Directional vs Omni question

In THEORY would a 6 dBi directional antenna send a signal at the same
strength in a given direction as a 6 dBi omni directional antenna? If dBi
is the same would the only advantage to a directional be to remove noise for
receiving?

Pardon my question into basic theory but I have done lots of reading just
because I find it entertaining and still don't know understand this area
well.

"ahh" <(E-Mail Removed)> hath wroth:

>In THEORY would a 6 dBi directional antenna send a signal at the same
>strength in a given direction as a 6 dBi omni directional antenna?

Yes. 6dB of gain is exactly the same no mattery what is used to
create it. However, that's only at the maximum gain point. As soon
as you get off axis or away from the maximum gain point, the omni and
the directional antennas work very differently.

>If dBi
>is the same would the only advantage to a directional be to remove noise for
>receiving?

That's really the big advantage. Sources of interference that are NOT
along the maximum gain axis of a directional antenna rapidly disappear
with a directional antenna. However, it's not perfect. If you're
unlucky enough to have a source of interference that is along the
line of sight axis, the directional gain antenna will actually make
the interference worse. It's not unusual for someone to install a big
24dBi dish antenna, and suddenly discover that another wireless
network that's located miles away, along the line of sight, is now a
source of interference.

Other benifits of directional antennas are:
1. Larger vertical radiation angle for directional versus omni (for
the same gain) which makes keeping the antenna vertical less of a
headache.
2. Directional antennas also reduce the effects of reflections, which
can be a serious problem in some highly reflective locations.
3. Directional antennas are much easier to mount.

There's also a big problem with end fed vertical collinear omni
antennas, which are what most omnis are inside. The patterns shown in
the data sheets look like a squashed donut with the major axis exactly
90 degrees (perpendicular) to the antenna. If such an antenna
actually worked like that, it would be idea with the bulk of the
signal pointed at the horizon. However, that's not reality. Those
patterns are the free space antenna pattern and totally ignore the
effects of the mounting structure or any metal directly under the
antenna. Both of these tend to make the squashed donut move upward
causing what's called uptilt. Typical is about 3-5 degrees. That may
not seem like much but when combined with an antenna that perhaps has
only about 11 degrees of beamwidth, it means that at least half the
signal is going towards the sky instead of downward towards the users.
It's especially a problem with rooftop and mountain top systems where
talking to airplanes is not the intent. There are high gain omni's
that don't have intentional downltilt that are a considerable help.

Directional antennas generally don't have this problem with one
exception. Sector antennas are designed to have a very broad
horizontal pattern, and a narrow vertical pattern. The idea is to
cover as many square miles of area as possible. This is done by
trading vertical coverage for horizontal. The result is that the
vertical radiation pattern of a sector antenna is quite narrow.
Pointed exactly horizontally, it will send the signal over everyone's
head, which is not exactly desireable. So, sector antenna always have
a mounting contrivance that allows for downtilt.

>Pardon my question into basic theory but I have done lots of reading just
>because I find it entertaining and still don't know understand this area
>well.

When I was much younger, I always wanted a magic wand. Antennas are
magic and will suffice.


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

I'm still trying to figure out how to point/tilt/aim/angle the two big
honkin 9dB booster antennas I put on the back of the WRT44G router, or
the standard antennas, for that matter. Do they point vertically,
horizontally, or somewhere in between? I

s the max signal boost at right angles to the two antennas (back of
router box pointing towards maximum signal lobe), or parallel to the
plane of the antennas(side of router box pointing towards maximum signal
lobe)?

I can't seem to find any literature on antenna positioining - it's not
mentioned anywhere in the Linksys router manual.

Sheesh, they give you all these rotation options, but not a clue as to
where to point them. Are they omnis, or dipoles? If omni, why two of
them?

On Fri, 07 Jul 2006 15:18:27 -0500, Harvey Gerst <(E-Mail Removed)>
wrote in <(E-Mail Removed)>:

>I'm still trying to figure out how to point/tilt/aim/angle the two big
>honkin 9dB booster antennas I put on the back of the WRT44G router, or
>the standard antennas, for that matter. Do they point vertically,
>horizontally, or somewhere in between? I
>
>s the max signal boost at right angles to the two antennas (back of
>router box pointing towards maximum signal lobe), or parallel to the
>plane of the antennas(side of router box pointing towards maximum signal
>lobe)?

Max signal from a "stick" antenna is at right angles to the axis of the
stick.

