On 20 Oct 2005 06:44:55 -0700, "CD" wrote:

Hi folks,

I have some questions. If I had two power amps at 15W each, 2 feet
apart, and they are connected to two similar isotropic antennas, would
the two far-field patterns add up? How would I calculate the power
received by a receiver at a certain distance?

I wonder since each antenna will have 0dB gain, then ideally the loss
that I would need to take into account would just be free space path
loss, eh? Will the antenna patterns change in terms of beamwidth and
gain? What other changes/factors do I need to know about?

Thanks!

Simple answer. First simplify. Two power amplifiers only cloud the
issue. We could say they are perfect ones and leave it at that but
it makes no difference if we just leave them out.

Assume those same isotropic radiators are feed by an inphase (0degree)
power splitter. What we have then is a pair of isotropic antennas as
an array. at low frequencies it will appear to be a fat radiator, and
at some frequency it will start to appear directional.

Assuming the frequency is high enough (around 5mhz) it will have
measurable directionality. At various points in space around those
two antennas the measurment antenna will recieve different amounts of
RF with varying phase. Why, the distances are with notable exception
unequal. The only place in space that will have the full radiated
power is a plane center between the two radiators and perpendicular
to the line between the two radiators. All other places there will
be a phase difference due to time/distance from the radiators and
measurement antenna. Those places will recieve less than full radiated
power due to the phase difference. The exact power is related to
antenna seperation and operating frequency.

Now if we pick a frequency, say 123mhz, which has a wavelength of
around 8 feet. The pattern (and gain) of the phased array (two
antennas feed in parallel are a phased array) will be similar to a
dipole in free space. For other frequencies the results will differ.

The exact answer obviously is frequency dependent and position
dependent. The exact field strength at a point in space is power
and location dependent. There is a standard calculation for path loss
that can be applied one you know the "gain" or "loss" of the array of
radiators in a particular direction


Allison
KB!GMX