Hello,

Am a real novice when it comes to antenna theory, but am trying to learn.
if anyone could explain the following for me, even though I admit it's
really awfully basic, would be most appreciative.

Am interested in receiving only, around the Marine VHF bands of 156 MHz, or
so.

I understand that "Gain" is achieved in many antenna configurations by
making them very directional.
My understanding is that you in effect re-shape the lobes to be prominent
along one axis, and minimal at right angles.

So, how is Gain achieved in a
vertical whip (the specs usually mention 3 or even 6 db), or the
rubber-duckie, types of antennas used so often on handheld scanners (or
mounted to recreational boats) ? They are, I believe, truly omni
directional.

Also, are there vertical whip antennas that are directional, with "gain"
perhaps ?

Thanks,
Bob

On Wed, 8 Aug 2007 12:07:27 -0400, "Robert11"
wrote:

So, how is Gain achieved in a
vertical whip (the specs usually mention 3 or even 6 db),

Hi Bob,

This is usually achieved through co-linear elements. That is, one
vertical element stacked upon (above) the other, inline. Many such
"gain" antennas' details of construction are obscured by a fiberglass
shell.

or the
rubber-duckie, types of antennas used so often on handheld scanners (or
mounted to recreational boats) ? They are, I believe, truly omni
directional.

Quite so. Unfortunately there is more to "gain" (or effective
sensitivity) than co-linear elements at the frequencies you are
interested in. More important is height which can make a substantial
difference in perceived "gain." If you invest any of your interest in
raising an antenna, it would reward you to also hoist a co-linear
design instead of a rubber duckie.

Also, are there vertical whip antennas that are directional, with "gain"
perhaps ?

As stated, the co-linears. They are common.

Now, as to the need for gain. The marine band is principally limited
to line-of-sight transmissions - hence the advice for height. The
higher you are, the further you can see. The earth's horizon can be
expressed as being the square root (twice the height in feet) miles
away. So, if you are using a rubber duckie, that horizon would be
roughly 3.5 miles away (barring obstructions). If you hoisted it 30
feet in the air, the horizon would be about 8 miles away, and less
likely to be obstructed (except at the far end). If your 30 foot
antenna is listening to a ship's antenna at 30 feet, your greatest
range would be roughly 16 miles. An airplane at 30,000 feet would
stay within your range out to 245 miles.

Without obstructions, you could probably hear them (airplanes or
ships) quite clearly even if they transmitted only 100mW of power.
Gain would be unnecessary.

However, we can anticipate variables to this such as those neglected
obstructions, and what would be called propagation. The signal
becomes weaker by degrees, or by huge plunges. Some of this can be
made up for by more power by the transmitter, or more gain in
either/both antennas.

73's
Richard Clark, KB7QHC

In article ,
Robert11 wrote:

Am interested in receiving only, around the Marine VHF bands of 156 MHz, or
so.

I understand that "Gain" is achieved in many antenna configurations by
making them very directional.

My understanding is that you in effect re-shape the lobes to be prominent
along one axis, and minimal at right angles.

That's correct. However, it's important to realize that the antenna
pattern is three-dimensional, not two-dimensional.

Antenna (directional) gain is achieved by (in effect) shaping or
compressing the lobes on both the vertical and horizontal axes.

So, how is Gain achieved in a
vertical whip (the specs usually mention 3 or even 6 db), or the
rubber-duckie, types of antennas used so often on handheld scanners (or
mounted to recreational boats) ? They are, I believe, truly omni
directional.

They are omnidirectional in the plane which is perpendicular to the
length of the antenna. That is to say, they have the same gain or
sensitivity "on the horizon", no matter which compass direction you're
looking in.

They are *not* omnidirectional in elevation.

The gain pattern of a vertical whip looks something like a donut. The
pattern shows the greatest sensitivity in the direction of the
horizon. At angles above or below the horizon, the sensitivity
becomes less and less. A theoretically-perfect vertical antenna, in
free space, has an extremely deep "null" in the direction along its
axis... it's very insensitive to signals arriving from overhead or
below.

The way that a vertical antenna achives gain, is to compress the
"donut" pattern vertically. Compared to a standard reference dipole,
such a "gain vertical" is more sensitive in the direction of the
horizon, and for some number of degrees above and below the horizon.
Once you reach a certain elevation, the gain antenna's sensitivity drops
down to the point at which it's equal to a reference dipole... and at
higher elevation angles, it's _less_ sensitive than a reference dipole.

The word "omnidirectional" can be a bit confusing, as to a beginner
this can convey the idea that the antenna radiates equal power in all
directions of the sphere (and has equal receive sensitivity in all
directions). That isn't the actual meaning of the term, when
discussing antennas.

