Comparison of Beverage antenna,magnetic loop antenna,and phased vertical receiving antennas
Low Noise Receiving Antennas and Arrays
Other related pages:
Noise
and common mode
noise.
Power
line and other noise
sources.
Note: The top of this
page has links
to various receiving
antennas such as
Beverage, “magnetic” loop, and
vertical low-noise
DX receiving antennas.
How
Low-noise
Receiving Antennas
really work
This area deals
primarily with low
noise antennas, and
discusses effect of
antenna directivity
on weak-signal
reception.
|
My local wintertime 350Hz Bandwidth noise as compared to a
The above signal levels may not be typical of every The signal level difference between noise floor and W4ZV Before talking about receiving antennas for lower |
Directivity Comparison
of
Receiving Arrays or
Antennas
The table below rates
receiving antennas in order of increasing performance. It uses directivity, with
results based on noise being
evenly distributed
in all directions.
These rankings are
most accurate in the
frequency range of
AM broadcast, 160 meter, or
80 meter bands when:
1.) The
receiving location
shows a nighttime
increase in noise
level. In other
words, the system is
not limited by local
or internally
generated noise,
instead being
limited by skywave
or propagated distant noise.
2.)
Thunderstorms or
other local noise,
such as power line
noise from specific
directions, does not
dominate the receive
system noise floor.
There will be
occasional
exceptions, but as a
general rule the
ratio of peak
response in the
direction of the
signal to average
response in all
directions determines how well
a receiving antenna works. In
virtually all
installations
without clearly
dominant direction
or directions of
noise arrival, RDF
(receiving
directivity factor)
accurately
predicts receiving
antenna
performance.
RDF (receiving directivity)
will be an almost
perfect indicator of
what you can expect
from your antenna so
long as:
- Noise is
not from the
same general
direction as the
desired signal - Noise field
strength is not
greater than the
ratio of peak
antenna response
to depth of the
pattern in the
direction of
noise - Noise is
not coming from
within the
antenna’s
nearfield or
Fresnel zone
If antennas are
within two dB of
each other in directivity (RDF), a
lesser ranked
antenna may outperform a better ranked antenna.
This is
because:
- Direction
and polarization
of arriving
signals
and noise
constantly vary,
so the
relative
relationship between any two individual
antenna’s
responses will
vary. - Through
various
unavoidable
errors or
omissions,
antennas in the
real-world may
not work
precisely as
predicted in a model.
In a
majority of cases,
the following RDF
(receive directivity) table shows
relative performance
of antennas in ascending order:
|
Antenna |
RDF |
20-degree |
Average |
|
1/2λ |
4.52 |
-20.28 |
-24.8 |
|
Vertical |
5.05 |
1.9 |
-3.15 |
|
(Ewe |
7.39 |
-36.16 |
-43.55 |
|
K9AY |
7.7 |
-26.23 |
-33.93 |
|
1/2λ |
7.94 |
-20.5 |
-28.44 |
|
1λ |
8.64 |
-14.31 |
-22.95 |
|
two |
9.14 |
-22.46 |
-31.6 |
|
two |
10.21 |
-15.45 |
-25.66 |
|
Small |
10.70 |
-15.79 |
-26.49 |
|
1-1/2 |
10.84 |
-10.88 |
-21.72 |
|
Small |
10.97 |
-30.28 |
-41.52 |
|
Single 1.75λ |
11.16 |
-6.50 |
-17.66 |
|
2 Broadside 1.75λ |
11.36 |
-3.51 |
-14.87 |
|
2 Broadside 1.75λ |
11.91 |
-3.50 |
-15.41 |
|
.625λ x .125λ |
12.5 |
-19.5 |
-32.0 |
|
2 Broadside 1.75λ |
12.98 |
-3.50 |
-16.48 |
|
2 Broadside 1.75λ |
13.48 |
-3.49 |
-16.97 |
Gain vs.
Directivity Myth
One common rumor
or myth is that
higher antenna gain
results in improved
reception. Gain is
an unreliable way to
predict receiving
ability on
frequencies below
upper UHF!
A clear
example is
illustrated in the table above. In the aqua colored areas, we can follow
comparisons between a single 1.75λ
Beverage and various spacing pairs
of 1.75λ phased
Beverages. In a case where spacing is .2λ, the
single Beverage has
a gain of -6.5dB. A pair of
Beverages spaced .2λ
has a gain
of -3.51dB. This is a gain
increase of about 3 dB.
Despite the gain
increase, antenna
directivity and
pattern do not
change a noticeable
amount. RDF (directivity) only
increases 0.2dB, an
undetectable
difference. Pattern
remains essentially
the same, so
reception remains
essentially the same. Significant new nulls, or deeper nulls, are not created at
close spacing.
