Electrolytic caps are leaky (measured in microamps),
have internal resistance that may be significant in some
applications, and vary with temperature (±20% typically)
and frequency (not useful at high frequency).
After six or more months on the shelf, aluminum
electrolytic caps should be re-formed to regain their voltage
Tantalum caps are also an electrolytic but smaller than
aluminum, lower leakage, have a better temperature
coefficient, and operate at higher temperature. Cost is more
than aluminum but does not de-form (lose its voltage rating)
There are other capacitor types to consider: mica,
polypropylene, metalized polyester, polystyrene, and paper.
Each has pluses and minuses to consider.
[#8133 - August 2013]
Can someone explain how adding inductance or
capacitance to an antenna changes the length?
An antenna becomes resonant when the energy that is
racing down the wire hits the open end and is reflected
back to the sending end; the transit time is equal to the time
of one cycle of frequency. The open end has high voltage
and low current (there can’t be any current at the open
end), and the sending end has high current and low voltage.
Adding inductance at the sending end will lower the
resonant frequency, thus making the wire appear longer.
Adding capacitance to the open end will also reduce the
resonant frequency. Since radiation occurs from the wire,
adding these L and C elements will reduce the antenna
efficiency because it is shorter than it would be if the L or
C were not added.
[#8134 - August 2013]
I would like to know how you can get a positive and a
negative part of a sine wave from a circuit that runs on a nine
You ask how to get a positive and a negative part of a
sine wave from a circuit that runs on a nine volt battery. It is
not clear how you are going to use that output, so I'll
presume this is to satisfy your curiosity (or perhaps to help
on your homework). The circuit suggested is not efficient,
and would not make a good power supply.
First, a nine volt battery produces direct current, DC,
with a steady amplitude of nine volts (gradually decreasing
as it is drained). One simple circuit that produces a fair
approximation of a sine wave from DC is a phase shift
oscillator, shown below on the left side (adapted from
www.learnabout-electronics.org). The sine wave can be
observed between the Out and the 0V test points.
Second, the sine wave must be separated into positive-and negative-going signals, which is done by the half-wave
bridge rectifier. The two half-sine waves can be observed
between the + or - test points and the 0V test point. The
waveforms are shown below, (adapted from http://macao.
communications.museum). The negative half-wave should
look like the second graph flipped upside-down.
If this did not answer your question, you might provide
some more information as to how the signals would be
80 September 2013