>I can't seem to find any literature on antenna positioining - it's not
>mentioned anywhere in the Linksys router manual.

See
<http://en.wikipedia.org/wiki/Dipole_antenna#Radiation_pattern_and_gain>

>Sheesh, they give you all these rotation options, but not a clue as to
>where to point them.

They expect you to make them vertical, like in the pictures, which
optimizes horizontal coverage, the usually desired result.

>Are they omnis, or dipoles?

Omni (horizontal).

>If omni, why two of
>them?

Diversity (space). See
<http://en.wikipedia.org/wiki/Diversity_reception>

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_How_To>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

On Fri, 07 Jul 2006 15:18:27 -0500, Harvey Gerst
<(E-Mail Removed)> wrote:

>I'm still trying to figure out how to point/tilt/aim/angle the two big
>honkin 9dB booster antennas I put on the back of the WRT44G router, or
>the standard antennas, for that matter. Do they point vertically,
>horizontally, or somewhere in between?

I couldn't find the "Big Honkin" company or a WRT44G.
I think you might mean one of the aftermarket Linksys omni antennas.
Perhaps this one?
| http://www.linksys.com/servlet/Satel...VisitorWrapper


>Is the max signal boost at right angles to the two antennas (back of
>router box pointing towards maximum signal lobe), or parallel to the
>plane of the antennas(side of router box pointing towards maximum signal
>lobe)?

Well, here a differ from orthodoxy. Somewhere in the docs, it says to
point both antennas straight up. That gives the maximum gain
horizontally (perpendicular to the antennas). Only one antenna is
"on" at a time, so it really doesn't matter (much) which way the
WRT54G is pointed. I suppose there might be a tiny bit more signal
towards the back of the router because the router itself is not
blocking the signal.

However, most houses are not built flat. They have upstairs,
downstairs, and basements. If you need to cover these areas, what I
recommend is to point both antennas to form a "V" or about 45 degrees
from vertical. The idea is point the antennas up and down somewhat
and also make it somewhat polarization insensitive. Unfortunately,
there's no free lunch, so you're going to lose some gain in the
horizontal plane.

>I can't seem to find any literature on antenna positioining - it's not
>mentioned anywhere in the Linksys router manual.

I think there was some documentation on the back of the antenna box.

>Sheesh, they give you all these rotation options, but not a clue as to
>where to point them. Are they omnis, or dipoles? If omni, why two of
>them?

They're omnis. You want two of them for diversity reception. The
problem is called frequency selective fadeing, which I don't wanna
expound on right now. Basically, having two antennas improves the
probability that at least one of them is going to receive the signal.
The router picks the antenna with the best signal.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

On Fri, 7 Jul 2006, in the Usenet newsgroup alt.internet.wireless, in article
<_(E-Mail Removed)>, ahh wrote:

>In THEORY would a 6 dBi directional antenna send a signal at the same
>strength in a given direction as a 6 dBi omni directional antenna?

Are you familiar with a common balloon? Think of one that is so strong,
that it won't break no matter how to squeeze or pinch it.

Fill that balloon with water so that is two units in diameter (inches, cm,
feet, meters, miles, whatever) - nice and round, and every point on the
surface is one unit from the exact center. That is an isotopic balloon -
the 'i' in dBi. Now, squeeze the balloon from the top and bottom. Notice
that as you squeeze in there, the surface of the balloon pushes _out_ all
around the sides. This is the same as an "omni" antenna with gain. It
gets that gain by reducing the signal out the top and bottom, and adding
that signal to go further out the sides.

Now, squeeze the top and bottom, and both sides. Notice how the balloon
extends further in two directions (call it "front" and "back") as you do
so. That's a "bi-directional antenna. Now, while you continue to squeeze
the top and bottom, and both sides, squeeze the back as well. Notice how
the front extends even more - a uni-directional antenna.

Some people also use the idea of a ball of "Play-Doh" (a clay like soft
plastic sold as a toy). You can squeeze that material into a ball of two
units in diameter. You can also take a board, and flatten that ball into
a disk - the disk will be thin, but much larger in diameter. Same idea.

>If dBi is the same would the only advantage to a directional be to remove
>noise for receiving?

