"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