"Cecil Moore" wrote in message
...
JB wrote:
The accuracy of a Bird for measuring power goes out the window for an
SWR
other than 1:1. Are you saying that if you put an unterminated 3 inch
piece
of coax on the Bird you have a 50 ohm system? That sounds like
semantics
rather than Math. In fact you would have a 50 ohm system if you left
the
antenna port disconnected, Then you could say you had an unterminated
50
ohm system. Real handy huh?

The Bird readings are consistent. In such a case it will read
equal forward power and reflected power (within its accuracy).

This same subject was thrashed out on one of these discussion
groups a few years ago. (I have had a hard disk crash since then
and lost all my notes.) Back then I thought like you are thinking
now. A person a lot smarter than I came up with the equations to
back the proof that a 50 ohm environment is established in only
a few inches of ideal 50 ohm coax. If I remember right, the
necessary length to establish the Z0 of coax was less than ten
radii. It really surprised me.

He proved that a Bird directional wattmeter didn't need any
surrounding coax to establish the 50 ohm environment - that
the internal Thruline establishes a 50 ohm environment all
by itself. Perhaps someone smarter than I who is reading
this exchange can do the math for you.

When light is emerging from glass into free space, how long
does it take space to establish the 377 ohm characteristic
impedance? I suspect that all necessary reflections take place
at the impedance discontinuity and the characteristic impedance
is established immediately.

That is why we can calculate the reflection coefficient at
the junction of two transmission lines with differing Z0s
as (Z02-Z01)/(Z02+Z01).
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Yer yankin my crank.

First off, lets get the Bird manual so that my practical generalizions are
set aside, and we can be more specific.
http://www.bird-electronic.com/produ...pmanual/43.pdf

Originally we were talking about SWR shutdown circuits AND wattmeters
although there may be a bit of difference how that is derived, the
measurement is usually about a sensing inductance with canceling capacitance
in the reverse mode = 0 volts with a 50 ohm termination.

The Bird 43 is a 50 ohm line section. (Page 5) the sensing isolation of the
forward and reflected wave are given as "better than 25 db" (page6,7). It
tells how the slug is calibrated in the reverse mode. Same as above.

It goes on about Load Power and states: "For loads with a VSWR of 1.2 or
less, the power dissipated in a load
(Wl) is equivalent (with less than one percent error) to the forward
power (Wf). When appreciable power is reflected, as with an antenna,
it is necessary to use the exact load power which is given by:
Wl = Watts into Load = Wf - Wr" -- We know there is something going on
here.

Now on to page 8 and here is where it makes plain that the meter and the
shutdown circuits really don't measure SWR but forward and reflected wave
and I don't really think there is a whole lot of math going on in the
protection circuits of the radios.

The Bird can be used in other than 50 ohm systems (page 16,17) but now you
have to figure out the actual power dissipated in the load, and the power
dissipated in the coax. Examples are given.

If you use the (Z02-Z01)/(Z02+Z01) for the reflection coefficient, Fine!
But the line must be properly terminated resistively. Great for discussion
but real world gets complicated fast. As soon as the load gets reactive, the
phase between the voltage and current are different and the length of the
coax and it's velocity factor now contribute unless it is really short
compared with the wavelength.

What we are really talking about here is the short piece of RG-174 (or
whatever) from the radios' sensing circuit to the antenna connector where
there was a dummy load to set the reflected nulls and output leveling,
reflected indication and/or trip point. Now we remove the dummy load and
there is a length of coax and an antenna there.

Now tell me the truth, if the SWR is above 1.2:1, does that mean the radio
is making more power? No. Because I can read 100 watts out of a radio into
a tuner and 195 watts forward and 175 watts reflected out of the tuner on
80m. I KNOW my MFJ tuner isn't a passive amplifier. Go back to Pp 6 and 7.
It actually reads scaler because it is reading inductive (magnetic) and
capacitive (electric) so the output "power" is calibrated for known forward
voltage scaler after being calibrated for 0 reflected voltage scaler at 1:1
SWR. Beyond the 1.2:1 case, the power reading is NOT CALIBRATED or
accurate, but we can make inferences and guesses, and the ratio is useful.
There are also inaccuracies due to the 25 db directivity isolation which
adds some forward to the reflected and vice versa.

