On Nov 30, 9:23*pm, Richard Clark wrote:
On Tue, 30 Nov 2010 13:50:35 -0800 (PST), JIMMIE
wrote:
Owen, the meter is just a 1 to 100 linear scale meter on on 100 uA
movement. Ive used it for years just to "guesstamate" VSWR. Not really
as inaccurately as one may think. My thought was to cal a scale for
the "full scale set" knob so that when the *meter is adjusted for full
scale the knob position will indicate power. Part of the reason for
doing it this way is that it is a nice large good quality meter and I
dont want to risk damaging it by taking it apart. I may have other
uses for it later on. Im thinking "linear scale capacitance meter
similar to the heathkit model. I intend to use an NE555 osc instead
of *vacuum tube osc should I do this .
Jimmie
Hi Jimmie,
On reading this, several thoughts came to mind. *You write about
having used a linear scale to guesstimate SWR. *Sounds good and it
immediately leads us to an existential question:
* * * * "What value is there in knowing the value of SWR?"
The first motivation following a glance at the SWR meter is to LOWER
the SWR, for whatever value it may reveal. *In this sense, the value
is a trivial consideration - relative indications are enough to
achieve the goal.
Using the feed from a Bruene style detector pair into a Log-Amp will
give you a power response in a linear scale. *If you want to know the
SWR with accuracy, you can throw the reverse switch for the second
power reading and compute, or you can throw the reverse switch and
start twisting nobs to make the power indication go away.
consider:http://rfdesign.com/mag/503rfd33.pdf
Says it all in one page - with pictures, schematics, and math too.
I don't quite catch the drift of the "capacitance meter" paired up
with NE555 osc. *I presume you mean to construct a DC-Freq converter?
73's
Richard Clark, KB7QHC
Heathkit made a linear scale capacitance meter years ago dont remember
the model #. CM? I think. Circuit use a tube in a multivibrator
configuration the NE555 is just a more modern approach to th same
circuit. Cap to be tested is in series with the osc and meter. Current
through the circuit is a linear function of capacitance or for that
matter frequency. I never got around and may never still build the
circuit because I can always bread board a little circuit to use with
my function generator when I want to measure a capacitor. I cal the
circuit using some precision caps before use. I agree with your view
of usefulness of knowing SWR. I use it mostly as a relative health
indicator of my antenna system. . A high SWR on my 40M dipole may
indicate something is not well and I should look out the window while
an unrealistically low reading on 2M probably means I have water in my
coax.
BTW the reason I bought the little home brew SWR meter was to source
the meter and box for this project. My lack of real need for it has
kept the little hamfest treasure an SWR bridge.
Jimmie