To: <topband@contesting.com>
>Date: Mon, 17 Nov 1997 20:29:53 -0500
>From: jbmitch@vt.edu (John Mitchell)
>
>At 08:58 AM 11/17/97 +0000, w8jitom@postoffice.worldnet.att.net wrote:
>>> From: Rich Measures <measures@vc.net>
>>
>>> >question2: It is my understandiung that the net output power as I
>>> >reported above ie 1800 fwd, 300w ref (birdetc) is 1500 watts? Correct
>>> >or not??
>>>
>>> Not correct. Wattmeters are calibrated for a 50 +/- j0 ohm load. If the
>>> load is other than that, the readings are bogus.
>>>
>>> Rich---
>>
>>Your reply is incorrect Rich.
>>
>>True directional couplers, such as used in the Bird wattmeter, remain
>>calibrated very well in spite of impedance excursions.
Tom, I think you meant to say "in spite of _load_ impedance
excursions" here. The directional couplers are calibrated for a
specific value of _line_ impedance. I realize this doesn't
change your point. But I think that whether we are talking about
the load or line impedance is one source of potential confusion
here. - Eric
>>
>>The method of subtracing reflected power from forward power is
>>perfectly acceptable, as long as the user remembers the tolerance of
>>a Bird 43 meter is +- 5% of FS anywhere on the scale.
>>
>>You can find this described in many publications, as well as Bird's
>>literature.
>>
>>73, Tom W8JI
>
>Hi Tom,
>
>In that case, what happens to the 300 Watts that is measured as reflected?
>
>According to at least one book I've read (Maxwell's Reflections), that power
>is re-radiated
Not re-radiated, but _reflected_.
>down the feedline, where the same proportion of RF bounces back,
>and is again re-radiated
Again reflected...
>, until it finally either all goes out the antenna, or gets
>eaten up by feedline loss.
>
>So, the power isn't lost, but is it measured?
It isn't lost. It just isn't power.
>Does the Bird directional meter account for this re-radiation
>due to lack of a conjugate match?
>
>73, John K4IQ
>
>
John,
Read "Reflections" again, more carefully. The power reflected
from the load is not "real" power. It does not have the voltage
and current in phase. The Bird meter is measuring either current
or voltage (I can't remember which right now) but not true
(current in phase with voltage) power. It is making an inference
about power due to the assumption about circuit (matched line)
impedance. When the reflection at the source occurs, the phase
relatiuonship is restored. So all power going towards the load
is real power. Except for the fraction lost in the feedline due
to heating the feedline, all the incident power is radiated (or
at least delivered to the antenna).
It turns out that the error in the power measurement caused by
sampling only current or voltage and making an erroneous
assumption about the load impedance is neatly resolved by
subtracting the reflected _indication_ from the forward
_indication_. At least for some reasonable range of error in
load impedance.
73, Eric N7CL
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