On Fri, 4 Apr 2008 20:01:30 -0600, "Bob Crantz"
wrote:


"Goofball_star_dot_etal" wrote in message
.. .

Alright, you got me. Is that 6 dBi?

No, 'regular' common or garden dB.

dB is a relative measurement. Is it relative to isotropic or dipole?

Transmission over a ground plane compared to free space. It is
irrelevant whether the antenna is isotropic or dipole or A. N. Other,
provided there is symmetry about the plane parallel to the ground.



f so, then I agree. Otherwise I don't
see how. Do you have a reference, example or link showing this 6
dB(dipole)
of gain for two end to end antenna separated by multiple wavelengths.

Does this help?
http://books.google.co.uk/books?id=o...hbV18#PPA91,M1

(Foundations of Antenna Theory and Techniques By Vincent F. Fusco.
page 91 )
" ....this leads to a 6dB power gain."


If one transmitter gives 1mV into a receiver the addition of a second
identical transmiter at the same distance and in phase, will give 2mV.
This is a 4 x increase in power, 6dB.

Let's say one transmitter is 1 watt. The second transmitter is 1 watt, both
total 2 watts. The receiving antenna then sees a 4x increase in power by
doubling the transmit power, by the law of superposition.

Spooky!

Got it! So the
Friis equation must be wrong:

http://en.wikipedia.org/wiki/Friis_t...ssion_equation

The Friis equation does not apply to two well separated transmitters
transmitting so the (coherent) signals arrive in phase or a single
transmitter and antenna and its image.
It would apply if they were transmitting uncorrelated noise (random
relative phase.
It is the same as the difference between combining light from two
light bubs or two lasers.

Your links pecifically states:

"The antennas (tx, rx) are in unobstructed free space, with no
multipath"


If the path, frequency etc are unchanged the Friis equation shows that
doubling the transmit power only doubles the receive power.

You have doubled the total tx
power (3dB) so you have an antenna gain of 3dB.

6dB (rx) -3dB (tx) = 3dB (antenna gain)


So, if the transmit power was quadrupled the receive power would go up by a
factor of 16 and the antenna gain becomes 12 dB. I never realized antenna
gain was determined by signal strength.


Never mind...

In the case of an
image in a 'ground plane mirror', there is no extra tx power and still
the same 6dB gain. The missing 3dB that came from the second
transmitter comes from the power that would have gone into space,
below the ground plane.

Your principles are correct but the numbers are wrong, unless you can state
3 dB relative to what?

On Wed, 02 Apr 2008 21:17:09 -0600, Bob Crantz wrote:

"Glenn Ashmore" wrote in message
...

By far the best solution is a waterproof wifi bridge at the masthead
connected by Ethernet with POE to a hub. Long range because there is
minimal coax loss and a wide Fresnel zone.

The Fresnel zone does not become any wider with that mounting
installation, it is just that there are less object to protrude into it.

Like that big motor yacht parked next to you? Whatever works...

Anyway this sounds good to me.

Glenn -- how long a cable will POE work with, without extra stuff?

If I can't get to the masthead I may try where the steamer light is
mounted, or the radar and TV antenna mounts just below that. It's
probably high enough to be helpful (20-25' above WL) and there are
convenient holes there.

Matt O.

"Wayne.B" wrote in message
news

The Fresnel effects are applicable in some circumstances, but in the
real world, gaining an unobstructed path via antenna height almost
always trumps everything else.


Somebody already said this. The point of the discussion at hand is whether
Wilbur sees an increase in signal by mounting his wifi closer to the water.
Glen implied this was not so and cited the Fresnel effect and said further
that he is an expert installer with 60 systems to date. Upon checking facts
that can be checked we find, in fact, that Wilbur's signal can be increased
by mounting the antenna closer to the water - this was stated under
descriptions of the Fresnel effect (multiple references), contrary to Glen's
implications. Glen has not demonstrated professional courtesy and stepped
forward to admit that this is indeed so a condition of the Fresenel effect
and can happen and probably is what Wilbur is observing. It seems people are
more concerned with attempting to cast Wilbur in a poor light at the expense
of truth.

This contrasts sharply with Roger. If he makes a mistake or hears of
something new, he checks it out. His primary concern is the joy of learning,
he has no inflated ego to keep pumped up. Roger may have made many mistakes,
but he has done things correctly by a much greater factor. The love of what
he does comes before all things, that is why he can harbor no ill towards
others and he judges a book by its content, not its cover. He can be honest
about a mistake of his because he views mistakes as part of the learning
process, not an attack on his self esteem. A person who won't admit mistake,
attempts to sidestep it or cites irrelevant experience to somehow make
claims in science books false is a person who has never made many mistakes
and as a consequence has done very little right.

Looking at individuals in a newsgroup: the ones with a healthy perspective
can laugh with/at an idiot, ignore garbage and keep good things going; the
ones with a lesser perspective are the ones whining (over nothing), the ones
yelling "plonk", the ones yelling "ignore them" etc. For those who served in
the military think of the types in your unit that caused the most grief and
trouble for everyone else. It certainly wasn't those who were well adjusted
was it?