Originally posted by Gilbo303
Just to help stem said misconception, I have a question! What is the difference? IE: What is the definition of 'Effective radiated power, against
total power output?
Hey, Gilbo. That's actually a really good question, and the cause of some weird misconceptions about any transmitting facility, but in the case of
ionosphere heaters like HAARP, it really gets played up in weird ways.
First, someone said upthread (I'll just roll it into this answer as well) that HAARP can transmit 5TW, not at all. I don't know where that number
comes from, but it's not legit. I'm sure they'd like to have that much juice - they don't.
Ok. Here's my 5 minute tutorial on various power numbers you hear bandied about as regards transmitters.
First, you have the power delivered to the finals. This is the transmitter input power. In the case of HAARP, there is an array of diesel generators
that has a total power output a bit over 10MW. This feeds the final amps, which are a big ol' array of D616G's, two per cabinet, 360 of which which
put out 10kW each (thus the 3.6MW number). D616G's were custom designed for the Gakona facility, although you also see them used a lot at other
ionospheric heater sites. The amps are a mixed bag, built by Continental Radio or DRS for the most part. There are a few other qualified vendors but I
don't think they have more than a handful that aren't CR or DRS.
So, the first number to be aware of, is the power delivered to the final amplifiers. That's the total input power, and for HAARP it's 10 million
Watts, more or less, at peak.
You don't get all that power to the output. There are inefficiencies everywhere in a big high output rig like HAARP. The first and worst is that they
can't operate class C in order to optimize final amp efficiency, because it has to be frequency and phase agile. Meaning, in order to steer the beam,
instead of moving the antennas like an old parabolic radar dish, you move the phases around that are being delivered to the output amps. And you have
to pulse it on and off with really fast attack and decay times, and you have to be able to change the frequency really fast. Class C amps are great
for efficiency, but because they use a tuned output tank, you can't do any of that. So in order to do all the things you need for an agile array like
HAARP, you have to use a linear amplifier class. D616G's are class AB. It would have been nice to run at least class B, but the crossover distortion
adjustments on a field of amps is way too fiddly. So, you have to accept the power losses. A class AB amp can be no more efficient than 78.5% by
design: Gakona's D616G's tend to be in the 35-40% range, so of that 10 million Watts of input power, you can't deliver more than, say, 4MW to the
coax on average, and at rated output it's that 3.6MW number you often hear. There are other losses, of course, in the antennas and transmission lines
because the antennas are NOT optimal for their entire design range. There are frequencies that they are good at, but away from that they're less and
less efficient, it's one of those tradeoffs you have to live with.
Here's where you get that number - 3.6MW. It's 360 D616G's at full output at 10kW output each. This is the maximum RF input power to the IRI
averaged over the full operating frequency span, if everything's switched on to full power. That's not a normal operating mode, btw, because they
tend to run one or the other dipole but not both on each antenna, so usually you're doing 1.8MW max - it's all in what they're using for antenna
choices. That's what's going into the coax at the arse end of the D616G. You don't actually get that total amount off the antenna, because of
losses in the coax, the baluns, the tuning network, and the fact that the antennas aren't uniformly efficient over their entire operating range.
Here's the next number - the EIRP, or effective isotropic radiated power. EIRP hasn't got bollocks to do with power being transmitted - it's a
composite of different things, which I'll try to explain. You'll also hear the term "ERP" used, and it's sort of an ambiguous description unless
they state ERP compared to what. In the case of HAARP, that 1GW number is an EIRP number. This is going to be hard for me without being able to embed
little drawings, but I'll try to do it verbally.
Ok. Envision a magic lightbulb. This lightbulb is just a sphere - no lamp, no wiring, no base, just a little round ball of light, hovering without
support in the center of a really big room 100 feet tall, the walls of which are painted flat black. The bulb emits the same light intensity in every
direction, with total uniformity, and the total power output in light of this magic light is 1 Watt total.
If you had a light meter, you could measure the light's power density in Watts per square meter at any point in this big flat-black room you wanted.
