50 to whatever impedance coax is now presenting (coax length,
transformed 25 ohm from antenna, ...), get SWR 1:1.
Does it now mean that SWR is 1:1 at the antenna end as well?
Asked differently, if SWR is low at one end, does it mean it's low on the
other end as well (again, lossless components used!)
