RADIATION PATTERNS, WHAT ARE THEY TRYING TO SAY ? AND WHAT CAN WE DO WITH THEM?
On this page we will try to provide some basic insight in all those antenna plots we see on the internet.
You should be able to get an idea:
- What freespace is, and the influence of earth.
- Where gain comes from.
- What the azimuth plot is.
- What the elevation plot is.
- The difference between front to back (FB) and front to rear (FR).
- What a take-off angle is.
- What side lobs are,
- To know how accurate should aim your antenna
- etc.
So lets start at the beginning :
The radiation pattern of an isotropic in free space.
Free space is the world without “earth or other things”. There is nothing for an antenna to radiate or “bounce” against. You can really look at it as “in freespace”. This world does not exist for us in the “antenna world”. Though we often like to refer to “free space” when speaking about the performances of antenna. As it is an ideal situation to compare antennas and to make calculations. The fact there is nothing also means there is “nothing” that can influence our antenna (negative or positive).
Lets say we have a single small spot which radiates in all directions equal in a free space environment. The antenna pattern would look like the one above. The shape of the pattern is similar to that of a round balloon or a “round” earth.
We call such an antenna: The isotropic antenna. This is an antenna which radiates in all directions equally. Thus, sadly that ideal “all direction” radiating antenna does not exist and besides: In our real live…the earth will always have effect on the antenna pattern.
Above the pattern of A dipole placed in that ‘free space”. The red line indicates the physical location and lenght of that dipole. As you can see at the ends of the dipole the pattern are pushed inwards.
Please notice; there is NO Round pattern anymore. A dipool doesnt radiate in all directions…it radiates mainly along its ax at 90 degree angle from it…
So if it isnt round….but more or less “squeezed” to a donut shape…… If we start to apply force on a balloon the balloon becomes smaller in one direction but bigger in another, we can see that balloon becoming bigger in one direction and smaller at the points where we are pushing it.
And just like that we have the basic of gain.
The total amount of energy which went “into” it…hasnt changed, we just focus that energy where we want it to be !
If we have “gained” something somewhere….that means..we loose something somewhere else.
THE DIPOLE HAS ALSO LOSS IN THE OPPOSITE DIRECTION!
As the total amount of energy always remains the same !
An antenna is not a amplifier, it just focusses the presented energy.
The above omni-directional pattern of the isotropic and the “pushed Pattern” of the dipole are both still in free space. That dipole has gain (green) in reference to the isotropic.
A dipole has 2.14 dBi gain (compared to that isotropic).
If we compare the gain to an isotropic we call it “dBi”.
If we compare the gain to a dipole it would be “dBd”.
The i stand for isotropic and d for dipole. Sometimes you will see gain expressed just as “dB”. That is not useful, as it doesn’t tell us to what its compared. It could be that needle in a haystack or compared to the biggest antenna you have ever seen…. we dont know… so just “dB” is meaningless.
We are now at our first step of conflict. You will hear many people complain about false gain figures. You will hear people say gain should be expressed in dBd since that is the only “real” antenna we can compare to.
But…a dipole has gain in one direction and loss in the other.
Another thing is that a dipole “gains” ground gain which could reach an as high as 6 dB
(we will explain that later)
… So “a bad person/salesman etc” can come up with different dBd figures. It would be logical that we compare gain at the “maximum” points of both antennas and at equal heights etc etc. But not all people/companies do that!
That’s why I prefer dBI…no where to hide, you can’t “cheat”
And if you wish to know the dBd figure you just subtract 2,14 from the dBi figure and you have your gain compared to that dipole.
LET US INTRODUCE THE EARTH !
At the above picture nr 1, we have placed the dipole above real earth. We might have noticed not only the “sides” are pushed outwards, from our beautiful round shaped isotropic pattern, but also the “top/bottom”….
YES, hold that thought! You are right! If the pattern is “more” pushed outwards…there must be additional gain somewhere else! And loss at an other point!
Super !, you have now learned, in real live (above real earth!) a dipole produces more gain than it does in “free space”. A valuable lesson which most people do not realize.