An antenna which does have equal sensitivy in all directions of the
sphere is an "isotropic" antenna. The sensitivity / gain of an
isotropic antenna is often used as a reference point for measuring or
describing other antennas... if you see antenna sensitivity referred
to in "dBi", you know that this is the type of reference being given.

Isotropic antennas don't (and can't) actually exist in practice...
the EM field equations don't work out. It's just a convenient
theoretical reference.

The other antenna gain figure you'll see is "gain, as compared to a
reference half-wave dipole". This is denoted by "dBd".

For any given antenna, its gain dBi will be 2.15 dB higher than its
gain in dBd (in other words, a reference half-wave dipole has a gain
of 2.15 dBi or 0 dBd).

When you look at antenna descriptions or ads, make sure you know which
reference is being used. Antenna manufacturers sometimes like to
simply say "dB" without saying which reference... and in this case
it's likely to be dBi, because it makes the gain number larger and
makes the ad read better :-)

Also, are there vertical whip antennas that are directional, with "gain"
perhaps ?

A single vertical antenna, with no other active or parasitic element,
is going to have the same pattern in every direction towards the
horizon. It's rotationally symmetrical, and therefore its sensitivity
pattern is also symmetrical around the vertical axis.

If you want greater sensitivity in one direction, you either need a
second element, or you need to bend the antenna somehow so that it is
no longer symmetrical around the vertical axis.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Aug 8, 9:07 am, "Robert11" wrote:
Hello,

Am a real novice when it comes to antenna theory, but am trying to learn.
if anyone could explain the following for me, even though I admit it's
really awfully basic, would be most appreciative.

Am interested in receiving only, around the Marine VHF bands of 156 MHz, or
so.

I understand that "Gain" is achieved in many antenna configurations by
making them very directional.
My understanding is that you in effect re-shape the lobes to be prominent
along one axis, and minimal at right angles.

So, how is Gain achieved in a
vertical whip (the specs usually mention 3 or even 6 db), or the
rubber-duckie, types of antennas used so often on handheld scanners (or
mounted to recreational boats) ? They are, I believe, truly omni
directional.

Also, are there vertical whip antennas that are directional, with "gain"
perhaps ?

Thanks,
Bob

Gain is commonly referenced to "isotropic" or pure omnidirectional
radiation. Even without using collinear elements, there's gain in a
vertical whip, just as there is in a horizontal dipole. In fact, even
an "omnidirectional" antenna operated over a ground plane has gain,
because of the reflection from the ground plane -- the surface of the
water for a marine antenna. At the very least, the energy radiated is
confined to half a sphere instead of a whole sphere.

You can download the EZNec demo program and look at, for example, a
half wave vertical dipole in freespace--just open the dipole1.ez file
(which has 2.16dB gain over isotropic), and a vertical quarter wave
fed against an infinite ground plane (which has gain over a dipole in
freespace; 3dB gain if the ground is infinitely conducting, since the
pattern is exactly half the dipole pattern, and all the power is in
that pattern, thus twice the power per unit area at a distance).
However, ground loss will kill much if not all the gain of a vertical
operated against real ground; but that may not be reported by the
antenna manufacturer, since the loss is not in the antenna itself (and
of course the numbers look better if they don't include the ground
loss, inevitable though it may be). You'll see a much different
effect if you simulate a vertical dipole spaced a few wavelengths
above ground, be it a perfect or imperfect ground.

Vertical collinears operate by having several vertical elements
radiating generally in phase, so in the horizontal plane, you the
reinforcement of all of them, but as you go up or down from
horizontal, the phases no longer match (it's further to an element at
one end than at the other). So you get a "flat pancake" effect. You
can adjust the design so the phases are not all the same, but progress
uniformly along the antenna, causing a cone-shaped pattern, useful for
an antenna on a high mountain, for example.

Making an antenna have purely omnidirectional radiation, even in
freespace, is extremely difficult. So in that sense, pretty much all
antennas have patterns which have "gain" over the theoretical
omnidirectional. System losses may negate that gain, but the pattern
at least will show some directionality.

Cheers,
Tom

On 8 Aug, 11:44, K7ITM wrote:
On Aug 8, 9:07 am, "Robert11" wrote:





Hello,

Am a real novice when it comes to antenna theory, but am trying to learn.
if anyone could explain the following for me, even though I admit it's
really awfully basic, would be most appreciative.

Am interested in receiving only, around the Marine VHF bands of 156 MHz, or
so.

I understand that "Gain" is achieved in many antenna configurations by
making them very directional.
My understanding is that you in effect re-shape the lobes to be prominent
along one axis, and minimal at right angles.

