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