The amount of this “additional ground-gain depends on the height above ground, polarization and ground circumstances.
Most plots so far were nice 3D plots. But that’s not what we always see from our manufacturers. We often see plots like nr “4” and “5”.
THE AZIMUTH PLOT
IF we look from the sky, down at our antenna the pattern seen won’t be a 3D anymore. Just a 2D plot. This is called the “AZIMUTH PLOT”. It’s what we use with our rotator…”ill turn the beam towards you! You will move the azimuth plot of the antenna.
The azimuth plot can be seen in picture 2 (the round circle) and picture 4.
The azimuth plot contains 360 degrees.
THE ELEVATION PLOT
IF we look from the side to the antenna, the pattern we see is called the “ELEVATION PLOT”. This plot can be seen in picture 5. In real live the elevation plot is always 180 degrees as the earth is underneath the antenna “blocks” the other 180 degrees.
We will investigate both plots to more detail!
THE AZIMUTH PLOT:
Back to the azimuth plot. We look from above, at the antenna…but remember, its 3d in real live. The antenna pattern isn’t flat … it was a big beam …just like a flashlight !
Nr1 (in red of the above azimuth plot.)
This indicates the maximum gain we have, most of our energy is directed in that direction.
In the above eznec plot we can find in this case the maximum gain is 7,11 dBI.
NR 2 (in red of the above azimuth plot.)
You can see two times the number two. These are the -3dB beam width points. The points where the gain has dropped with -3dB compared to the maximum gain number of the antenna. The beam width is expressed in degrees. If your beam width is rather small, you will need to beam very accurate this is the case for very big yagi’s!
The -3dB points exist in the azimuth plain but also in the elevation plain. Besides this, it tells us how “exact” we must aim our yagi. It can also be used to calculate stacking distances. Which is on another chapter on our website
HOW ACCURATE SHALL I BEAM?
Well, 6 dB is one S-unit. 3 dB is 0,5 S-unit. With the above beam (a small 3elements) the -3dB beam width is 66,4 degrees. So there’s either 33,2 degrees to the right and to the left before a station drops a half S-unit….in that situation is doesnt really matter if you point your antenna a couple degrees to the left or to the right.
We are now going to talk about the back of the beam:
Nr 3, in the above picture, is the front to rear.
The front to rear figure, is the amount the signal is suppressed over the entire back of the antenna,
over the full 180 degrees!
NR 4, in the above picture, is the front to back.
This is the figure which expresses how much the signal is reduced in dB, opposite to the front (maximum gain, nr.1)
WAKE UP AND PAY ATTENTION!
WE LOVE TO SEE A HIGH FRONT TO BACK FIGURE IN OUR MANUFACTURERS ADVERTISEMENTS!
But more importantly would be to know what the entire back of the antenna does. (the so called: front to rear ! )
Some manufacturers don’t show antenna modeling plots. Any good manufacturer will do this, as it is proof of how “good” the antenna is.
In this case, the manufacturer could have advertised”: front to back over 40 dB! for the above yagi.
Most costumers are only interested in the front to back figure. But if we take a closer look at the second yagi.
We notice the front to back of this one is only -23 dB, though the front to rear lobs are down to almost -30dB.
You may be the judge, of which you find a better “back” of the antenna.
FIRST SIDE LOBS.
You might have noticed the second plot with the words “fist side lobs”.
As soon as a directional antenna becomes rather large, roughly above a 6 elements yagi. The antenna pattern will establish “side lobs”. You might come across ads or other written documents which say for example … the first side lobs are down -13 dB. This means when you change your azimuth of the beam the signal will drop down and come up again to a
maximum of -13 dB (compared to the maximum gain direction).
You will often read stories about first side lobs when it concerns stacking. Stacking for maximum gain produces very large first side lobs. It might sometimes be wiser not to stack for maximum gain but just slightly below, so you will have smaller side lobs. We don’t want side lobs cause they brings in “other stations” as well.
THE ELEVATION PLOT
First a quick reminder. The elevation plot was when we were looking at the side of the antenna. In the above plot you can see two “lines” one would be the free space elevation plot (the black line) and the blue line indicates the “real elevation plot” as we use in our daily life.
