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DA25 • 8
What About Impedance Matching and VSWR ?
Another consideration with electromagnetic wave antennas is the “match”
presented to either the receiver or transmitter. In our discussion of wavelength
and resonant frequencies it became apparent that the length of the antenna is
critical to match that of the desired frequency. A small error in length can
detune an antenna significantly and inhibit the antennas performance.
For many communications systems, 50 or 75 Ohms are the desired “magical”
impedance values desired for the antenna systems. The proper impedance
allows for maximum power to transfer to or from the antenna system with a
minimum of loss. Even with the high frequencies being used we want the
antenna to appear as a proper load. In this way the antenna presents a good
match to the receiver. Luckily for us, the discone antenna exhibits exceptional
performance in the impedance matching department.
The Voltage Standing Wave Ratio (VSWR) of an antenna system is another
measure of this impedance match. At RF frequencies, if the load at the end of
the transmission line is not the desired impedance, the signal will actually
reflect back down the line and precipitate a high VSWR. Typical usable VSWR
ratios are in the “3.0 : 1.0” range for commercial available communications
equipment, while the robust discone design actually outperforms these at
many frequencies with a typical ratio of “1.5 : 1.0” or better. A “1.0 : 1.0” ratio
indicates the best match possible resulting in no wasted signal reflection.
DISCONE DESCRIPTION
Getting back to the kit at hand, let us apply some of the theory we just
discussed. Notice that the discone antenna is predominately two sections, the
upper “disk” and lower “cone” section.
To allow for the large frequency range of the antenna, notice how the lower
cone section of the antenna slopes away from the top disk section. This
design allows for a smooth transition between the multiple wavelengths that
we hope to receive without any discontinuities in between. This is an ideal
configuration for an omni-directional antenna response pattern.
The coaxial cable mounts directly to the provided circuit board plate, which in
turn will be connected to both the conic section as well as the top disk of the
antenna. The cable has been supplied with a crimped BNC type connector at
one end for ease of connection to your receiver.
The discone dimensions have been calculated such that the usable
performance range is between 450 MHz to 2500 MHz with a typical VSWR of
“2.0 : 1.0” or less.