So, how is Gain achieved in a
vertical whip (the specs usually mention 3 or even 6 db), or the
rubber-duckie, types of antennas used so often on handheld scanners (or
mounted to recreational boats) ? They are, I believe, truly omni
directional.

Also, are there vertical whip antennas that are directional, with "gain"
perhaps ?

Thanks,
Bob

Gain is commonly referenced to "isotropic" or pure omnidirectional
radiation. Even without using collinear elements, there's gain in a
vertical whip, just as there is in a horizontal dipole. In fact, even
an "omnidirectional" antenna operated over a ground plane has gain,
because of the reflection from the ground plane -- the surface of the
water for a marine antenna. At the very least, the energy radiated is
confined to half a sphere instead of a whole sphere.

You can download the EZNec demo program and look at, for example, a
half wave vertical dipole in freespace--just open the dipole1.ez file
(which has 2.16dB gain over isotropic), and a vertical quarter wave
fed against an infinite ground plane (which has gain over a dipole in
freespace; 3dB gain if the ground is infinitely conducting, since the
pattern is exactly half the dipole pattern, and all the power is in
that pattern, thus twice the power per unit area at a distance).
However, ground loss will kill much if not all the gain of a vertical
operated against real ground; but that may not be reported by the
antenna manufacturer, since the loss is not in the antenna itself (and
of course the numbers look better if they don't include the ground
loss, inevitable though it may be). You'll see a much different
effect if you simulate a vertical dipole spaced a few wavelengths
above ground, be it a perfect or imperfect ground.

Vertical collinears operate by having several vertical elements
radiating generally in phase, so in the horizontal plane, you the
reinforcement of all of them, but as you go up or down from
horizontal, the phases no longer match (it's further to an element at
one end than at the other). So you get a "flat pancake" effect. You
can adjust the design so the phases are not all the same, but progress
uniformly along the antenna, causing a cone-shaped pattern, useful for
an antenna on a high mountain, for example.

Making an antenna have purely omnidirectional radiation, even in
freespace, is extremely difficult. So in that sense, pretty much all
antennas have patterns which have "gain" over the theoretical
omnidirectional. System losses may negate that gain, but the pattern
at least will show some directionality.

Cheers,
Tom- Hide quoted text -

- Show quoted text -

Bob,
First of all we have to determine what your needs are before we talk
about antennas.
For instance, you state it is for listenning purposes only, very
important, and then you state
it is for listening to a marine band. It is here where your physical
position is important.
If you are on board a ship you might want to hear everything around,
all directions. If you are on shore then you probably
do not want to hear anything behind you and concentrate on a beam form
of antenna. The antennas for these two different situations are quite
different so this is a descision you must make before deciding on the
antenna.
Art

In addition to Richard's words.

One thing to also keep in mind is that once you reach the maximum range
of a "lower" gain system, changing to a higher (antenna) gain system
doesn't realize a huge increase in coverage. (Ignoring the gain effects
of height that is)

The theory of course says that if you have 6dB more gain you will cover
twice the distance, simple inverse square law stuff. What happens though
is once you are at maximum distance with the lower gain system, the path
losses past that point are much higher than what you get from simple
inverse square law. I forget the actual numbers now but whilst still in
line of sight of the antenna (and a little more) you'll get roughly the
6dB loss for every distance doubling. Past LOS and out to some distance
(500km?) you lose a huge amount more, maybe 30dB per distance doubling.
After 500km the loss curve is steeper still. I'll admit I cant remember
the numbers nor the distances well but you can see that once past LOS,
where you will be at maximum low gain coverage, an extra few dB of
antenna gain wont make a lot of difference. (You can model this if it is
important to know)

The best analogy I ever heard of for describing antenna gain was those
foam rubber stress balls. In its spherical state it represented a point
source radiator. As you squash down on the ball, it becomes more like a
pattern for a normal dipole . ie the diameter in the horizontal gets
larger. Higher and higher collinear gain is represented by pushing down
harder on the ball still. You could eventually make it into a very thin
pancake with a large diameter (and be very stress relieved!). You can
also see from this analogy that in its spherical state a lot of
radiation also goes up and down, where it isn't much use normally. As it
flattens you get less and less up or down angle radiation. There is a
downside to that for example when you are skating up and down waves,
your pattern is skying one side and burying in the water in the other.
It sometimes pays to not have a too high gain collinear on a boat for
that reason.

Hope you find this useful.

BOB W5/VK2YQA

Richard Clark wrote:

Quite so. Unfortunately there is more to "gain" (or effective
sensitivity) than co-linear elements at the frequencies you are
interested in. More important is height which can make a substantial
difference in perceived "gain." If you invest any of your interest in
raising an antenna, it would reward you to also hoist a co-linear
design instead of a rubber duckie.