Again we notice above real earth there is additional “ground-gain” In this case the blue pattern has maximum gain at approximately 24 degrees and is 11,75 dBI at that spot. The free space gain of this antenna is only 7 dBI. So the “additional ground gain in this case at a 24 degree angle would be 4,8 dB.
And now back to the elevation plot.
The elevation plot has a couple highlights of interest.
The -3dB BEAMWIDTH is equal to the azimuth plot but now from a “sideward’s” perspective. In the above picture the “main” force was at a 24 degree angle and 11,75 dBI. The bottom pink line is at 11,4 degrees and since it is a -3dB beam width indicator the gain at 11,4 degrees is still 8,75 dBI!!
There are “side lobs” (blue). In this case it is only a minor one, producing 13,84 dB straight up. If we had a high placed yagi or a vertical antenna you might have seen many more side lobs. In order to keep things straight I have picked one with minor side lobs. These side lobs are more or less equal to the ones in the azimuth plain. They are now seen from a “vertical” perspective. Side lobs can come in handy as well.
THE TAKE OFF ANGLE:
If we stand outside and take a look at the horizon, the angle between earth and the angle we see is more or less 0 degrees. If we rise our point of sight slightly upwards from the horizon (say looking at the sun a hours after sunrise) we have an angle between the earth and the point where at that specific moment the sun is.
The elevation angle or take-off angle works the same. At some point we are looking towards the horizon… We adjust our spot slightly upwards…until we reach the point where the antenna produces maximum gain. The angle between the “earth” and that point is called THE TAKE OFF ANGLE.
The abbreviation for take off angle is TOA. On 27 MHz the take off angle is often used with earth as one point and the angle of maximum gain at the other. It can however be that the take off angle is given from another “point”. In the above plot the TOA of the antenna is 24 degrees and the antenna produces 11,75 degrees at that angle.
Above we have a 5/8 wave vertical at some height above earth.
The TOA is only 8 degrees, and maximum gain at that angle is 4,35 dBI. At 11 degrees the antenna produces 3,74 dBI. Which was much lower compared to our yagi at relative low height.
It would be important to remember that:
AN ANTENNA CAN HAVE A LOWER TAKE OFF ANGLE COMPARED TO ANOTHER, THIS DOESN’T MEAN IT HAS MORE GAIN AT THE SAME ANGLE!
Why is the take off angle so important?
Well, it determines how strong you are at different locations around the globe.
On 27MHz the propagation forms we use most often are F2 and E-skip (sporadic E propagation). Around the earth there is a imaginary mirror reflecting our waves back to earth.
My apologies for the bad drawing, but infact it is quite spot on! This is what is actually happening. At the left side we have a black vertical line which indicates our transmitting position. At the opposite right there is a receiving point. Now the picture is “straight” but should be bend in order to reflect the “real” world…but for now we have work with this image The bottom black line represents earth’s ground and the blue bar is the imaginary mirror (F2 layer for example).
I have drawn two “radiation angles”. A red and a green one.
We see that the angle between the green line and earth is small and the angle between the red line and earth is large.
The red line needs to travel a much further distance in order to arrive at the location. More distance is more loss.. there for the signal will be much weaker and perhaps in real live not even audible.
A LOW take off angle WILL PRODUCE A STRONGER SIGNAL FURTHER AWAY
If you want to know more about how to change that take-off angle to your favour please read:
How high should I place my beam ?
and
How high should I place my vertical ?
NOISE ?
Imagine your transceiver without the coax connected…let’s wait for a moment…yes, actually nice isn’t it? The noise is low, no QRM no irritating noises, no QRM makers etc, etc. Now let’s connect the antenna. We notice the “noise” floor rises.
It is the ENTIRE ANTENNA PLOT, WHICH BRINGS IN THOSE NOISES.
Just Imagine if we had no side lobs, imagine if that -3dB beam width was really small, and our beam had a fabulous front to rear.
What would that bring us…..?
Yes…less noises, hence more weak signals … DX!
And that forms the basic of the so called “low noise” yagi antennas these days.