Don, all you say is correct and good. BUT (seems there always is a but), as
a practical matter, the input R+j of an amplifier varies with drive level
and usual test equipment will not provide useful data. If you drive it with
a transmitter there is no way to extract R+j data at that power level.
Without good R+j data the smith chart is unusable for this problem.
73,
Gerald K5GW
In a message dated 6/11/2017 1:13:19 P.M. Central Daylight Time,
donroden@hiwaay.net writes:
"True SWR" doesn't change, but *WHERE* it is measured will vary widely.
exception : perfect Z load for perfect Z line where Zload=Zcoax
You are measuring VOLTAGE inside the coaxial line and the voltage
varies along the length of the line like a sine wave.
Some distances from the wrong Z load = peak voltage
Some distances from the wrong Z load = minimum voltage
Some distances make for a close-to-perfect input match.
1/4 wave away from the above perfect match is a point that
your transmitter probably won't like.
In other words, changing the coaxial length *COULD* improve
your situation, but it could also make it worse.
The Smith Chart is your friend, or just make five jumpers of
1,2,4,8,16 feet with a handful of couplers and start substituting
jumpers between the meter and the load. One combination will *MATCH*
or make a length that the transmitter likes. Others will make it
worse.
This is why it's important to measure the Z *AT* the load and not
through an arbitrary length of coax.
Don W4DNR
>> ## On a related note, why is it, when I increase the length of 50
>> ohm coax from my drake amps to the xcvr, that the
>> input swr rises ?? Its flat with 4 ft or less.... but rises
>> when say 5-8 ft is used. Thats on all bands.
>
> <REPLY My understanding is that's a false reading caused by current
flowing
> on the outside of the coax braid. The true SWR does not change.
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