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!

CD wrote:
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?

Will the two outputs of the "power amps" be phase-locked?
--
73, Cecil, http://www.qsl.net/w5dxp

If they are ABSOLUTELY in phase, the far fields will add.

Typically, the fields will not be in phase, unless the signal SOURCE and
ALL cables are precisely controlled [phase locked and absolute phase
velocity controlled], and the resulting pattern will be the vector sum
of the individual fields. a distorted pattern will result.

AK

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!

Making my brain work again!! (So some of this may be through my hat!)

The far field pattern would no longer be isotropic. You didnt specify
the frequency in use but genenerally speaking you would get more
radiation perpendicular to a line between the two antennas than in line
with them. As you go lower in frequency this effect becomes less and
less. As you go higher than frequencies where the distance between
become more than a wavelength (or so) the pattern will tend to break
into more than one lobe in each (general) direction. This assumes of
course that the antennas are also being fed in phase, probably from the
same transmitter.

This pattern is a result of the addition and cancellation of the
voltages due to the phase differences. If your antennas dont run from
the same TX and the freq/phase is changing the pattern will be changing
at the rate of the freq difference.

Calculate the power in theoretical terms of an AC voltage either
perfectly in phase (ie twice) or if some phase difference occurs, being
somewhat less.

I am sure that you know about the squishable rubber ball analogy of
antanna radaition patterns.

Modeling your setup through 4NEC2, EZNEC etc will give you a far better
appreciation of the outcome. I'll admit I dont know how to input an
isotropic in these pgms though.

As a general rule of thumb when one combines two antennas (usually ones
with gain) at the correct spacing you get inbetween a 2dB and (just
under) 3dB power gain increase in the desired direction. As you get
close to the maximum gain possible though you also see "extra" sidelobes
that depending on your use may be undesirable.

Hope you find this of some use and not too confusing!

Cheers Bob VK2YQA in W5


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'm not sure if they are phase-locked. I think my friend just added a
coupler or splitter (?) from his transmitter and fed them off the two
power amps.

CD wrote:
I'm not sure if they are phase-locked. I think my friend just added a
coupler or splitter (?) from his transmitter and fed them off the two
power amps.

That's probably asking for trouble in the far-field.
--
73, Cecil, http://www.qsl.net/w5dxp

On 20 Oct 2005 06:44:55 -0700, "CD" wrote:
would the two far-field patterns add up?

Yes.

How would I calculate the power
received by a receiver at a certain distance?

With considerable contortions of phase math, or simply by using EZNEC.

Unfortunately, EZNEC does not model a isotropic radiator (but you
don't really have two of them anyway, do you?), so you may have to
slog through the vector additions. They are not that very difficult,
conceptually (although it seems to have baffled the crew so far).

Phase at a frequency is equivalent to distance, and you already know
both the frequency and the distances. Beyond this, it is no more
difficult than making a three bank shot in billiards.

73's
Richard Clark, KB7QHC

"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

"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