The characteristics of the cap you need to tune an inverted L to
resonance on 160 has a lot to do with the shape of the antenna. In some
cases the cap may be very large value, have a fairly low voltage across
it and high current thru it. In this case the equivalent series
resistance may be important.
In other cases the required cap may be a small value, have a large
voltage impressed across it, and low current thru it. In this case
avoiding voltage breakdown may be the most difficult parameter to meet.
In general if you use only a series cap for matching a long inverted L,
the shorter the vertical section of the L, the smaller the cap that will
be needed. That is because with a low horizontal wire, the feedpoint
impedance will be very low, so you have to make the antenna longer to
raise the real part of the impedance to 50 ohms. The longer you make
the antenna, the lower the resonant frequency goes, and you need a
smaller cap to bring it back in band.
If you live in an area where there is a lot of lightning I would not
recommend fixed value capacitors with dielectrics like mica,
polypropylene, etc, unless you provide some protection against
overvoltage. At my old location in Florida these would last about one
to two months before lightning would take them out (without
protection). I like to use a cap that can recover from an arc-over if
it happens (plus I usually add a spark gap around them). A relay to
short them out when not in use also helps a lot.
Jerry, K4SAV
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