Ian, G3SEK wrote:
"Therefore the Bird does not directly sense RF power."

Problem with that statement is that to the uninitiated it makes the
indication suspect. A voltmeter doesn`t measure volts directly. The
ordinary voltmeter measures current scaled by a multiplier resistor.
Wish you and Walt would knock off the nonsense.

A Bird senses all it has to sense and legitimately converts it to a
reasonable indication of watts. The Bird can do so thanks to Ohm`s law,
and that`s that.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
A Bird senses all it has to sense and legitimately converts it to a
reasonable indication of watts. The Bird can do so thanks to Ohm`s law,
and that`s that.

Let's approach it a little differently. Assume a signal generator equipped
with a circulator load driving a lossless 50 ohm feedline. The load is
variable.

100W SGCL----50 ohm lossless coax--------variable load

Are there any locations up and down the line and/or any value of
load that will cause the Bird wattmeter not to read 100 watts
forward power? In the above example, I can show that the two
voltages that get added inside the Bird always yield a constant
value of voltage, i.e. and therefore a constant power indication.
--
73, Cecil, W5DXP

Cecil, W5DXP wrote:
"Are there any locations up and down the line and/or any value of load
that will cause the Bird wattmeter not to read 100 watts forward power?"

Cecil has eliminated a re-reflection with his circulator at the
generator.

Therefore, the Bird, being insensitive to power traveling in the
opposite direction to that sensed, should read 100 watts forward power
at any location in the line.

There must be enough transmission line to enforce Zo. The width of the
Bird case is likely insufficient line length.

Another caveat is that the Bird elements were designed to balance using
a 50-ohm load on the line, but were intended and have proved workable on
mismatched lines.

Another frequent question is, "How about a large reactance termination?"
Not to worry, again, given sufficient line. Terman and other authorities
report that this reactance only displaces SWR patterns on a line. That
is logical considering lengths of transmission lines as reactances
themselves. So as Cecil would likely be quick to point out, adding a
reactance is tantamount to adding a length of transmission line, and
vice versa.

SWR should have little effect on power determination with the Bird
wattmeter which is performed as : Forward power minus reflected power,
period! With the circulator, disposition of all power is clear. The
forward power must contain all load and reflected power which only makes
one round-trip.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Ian, G3SEK wrote:
"Therefore the Bird does not directly sense RF power."

Problem with that statement is that to the uninitiated it makes the
indication suspect. A voltmeter doesn`t measure volts directly. The
ordinary voltmeter measures current scaled by a multiplier resistor.
Wish you and Walt would knock off the nonsense.

I wish you would credit "the uninitiated" (as you disdainfully call
them) with the intelligence to understand things when they're explained
to them.

A Bird senses all it has to sense and legitimately converts it to a
reasonable indication of watts. The Bird can do so thanks to Ohm`s law,
and that`s that.

No, that's just a half-truth. "The uninitiated" deserve better.

--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek