How does an Omni antenna have gain?

I see omni antennas advertised as having gain. How does an omni have gain
if by definition it radiates in a perfect sphere? How is the gain achieved?
What is the baseline for gain, is it 0 gain or 1db gain? Is there a
theoretical maximum gain and what determines gain?

Thanks, if there are websites you know about that explain please provide
links.

Marvin Cohen wrote:
> I see omni antennas advertised as having gain. How does an omni have gain
> if by definition it radiates in a perfect sphere? How is the gain achieved?
> What is the baseline for gain, is it 0 gain or 1db gain? Is there a
> theoretical maximum gain and what determines gain?
>
> Thanks, if there are websites you know about that explain please provide
> links.
>
>

There is no free lunch. Gain in one direction means loss in another
direction. It's been a while, but I think the term you're looking for
is isotropic, radiating in a sphere. Gain can't be had without
changing the pattern.
Omnidirectional antennas compress the vertical dimension to achieve
gain in a horizontal plane. Theoretically, I think you can have as much
gain as you want if you compress all the energy into an infinitely
thin disk...you'll just not be able to capture it at the other end.
For real antennas, there's probably some optimum level of pattern
compression/maximum practical path gain. Do the math...I sure can't.
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

Entity Marvin Cohen spoke thus:

> How does an omni have gain
> if by definition it radiates in a perfect sphere?
There is no such radiator that makes a "perfect sphere", except for the Sun.
Specifically your omni radiates in a plane perpendicular to the radiator
isotropically, not a sphere.

When they say it has "gain" it means relative to some other antenna.
A dipole antenna has a gain of 2.15 dBi. An isotropic radiator has a gain of
0 dBi.

--Gnarlie

The way it gets its gain is by being configured as a Colinear or Collinear
with
elements in line and a phasing coil in between the two elements. Its gain is
not in the azimuth as a .i.e., Yagi, but in the vertical by "compressing"
the upper part of the lobe. In other words if you are "flying" over a
vertical co-linear you wou;ld theoretically receive less signal than with a
plain vertical. They are not easy to make work right. Regards


"Marvin Cohen" <(E-Mail Removed)> wrote in message
news:1jEwb.491000$pl3.182623@pd7tw3no...
> I see omni antennas advertised as having gain. How does an omni have gain
> if by definition it radiates in a perfect sphere? How is the gain
achieved?
> What is the baseline for gain, is it 0 gain or 1db gain? Is there a
> theoretical maximum gain and what determines gain?
>
> Thanks, if there are websites you know about that explain please provide
> links.
>
>

The higher the gain, the more we horizontally squeeze our doughnut until it
starts looking like a pancake, as is the case with very high gain antennas.





"Marvin Cohen" <(E-Mail Removed)> wrote in message
news:1jEwb.491000$pl3.182623@pd7tw3no...
> I see omni antennas advertised as having gain. How does an omni have gain
> if by definition it radiates in a perfect sphere? How is the gain
achieved?
> What is the baseline for gain, is it 0 gain or 1db gain? Is there a
> theoretical maximum gain and what determines gain?
>
> Thanks, if there are websites you know about that explain please provide
> links.
>
>

"Marvin Cohen" <(E-Mail Removed)> ha scritto nel messaggio
news:1jEwb.491000$pl3.182623@pd7tw3no...

> Thanks, if there are websites you know about that explain please provide
> links.

Try this:
http://www.wireless-italia.com/modul...iewlink&cid=14

There are many free projects and theory.

--

Il FORUM sul Wireless, legislazioni, antenne, installazione,
sperimentazione:
http://www.wireless-italia.com/~forum/index.php

On Tue, 25 Nov 2003, Marvin Cohen wrote:
> I see omni antennas advertised as having gain. How does an omni have gain
> if by definition it radiates in a perfect sphere? How is the gain achieved?
> What is the baseline for gain, is it 0 gain or 1db gain? Is there a
> theoretical maximum gain and what determines gain?

....Because they don't radiate in a sphere, but more like a flattened one where
all horizontal crosssections are circles and vertical ones (that include the
center) are elipses. A sphere would have all such crosssections as circles,
regardless of orientation.

The baseline for gain is 0dBi. There is no theoretical maximum because there is
no maximum on size (if the universe's border expands at the speed of light, then
one can never get there).

> Thanks, if there are websites you know about that explain please provide
> links.

I leave that for you to find.