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

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

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?

In article ,
Bob Bob wrote:

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

what is this theory??

A rubber ball represents the radiation from an antenna. ie perfectly
spherical for an isotropic antenna. You can think of the field strength
of the signal as being proprtional to the ball's radius.

When you put gain into the system you take the same amount of "power"
and "squash" it into specific directions. You take from the direction
you dont want radiation in and put it into the direction you do. You
never get something for nothing and gain always implies directivity,
even if that directivity is undesirable.

Okay?

ml wrote:

what is this theory??

In article ,
Bob Bob wrote:

A rubber ball represents the radiation from an antenna. ie perfectly
spherical for an isotropic antenna. You can think of the field strength
of the signal as being proprtional to the ball's radius.

When you put gain into the system you take the same amount of "power"
and "squash" it into specific directions. You take from the direction
you dont want radiation in and put it into the direction you do. You
never get something for nothing and gain always implies directivity,
even if that directivity is undesirable.

Okay?

ml wrote:

what is this theory??

Ahh yeah ok i kinda knew this but spazed out thank you for helping
appreciate it

m

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