"CD" wrote in message
ups.com...
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?


Interesting proposal.
Interaction between the 2 would depend on several factors.
Your only stipulation is that they are 2 ft apart, so...
The frequency would matter and [[Relative]] lengths of coax to each antenna
would also matter.
Its possible you could have a phased array.
Under certain conditions gain could be broadside to the two,
in some other conditions, gain could be endfire.
Its probable, though, that all you have

is a mess.

unless

the builder is

one

of


us.

hehe

Hal Rosser wrote:

SNIPPED

Interesting proposal.

Its probable, though, that all you have is a mess.

unless the builder is one of us.

hehe


Then, no matter how it works, it will be debates to death, argued about,
condemned, praised, and finally you will be judged a fool.

We do have some strong willed 'Know-it-Alls' who read this list.

We also have some very knowledgeable contributors.

"By their fruits ye shall know them."

"CD" bravely wrote to "All" (20 Oct 05 06:44:55)
--- on the heady topic of "Two antennas and two power amps"

CD From: "CD"
CD Xref: core-easynews rec.radio.amateur.antenna:219123

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

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


I'm not sure of the physics involved but when 2 oscillators of nearly
the same frequency are brought within a certain maximum distance of
each other, their frequency and phase will lock together suddenly. You
can experiment the effect with walkie-talkies. I'll bet the same
effect will occur with your transmitters.

A*s*i*m*o*v

.... A seminar on Time Travel will be held 2 weeks ago

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

Debate all you want, here's the scoop.

The answer to your first question (add?) is Yes. That is how multi-element
antennas work, regardless of the element shapes.

For your second & third question (how to calc &pattern, beamwidth, gain):

With the described "one transmitter feeding two antenna" system you have
nothing more than a phased array. Using two amplifiers is not important as
long as you use good techniques connecting everything and match impedances
_when both amps are transmitting_. That is if, in the first approximation,
you have all the power from each amp going into the respective antenna. I
suspect this is a valid assumption for the original poster. I'll speculate
that this will be harder with only a splitter (no amps) due to the mutual
coupling in the antennas and the impedance change therefrom, but matching
things removes this complication.

Gee... do isotropic antennas have any mutual coupling / impedance??
Seems there could be for voltage only. anyway...

The relative phase of the two antenna currents will create the pattern
(by summation in the farfield) [[regardless of the shape of the "iron in the
sky" as I call it]] I believe (and Lew can corroborate or dispel) you can
use EZNEC to get an idea what will happen by looking only at the horizontal
plane, or vertical view of two verticals; 1/4 wave should do. This is the
"plan view" or the view from above, where the verticals look like two dots.
This should represent the view of the two iso's that looks the same (two
dots).

With equal power and in phase current, you'll have a figure eight with
the nulls broad-side and the depth of the nulls will depend on the
frequency... starting from lower freq, at a frequency where the spacing is
1/2 wave, the nulls are deepest and as frequency increases, the pattern will
develop more and more lobes/nulls.
IF the two antenna currents/powers are not the same, you should also be
able to model this in EZNEC as described and the nulls will not go to zero
as expected (well, by me at least).

Looking at the two iso's from the endfire direction (the two iso's look like
one dot)..lemme think here...
Oh yea, you'll see a round pattern ( perfect circle) of varying diameter as
you vary the relative phase/amplitude. This will be equivalent to looking
at it at only one angle from the plane equidistant from the two dots. Hard
to describe, easy to imagine. As you change the angle of this cone (similar
to viewing pattern at differing radiation angles), the circle also changes
diameter.

SO... The "gain" will vary depending on Frequency, relative phase of the
two feeds and relative power of the two feeds. I don't thing path loss is
considered in pattern calculations since it is the same for all components
in the field by the time you get to any distance.
Post some pix.

OK, dispute away, I'm done.

Any comments Lew?

Regards, Steve, K,9.D;C'I



"CD" wrote in message
ups.com...
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!