As you did so, you would find that the power density fell off as the inverse square of the distance from the magic light - at 2x the distance, you'd
have 1/4 the power density, at 3x, 1/9 the power density and so on. This would always be true - it's a basic truism of any EM source, and you will
see it called the inverse square law if you want to go look it up. I see some sites with really nice graphics which I can't do here that you might
want to hit.
So, that's "inverse square law" - it's important, so put a thumbtack in that one.
Now, the magic light - since it radiates in every direction with perfect uniformity - we will call THAT an isotropic radiator. Isotropic means "equal
in all directions" from the Greek words 'iso' and 'tropos', which oddly enough mean "equal" and "direction". In real life, there probably
aren't any actual isotropic radiators - but it's a convenient mathematical fiction for this sort of thing. So, pin that one too - an isotropic
radiator of EM radiates exactly the same amount of power in any direction you look at it - like a perfectly uniform sphere of light.
Ok - we're getting there, bear with me, this is all prerequisite for the explanation.
NOW. When dealing with real radio antennas, you will find that none of them are actually isotropic - instead of emitting radio waves in a big uniform
sphere, they emit more power in some directions, less in others. This is either pesky or intentional, depending on what you want for an outcome. For a
vertical antenna like the one on your walkie-talkie, the shape looks a bit like an apple with the antenna rammed up the core. If I had a dipole
antenna - that would be two rods in a straight line with the connection in the center - the pattern over the ground would look a bit like two tear
drops point to point - it's directional in those two directions. A parabolic radar antenna has a pattern of radiation that looks a bit like a long
skinny teardrop in the center with a bunch of short flower petals near the antenna (those are sidelobes and we hate them).
Since the radio waves are stronger in one place and less in another, these antennas exhibit directionality - the power going to the antenna isn't
leaving uniformly in all directions. Sometimes we want that, like in the case of the parabolic radar antenna.
Now, back to the light bulb room. You're standing 20 feet away, and you're measuring a certain light power density at that distance. The light
leaving the bulb in other directions, say directly away from you, is being absorbed by the flat black walls and turned into heat, so you don't see it
in your measurement. However, a man has just walked into the room with a big parabolic mirror, and put it behind the light on the far side. Wow, it's
going to get brighter, right? The light that was going away from you is now all coming towards you, as well as the light that left your side of the
bulb. Your meter reading will definitely go up. The magic light is now exhibiting directionality just like the radar antenna. It's emitting more on
one side, due to the reflector. The light appears
brighter, although it's putting out the same total amount of light in Watts as before.
Effectively, the light is brighter, because now it's directional. If you used your old table of measurements you took before, that gave you a certain
power density at a certain distance, you're going to find that now all the readings are higher. They'll still fall off with the inverse square of
the distance, but they're all higher because of the mirror.
If your reading were twice as high, it would be as if the bulb was now putting out 2 Watts instead of 1, although it's still only putting out 1 Watt,
it's twice as bright due to the mirror. So effectively, as far as the meter goes, the bulb is putting out 2 Watts. It's not - you're getting a
higher reading due to the mirror because some of the "wasted" light is being bounced your way by the mirror that was being lost to the walls before.
But as far as your meter goes, it's giving you the same readings as a 2 Watt bulb without the mirror.
Now, remember the word isotropic? That's what your bulb was before the mirror. Now it's directional. But effectively, the readings you get seem like
an isotropic 2W bulb, even though it's really only putting out 1W. 2W is the Effective Isotropic Radiated Power of the bulb, when you include the
What EIRP is telling you is, if that were a magic isotropic light bulb, in order to get the readings I'm getting at this distance, it would have to
be a 2W bulb. It's not - you are using a mirror to concentrate the wasted light. So the total output power of the bulb is 1 Watt, and the EIRP is 2
Watts. The difference is caused by the mirror, and although mirrors aren't rated this way, in a radio antenna, we'd call the improvement you got the
"antenna gain factor". The better and larger that mirror, you might get a magic light bulb EIRP of maybe 5 Watts, still with a 1W total light
output, because the mirror would be capturing more and more of the wasted light and sending it your way. So at 10 feet, as far as your meter knows,
that's a 5W bulb, if it were shining isotropically. Even if you could capture all the light bulb light and send it in a perfect 1 meter square block
at that 10 foot distance, the power would never really be more than 1 Watt, that's all you've got to work with, but it would take an equivalent
larger light bulb shining isotropically to have that apparent brightness.
