On Mon, 10 Sep 2007 08:51:56 -0500, "amdx" wrote:

I see that height above ground affects the gain and the angle of the
main lobe. (actually all lobes) The info I've seen shows best gain and lobe
formation somewhere between 1/4 and 1-1/2 wavelengths above ground.
So I have a 2.4Ghz yagi, does that mean I should put it 1/4 to 1-1/2
wavelengths
above ground? That means it would be 1-1/2" to 6" above ground.


Hi Mike,

All very true. However, the best gain aimed through your neighbor's
hubcaps isn't very useful.

Another point, at the frequency you are talking about, that "gain"
coming from the reflection from ground can be absolutely useless, or
worse, render a poor bit rate throughput (the appropriate topic thus
turns on the term "Fresnel Zone"). The principle mode of
communication is line-of-sight. In other words, you want both
antennas to be literally within "sight" of one another. For greater
range, you don't necessarily want more gain, you want more height.

More height gets you above the hubcaps and kneecaps of the world, and
out into blue sky where the sun, moon, and the stars are seen by
everyone. Very little power is required to transmit to the furthest
point on the terrestrial horizon. With an antenna mounted 100 feet
high, that is only 14 miles away and 100mW will give a clear signal
(at least at VHF). Adding more gain won't make that signal any
clearer, but you might want gain if the transmission power is in the
microwatts. Then you add elements.

Returning to the allusion to Fresnel Zones, this is where reflections
from the ground (which raises the "gain" you noticed) or nearby
buildings add out of phase and distort the bit shape of digital
transmissions. If your location is susceptible to such problems (and
forces a lot of retries in packets); then you want more "gain" in the
sense of actually achieving more selectivity. The antenna's added
elements react to these blurring reflections less, and focus on the
line-of-sight signal more.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 10 Sep 2007 08:51:56 -0500, "amdx" wrote:

I see that height above ground affects the gain and the angle of the
main lobe. (actually all lobes) The info I've seen shows best gain and
lobe
formation somewhere between 1/4 and 1-1/2 wavelengths above ground.
So I have a 2.4Ghz yagi, does that mean I should put it 1/4 to 1-1/2
wavelengths
above ground? That means it would be 1-1/2" to 6" above ground.


Hi Mike,

All very true. However, the best gain aimed through your neighbor's
hubcaps isn't very useful.

Another point, at the frequency you are talking about, that "gain"
coming from the reflection from ground can be absolutely useless, or
worse, render a poor bit rate throughput (the appropriate topic thus
turns on the term "Fresnel Zone"). The principle mode of
communication is line-of-sight. In other words, you want both
antennas to be literally within "sight" of one another. For greater
range, you don't necessarily want more gain, you want more height.

More height gets you above the hubcaps and kneecaps of the world, and
out into blue sky where the sun, moon, and the stars are seen by
everyone. Very little power is required to transmit to the furthest
point on the terrestrial horizon. With an antenna mounted 100 feet
high, that is only 14 miles away and 100mW will give a clear signal
(at least at VHF). Adding more gain won't make that signal any
clearer, but you might want gain if the transmission power is in the
microwatts. Then you add elements.

Returning to the allusion to Fresnel Zones, this is where reflections
from the ground (which raises the "gain" you noticed) or nearby
buildings add out of phase and distort the bit shape of digital
transmissions. If your location is susceptible to such problems (and
forces a lot of retries in packets); then you want more "gain" in the
sense of actually achieving more selectivity. The antenna's added
elements react to these blurring reflections less, and focus on the
line-of-sight signal more.

73's
Richard Clark, KB7QHC

Thanks for the reply,
So, I need to get it off the ground, or in my case the boat.
I'm mounting it onto the aluminum roof of a boat.
How many wavelengths above ground (roof) would be considered Free Space?

Regarding transmission power: it's not microwatts but it is 18 milliwatts.

Fresnel Zone; I didn't really understand the why, but jist of it is, an
object
can be outside the line of site, but be in what is called the Fresnel Zone
and this object will attenuate the signal.

Thanks, Mike

On Mon, 10 Sep 2007 20:01:16 -0500, "amdx" wrote:

How many wavelengths above ground (roof) would be considered Free Space?

Hi Mike,

I'm not sure that is very important in the scope of practicality. More
about what can go wrong below.

Regarding transmission power: it's not microwatts but it is 18 milliwatts.

Sufficient for a lot of applications. Given you are on a boat, you
have your limitations. I don't suppose the horizon is more than 10
miles away if it is a sail boat with a tall mast. To another,
similarly equipped boat, you can roughly double your range. Standing
on the deck of a small boat, you probably can only see out to 3 or 4
miles.

Fresnel Zone; I didn't really understand the why, but jist of it is, an
object
can be outside the line of site, but be in what is called the Fresnel Zone
and this object will attenuate the signal.

No.

Fresnel also goes by the name "picket fencing." This means that
reflections from nearby objects, and the direct signal cancel each
other. There are dead zones for reception. Shift things a half wave
(not very far for the band we are talking about) and they add
constructively. There are also intermediate zones which for digital
transmission can be problematic. For mobile operations like VHF in a
car, the signal from a fixed source will alternately fade and build as
though you are listening through a picket fence rushing past the
window. A choppy water can really create a witch's brew of wave
mixing (no pun intended).

Here, the solution is a gain antenna that is focused (aimed) at the
source giving it a stovepipe view, so to speak. This cuts down on all
reflections contributions and you do this not for the gain, but for
the directivity (which is euphemistically called gain). However, a
highly directive antenna, up high on a boat, in the same chop, will
bob and weave and runs the risk of that stovepipe view looking
somewhere other than at the source. Mixed bag.

73's
Richard Clark, KB7QHC

Fresnel Zone; I didn't really understand the why, but jist of it is, an
object
can be outside the line of site, but be in what is called the Fresnel Zone
and this object will attenuate the signal.

No.

Fresnel also goes by the name "picket fencing." This means that
reflections from nearby objects, and the direct signal cancel each
other. There are dead zones for reception. Shift things a half wave
(not very far for the band we are talking about) and they add
constructively. There are also intermediate zones which for digital
transmission can be problematic. For mobile operations like VHF in a
car, the signal from a fixed source will alternately fade and build as
though you are listening through a picket fence rushing past the
window. A choppy water can really create a witch's brew of wave
mixing (no pun intended).

Here, the solution is a gain antenna that is focused (aimed) at the
source giving it a stovepipe view, so to speak. This cuts down on all
reflections contributions and you do this not for the gain, but for
the directivity (which is euphemistically called gain). However, a
highly directive antenna, up high on a boat, in the same chop, will
bob and weave and runs the risk of that stovepipe view looking
somewhere other than at the source. Mixed bag.

Thanks, Richard,
I found additional explanation of Fresnal Zone at
http://www.zytrax.com/tech/wireless/fresnel.htm
Mike