Keep in Touch Series
The Modern R/C Radio System Explained, Part 4

The Antenna and VSWR



In days of old, when knights were bold And radio was not invented They sent their thoughts on lumps of quartz And their envelopes were cemented.
You probably have a good idea on how a radio signal gets from a transmitter (TX) to a receiver (RX), but have you ever thought about the role that the antenna plays?  VSWR (Voltage Standing Wave Ratio) is what it’s all about.  Without the correct VSWR, your TX is worthless. First, let’s consider what happens between a transmitter and a receiver.  In the early days, it was discovered that a coil of wire with a voltmeter attached to it registers a voltage across the coils whenever a spark occurred near it.  In fact, the spark transmitter, as it became to be know, became a workbench for ongoing experiments that led the great inventors like Faraday, Hertz and Marconi to the wireless technologies that we have today. Essentially, a radio transmission occurs when electrical energy get imparted in to the air and it becomes electro-magnetic.  To do this the electrical signal has to be constantly changing in voltage and current flow.  A spark, of course, is a good example of an electrical current in violent and instant change.  These days, transmissions induced by sparks are often noticed as annoying interference, like the spark of a spark-plug in  our automobiles. A modern radio transmission signal is developed as an electrical current that constantly alters from its maximum voltage in one direction (positive) to its maximum voltage in the other direction (negative).  A bit like constantly reversing the battery connections to a motor.  The voltage is changed smoothly in what is known as a sine wave. This type of Alternating Current is known as AC.  The household current is AC and it alternates positive to negative and back to positive 60 time per second (60 Hertz, 60 Hz) So, does household current transmit?  Yes it does.  But because it is such a low frequency (60 Hz) it doesn’t do it very well.  It doesn’t do it well also because it always drives a load (a TV, a vacuum cleaner) that absorbs all of the available energy. A radio signal is usually a much higher frequency (In R/C it’s around 72,000,000 Hz, 72 Mhz -- Mhz is pronounced “Megahertz”). The radio signal is sent to an antenna rather than to an appliance such as a TV.  So how does an antenna work.  That’s the subject of this article. If did your homework assignment described in Part 3, you may have noticed that the ripples in a pond radiate from the point where the stone fell into the water.  An alternating current also does this.  When the voltage is high going into a wire, that high voltage does not instantly get to the other end, rather it travels down the wire at a finite speed at approximately 2/3 to 3/4 the speed of light (for copper wire).  That means that by the time that high voltage gets to some point along the wire, the voltage at the end has altered (remember an AC voltage is constantly changing) and is now a different voltage.  So, if you could take a voltage snapshot of the length of wire with an AC current in it you would see different voltages all along the length of the wire in a sine wave picture.  There are instruments that allow you to tap into a wire and move it along to actually read the different voltages. If you can picture this easily, you will see in your minds eye, that if you take another snapshot an fraction of a second later, the sine wave will still be there, but it will have moved down the wire.  This is what is called a “Traveling wave”.  It is almost exactly the same as the ripples in the pond.  The wave always looks the same, but the crests and troughs constantly travel outwards towards the pond’s edge. The traveling wave in a wire assumes that the wire, for all intents and purposes, is infinite in length, or it is terminated in an absorbing load.  If we were to consider the wire to be connected to an antenna, then we would see that it is not infinite in length.  The ripples in the pond assume the pond is an infinite size - they know nothing about the pond’s edge until they reach it. For an antenna to work efficiently, it must be of a length close to an even multiple of a quarter wave length.  Going back to the ripples on the pond.  The waves had crests and troughs, obviously.  How far apart were the crests on your ripples?  6 inches, maybe.  Well, what ever it was that is the wave length.  It’s the same with a traveling wave in a wire.  If you divide that length into quarters, that is a quarter wave length.  An antenna, therefore, should be 1/4, 1/2 of a full wave length long to be efficient. Going back to the ripples, let’s consider what happened when one of the ripples reached the pond edge.  Since they could go no further, but it still contains an amount of energy, the ripple seems to “Bounce” off the edge and it gets reflected back into the path of new ripples coming out from the center of the pond.  The new ripples and the older reflected ripples collide;  their crests and troughs mix.  When a new crest and an old reflected crest mix, the resulting crest can actually become larger than either of them.  When a crest meets a trough, they cancel  each other and the wave actually decreases.



Well, since an antenna is not connected to an absorbing load, it acts like the edge of a pool. Just like the ripples in your pond, the water traveling wave has nowhere to go. When the electrical traveling wave reaches the end of the antenna, it, also, has nowhere to go.  So it does the same as those ripples; it reflects back down the antenna wire towards the TX.  The reflecting waves colliding with the new waves coming along from the TX. Because this is a dynamic environment, it’s always changing, I’m going to ask you to use your mind’s eye, again.  Imagine, if you will, if you could move the tip of the antenna with respect to its base (make it longer and shorter).  That would affect the timing of the reflected signal vis-à-vis the signals from the TX.


If you adjust it so that it was in perfect synchronization with the new wave coming from the TX, the reflected wave crests and the new crests would collide at exactly the same spot on the antenna.  This would cause the traveling wave to stop (or to appear to do so).  To become stationery.  To become a “Standing wave”.  If we were to use that device that measure crests and trough (I mentioned it earlier), and move it along the length of the antenna, we would see the crests of energy always at the same place. The synchronization of the new signal and the reflected signal will occur when the length of the antenna is 1/4 wave length. Assume a 72 Mhz signal traveling in an antenna at 2/3 speed of light (200meters per microsecond).  This will have a wavelength of 120 -  inches.  Therefore, a 1/4 wavelength is 30 inches.  (Is the antenna on your TX about 30 inches?  If so, this is why).  Some internal electrical circuits used in modern transmitters can alter the apparent speed of the signal, thus changing the required antenna length. Although there are many other factors that affect this, we will consider only antenna length.  If the antenna length changes from 1/4 wave length, the amount of non-synchronized reflected waves increases, causing the “Standing Wave” to move.  This error can be measured as a percentage of the transmitter signal. A perfect reflection is in exact synchronization and is therefore equal to the transmitted signal.  The ratio of these signals is therefore 1:1 (referred to as a ratio of 1). As the reflected wave becomes less synchronized, the two signals become less equal and the ratio, therefore, changes.  this ratio is called the Voltage Standing Wave Ratio, VSWR.  A VSWR of 1 is perfect.  A VSWR of 1.2 is pretty good.  A VSWR of 1.5 is very marginal.  A VSWR of greater than 1.5 is  usually unacceptable. Now you’ll make sure your TX antenna is fully extended before you fly, won’t you?? Just one more point, you’ve probably seen those short flexible antennas that are nowhere close to 30 inches in length.  The way they do this is to place a coils of wire at the base of the antenna.  This “Fools” the signal from the TX into thinking the antenna is longer than it really is.  This, of course, is a very simplistic description.  There are many very good books available if you want a more detailed explanation. This concludes my four part series on R/C radio theory.  The detailed technology is much more complex than I have indicated in these articles.  I deliberately kept the explanations as simple as I could to illustrate the general concept of how an R/C radio system works. Fly high and enjoy.