In message , rickman
writes
On 10/5/2015 4:17 AM, Ian Jackson wrote:
In message , rickman writes



You keep saying that the 1:1 match between the TX and the ATU prevents
any power from being sent to the TX which is not true. You are
confusing the power from the TX which is not reflected and the power
reflected from the antenna which passes through the ATU to the TX.

If the SWR meter between the TX output indicates a 1:1 SWR, then there
can be NO power travelling between the ATU input and the TX output - ie
there IS no reflected power. QED, surely?

If you ignore the losses in the ATU, all the power that the mismatched
antenna reflects, and that makes it back to the ATU output, MUST be
re-reflected by the ATU output impedance, and head off back towards the
antenna. This is because the reflected signal cannot heat up a lossless
ATU, and the SWR meter says it isn't coming back through the ATU. It
simply has nowhere to go except back down the coax.

You saying something is true or imagining a SWR reading is not the same
as understanding what is going on. What SWR reading are you imagining?

A typical HF station setup is TX - SWR meter - ATU - feeder - antenna.
[Please no one start saying they don't use this configuration.]

Can you explain this in terms of the circuit analysis?

The only analysis required is to ask yourself why, after much careful
twiddling with the knobs, you eventually get the SWR meter to show 1:1.
Does this not indicate that no power is coming back to the TX from the
ATU? [If not, what does it indicate?]

The ATU consists of what circuit?

I haven't a clue. It's a large shiny black box with three silver knobs
and two RF connectors. All the spec says is "Insertion loss 0dB". [Mind
you, it was very, very expensive.]

The TX has some source impedance, what would that be?

Somewhere between not-a-lot and probably not-too-high (because, in
operation, the TX doesn't get unduly hot). It really doesn't matter what
it is. The SWR meter shows that zero power is coming back out the ATU
input, so there is nothing to get re-reflected from the TX output.

I don't think you can design an ATU circuit that will isolate the
real source impedance of the TX from the reflected wave from the
antenna.

The purpose of what many of us (often slightly incorrectly) call an ATU
is to present the TX output with its design load impedance. In most
cases, this is 50 ohms resistive.

The TX output impedance is really of no concern to the designers of the
ATU. As long as they can produce an ATU that will present the TX with a
load of 50 ohms resistive, then it will do its job perfectly - and this
will be indicated by the 50 ohm SWR meter between the TX and the ATU
showing 1:1. And when an SWR meter shows 1:1, it means that there is no
power coming back towards the TX output.

--
Ian