ARLA/CLUSTER: Polarização da antena e você

[João Costa > CT1FBF]

Foundations of Amateur Radio #172


*Antenna Polarisation and you*
The first time I came across the concept of antenna polarisation was a
decade before I became a radio amateur. To connect to the internet while
driving around Australia I became the proud owner of a portable satellite
dish. Portable in the broadest sense of the word, 150 kilos with a dish
that's 2.4m high, 1.8m wide, steel base, electronics, power and patience to
erect and point.

The dish has a receiver and transmitter component that needs to be aligned,
just so, in order to be able to have two-way communications using 5 Watts
into geosynchronous orbit. The transmit and the receive are exactly 90
degrees offset from each other. One is called horizontal polarisation, the
other vertical.

The first thing to observe is that if you're using the wrong polarisation,
it doesn't really work well. We'll get into what is right in a moment.
Depending on where you you ask, the definition of not working well can be
as bad as 40 dB loss.

Just let that sink in for a moment.

If you want to punch through with more power, you'll need to bring 10
kilowatt with you for the receiving station with the opposite polarisation
to hear 1 Watt.

If you're using a VHF or UHF FM radio in your car, you're likely to have a
vertical antenna. The combination of a repeater on a hill and a radio in a
car adds up to much more than the the two alone. The line is blurred today
because repeaters are very popular and home-base stations are becoming
smaller and smaller by the week, so vertical antennas for VHF and UHF at
home are today just as common as they are on cars.

It wasn't always that way. In fact, in HF, it's almost never that way and
if you're a fan of Tropospheric Ducting or long distance VHF, then you'll
also shy away from vertical antennas.

Let me explain.

If you want to erect a HF antenna and you want it to rotate and you want it
to be high enough off the ground, you'll build the simplest mast you can
get away with. Imagine a HF Yagi. It's got several elements, long to short
along a boom, rotator somewhere in the middle. If you mount this Yagi
horizontally, your mast will be around half a wave length in height.

If you mount the same Yagi vertically, aside from the height discussion -
should it be mounted higher or not - now your mast becomes another
interfering element within your Yagi. The steel wires that keep your mast
standing will also interfere with the Yagi elements and your elements will
be closer to the ground where they can potentially cause harmful radiation.

So from a mechanical perspective, putting a Yagi on a mast vertically is
not trivial.

>From a radiation perspective you may theoretically get some gain, but
adding an element or two will make up for any potential gain that a
vertical arrangement interacting with Earth might assist with.

There's another reason. The ionosphere. It sounds like a smooth billiard
ball, it's drawn as a uniform layer around the earth, but in reality,
clouds and their appearance are much more likely to represent the actual
surface shapes that the ionosphere presents to your radio waves.

A signal coming in one way is unlikely to come out at the other end in the
same way and vice versa.

That's HF. On VHF and UHF a horizontal signal and a vertical signal when
they're used with line of sight are pretty similar, but once you get beyond
that, a horizontal signal will travel further, how exactly is a story for
another day. If you're doing point to point VHF or UHF contesting,
horizontal is the way to go.

What about a single HF vertical?

It's excellent for a portable station, it is simple to set up, works in all
directions, but it means you'll be able to hear all the local man-made
noise as well, so find a quiet spot near the beach if you can.

So what's the right way? Almost always horizontal, except on cars or when
you're on a DXpedition on a beach sipping pina collada and getting caught
in the rain.


I'm *Onno VK6FLAB*

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