Antenna as part of the radio system
by Jim Gunderson, AD0ZM
We can think of a functioning amateur radio ‘station’ as a tripod – the three legs are Power, Transceiver, and Antenna. If any of these are missing or out of whack, the tripod falls over, and the ‘station’ is inoperable or very inefficient.
In this blog, we will focus on the antenna portion of the tripod, what it does, how does it do it, and how can it go wrong. Now don’t worry – I’m not going to fill the rest of the blog with equations, curl operations, or any calculus. We will talk about the physics, but more by way of metaphors. Our focus is on Amateur Radio for Emergency Communications (EmComm), and you are not going to be calculating any derivatives in the field. So, let’s get started!
What does an antenna do anyway?
Essentially, an antenna is a transducer – it accepts energy in one form and changes it into energy in another form. Specifically, it takes alternating current electrical energy in and spits electromagnetic waves out – or the other way around, it can take electromagnetic waves in and spit alternating current electricity out (this makes it a bidirectional transducer). That’s it, that is all it does. “But Jim, that sounds like any old piece of wire is an antenna,” you say. Yep – it is! But there is one key thing: an antenna is a passive element (which makes it a passive bidirectional transducer) – it doesn’t add any energy into the system, but it can be really inefficient and waste a lot of energy by turning it into heat. That heat doesn’t get the message out.
Everything about antenna design, construction and use comes down to two things – make the antenna as efficient as possible to maximize the energy transfer, and make the energy go where you want it to go. This latter point is important. A perfect ‘isotropic’ antenna sends the radio waves out in all directions equally, think about the sun or a light bulb. But, you don’t want to talk or listen in all directions. You want to hit that repeater on top of the building downtown a mile due south of you, or send a message to the Emergency Operations Center which is 15 miles to the north-west.
We know that an antenna can’t add any energy to the radio signal – but it sure can change the shape of the radiation pattern, for better (good) or for worse (not good). So the size, position, orientation, and height above ground all affect that radiation pattern, as do nearby objects, buildings, and terrain. These all change how the electrical energy is sent (or received) by the antenna.
In this post we will focus on the resonance of the antenna and a later post will look at the ways to change the radiation pattern.
How does an antenna do this wonderful thing?
We agreed that we would skip the equations, so we are going to explain this in words. By definition, it won’t be a precise, nor will it be as accurate, but it will get the message across, I hope. We will start with the transmission side of the transducer, what happens when you push the Push To Talk button? Through the engineering inside your radio (whether it is a little HT or a huge $10,000 rig) the sound of your voice is changed into alternating electrical current coming out the antenna connection on the radio. That signal arrives at the antenna.
The antenna is just a conductor, and so it conducts the current. Since it is alternating current it flows into the antenna for half a cycle, and flows back out during the other half. You can think of it like a wave sloshing in a bathtub, back and forth, back and forth. You might think it would just keep doing this forever, but we have a funny old universe.
It turns out that when an electrical current flows in a conductor, it causes a magnetic field to form, it transfers some of the electrical energy into the magnetic field energy. Well, that’s not so bad is it? The energy is still all there, just in a different form. But we have alternating current, so after half a cycle the current flow reverses, and the magnetic field begins to collapse. So what happens to the energy, you ask? Well, some really smart physicists figured out that when the magnetic field collapses it turns the energy into an electric field. So when we have alternating current in a conductor, we get a series of electrical and magnetic fields playing catch with all that energy, back and forth between alternating electrical and magnetic fields. And that process generates electromagnetic waves that take all that energy you pump in from the transmitter for a ride – traveling at the speed of light away from the antenna. You just made a radio transmission.
What affects its efficiency?
