"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?


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. Got it! So the
Friis equation must be wrong:

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

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.

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.

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 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?