On 10/6/2015 3:26 AM, Ian Jackson wrote:
In message , rickman writes



When you tune for VSWR of 1:1, the impedance of the TX side of the ATU
will match the TX. You can't say it is 50 ohms unless the TX is 50
ohms, no?

No, no, no.

As I keep saying, the reading on the SWR meter has nothing to do with
the output impedance of the TX feeding RF into it. It is determined by
the reference resistors in the meter's directional coupler circuits and
the impedance of the load attached to its output. If the load is 50
ohms, a 50 ohm SWR meter will read 1:1, regardless of the TX output
impedance.

The purpose of the ATU is not to match the TX output impedance to the
outside world. It is to convert the impedance of the outside world to 50
ohms - which is the impedance the TX is designed to work into.

So there *can* be reflected power back into the TX by the ATU if the
impedances don't match. I guess this is not so much an issue in that
the TX is never supposed to drive any other impedance. Rather than talk
about power reflected back into the TX, the power delivered into a 50
ohm load is considered the point of reference. But if a resistive load
of less than 50 ohms is connected to the output, it would dissipate
*more* power than the 50 ohm load would. If the TX output were also 50
ohms, max power would only be delivered into a 50 ohm load.

--

Rick

On 10/6/2015 6:31 AM, Jeff wrote:

The whole function of the ATU is to provide a 1:1 match for the Tx, when
that is the case NO POWER GOES BACK TO THE TX.

That is not technically correct. A 1:1 match prevents TX power from
being reflected, it does not prevent other power from being sent to
the TX.

Depends how pedantic you are being, we are only considering the wave
that is originated by the Transmitter at one particular frequency. If
the ATU is set to provide a 1:1 match at that frequency then NO PART of
that forward wave will find its way back to the Tx.

Who is being pedantic? It is not about the reflection of the power from
the TX at that point. The issue being discussed is how much power
reflected from the antenna will find it way back through the ATU to the TX.

Just as all power from the TX will cross the 1:1 impedance match (if
there *is* a 1:1 impedance match) all the power passing through the ATU
from the antenna will find it's way into the receiver. The question at
this point is if there is *any* power that is not reflected by the ATU
*back* to the antenna to pass through the ATU.


That is not to say that
....snip without changing meaning...
a signal that originates at the antenna will
not pass to the Rx.

That is my point.


Anyway this discussion has now got to the silly stage so I am off.

Ok, enjoy.

--

Rick