An antenna produces the same gain in transmit and receive. It does this
by not transmitting (or receiving) in "that" direction over there, but
increases the signal in "this" direction. Think about a flashlight
bulb - the bulb in a 2xD cell (NEDA 13F) is rated at about 0.8 candela
(close enough to the light output of a candle) or 1.2 Watts. Compared to
a common "night light" which is rated at 4 or 7 Watts, it shouldn't be
very bright. In fact, the night light produces about 3.3 or 6 times as
much light. Compare that to the light of a single birthday candle. But
you notice that the flashlight is "directional". Off to the side or
behind, there isn't that much light. But when the flashlight points
directly at you, it's often as bright as a much larger lamp. The
flashlight concentrates the light in one direction (directional) while
the night light spreads the light in all directions (nearly isotopic).

Old guy

On Fri, 07 Jul 2006 19:43:33 -0500, (E-Mail Removed)
(Moe Trin) wrote:

>Now, squeeze the top and bottom, and both sides. Notice how the balloon
>extends further in two directions (call it "front" and "back") as you do
>so. That's a "bi-directional antenna. Now, while you continue to squeeze
>the top and bottom, and both sides, squeeze the back as well. Notice how
>the front extends even more - a uni-directional antenna.

Good analogy. However, the last time I used it in front of a
knowledgeable audience, some clown ask "Does it extrude linearly or
logarithmically like in the antenna plot"? Oops. I instantly lost
the audience when I stated explaining why antenna plots use
logarithmic scales.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann wrote:
> "ahh" <(E-Mail Removed)> hath wroth:
>
> >In THEORY would a 6 dBi directional antenna send a signal at the same
> >strength in a given direction as a 6 dBi omni directional antenna?
>
> Yes. 6dB of gain is exactly the same no mattery what is used to
> create it. However, that's only at the maximum gain point. As soon
> as you get off axis or away from the maximum gain point, the omni and
> the directional antennas work very differently.
>
> >If dBi
> >is the same would the only advantage to a directional be to remove noise for
> >receiving?
>
> That's really the big advantage. Sources of interference that are NOT
> along the maximum gain axis of a directional antenna rapidly disappear
> with a directional antenna. However, it's not perfect. If you're
> unlucky enough to have a source of interference that is along the
> line of sight axis, the directional gain antenna will actually make
> the interference worse. It's not unusual for someone to install a big
> 24dBi dish antenna, and suddenly discover that another wireless
> network that's located miles away, along the line of sight, is now a
> source of interference.
>
> Other benifits of directional antennas are:
> 1. Larger vertical radiation angle for directional versus omni (for
> the same gain) which makes keeping the antenna vertical less of a
> headache.
> 2. Directional antennas also reduce the effects of reflections, which
> can be a serious problem in some highly reflective locations.
> 3. Directional antennas are much easier to mount.
>
> There's also a big problem with end fed vertical collinear omni
> antennas, which are what most omnis are inside. The patterns shown in
> the data sheets look like a squashed donut with the major axis exactly
> 90 degrees (perpendicular) to the antenna. If such an antenna
> actually worked like that, it would be idea with the bulk of the
> signal pointed at the horizon. However, that's not reality. Those
> patterns are the free space antenna pattern and totally ignore the
> effects of the mounting structure or any metal directly under the
> antenna. Both of these tend to make the squashed donut move upward
> causing what's called uptilt. Typical is about 3-5 degrees. That may
> not seem like much but when combined with an antenna that perhaps has
> only about 11 degrees of beamwidth, it means that at least half the
> signal is going towards the sky instead of downward towards the users.
> It's especially a problem with rooftop and mountain top systems where
> talking to airplanes is not the intent. There are high gain omni's
> that don't have intentional downltilt that are a considerable help.
>
> Directional antennas generally don't have this problem with one
> exception. Sector antennas are designed to have a very broad
> horizontal pattern, and a narrow vertical pattern. The idea is to
> cover as many square miles of area as possible. This is done by
> trading vertical coverage for horizontal. The result is that the
> vertical radiation pattern of a sector antenna is quite narrow.
> Pointed exactly horizontally, it will send the signal over everyone's
> head, which is not exactly desireable. So, sector antenna always have
> a mounting contrivance that allows for downtilt.
>
> >Pardon my question into basic theory but I have done lots of reading just
> >because I find it entertaining and still don't know understand this area
> >well.
>
> When I was much younger, I always wanted a magic wand. Antennas are
> magic and will suffice.
>
>
> --
> Jeff Liebermann (E-Mail Removed)
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

Probably not completey relevant, but I've been studying the demod
output of trunk radio systems and noticed that the signal quality with
directional antennas is better, even at similar gain. You can see
multipath in the digital signal. If there isn't too much, the effects
of multipath will get removed in the slicing, but for noisy signals,
directional antennas do have an advantage.

