Part 2 - Aerials (Antennas)

Let's re-state that the point here is to keep it simple, if you are a theory buff then I apologise but there are plenty of pages discussing if electron acceleration causes radiation etc. Here we just need to get over some simple ideas so people can do a bit more of their own research and learn more.

Can a piece of wire be that complicated?

The answer seems to be yes – but only if it’s called an aerial (or antenna – see more confusion!).

So what makes this wire so confusing?

Well it has a job to do and that is to convert moving electrons inside it into radio-waves (Radio Frequency - RF) travelling from it. If you want to understand that then please have look elsewhere as it’s quite a complicated subject. for now, let’s just accept that it happens!

There are three main things we want the aerial to do:

1 Get as much of the transmitter’s power into the aerial so it can be radiated.

2 When it is radiated we want to get as much of it sent in the direction we need it to go

3 We need it to fit the space we have available and our circumstances

How it does this is where the complication starts!

But then number 1 is not really the aerial’s job alone, it is the feeder and the aerial – the complete system.

So let’s look at this first (makes sense to me) – how to get as much energy as possible into the aerial? Well, the transmitter has an output socket we connect to a cable and that connects to the aerial. The cable we call a feeder because it feeds the aerial.

Have you heard of impedence? Well it can be complex (no pun intended!), or we can treat it simply so let’s do the simple way first. Think of it as a resistance, it isn’t ,but tow the line for a bit.

We go back to water again here – if you join the tap (transmitter output) to the feeder (pipe) and then the aerial (drain) and turn the tap on water flows out the drain – simple. The most consistent flow is if they are all the same diameter like a hose pipe and its attachments.

Right, now let’s change the diameters of the pipe or the drain, it doesn’t matter what combination we use, narrower, fatter we will alter the flow of water and it won’t be as efficient.

Now let’s look at that aerial SYSTEM. Transmitter - 50 Ohm impedance, Feeder 50 Ohm impedance, Aerial – 50 Ohm impedance. All the same – efficient! Change any of the impedances in the system and the efficiency suffers, we don’t need to know by how much yet, just that it does.

Why 50 Ohms? Well a half wave dipole (a what?! – I’ll get onto that) aerial has an impedance of about 50 Ohm in the real world, someone must have plumped for it over 75 Ohms at some time.

What if something in the system isn’t 50 ohms?

Well let’s change the aerial so it has an impedance of 100 ohms. You need to put a matching circuit in the system to change the 100 Ohm to 50 Ohm or things go horribly wrong at the transmitter. The Tramsmitter expects that all its power output will be taken away (absorbed) by the aerial system, if you change an impedence some will get sent back (reflected) to the transmitter and it will not be happy!

So now we have another component the matching circuit – and true to form this has loads of variations and lots of names for the same thing - Antenna Tuning Unit (ATU) or Antenna Matching Unit (AMU) it doesn’t matter, they all do the same thing.

Remember our Nirvana of a system, all the same impedance throughout so let’s all use a half wave dipole, we don’t need an ATU or AMU so here endeth the story.

You must be kidding!

Let’s move onto our next items – number 2 & 3 – getting it the signal going in the ditrection we need it to go from the confimes of our garden (or field!)

Lets' imagine we have decided on a half-wave dipole (we'll soon come onto what that really is) as an aerial because it is the simplest and has a 50 Ohm impedance .

I could just look it up on Google, find what length it needs to be for the frequency we want to use and that's it...? Well it would be if we could raise it high enough!

The half-wave dipole is only at it's best when its placed horizontally and at a height equal to half of the operating wavelength. So we want to transmit on 7 MHz which has a wavelength of 40 m, half of that is 20 m, so the dipole will operate very well if it is mounted 20 m high (see Part 3 to see this isn't entirely true, did I mention this can be confusing?)

Is your house 20 m high?

Do you see where the 'half-wave dipole' comes from now? The 7 MHz wavelength is 40 m so a half-wave at this frequency is 20 m. A half-wave dipole for 7 MHz is therefore 20 m long. The Dipole bit means it is two (di) poles (poles is a technical term like magnetic poles, opposites) in practice the poles are seperate pieces of wire. So the half-wave dipole is a piece of wire cut in the middle to make two 'poles'. We feed the aerial in the middle, where the wires were split - more on that later! For ease we will just call the half-wave dipole the 'dipole' from now on. [when I was young I took my FT290 2m rig on a family holiday to canterbury, I was 14 and camped in my own tent, with a lead to the car battery and a coax feeder to a half-wave dipole for 2 m (yes 1 m long, split in the middle - well done) With the dipole on a pole oustide the tent it must have looked like a small cross and I was known for the rest of the holiday as 'Bishop Dipole' - I digress].

A dipole aerial and its measurements in meters.....

To get the wavelength we divide the speed of light by the frequency and then times by a correction factor for 'reality' which is usually 0.95. This is the same as dividing 142.5 by the frequency in MHz (play around with a calculator and you should get it), rounded up to 143 is easy to remember and gives a little extra wire to trim.

If the dipole is mounted high enough it will radiate the radio signal in a doughnut type shape around the middle of the dipole.

That's great, we get a really large signal firing off to the sides and at a low angle (a low angle is good for long haul communications - DX - trust me).

But I want to work stations in the opposite direction, and I can't get the dipole that high.

Well to work other directions you can turn the dipole - is your garden wide enough?

If the dipole can't be mounted that high then the doughnut shape changes and the RF signal is transmitted higher into the sky (not good for DX).

Summary

Already we are seeing that the dipole aerial is simple but where we place it and what we want it to do are putting constraints onto it. Overcoming these constraints has invoked the 'necessity is the Mother of all invention' quote and my, have they been inventing different solutions ever since!!!

Just remember that nearly all antennas are either a half-wave dipole or a poorer relation (now that will open a can of worms!). Even a Yagi beam with loads of gain usually has a dipole as its radiator. See those Mobile phone masts with funny aerials? They are just lots of dipoles with a plasic cover (they are aligned - phased - to make the most of the situation but they are still dipoles).

Just to add another fly to the ointment - The half-wave dipole only works properly on the frqeuency it has been designed for (and also on 3 times that frequency - 3rd harmonic - a later story). So if we want to tranmsit on 28 MHz or 3.5 MHz, or any other frequency we need another dipole! Why we would want to transmit on those other frequencies will be covered in the propogation section when I've written it.

Let's try to see what height really does to a dipole in part 3