J.B. Wood wrote:
On 06/11/2014 11:00 AM, Ralph Mowery wrote:
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.


Hello, and in order for a directional wattmeter (DWM) to give the
correct incident (forward) and reflected readings from the load, the
source (transmitter) output impedance and the design characteristic
impedance of the DWM should ideally be equal. For example, consider a
"classic" Bird model 43 designed for a 50 ohm system connected between a
75 ohm transmitter source and some arbitrary load impedance. Any
reflections returned from the load to the transmitter are re-reflected
rather than absorbed. These re-reflections add to the sampled transmit
power in the incident 50-ohm termination in the Bird DWM and also affect
the sampled load reflected value. The result is incorrect incident and
reflected indications on the DWM.

The assumption probably is that the length of the piece of 50 ohm line
in the wattmeter is short relative to the wavelength, and has little
influence on the system. That should be true below 200 MHz or so.

Once the line length is approaching a quarter wavelength (at about 900 MHz),
there will be considerable influence of the mismatch.