Foundations of Amateur Radio

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Antenna gain and polar chart magic

Posted: 03 Feb 2018 09:00 AM PST


Foundations of Amateur Radio 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...
This posting includes a media file:
http://podcasts.itmaze.com.au/founda...teur-radio.mp3