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transmit antenna, we want maximum possible field strength in
a given direction (or directions) at the most useful elevation
(wave) angles. We cannot tolerate unnecessary power loss in
a transmit antenna, because any amount of transmitting loss
decreases signal-to-noise ratio at the distant receiver. Antenna
efficiency is an important issue for transmitting. It is obvious
that for a given elevation angle and direction the highest gain
antenna will deliver the strongest signal to the target area. We
really do not care if we are being heard in other directions
(areas) or not, we are only interested in the target direction.
Choosing a transmit antenna is a matter of properly
positioned gain. Transmitting antennas require high directiv­
ity to achieve high gain, not directivity just for the sake of
eliminating transmitting signal in unwanted directions. Tom,
W8JI, at www.w8ji.com/ adds to that: “Takeoff angle is not
important, what we actually need is maximum possible gain
at the desired angle and direction. After all, we don’t care
where the peak is as long as the antenna we pick has more
signal (gain) at the desired spot than other antenna choices!”
A receiving antenna on the other hand has a different
design priority. The goal is obtaining a signal that can be read
comfortably, which means having the minimum possible
amount of QRM and noise. The important issue when receiv­
ing is signal-to-noise ratio (S/N). The receiving antenna
providing the best performance can and will be different under
different circumstances, even at the same or similar locations.
There is no such thing as a universal “best low-band receiving
antenna.”
Why doesn’t the reciprocity law apply to signal-to-noise
ratio as it applies to signal level? It’s easiest to explain this
with an example: Consider a high-band Yagi with 7-dBd gain,
including ground-reflection gain. This antenna will improve
the transmitted signal by 7 dB over a dipole, provided both
have peak gain oriented to the target area. Does a 3-element
Yagi with the same efficiency as a dipole improve reception S/
N by the same amount as it improves transmission?
The answer is simple: Probably not! S/N will improve
much more than the 7-dBd gain when very strong noise
sources are located in a pattern null. If the null is –25 dBd,
S/N can increase as much as 32 dB (+7 dBd signal to –25 dBd
noise). Of course, the improvement will normally be less than
32 dB, since that is extreme.
If the noise comes from exactly the same direction as the
desired signal, the Yagi’s 7-dBd gain will not improve S/N at
all. The Yagi will deliver equally increased signal and noise
power, both being 7 dB stronger than the dipole.
The Yagi also might have decreased efficiency. This is
actually very common, because of losses caused by increased
element current. In reality, a 7-dBd Yagi often has more than
7-dBd directivity. If Yagi efficiency were only 50%, 7-dBd
gain would require having 10-dB directivity increase. These
are all reasons why gain does not determine receiving S/N
improvement, and why the higher-gain antenna very often
does not provide the best reception.
There is one predictable effect of gain. Signal levels will
be increased by the amount of gain, both in transmitting and
receiving. (Keep in mind that signal level is not the same as
signal-to-noise ratio.) Continuing the example above and
assuming perfect lobe alignment with the path, the Yagi’s
signal level will be 7 dB above the dipole in the target area.
The distant receiver will always have 7-dB more S/N when the
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Chapter 7.pmd

Yagi is used. This is true regardless of any S/N improvement
we might or might not observe when receiving with the same
Yagi. What counts for improving communications is the ratio
of signal-to-noise on both receiving ends of the circuit. In
practice this means there is no reciprocity “in readability”—
reciprocity only applies to signal level. This is not “one-way”
propagation, although it sometimes may cause people to think
this is happening.

1.2. Gain Versus Directivity
Gain is a function of efficiency and directivity. High gain
means an antenna has high directivity and reasonable efficiency.
The increased field strength comes with a price. The extra energy
found in the main lobe is energy that was removed from other
directions (also see Chapter 5, Section 2.1.).
The answer to improved receiving can be the same as
transmitting. Installing a highly directive transmit antenna
results in high-performance receiving, so long as the antenna
is not aligned with or installed near noise sources. Unfortu­
nately, the physical size and height of efficient antennas—
especially on 160 and 80 meters—often makes high-gain
transmitting antennas prohibitively expensive.
Fortunately, high or even modest efficiency is not a
direct requirement for directivity and receiving. This chapter
will show it is possible to build relatively small receiving
antennas that exhibit excellent directivity and greatly improve
receiving, even though the antennas are useless for transmit­
ting because of high losses and low gain.
Directivity is not the same as gain. It is possible to
construct very directive antennas that actually have negative
gain but that provide phenomenal receiving improvements. It
is worth repeating: We need directivity—not gain—for a good
receiving system.
The next question is: How much negative gain can we
live with? The answer is fairly simple once an antenna is
installed. If you can easily detect a background noise increase
when a dummy load is removed and the antenna connected
under the quietest operating conditions (usually winter day­
time within a few hours of sunrise or sunset) with the narrow­
est IF filter selected, gain is OK! As Tom, W8JI, puts it with
regard to preamps and matching devices in particular: “Once
you clearly hear external noise, amplifiers or impedance
matching won’t help. Just be sure you can hear noise at the
quietest time you expect to operate.”
We learned in Chapter 3 (Section 1.2) that our present­
day receivers have a large sensitivity margin when used with
reasonably efficient antennas, especially considering the large
amount of noise on the low bands (unless you live on a desert
island or in the wilderness). Many receivers are sensitive
enough to use with antennas having –10 to –20 dBi gain,
depending on various factors. (See Section 1.2 in Chapter 3.)
For the rest, we can always use a preamplifier to boost the
signal to a more comfortable level.
In very quiet locations, with 250-Hz selectivity, a mini­
mum discernable signal sensitivity of −140 to −145 dBm
might be required while using narrow-pattern, low-efficiency
receiving antennas. In suburban locations, −125 to −135 dBm
sensitivity is often adequate. (See also Chapter 3.)
Very directional antennas and narrow selectivity reduce
noise power, requiring less receiving system sensitivity to
yield a satisfactory output S/N. Since noise power is propor­

Chapter 7

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2/18/2005, 9:24 AM