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.

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.

As an aside, back in the analog days of TV this same phenomenon could
result in ghosting on the TV screen when 50-ohm power
splitters/combiners or directional couplers were used on a 75-ohm TV
signal distribution system. Sincerely, and 73s from N4GGO,

--
J. B. Wood e-mail:

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.

Jeff wrote:
From the Bird 43 manual:

"There may be cases where it is necessary to use the Bird 43 with a
non-50 ohm
transmission line. If the reflected power is less than 10% and the
frequency is
below 200 MHz, the resulting mismatch will not be too serious. At higher
test
frequencies and/or higher reflected power levels, the load impedance will
change when the wattmeter is removed from the circuit.

Ah I did not even read that manual, but my guess from the width of the
Bird 43 and the relative wavelength was completely correct :-)

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
Please remove abc first in case of PM

On 06/12/2014 09:59 AM, Wimpie wrote:

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.

Hello, and I think it would be more accurate to state that the
definition of load VSWR ("SWR") is source impedance agnostic but it is
always referenced to some value, say 50 or 75 ohms. If the load VSWR is
being calculated from incident and reflected readings from a mismatched
directional WM (which is just a directional coupler with terminations
and an indicator(s))then errors will occur, as has been pointed out
already. Sincerely, and 73s from N4GGO,

--
J. B. Wood e-mail: