Observations
 

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

Observations
 

JB wrote:


Now if you take 1000' of RG-174 and left it dangling over the end of a
cliff, the reflections wouldn't be able to climb back up the coax!


Frankly, the VF would be what I would be concerned about. Obviously,
the stress of the weight would distort the line from factory specs and
shoot 'em to hell ... not to mention power handling capacity would be
reduced ...

If I were you, this is what I would consider ... but hey, you will
eventually pick it up ...

Regards,
JS

Observations
 

Jeff Liebermann wrote:


Yeah, yeah. I seem to have contracted a temporary case of dyslexia.
Not only am I getting power and voltage backwards but I'm also
spending money instead of receive it, and voting Democrat instead of
Republican. Probably caused by too much exposure to RF which is
hopefully reversible.

Back to sulking... I just hate making misteaks.

I am jealous!

I can't even tell the difference between black and white! But, I do
know a senile old man with a young girl, when I see one, and a puppet
who awaits his strings to be pulled ... blank stare

Regards,
JS

Observations
 

JB wrote:

...
Just remember: What's bad for the country is good for the democrats. Maybe
...

Never heard of 'em; so, really can't say. Is there an official 3rd
party now?

All I am really familiar with is the republicrats and the democans. :-(

Gawd, I know, my ignorance is showing ... sorry all.

Regards,
JS

Observations
 

JB wrote:

...
I'm afraid these things don't always jump out at you from the General Class
license training seminar.


JB:

Please, don't take offense easily ... this is but one of my hobbies,
some others a growing old and cranky, becoming cynical, practicing my
"high school smart a$$ routine", doing my warped sense of humor,
displaying my ignorance, etc. ... I mean no real harm here ...

Warm regards,
JS

Observations
 

Cecil Moore wrote:

I suspect that all necessary reflections take place
at the impedance discontinuity and the characteristic impedance
is established immediately.

Cecil - you might like this.

What I got out of Feynman's QED was that the probability for a
reflection to take place would be highest at the discontinuity. But
that reflection would take place at any point thereafter with some
lesser probability - inverse with distance (smaller arrow, different
angle). Summing all the little probability arrows (vectors) for various
distances results in a magnitude equivalent to a reflection occurring
solely at the discontinuity. He used the reflection from a pane of
glass as an example. But as you know a change of index of refraction is
directly analogous to a change in impedance.

In relation to this discussion, there would be some distance from the
discontinuity where the probability of reflection is so low, that beyond
that point, the sum total contribution of all the reflections is
immeasurably small. I believe it may have been Reg Edwards (sk) who
spoke of the distances you refer to. The brilliant thing is that Quantum
Electrodynamics substantiates his argument.

73, ac6xg

Observations
 

"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

Observations
 

"JB" wrote in
:

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

If by "measuring power" you mean determining the average rate of flow of
energy past the sensing element, you are not correct. The power can be
calculated as the indicated 'forward' power - indicated 'reflected'
power.

This is a property of any directional wattmeter of this type when the
sensor is calibrated for an impedance that is real (ie non reactive),
irrespective of the load or the type of coax that might be attached to
either side of the instrument.

Of course, practical circuits are not ideal, and errors are introduced in
all measurements. Some ham grade instruments are even less perfect.

The reason that Bird suggest that you can get an answer for power for
VSWR 1.2 by ignoring 'reflected' power, is that 'reflected' power is
less than 1% of 'forward' power and ignoring it does not introduce
significant error compared to other system errors.

Owen

Observations
 

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

Observations
 

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