ARLA/CLUSTER: Episódio n.º 139 da série " Foundations of Amateur Radio "

[João Costa > CT1FBF]

Foundations of Amateur Radio #139

Antenna gain and polar chart magic

If you've ever been on the hunt for an antenna, and let's face it, in
amateur radio that's pretty likely, you'll get information about the
gain of an antenna. Often someone will tell you that this one has 12
dB gain, versus that one which only has 9 dB.

As an aside, I've seen a few videos where people are comparing sound
levels and mention that without the fan, there is only 3 dB less
noise. What they don't realise is that 3 dB means HALF the noise.

The same is true with an antenna. That 9 dB antenna has half the gain
of a 12 dB antenna.

In the past I've talked about gain. It's always in comparison to
something else. If I say "that antenna has 12 dB gain", I'm actually
saying: "that antenna has 12 dB gain when compared with an isotropic
source". To jog your memory, an isotropic source is a theoretical
source of electromagnetic radiation. It cannot actually exist. It
radiates uniformly in all directions.

Now when we talk about gain, we're saying that our new funky antenna
radiates better in some or other direction than an isotropic source.

As a consequence of this, it also means that it radiates worse in
other directions.

So antenna gain is a trade-off between radiating everywhere like an
isotropic source, and only radiating in one direction like a laser
beam. As an aside, a laser beam could be seen as an antenna for light.
It radiates much better in one direction than in any other, and given
that light is also an electromagnetic radiation, we're still playing
in the same area of physics.

If you've ever shone a torch light onto a wall, you'll have noticed
that the light isn't uniform. There are brighter and darker areas.
It's the equivalent of differences in gain. Some bits of the light are
amplified more than other bits. If you compare it to something like a
candle, not exactly an isotropic source, but remarkably close, you'll
notice that the light is uniform.

A torch doesn't shine from the rear, the energy from the light that's
missing from the rear comes out the front and that's gain.

Radio antennas do the same thing.

In order to compare antennas with each other we've devised several
tools, the most common is a polar plot. It's a circle that is divided
into 360 degrees, and inside the circle are concentric circles with
gain numbers attached to them. Often, but not always, the outside
circle has 0 dB as a value and you'll see -10 dB, -20 dB and so-on as
you get closer to the middle.

Weaker signal is drawn away from the outer edge, stronger towards the
edge. No signal in the middle.

As you walk around your torch, you could record the strength of the
light. Where it's strongest you'd make a mark on the edge of the
chart. Where it's weakest you'd mark towards the centre of the chart.

If you were to take your torch and take a slice through the middle of
your battery, through the reflector, through the globe, through the
lens and out to the wall, you'd end up with what a polar chart is
displaying.

Of course you can slice through your torch in any direction and make a
chart, but traditionally, you'd slice it horizontally and vertically,
or azimuth and elevation - and if you can't remember which one is
which, an elevator goes up.

A torch is generally symmetric, so both charts should be the same,
unless your reflector is a weird shape at which point the two charts
will likely be different. Antenna charts work the same way. The polar
graph is showing the signal strength as you walk around the antenna -
twice - once for the horizontal slice and once for the vertical one.

As I said, the outer edge of the chart is set at 0 dB. This is because
you need to compare full signal to less signal. If you are comparing
multiple antennas and they all have the same 0 point, you can draw
them over the top of each other and see their differences. This allows
you to compare wildly different antennas with vastly different amounts
of gain.

I must also point out that you can get more signal strength in two
ways, more gain from the antenna, but also, more power into the
antenna. This means that your choice of antenna is dependent on what
gain you want and how much you're prepared to pay for it. I could
light up an omni-directional antenna with 300 kilowatts, or I could
use a very high gain antenna and use 5 Watts. It all depends on your
purpose.

Final comment. Beam-width of an antenna, the main direction of
radiation, is often based on where the signal strength is half, so 3
dB less than the maximum gain. That location will determine the angle,
remember the chart is expressed in 360 degrees, so you'll be able to
see the beam-width on the same chart.

Polar charts, lots of hidden meaning inside a pretty picture.

I'm Onno VK6FLAB

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