Here is the same table showing only 1.75λ
Beverages:
| Antenna | RDF (dB) | Gain | Directivity Change dB |
Gain Change dB |
|
Single 1.75λ |
11.16 |
-6.50 |
0 |
0 |
|
2 Broadside 1.75λ |
11.36 |
-3.51 |
+.2 |
+2.99 |
|
2 Broadside 1.75λ |
11.91 |
-3.50 |
+.75 |
+3.00 |
|
2 Broadside 1.75λ |
12.98 |
-3.50 |
+1.82 |
+3.00 |
|
2 Broadside 1.75λ |
13.48 |
-3.49 |
+2.32 |
+3.01 |
Gain of any spaced pair is around 3dB more than a single Beverage, but
reception improves and antenna pattern changes only with relatively wide
spacings. Spacing must be
at least be 1/2λ
or more for phased
Beverages to add
a reliable
improvement in
reception quality. Wider spacing improves null depth off the sides, and narrows
front lobe beamwidth. At
3/4λ
spacing
directivity
improvement for evenly distributed noise and QRM falls
short of 3dB, although side suppression of signals improves greatly!
Of
nearly equal
importance, many end-fire
arrays actually work
better with closer
spacing. For an
example, compare the
1/8th wl four-square
RDF with the 1/4-wl
four-square array.
How well does
the above hold true?
Over the years, I
have had virtually
all of the above
systems. I always
have multiple
phase-locked
receivers on
multiple antennas
listening in stereo
or a very fast way
to “A-B”
antennas. When an
antenna sits unused
most of the time, I
replace it with a
more useful antenna.
My single Beverages
are now virtually
all eliminated, my
last phased loops
were in the 80’s
(when I had four
end-fire diamond
terminated loops).
Even on 80 meters,
my large arrays with
300-350 foot spacing
almost always beat
my single long
Beverages. I’ve
migrated towards the
bottom end of the
chart with all my
antennas because
they actually do
receive better.
If you ask
operators who visit
for contests,
everyone prefers the
large vertical or
wide-spaced Beverage
arrays. Guest
operators, given a
choice, almost never
not use single
Beverages or
close-spaced
Beverages.
You can listen to
directivity examples
on my DX
Sound files
page.
Horizontal vs.
Vertical
One popular claim
is that vertically
polarized antennas
are noisy, while
horizontally
polarized antennas
are quiet. Another
myth tied to the
claim verticals are noisy is noise
sources are
predominantly
vertically
polarized.
There is some
truth to the claim
that a horizontally
polarized antenna
can be quieter than a vertically polarized antenna, but
this requires a
special condition or bondary. The special condition occurs when the predominant system noise is local
extended groundwave
noise. For example
my dominant daytime
noise on 160 meters
comes from
Barnesville, GA and
Forsyth, GA. Both
towns are about seven
miles from my location. If I
use a tall vertical
antenna on 160 meters, the
daytime noise from these
towns and other groundwave sources is almost 20
dB stronger than
noise from my
300-ft high 160-meter dipole.
The vertical has more noise for comparable antenna gains because
the vertical
responds better to
extended groundwave
than the
horizontally
polarized dipole. The horizontally polarized dipole has virtually no groundwave or zero elevation angle response at all.
At night time,
when the band opens
and the dominant
noise
propagates via skywave, there is
absolutely no signal-to-noise advantage
in using the dipole!
This is explained
in some detail on my
NOISE
page.
DC
Grounded vs. Open
Antennas
Another myth is
that dc grounded
antennas are
quieter, filtering
noise by shunting it
to ground. This
would require the
antenna to short a
1.8 MHz noise, while
NOT shorting a 1.8
MHz signal!!! That
would be pure magic.
Loop vs. Open End Antennas
Loops are commonly rumored to have less noise than other antennas. There is a thread of truth to this, but that truth is not applicable to anything except very specific cases.
First, in inclement weather the earth and sky can develop a more concentrated voltage gradient. That voltage gradient causes corona to leak off earth objects. We call that corona P-static.
The loop is a relatively blunt-ended antenna. The “blunt” high voltage areas mute the electric field right where the antenna is most sensitive to such discharges. An open ended antenna, especially an antenna without a large hat or other end termination, has a very strong electric field response at the open end. It couples better to the areas where high impedance corona fields cluster. This is a poor weather issue.
Second, a loop typically has a very high common mode impedance. The high common mode impedance does not drive common mode feeder currents very well, nor does the loop with its high common mode impedance couple to the low impedance common mode feeder noise as well as antennas like dipoles or verticals. This is a poor feedpoint design or implementation issue, we get away with a “sloppier” installation with a loop.
Both of these loop advantages are easy to nullify.
Some of this is
explained in my
precipitation static
page, and also
touched on in the
quad antenna
and
loop antenna
pages.
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