Practical_Antenna_Handbook_0071639586

CHAPTER 14
Receiving Antennas
for High Frequency
Antennas are reciprocal devices. That is, just as transmitter power applied to the
feedpoint of an antenna results in a field of a certain strength at a remote point,
fields originating remotely can impinge upon that same antenna and create a
received signal voltage and power at that same feedpoint.
Under most—but not all—circumstances, the best antenna to use for transmitting is
also the best antenna to use for receiving. Sometimes, however, the use of a separate
receiving antenna (or antennas) can lead to better reception and an overall improvement
in communications reliability. This is especially true for the very low frequency
(VLF), low frequency (LF), medium frequency (MF), and lower high-frequency (HF)
ranges of the radio spectrum, for any of the following reasons:
• Dimensions and construction costs of effective and efficient transmitting structures
(including the radials beneath them) are relatively large; the vast majority
of transmitting antennas in use below 10 MHz consist of a single radiating element
fixed in one position and exhibiting a nearly omnidirectional pattern
around one axis. As a result, these transmit antennas provide very limited rejection
of interfering signals from directions other than that of the desired signal.
• Most communications receivers have more than enough sensitivity below 10
MHz. That is, atmospheric noise is almost always many decibels above the
noise floor of today’s receivers. Consequently, receive antenna efficiency is
relatively unimportant, and high-performance multielement directive receiving
array designs capable of being “steered” to multiple headings around the
compass can be built on much smaller land parcels and at substantially reduced
cost compared to transmitting arrays providing comparable received signal-tonoise
ratios.
• The ability to copy weak signals is often limited by atmospherics (lightninggenerated
noise, or QRN) or local noise sources that are stronger at the lower
frequencies and often have different arrival angles or wave polarization than
the desired signal(s). Consequently, it is often easier and more important to null
out the noise with an easily steerable receiving antenna than it is to boost the
incoming signal enough to override the noise.
• Two identical phase-locked receivers (such as are found in some of today’s
transceivers)—each connected to its own receiving antenna or one connected to
a receiving antenna and the other to the transmit antenna—provide enhanced
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