[Amps] Re: [Amps] Re: [Amps] Re: [Amps] BirdŽ 43 Ma nual

[Tom Rauch]

> 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.

It is electrically impossible to have a single-point measurement in a 
transmission line that measures forward and reflected power 
without sampling both voltage and current.

I am amazed that some claim current can flow two directions at the 
same instant of time in a conductor, or that a zero-crossing 
symmetrical alternating current has directivity!

> 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.

The original Bruene description will explain it.
73, Tom W8JI
W8JI@contesting.com