SG-2000 MANUAL
40
The SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA
©1995, SGC, Inc.
TEL: (206) 746-6310 FAX: (206) 746-6384
11.0 Antenna Systems
The function of an antenna is to couple radio energy into space and the most important
factor in considering an antenna is to realize an inescapable law of physics: big antennas
are capable of doing a better job of coupling energy into space than are small antennas
at any given frequency.
The single most important formula to know in dealing with antennas is that the size of a
1/2 wavelength dipole antenna is:
468 divided by the frequency in Megahertz.
(468/F MHz = 1/2 Wave in feet)
The second most important fact to remember is that, due to space considerations, often-
times the size of a MF or an HF antenna is a quarter wavelength.
This means for example that at 10 MHz (The frequency of the US National Bureau of
Standards time and frequency standard stations WWV and WWVH) a half wavelength
is 46.8 feet and a quarter wavelength is 23.4 feet. By the same token a broadcast station
at 1 MHz (1000 KHz) has a quarter wavelength of 234 feet while a quarter wavelength
at 100 MHz in the FM broadcast band is only 2.34 feet.
An antenna does not have to be a physical quarter wavelength in order to work
efficiently: shortened antennas can be used. But any time an antenna is less than a
physical quarter wavelength it will not have as much gain — or efficiency of coupling
energy into space — as will a larger antenna. Use of a high performance antenna
coupler, such as an SG-230 Smartuner™, Smartuner™ Pro, or SG-235, will improve
performance of all antennas dramatically.
Short antennas represent some compromise. However, in many cases they can give
good performance, especially in applications such as mobile HF work and in loops
which are used where space is limited and noise levels are high. In other words, there
are antennas which will provide performance better than a quarter wavelength antenna
for a specific application.
Antenna length is not the only consideration. Another important factor is antenna wire
size