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.
>
>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 down the feedline, where the same proportion of RF bounces
back, and is again re-radiated, 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? Does the Bird directional
meter account for this re-radiation due to lack of a conjugate match?
73, John K4IQ
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|