El 11-06-14 17:00, Ralph Mowery escribió:
I ran across some statements in a magazine artical a while back that has me
wondering if true or not.

One example is that a directional wattmeter such as the Bird does not make
any differance as to the line impedance. That is if you have a 50 ohm line
and 50 ohm antenna or a 70 ohm line and antenna, the swr will calculate to
the same which in this case would be 1:1. Also if the line and load
impedance is differant, the swr will still calculate out to the same. that
is say you have a 50 ohm line and 100 ohm load or a 70 ohm line and 140 ohm
load the same directional wattmeter such as the Bird or a Drake w4 will
still calculate the same 2:1 swr even if they are not set up for the
differances in impedance. All that is asuming a line say 50 to 100 feet
long so the standing waves can really form.

I do know that the transmitter tuning will be differant due to the 50 or 70
ohm impedance even if the swr shows 1:1.



A directional coupler is designed for a certain reference impedance.
A 50 Ohms coupler reads zero reflected power when terminated with 50
Ohms. It doesn't matter how you generate the 50 Ohms load.

Using 75 Ohms cable gives large uncertainty if you want to measure the
VSWR inside the 75 Ohms cable.

An example:
A 75 Ohms cable terminated with 112.5 Ohms has VSWR=1.5 inside the 75
Ohms cable.

When you connect this combination (that is 75 Ohms cable plus 112.5
Ohms termination) to a 50 Ohms coupler, VSWR reading on the coupler
will vary between 0 and 2.25 (depending on length of 75 Ohms cable
between load and coupler).

So you can't use a 50 Ohm referenced coupler with scalair outputs to
measure the VSWR inside a 75 Ohms cable.

You can use the 50 Ohms coupler to measure the net power flowing
through the coupler. The net power equals Pforward - Preflected. Of
course the electrical length of the coupler should be well below 0.1
lambda, especially when there is large relative deviation between
cable impedance and the coupler's design impedance.

When the source impedance doesn't match the coupler reference
impedance, the forward power reading on the coupler can be more than
the net power supplied by the source. This is due to multiple
reflections. VSWR readings do not depend on source impedance.

--
Wim
PA3DJS
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