So, in a perfect world all the energy from your transmitter gets sent out as radio waves, whether it is 5 watts from an HT or 1500 watts from a massive power amp. Of course the world isn’t perfect – you get some losses because the conductor in your feed-line, and in the antenna itself are not perfect conductors. So you lose a little bit of energy every time the wave runs out to the end of the antenna and makes the round trip back. If the antenna is perfectly resonant, the energy gets transduced into EM waves completely on one trip – you have a standing wave with no reflected signal: and standing wave ratio (SWR) of 1.000000. That is a good thing. If the antenna is not resonant however, some of the energy gets reflected back along the antenna, and (like the bathtub wave) when it hits the other end (at the transmitter) it gets reflected once more out to the antenna, back and forth, back and forth.
But every trip causes a little more energy to be wasted by the resistance in the conductor, and turned into heat. So the worse your SWR – the less power you get out the antenna. It also can do bad things (think unpleasant smells, smoke, blown parts, trips to the radio store for new equipment) to the final stages in the transmitter, so keep the SWR low.
That’s not the end of the process, you need those radio waves to arrive at your intended target, so they have a long trip to make. Okay, not really long for electromagnetic waves – in an ideal environment (like free air or outer space) they will keep going forever. Think about looking up at the stars – if you manage to get a glimpse of the Andromeda galaxy those light waves have traveled 2.4 million light years and you can see them. But that’s a galaxy, you’ve got an HT, and that’s in free space, you are on a planet. And practically everything between you and your target will eat your radio transmission.
Trees, buildings, hills, almost anything will attenuate your signal to some degree. Water is bad. But you say, “Jim, I am not going to transmit through water!” Think about that HT clipped to your belt. On the one side the antenna is facing open air – perfect for sending a radio transmission to infinity and beyond. But, on the other side it is right next to you – and you are 70% water. Figure on about a 6 dB loss going through you – less than 1/4 of the power makes it out the other side. So think about which direction it is to the target, or better yet, pull the HT off your belt and raise it up to head level.
How can you make it better?
Well, it is not so much how you can make it better, as much as what can make it worse. Our reference antenna is a perfect 1/2 wave dipole with an SWR of 1.0, everything else compares to this. Of course it may not be possible to use a perfect antenna in some cases – imagine hiking through a festival with a Dipole on your back! You will most likely use a much smaller antenna.
The effect of using a smaller antenna is significant. Yes, the electrical characteristics can be adjusted so that the transceiver ‘sees’ it as a perfect 50 Ohm connection, but there are costs. Let’s look at the ‘rubber ducky’ that came with your HT. It is probably about 4 inches long and covered with a rubber coating.
The antenna is basically a coil of wire (an inductor) that is tuned for the center of the band. So on that frequency it is resonant, and has its lowest SWR. But there is another factor besides resonance. To be an ideal radiator – the length of the antenna comes into play. This is known as the aperture. The closer the aperture is to the wavelength the more efficient the radiation is. While electrically the rubber ducky is resonant, the length is far short of the ideal aperture, so it looses efficiency. That means more of the signal bounces back and forth – so that means a higher SWR, and that means more loss of radiated power.
There is also one other effect – the range of frequencies with good SWR gets smaller and smaller. So, with a 1/2 wave dipole you can get a low SWR across the entire band, the short antenna gives you a very narrow range where the SWR is good, and over the rest of the band it can be really high.
So, to summarize
The antenna is a key part of your radio system – it is the transducer that changes electrical signals into electromagnetic (radio) waves and vice versa. In a perfect setup 100% of the electrical energy going in comes out as radio waves, but the world isn’t perfect.
SWR is one measure of the inefficiency of the antenna, the higher the SWR the more round trips the electrical energy makes, the greater the losses to heat.
As second aspect that can affect the efficiency is size – while shorter antennas can be made to ‘look like’ a perfect 50 Ohm antenna, the coupling between the antenna and the outside world (the aperture) is less than perfect. As a result the antenna has lower gain and is less efficient.
Choosing the right antenna for the mission is key and will be covered in a following post.
If you want to dig into more about the effects of size – check out this Wikipedia article: Rubber Ducky Antenna