For more on that see:
http://www.bird-technologies.com/tec...irectivity.pdf

JB wrote:
If you use the (Z02-Z01)/(Z02+Z01) for the reflection coefficient, Fine!
But the line must be properly terminated resistively. Great for discussion
but real world gets complicated fast. As soon as the load gets reactive, the
phase between the voltage and current are different and the length of the
coax and it's velocity factor now contribute unless it is really short
compared with the wavelength.

You need to wade through the math. No matter what the phase,
the Bird directional wattmeter reads the forward or reflected
power referenced to its 50 ohm Thruline plus or minus 5%. If
that were not true, the Bird directional wattmeter would not
meet its published specifications. The phase is taken care of
when the sampled voltage and sampled current are added for
forward power or subtracted for reflected power.

Now tell me the truth, if the SWR is above 1.2:1, does that mean the radio
is making more power?

I certainly hope that is a rhetorical question. In a matched
system, the reflected power is part of the forward power. In
a matched system, the reflected power is redistributed back
toward the load and becomes a percentage of the forward power.

Source Power = Forward Power - Reflected Power = Load Power

If the SWR is not 1:1, the forward power must necessarily be
greater than the source power. Maybe the following web pages
will help:

http://www.mellesgriot.com/products/optics/oc_2_1.htm

"Clearly, if the wavelength of the incident light and the thickness of
the film are such that a phase difference exists between reflections of
p, then reflected wavefronts interfere destructively, and overall
reflected intensity is a minimum. If the two reflections are of equal
amplitude, then this amplitude (and hence intensity) minimum will be
zero." (Referring to 1/4 wavelength thin films.)

"In the absence of absorption or scatter, the principle of conservation
of energy indicates all 'lost' reflected intensity will appear as
enhanced intensity in the transmitted beam. The sum of the reflected and
transmitted beam intensities is always equal to the incident intensity.
This important fact has been confirmed experimentally."

http://micro.magnet.fsu.edu/primer/j...ons/index.html

"... when two waves of equal amplitude and wavelength that are
180-degrees ... out of phase with each other meet, they are not actually
annihilated, ... All of the photon energy present in these waves must
somehow be recovered or redistributed in a new direction, according to
the law of energy conservation ... Instead, upon meeting, the photons
are redistributed to regions that permit constructive interference, so
the effect should be considered as a redistribution of light waves and
photon energy rather than the spontaneous construction or destruction of
light."
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

J.B. wrote:
"As soon as the load gets reactive, the phase between the voltage and
current are different and the length of the coax and its velocity factor
now contribute unless it is really short compared with wavelength."

The Bird wattmeter won`t affect tuning and loading of an antenna if it
is made part of a half-wavelength of line inserted for the measurement.
This may be important at VHF and higher frequencies. The inserted line
and its meter need to be specially tailored to make up a half
wavelength, if it is to be inserted in nonflat lines. For several
different frequencies, several line measurement assemblies are required.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
J.B. wrote:
"As soon as the load gets reactive, the phase between the voltage and
current are different and the length of the coax and its velocity factor
now contribute unless it is really short compared with wavelength."

The Bird wattmeter won`t affect tuning and loading of an antenna if it
is made part of a half-wavelength of line inserted for the measurement.
This may be important at VHF and higher frequencies. The inserted line
and its meter need to be specially tailored to make up a half
wavelength, if it is to be inserted in nonflat lines. For several
different frequencies, several line measurement assemblies are required.

Best regards, Richard Harrison, KB5WZI

Hence, impractical for multiband use, where the sampling line might just as
well be left in place and considered part of the whole line length even for
those times when you have to insert a 5K watt slug (because your 500 watt
amp pegs the 1000 watt slug) just so you can fully load for maximum forward
voltage with your Pi-Net.

Certainly useful for your 2m antenna being fed with surplus CATV hardline so
you can adjust the line length for a multiple and find a good SWR. But are
there still reflections? Yes.

JB wrote:
Now tell me the truth, if the SWR is above 1.2:1, does that mean the radio
is making more power?

A simple example should alleviate your confusion. Assume a
one second long lossless transmission line. Source power is
100 watts. Forward power is 200 watts. Reflected power is
100 watts. Source power is 100 watts.

How many joules are in the one second long transmission
during steady-state? Hint: it is *NOT* 100 watts.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

**Pop!**

Thanks- It's really about minima and maxima being more intense in the
complex vector addition of the reflections.

I see a lot of the same math for light. Polarization was interesting.