To: <amps@contesting.com>
> Date: Tue, 19 May 1998 23:54:07 +0100
> From: w7iuv@axtek.com (Larry Molitor)
> Subject: Re: [AMPS] Re: Parasitics
> To: amps@contesting.com
> Good suggestion and one that I sometimes use variations of in my lab at
> work. With the virtually unlimited resources at my disposal on the job, I
> could easily make repeatable measurements and judge the contribution each
> component has by simple substitution. Unfortunately, I do not have access
> to that equipment at home. I don't have a swept source or even a leveled
> source or good detection device. Like most of the HAMS out there, I am
> trying to do this stuff under barbaric conditions. So, as good as your
> suggestion might be, it's unuseable to me.
But you can easily build the necessary barbaric equipment. The source
does not need to be swept or accurately leveled if you plot points
and look for coupling maximums.
For example, I can use a MFJ-259 Antenna analyzer (or the new
MFJ Antenna Analyzer that measures R and X, return loss, reflection
angle, and other parameters) or any other signal source to excite the
cathode (or anode) and detect voltages with a zero bias schottkey
detector or any low impedance detector with a broad band high Z
preamplifer. A J-310 source follower mounted on a small PC board (as
a probe) driving a conventional broadband pre-amp makes a nice probe.
IMO, rather than spoon feeding personal opinions this reflector would
be a better place if we gave useful information.... like how to
measure something with almost nothing. I've had to live that way most
of my early years.
A homebrew guy is looking for peaks in power transmission through the
tube, not exact values of attenuation. Old vector RF millivoltmeters
are available for a few hundred bux, I saw about a dozen at Dayton.
I've measured equipment for hams for free, and I'm sure others would
do the same. But I believe building a few probes is no problem at
all for many homebrewers, and a 250 buck HP RF millivoltmeter is a
VERY useful piece of equipment that can be used to measure almost
anything necessary when used in conjunction with a signal source!
> >Resonance, as measured with a GDO, tells you nothing at all except
> >where something external to the GDO couples to the GDO. You have no
> >idea whether impedance is high, low or what.....or even if the
> >resonance is what the grid actually "sees".
>
> True, but it might just be all you have to work with.
That's fine, as long as we don't re-write engineering texts by
using a GDO.
73, Tom W8JI
w8ji.tom@MCIONE.com
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