Antenna types and antenna polarization
The most common, and perhaps the simplest, antenna is the half-wave dipole antenna. As the name suggests, it measures close to one half wavelength from one end of the antenna to the other. Because half-wave dipole antennas can be quite long—a 40m half-wave dipole antenna, for example, is about 66 feet long—they are normally mounted horizontally between two vertical supports.
QUESTION: Which of the following describes a simple dipole oriented parallel to the Earth’s surface? (T9A03)
ANSWER: A horizontally polarized antenna
When putting up a dipole antenna, you should consider the orientation of the antenna because it radiates better in some directions than others.
QUESTION: In which direction does a half-wave dipole antenna radiate the strongest signal? (T9A10)
ANSWER: Broadside to the antenna
So, for example, if you live in Kansas, you might want to orient the antenna so that the wire runs north to south. The antenna will then radiate best east and west, meaning that you’ll be able to cover most of the U.S.
The length of a dipole antenna is actually about 5% shorter than the value that you would calculate using the formula wavelength (m) = 300 / frequency (MHz). The reason for this is that there will be some stray capacitance between the wire and the ground and other objects near the antenna. Let’s take a look at an example.
QUESTION: What is the approximate length, in inches, of a half-wavelength 6 meter dipole antenna? (T9A09)
ANSWER: 112
Here’s how to make this calculation. One half wavelength is 3 m. 3 m x 39.4 in/m = 118.2 inches. Since the length of the antenna is about 5% shorter than this calculated value, the approximate length of the antenna will be 118.2 inches x 0.95, or about 112 inches.
Once you have built a dipole antenna, chances are it won’t be resonant on the frequency you want. To tune the antenna, you need to determine its resonant frequency and then make it longer or shorter.
QUESTION: How would you change a dipole antenna to make it resonant on a higher frequency? (T9A05)
ANSWER: Shorten it
Conversely, to make it resonant on a lower frequency, you lengthen the antenna. The final length will depend on the height at which you mount the antenna and any trees or buildings that are nearby.
Perhaps the second-most popular type of amateur radio antenna is the quarter-wave vertical antenna. The radiator of a vertical antenna is mounted perpendicular to the Earth. This makes it vertically-polarized, because the electric field will have the same orientation as the antenna’s radiator.
Like the half-wave dipole antenna, the length of a quarter-wave vertical antenna will be about 5% shorter than the calculated quarter wavelength.
QUESTION: What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz? (T9A08)
ANSWER: 19
The wavelength of a 146 MHz radio wave is approximately 2 m. One quarter of a wavelength is therefore 0.5 m. 0.5 m x 39.4 in/m = 19.7 in. 19.7 in x 0.95 ≈ 19 in.
Because HF antennas can be very long, many amateurs use a technique called “loading” to shorten them. You can use either inductors or capacitors to load an antenna, but the most common way is to use an inductor. In either case, loading an antenna makes it seem electrically longer to a signal at the antenna feed point.
QUESTION: Which of the following describes a type of antenna loading? (T9A02)
ANSWER: Inserting an inductor in the radiating portion of the antenna to make it electrically longer
While this technique does allow you to shorten an antenna, the shortened antenna will not be as efficient as a full-size antenna.
Many amateurs use directional, or beam, antennas. There are many different types of beam antennas, including the Yagi, the quad, and the dish antenna. They focus the power that is applied to them in a particular direction, and are therefore said to have “gain” in that direction. What this means is that they will output more power in a particular direction than say a dipole or vertical antenna.
QUESTION: What is a beam antenna? (T9A01)
ANSWER: An antenna that concentrates signals in one direction
QUESTION: What type of antennas are the quad, Yagi, and dish? (T9A06)
ANSWER: Directional antennas
QUESTION: What is the gain of an antenna? (T9A11)
ANSWER: The increase in signal strength in a specified direction compared to a reference antenna
Most handheld VHF and UHF transceivers come with what’s called a “rubber duck” antenna. Rubber duck antennas use inductive loading to make them shorter than a full-sized antenna. Loading them like this makes them less efficient than a full-sized quarter-wavelength vertical antenna.
QUESTION: What is a disadvantage of the “rubber duck” antenna supplied with most handheld radio transceivers when compared to a full-sized quarter-wave antenna? (T9A04)
ANSWER: It does not transmit or receive as effectively
QUESTION: What is a disadvantage of using a handheld VHF transceiver, with its integral antenna, inside a vehicle? (T9A07)
ANSWER: Signals might not propagate well due to the shielding effect of the vehicle
Think about it. If the antenna is inside the vehicle, the metal frame will prevent some of your signal from getting outside the vehicle and block some of the signal from a repeater or other station from reaching the antenna.
A better option is to use an externally-mounted antenna. A popular choice for externally-mounted mobile antennas is the 5/8-wavelength vertical antenna.
QUESTION: What is an advantage of using a properly mounted 5/8 wavelength antenna for VHF or UHF mobile service? (T9A12)
ANSWER: It has a lower radiation angle and more gain than a 1/4 wavelength antenna
Feed lines and connectors
Feed lines connect radios to antennas. There are many different types of feed lines, including coaxial cable, ladder line, twin lead, and open-wire feed line, but coaxial cable is the most common type.
QUESTION: Why is coaxial cable the most common feed line selected for amateur radio antenna systems? (T9B03)
ANSWER: It is easy to use and requires few special installation considerations
Perhaps the most important consideration when choosing a feed line is to match the impedance of the feed line to the output impedance of the transmitter and the input impedance of the antenna.
QUESTION: What is impedance? (T5C12)
ANSWER: A measure of the opposition to AC current flow in a circuit
QUESTION: What is a unit of impedance? (T5C13)
ANSWER: Ohms
Most amateur radio transmitters have an output impedance of 50 ohms, and most antennas have an input impedance close to 50 ohms. Because this is the case, most coaxial cable used in amateur radio stations has an impedance of 50 ohms.
QUESTION: What is the impedance of most coaxial cables used in amateur radio installations? (T9B02)
ANSWER: 50 ohms
RG-58 and RG-8 are two types of coaxial cable often used in amateur radio stations. Both have an impedance of 50 ohms.
Coaxial cable does, however, have some disadvantages. One of them is that it may be lossy at high frequencies.
QUESTION: In general, what happens as the frequency of a signal passing through coaxial cable is increased? (T9B05)
ANSWER: The loss increases
QUESTION: What is the electrical difference between RG-58 and RG-8 coaxial cable? (T9B10)
ANSWER: RG-8 cable has less loss at a given frequency
In general, the smaller the diameter of the coaxial cable, the higher the losses will be at higher frequencies. And, the longer the feed line, the higher the losses. So, for example, using RG-58 coax as the feed line for an 80 m dipole might be just fine, but you don’t want to use 50 feet of RG-58 to connect your 440 MHz FM transceiver to an antenna on your roof or on a tower.
Even RG-8 coaxial cable is not really the best choice for use at VHF and UHF frequencies. Most repeaters, for example, use air-insulated hard line coaxial cable for the feed line.
QUESTION: Which of the following types of feed line has the lowest loss at VHF and UHF? (T9B11)
ANSWER: Air-insulated hard line
Another disadvantage of coaxial cable is that it is more sensitive to moisture than other types of feed lines.
QUESTION: Which of the following is the most common cause for failure of coaxial cables? (T7C09)
ANSWER: Moisture contamination
One way that moisture enters a cable is via cracks in the cable’s outer jacket.
QUESTION: Why should the outer jacket of coaxial cable be resistant to ultraviolet light? (T7C10)
ANSWER: Ultraviolet light can damage the jacket and allow water to enter the cable
Air-insulated hard line is also susceptible to moisture problems.
QUESTION: What is a disadvantage of air core coaxial cable when compared to foam or solid dielectric types? (T7C11)
ANSWER: It requires special techniques to prevent water absorption
PL-259 connectors are the most common type of connectors used on coaxial cables in amateur radio stations.
QUESTION: Which of the following is true of PL-259 type coax connectors? (T9B07)
ANSWER: They are commonly used at HF frequencies
One problem with PL-259 connectors is that they are not the most suitable connector when operating at higher frequencies. Type N connectors are a much better choice for UHF frequencies.
QUESTION: Which of the following connectors is most suitable for frequencies above 400 MHz? (T9B06)
ANSWER: A Type N connector
One of the problems with PL-259 connectors is that they are not waterproof. If installed outdoors, water can get into the coaxial cable if it is not sealed in some way. This can lead to increased loss and eventual failure.
QUESTION: Why should coax connectors exposed to the weather be sealed against water intrusion? (T9B08)
ANSWER: To prevent an increase in feed line loss
Coaxial cable connectors can be a real pain to install properly and are a frequent cause of failure. When installing a feed line, make sure that your coaxial connectors are soldered properly and that they make solid connections with one another.
QUESTION: What can cause erratic changes in SWR readings? (T9B09)
ANSWER: A loose connection in an antenna or a feed line
Standing wave ratio and antenna measurements
Standing wave ratio, or SWR, is a term you’ll often hear when talking about antennas and feed lines. It is a measure of how well-matched a feed line is to an antenna. When we say that an antenna is matched to a transmission line, we mean that the impedance of the transmission line is equal to the impedance of the antenna.
QUESTION: What, in general terms, is standing wave ratio (SWR)? (T7C03)
ANSWER: A measure of how well a load is matched to a transmission line
In this context, the “load” is the antenna.
The higher the SWR, the greater the mismatch between the antenna and the transmission line.
