Tom Rauch wrote:
>> And think about it. You get the same reflected power for the
>> same
>> incident power if the load resistor is 100 ohms or 25 ohms.
>> Either produces a 2:1 SWR but a 100 ohm resistor would produce 1/4 the
>> current as a 25 ohm resistor. OHM's law still applies.
>> The meter does not precisely measure power for all load
>> conditions
>> but it is real close when the load resistance is close to 50 ohms.
>
>Except for that very last sentence Bill.
>
>The meter works VERY well for measuring power as long as you subtract
>the reflected power from the forward power, as my measurement examples
>have clearly shown. Theory says it should, and it actually does.
I'm not disputing Tom's measurement data, but the theory still is not
convincing.
Reviewing where we've got to so far: hopefully, we all now agree that
the capacitive coupling produces an RF voltage in the pickup loop that
is proportional to the voltage on the main line, and the inductive
coupling produces an RF voltage proportional to the current in the main
line. The inductive signal reverses phase when the slug is physically
reversed, but the capacitive signal does not.
Many other types of directional coupler such as the Bruene bridge work
on the same principle - the only difference is in the methods used to
get the capacitive and inductive signals, and to combine them with a
choice of phases.
In the Bird, these two separate RF signals add or subtract at the output
of the pickup loop, and the resultant voltage is detected by the diode.
The instrument is designed so that when it is terminated in exactly 50
ohms, and the slug is rotated to the 'reverse' position, the two
voltages are exactly equal and opposite, so the meter displays zero
reflected. That's what gives the instrument its directional properties.
So far, so good, but...
When the slug is turned round to the 'forward' position, the two RF
signals add in phase. We agree that one signal is proportional to
voltage, and the other is proportional to current - but the meter
displays the rectified SUM of these two signals. To give a resultant
that is truly proportional to power under all circumstances, they would
need to be multiplied - which they ain't.
That's why I still don't understand how the Bird can indicate a true
difference between indicated forward and reflected power with all kinds
of terminations. As I said at the start, I don't dispute Tom's
experimental data, but I still don't think we have a solid theoretical
backup for it.
Another point: whatever kind of theory is proposed, it has to work
equally well for two separate cases:
Case 1. Directional coupler terminated by a transmission line and
mismatched load; theory based on forward and reflected waves.
Case 2. Directional coupler terminated by a lumped-component load;
theory based on the relationship between voltage and current at a single
point - NO transmission-line concepts to be used in this theory!
So we actually need TWO separate theories which logically MUST confirm
each other.
Up to now we have mostly been thinking about Case 1, and getting
seriously bogged down in transmission-line theory. IMO Case 2 is much
easixxxx less difficult to think about, and should be a better way to
get into the problem.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
New e-mail: g3sek@ifwtech.co.uk
New website: http://www.ifwtech.co.uk/g3sek
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