With wifi, the higher frequency signals should have less multipath than
a UHF trunk system, but still, I think there would be advantages with a
directional antenna.

I've been thinking about this in relationship to war driving, where a
figure 8 pattern with the gain lobes firing out the sides of the car
and the nulls front and back. Should be possible with two verticals
spaced a half wave apart according to the ARRL antenna book.

On Sat, 08 Jul 2006, in the Usenet newsgroup alt.internet.wireless, in article
<(E-Mail Removed)>, Jeff Liebermann wrote:

>(Moe Trin) wrote:
>
>>Now, squeeze the top and bottom, and both sides. Notice how the balloon
>>extends further in two directions (call it "front" and "back") as you do
>>so. That's a "bi-directional antenna. Now, while you continue to squeeze
>>the top and bottom, and both sides, squeeze the back as well. Notice how
>>the front extends even more - a uni-directional antenna.
>
>Good analogy.

You notice I'm using a water filled balloon - that was after someone came
back when things didn't work as well as they thought using an air filled
one.

>However, the last time I used it in front of a knowledgeable audience,
>some clown ask "Does it extrude linearly or logarithmically like in the
>antenna plot"?

With a water filled one, it's linear. If your want the logarithmic
expansion, you have to add a tablet of metallic sodium to the water - the
tablet size relates to the size of the balloon of course. Do be careful.

>Oops. I instantly lost the audience when I stated explaining why
>antenna plots use logarithmic scales.

That's only a mental convenience to allow you to see more details. In
these days of pocket calcula^Wcomputers, people forget that adding and
subtracting is easier to do in your head than multiplying/dividing. The
HP 8552B IF section on the spectrum analyzer offered both log and linear
presentations. I don't think I ever used linear even though the display
graticule had 70.7%, 50% and 10% markings in addition to 2 or 10 dB per cm.

Smoke and mirrors? Wazzat? "Ignore the man behind the curtain..."

Old guy

On Sat, 08 Jul 2006 18:55:22 -0500, (E-Mail Removed)
(Moe Trin) wrote:

>You notice I'm using a water filled balloon - that was after someone came
>back when things didn't work as well as they thought using an air filled
>one.

Using water also adds considerable suspense to public demonstrations.
I've noticed that the front rows of the audience tends to empty as I
squeeze the balloon.

>With a water filled one, it's linear. If your want the logarithmic
>expansion, you have to add a tablet of metallic sodium to the water - the
>tablet size relates to the size of the balloon of course. Do be careful.

Metallic sodium in water creates lots of hydrogen gas and sufficient
heat to explode the hydrogen in air. I have some of the stuff if you
want to Learn By Destroying(tm).

>>Oops. I instantly lost the audience when I stated explaining why
>>antenna plots use logarithmic scales.

>That's only a mental convenience to allow you to see more details. In
>these days of pocket calcula^Wcomputers, people forget that adding and
>subtracting is easier to do in your head than multiplying/dividing.

I know the feeling. In 1970, I bought a TI SR-10 calculator in
college because I couldn't afford an HP-35. Anyway, my checkbook and
bank statements haven't balance since then. Even with the help of
various arithmetic accelerators, I still can't add and subtract.

Incidentally, I bought two K&E slide rules last weekend for $10 in
perfect condition. Don't ask me why.

>The
>HP 8552B IF section on the spectrum analyzer offered both log and linear
>presentations. I don't think I ever used linear even though the display
>graticule had 70.7%, 50% and 10% markings in addition to 2 or 10 dB per cm.

I have one of those in an HP140T main framis. It really impresses the
visitors. Ummm... my graticule isn't stock so it's missing the NTSC
references and linear confusion. I have used the linear scale to do
IF and xtal filter tuning. It's way more sensitive and I needed to
see ripple down to fractions of a decibel.

Another great antenna demonstration is to rotate the antenna while
measuring the detected signal on a polar display. The display shows
the antenna pattern (when run through a log amp). However, what was
normally suppose to be an outdoor demonstration turned into an indoor
demonstration because the parking lot had just been re-paved. I also
managed to mismount the 5 element VHF yagi off the center of gravity.
When the rotator motor got up to speed, the assembly took off in the
general direction of the audience. It was almost as good at clearing
the front rows as the water balloon antenna pattern demo.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS