Practical Antenna Theory - Part 1
One of the most often discussed topics in Ham Radio, and the most misunderstood, is SWR. If we were to take what is heard on the air as gospel, we would always be in pursuit of the elusive 1:1.00 SWR. Reasoning behind this pursuit is as varied as ham radio itself. But one thing is obvious when compared with the appropriate literature available from many sources, including the ARRL. Having a low SWR is no more an indication of antenna system performance than a high SWR is a guarantee of power being wasted. If you have only one source of technical information on antennas, make it the ARRL Antenna Book! We will refer to it quite a few times over the next few articles and the reading on the presented subject in the ARRL Antenna Book is thorough and illuminating.
In this series of articles we will learn some basics, some theory, and dispel a few myths that are generally perpetuated in the Ham Radio community. We want to take a practical, informed approach to antenna system problem solving, rather than “best guess” a solution, or folow the proliferation of urban ham radio myths. You will become armed with the knowledge needed to solve those previously “difficult” antenna system problems on your own.
The first thing to learn about the RF that flows in our coax is that it has unique characteristics. Yes it is an alternating current much like the 120 VAC coming from the outlet you plug your power supply into albeit at a much higher frequency. However, the primary difference in the reason for RF presenting characteristics that are quite unlike 60 Hz AC, is frequency. These characteristics are present regardless of the type of cable used as transmission line. You may use open wire twin lead as transmission line rather than coaxial cable. The use of the term “coax” in this study will be a general use term often interchanged for “transmission line” regardless of type or construction. Literally speaking, "coax" is an abbreviation for "coaxial" - referring to the construction being two concentric conductors on the same longitudinal axis.
We can describe, in general terms, RF characteristics by behavior: Pay close attention to these terms as they will be used throughout.
Skin Effect
This is a term that describes the theory that RF does not travel inside of a conductor as does DC from a battery or 60 cycle AC from the wall outlet. Instead, RF is believed to follow the outside of a conductor (hence the term skin effect). As we will see later, this is significant in terms of how fast RF is allowed to flow down the transmission line.
Traveling Incident Wave
This term is used to describe RF waves traveling up the coax to the antenna. Notice the term describes only the initial power traveling in the direction toward the antenna. This will be significant as we define other terms. Traveling wave simply indicates that an alternating cycle of power or voltage is moving with time along the transmission line. For mathematical simplicity, traveling waves are usually discussed for a specific frequency, as is SWR.
Traveling Reflected (Return) Wave
This term is used to describe the un-absorbed RF waves that flow on the coax away from the antenna (or load) to the source.
Velocity Factor
This term describes the delay RF encounters as a traveling wave, caused by the material used as dielectric in the transmission line. Some materials slow the traveling wave significantly, while some dielectric materials do not. All dielectrics are measured against the standard of air (sea level and 50% humidity) as a value of 1.00 or 100%. Generally speaking, the larger the velocity factor (i.e. numbers approaching 100% or 1.00) the closer the resemblance to open-wire feedline - which has a velocity factor range of .98 to 1.00 (virtually no delay of propagation). Do not confuse open-wire twin lead transmission line with the close cousin, twin-lead “window” or “ladder” line. Ladder line has a much higher velocity factor (i.e. lower numbers) than open-wire line.
When RF must flow on the coax and encounters a dielectric material such as polyurethane (PE) the rate of travel along the conductor is slower. How much slower is indicated by the velocity factor expressed in comparison with the velocity of an air dielectric (e.g. .66 or .87 as in round coax). More on this subject later.
Standing Wave and Standing Wave Ratios (SWR, VSWR, etc)
Standing waves are the result of the encounter between forward traveling waves and return traveling waves. This being said, it is also true that there is no standing wave if there is no forward power. The assumption is always made that the frequency of the incident and return waves remains the same as does the length of the antenna and transmission line. This encounter is best illustrated by the figure seen here.
Notice that where the traveling wave peaks approach the same polarity, so does the magnitude of the standing wave (even though the waves are traveling in opposite directions). The polarity of the standing wave magnitude is determined by the magnitude of the additive waves (i.e. two positive magnitudes produce a positive standing wave magnitude and two negative magnitudes produce negative standing wave magnitudes).
When the comparisons are of the voltage magnitudes, we speak of VSWR or voltage standing wave ratio. There are other comparisons that are similar, but are rarely used in ham radio. These will not be discussed here.
So now that we have defined some of the more common terms we use when talking about transmission line and antennas, let’s look at what causes SWR.
The most common cause of SWR on the coax is a mismatch of characteristic impedance. This choice of words is important in understanding SWR. A transmission line is not a simple resistive value as we would think of it in passive components in the rig we use. Nor is it a simple inductive or capacitive reactance. Not only that but it should not be characterized as a “length of shielded wire”. Transmission lines are actually complex networks containing the equivalent of all the three basic electrical components: resistance, capacitance, and inductance. Because of this fact, transmission lines must be analyzed in terms of an RLC network.
The two conductors that make up a transmission line (the two parallel wires of open-feed line or the shield and center conductor of a coaxial line) are arranged in such a way that the electrical field of one conductor is offset or nullified by the opposite electric field of the other conductor. The two conductors in such close proximity exhibit a complex set of characteristics that we call impedance. The illustration below is from the ARRL Antenna Book chapter on transmission lines.
This complex characteristic impedance discussed is expressed in writing it like this:
R +jX
It is expressed this way to show the complex nature of transmission lines (or antennas). The “R” term represents the “resistive” or often called the “real” part of the characteristic while the + or - “jX” indicates the reactive or “imaginary” part of the characteristic. It is possible to have a zero value for the reactive term, in which case the only important value is “R”. Both antennas and transmission lines exhibit this complex character. The theory underlying antennas and transmission line characteristics is explained very well in the ARRL Handbook and in the ARRL Antenna Book. It is well worth the reading to fully understand this complex subject. This discussion is merely food for thought.
A mismatch may occur in either part of the characteristic impedances of the source or load (i.e. coax or antenna). This mismatch gives rise to inefficiencies in the transfer of power from the delivery vehicle (your coax) to the consumer destination (your antenna). When the load is not able to consume all the power delivered, the undelivered portion is returned to the source. This returned and undelivered power is the reflected traveling wave defined earlier.
Impedance mismatches occur for various reasons. The most common are: transmission line impedance and antenna feedpoint impedance mismatch, open feedline circuit or shorted feedline circuit.
You may encounter any or all of these conditions as your ham radio experience grows longer.
Should we be concerned when a VSWR can be measured on our coax? That depends. Remember we discovered that your feedline and antenna are complex in character. So too are the conditions under which we may become concerned about VSWR. To simplify the answer somewhat, let’s look at a common situation.
You are installing a 2 meter mobile rig in your car. The antenna is installed and you ran RG58/U cable to the rig and you follow the manufacturer’s instructions to the letter. The VSWR you measure is 1:1.00. Perfect right? Why then does it not hear or talk very well?
We are assuming the manufacturer’s instructions include proper grounding instructions, which you follow. Is this 1:1 VSWR a problem? It is not supposed to be...right?
First let me dispel myth #1 - That you will burn out the final transistors in your rig due to a 1.5:1 or even a 2:1 VSWR. Most modern VHF rigs will handle a very wide SWR range without damage. This relatively minor VSWR will NOT harm most modern rigs on the market today. Higher VSWR situations may make RF power transmission very inefficient for the transmitter. The inefficiencies cause the final transistors to work harder to produce power – and yes (if the SWR is high enough, as in a short, and the rig is in transmit mode long enough, there could be thermal break down in the finals. But notice, this is due to the loss of efficiency, NOT reflected RF.
How about performance? Why does this 1:1 VSWR not affect performance?
Good performance is a result of removing as many inefficiencies as possible from the antenna system. Remember we discussed how impedance mismatches give rise to inefficiencies in the transfer of power to the antenna. That power must go somewhere.
It does not get transferred to the rig finals: goodbye Myth #1.
Power that is unused by the antenna will continue to travel on the transmission line back and forth from source to antenna until it is absorbed dissipated) by the actual (read it “real”) resistance of the transmission line wire in what is known as I2R losses. This amounts to an attenuation that can dissipate as much as 2-3 watts out of 50 watts leaving your transmitter (given the RG58 coax in our scenario) from only a 3:1 VSWR or lower.
More importantly, a 2.0:1 VSWR will attenuate incoming signals as well. The graph below illustrates the amount of loss due to attenuation by the coaxial feedline. It also makes clear that investment in high quality feed line could pay you back with very good weak signal performance.
On point of our discussion, the graph illustrates plainly that high SWR readings (i.e. above 3.0:1) are not as detrimental as some would have us to believe.
Since we are considering only the antenna system (leaving the rig or tuner out of it for now) we must consider the loss of efficiency to be due to the difference in characteristic impedance of the transmission line and the impedance of the antenna at the feed point. This difference could be due to an improper selection of coax type (RG59 instead of RG58 or vice verse). A mismatch could also be due to an improperly tuned antenna. Tuning an antenna usually involves changing the characteristic impedance at the feedpoint by changing the length or adjusting a matching network that is part of the antenna. We will look at proper selection of feedline type and antenna feed point matching later.
Now let’s change the scenario to 440 Mhz. The major factor in this scenario, (even if the antenna is perfectly matched) is the attenuation by the RG58/U coax. It is a whopping 10 db per 100 feet! That is an increase of more than 20 over the 2 meter scenario. Compare that to only 1.5 db at 3.5 Mhz.
This is a scenario where 2.0:1 is significantly high due to the amount of power that would be dissipated in the coax due to SWR induced I2R losses. Accordingly, signals being received could be attenuated to the point of unreadable, simply being lost in the coax.
So is it worth hours of cutting and measuring for that .5 SWR decrease in terms of signal received? Only you can make that decision. But given the information learned so far, you should think long and hard before spending tons of time for nominal improvements in lower SWR readings.
NEXT TIME:
What is radiating coax?
Proper selection of feed line type.
Antenna matching techniques.
Does a matchbox remove SWR?
Does my matchbox tune my antenna or my radio or both?