On 3/24/2021 12:21 PM, Don Kirk wrote:
I just went back and looked at the audio on Garys recording using a real
benchtop oscilloscope instead of a software based audio scope.
Back in the early '80s, Richard Heyser, a brilliant engineer/physicist
at JPL invented an instrumentation system called Time Delay
Spectrometry. That invention, and his extensive teaching, revolutionized
pro audio. One of his teachings was to use different ways of looking at
systems and data from measurements. For example, FFT of time data
transforms it to the frequency domain, giving us a different way of
looking at a single measurement. And conversely, an inverse FFT of a
frequency sweep gives us a broadband look at the time response. The
inverse FFT of a very wide frequency sweep of a transmission line yields
a high resolution TDR.
I used Heyser's "different way of looking" principal to study the effect
of height on horizontal and vertical antennas and their relationship to
soil conductivity to produce these tutorial app notes. My work was all
done in NEC, and when presented at several ham clubs, some very sharp
engineers in the audience gave me what I consider to be "peer review"
thumbs up.
http://k9yc.com/AntennaPlanning.pdf
For this work, my "different way of looking" was to present vertical
patterns for varying conditions (soil, mounting height) on the same
graph, as opposed to the ARRL standard (and NEC default) of setting the
amplitude scale to 0 dB. When you do this, the differences from one
condition to the others stand out NOT as a launch angle, but as
differences both in field strength at various vertical angles of
interest and overall efficiency. And with multiple plots on the screen,
the cursor can be moved from one curve to the next, and when on a
"recalled" curve, the readout will show the difference between that
curve and the reference curve.
Then another way of looking at the same data was to pick off data points
from each curve at elevation angles of 5, 10, 15, 20, and 70 degrees,
write them down on paper, and plot them on a new graph where the
horizontal axis was mounting height and the vertical axis was gain in
dBi (figs 33 and 36). These plots clearly show the value in dB for
another 10 ft of mounting height.
One of the lessons of Fig 36, for example, is to disprove the long held
myth that an antenna must be very low to be useful for NVIS. I used 70
degrees for Fig 36 with the assumption that the shorter distances
covered by 80-90 degree elevation angles are covered by ground wave. Fig
36 shows that 1/8 wave height is near optimum for 70 degrees, and we
lose only about 1 dB at 3/8 wave height.
Getting back to your measurement issue -- several things affect some of
the measurement methods used by that computer-based system, one of which
is resolution bandwidth, which in turn is related to sampling frequency
and sweep rate of the measurement system. That's a possible reason that
it didn't see the 60 Hz component clearly, where your traditional scope did.
There's a principle of measurement that, because time and frequency are
the inverse of each other, our choice of measurement parameters to favor
time detail proportionally reduces frequency detail, and vice-versa. A
traditional scope is frequency blind, while a sweep is time blind.
Another technique that greatly reduces the strength of random frequency
data in swept measurements is averaging a lot of sweeps. I use the
highest setting for averaging in my P3 to bring out weak signals.
Around 1986, I used TDS to embed 64 sweeps in music program material at
a level so low you had to know it was there to hear it to measure the
dynamic response of a then-popular FM modulator that did extensive audio
signal processing to maximize loudness. http://k9yc.com/AESPaper-TDS.pdf
Software that Heyser had written allowed either scalar (amplitude only)
or complex (magnitude and phase) average of up to 64 sweeps, which
averaged out the music and display the amplitude and phase response.
Scalar averaging yields 3 dB/doubling, complex yields 6 dB/doubling.
Techron, an industrial division of Crown (the power amp company then
based in Elkhart, IN) built a dedicated TDS instrument revolutionized
pro audio. I was one of the earliest owners (it cost $12K, delaying the
down payment for my first house by five years), and what I learned from
it (and from Dick Heyser) made my professional career. Gerald Stanley,
who brought the design of Crown's first power amp along with him as a
summer intern, designed that analyzer. Up to then they only made tape
recorders.
73, Jim K9YC
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