So what EIRP does, is it takes into account the total power of the bulb (1 Watt) and factors in the effect of the mirror, to tell you how bright an
isotropic bulb would have to be to look that bright at a certain distance. The more directional the mirror, the higher the EIRP will be for a certain
power input. You don't really get more power that way. It's just that compared to a bulb without a mirror, it's brighter by a certain percentage
than one without a mirror, as if you had a brighter bulb.
How does that work with HAARP? With any directional antenna, the more directional it is (the better a mirror), the "brighter" it will appear at a
given distance, compared to an isotropic antenna. HAARP uses a phased array instead of a parabolic dish, which wouldn't be physically practical for
this use. Instead of having a big Arecibo looking dish, HAARP's IRI has a bunch of identical antennas at very carefully fixed spacings. When you send
the power to the individual antennas, the signal is a little different for each antenna in the array. As the radio signals leave the antennas, they
add and subtract so as to form a long skinny teardrop shape, a lot like the radar dish does, through a magical mathematic nightmare called
constructive interference. Complete with the nasty flower petal leftover wasted power we call sidelobes, dammit. You just can't get rid of them.
Those flat radar antennas you see (go google 'phased array radar') work the same way. A lot of little antennas that are all sending a similar but
not identical signal so that they all add up to send the antenna's power in one direction.
Since the IRI manages to squirt the output power in one direction, to the extent possible limited by the number of elements, their perfection in
orientation and spacing, phase and amplitude noise from the finals, errors in coax feeder lengths, and the accuracy limits of the magically wonderous
device we call the "exciter" that makes 360 slightly different signals that are arranged in time so as to cause this directionality to happen (along
with a lot of other crap like phase noise from the baluns and matching networks), you get most of the power sort of going towards one place, although
it's a lot like herding cats. On top of which, there are other limits imposed by physics caused by the fact that the array isn't infinite, so you
get aperture induced fuzziness. But I digress.
The point is that it's directional. And given all the problems, it's REALLY directional. So much so, that at a certain distance, you'll get the
power density per square meter you'd expect from a 1 GW isotropic source. That is, in the focus it looks as bright at any distance you pick as a 1GW
magic light bulb radiating uniformly in all directions. That doesn't mean it HAS 1GW of power - you've got 3.6MW, no more, but since it's all
squirting in one general direction, it "looks brighter".
Why bother with an EIRP number then? Well, let's loop back to the beginning. Remember how, as you moved away from your magic bulb, the power density
dropped off as the inverse square of the distance? That gave you a way (had you done it) to calculate what the power density would be of a magic bulb,
given the total output power of the bulb, and the distance away from it.
EIRP numbers are a way to sort of roll-up the bulb and mirror effects, and standardize them so that your inverse square calculations still apply, no
matter what the nuts and bolts are. Given an EIRP number, you don't have to deal with antenna patterns or gain factors or actual power output or
whatnot - you just calculate the power densities based on what you'd get if you had an isotropic radiator with a certain EIRP power output. It's a
comm engineer's way of rating a system to make calculations easier, a sort of shorthand. It doesn't mean that magically you get an actual gigawatt
downrange for 3.6 megawatts into the antenna farm. It just means that if you had an isotropic antenna (they don't exist) it would have to have a
gigawatt input to seem that "bright".
ps - all this stuff is off the shelf, generators to antennas, if you had the money. The real magic is in the exciter, which was designed by a number
of people, some of whom are as Gods, to whom other comm engineers should bow in awe filled wonder. Also, using a phased array like this, you can pull
off a number of other things, among them, you could direct the exciter to form more than one beam at the same time, but that's for another post.