QUESTION: What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line? (T7C04)
ANSWER: 1 to 1
QUESTION: What does an SWR reading of 4:1 indicate? (T7C06)
ANSWER: Impedance mismatch
When the feed line impedance matches the antenna input impedance, the SWR will be low. The reason this is a good thing is that when the feed line impedance matches the antenna input impedance, the most power is transferred to the antenna and radiated.
QUESTION: Why is it important to have low SWR when using coaxial cable feed line? (T9B01)
ANSWER: To reduce signal loss
The bigger the mismatch is between the feed line and the load, the higher the SWR will be. The higher the SWR, the more power you will lose in the feed line.
QUESTION: What happens to power lost in a feed line? (T7C07)
ANSWER: It is converted into heat
Power converted into heat is not radiated by the antenna, meaning your radiated signal will be weaker.
You can measure the SWR of your antenna system with an SWR meter. You usually connect the SWR meter between the transmitter and antenna, near the output of your transmitter because it is important to have a low SWR at that point.
QUESTION: What is the proper location for an external SWR meter? (T4A05)
ANSWER: In series with the feed line, between the transmitter and antenna
Most amateur radio transceivers today will reduce output power should the SWR of the antenna system get too high. The reason for this is to protect the radio from being damaged by reflected power.
QUESTION: Why do most solid-state amateur radio transmitters reduce output power as SWR increases? (T7C05)
ANSWER: To protect the output amplifier transistors
An SWR meter is not the only way to measure SWR.
QUESTION: What instrument other than an SWR meter could you use to determine if a feed line and antenna are properly matched? (T7C08)
ANSWER: Directional wattmeter
When using a directional wattmeter, you first measure the forward power, then the reflected power, and using those two values, calculate the SWR.
Another test instrument that you can use to measure the SWR of an antenna system is the antenna analyzer.
QUESTION: Which of the following instruments can be used to determine if an antenna is resonant at the desired operating frequency? (T7C02)
ANSWER: An antenna analyzer
Some antenna analyzers will also let you measure capacitive reactance and inductive reactance, and also be used as an RF signal generator. These capabilities may be useful in applications other than antenna analysis.
If an antenna’s impedance is not 50 ohms, the impedance at the transmitter end of a feed line will not be 50 ohms. Don’t worry, though. You can use a device called an antenna tuner or antenna coupler to transform the impedance from whatever it happens to be to 50 ohms. We call this process impedance matching.
QUESTION: What is the major function of an antenna tuner (antenna coupler)? (T9B04)
ANSWER: It matches the antenna system impedance to the transceiver’s output impedance
In addition to instruments that make antenna measurements, it’s helpful to have an instrument that can simulate an antenna. That’s the purpose of a dummy load. Basically, a dummy load is just a big 50 Ω resistor that provides a known impedance to the transmitter and converts the transmitter output power into heat so that it does not get radiated. If a transmitter operates normally when a dummy load is connected to it, you can be reasonably sure that your transmitter is good.
QUESTION: What is the primary purpose of a dummy load? (T7C01)
ANSWER: To prevent transmitting signals over the air when making tests
QUESTION: What does a dummy load consist of? (T7C12)
ANSWER: A non-inductive resistor and a heat sink
Dave New says
Hmm. I was under the impression that the reason that antennas are somewhat shorted than free space is because the speed of light is slower in solids and liquids than in vacuum or near vacuum.
Dan KB6NU says
That’s what I used to think, too, but we had a discussion about this here on my blog a couple years back. See the post, Why are dipoles shorter than a half wavelength? That convinced me that other factors affect the length more than the velocity factor.
Dave New says
The actual resonant frequency of the antenna is mostly decoupled from other factors, like being close to other conducting objects (up to a point). The effect is that the impedance match moves away from 50 ohm resistive, turning reactive, but the actual resonant ‘dip’ stays pretty much the same. If you use a simple SWR meter, it looks like the resonance has moved, because the best match moves to the best complex impedance that comes closest to matching a 50 ohm non-reactive transmission line.
Dave New says
I think I stand corrected. I spent some time poring through the ARRL antenna book, 23rd edition.
Chapter 2, “Dipoles and Monopoles” comes at this from a couple of angles, but this is what I’ve been able to distill:
1) “End effect” does effect the physical length of the antenna.
2) l/d ratio (or thickness of conductor) does as well. This generally also affects the Q, or how sharp the tuning will be, plus or minus the center resonant frequency.
3) There is a table 2.2 on page 2-5, that gives the variance of the physical length of a 20 meter dipole at various electrical heights above ground. The resonant length column looks suspiciously out-of-order. The longest length is at 1/2 wavelength above ground, and decreases both as the antenna is moved lower and higher from that point. Maybe something ‘magic’ about a 1/2 wave antenna 1/2 wave above ground? The feed point impedance likewise has a local maxima, but at 1/4 wavelength above ground. Go figure.
There then follows an involved discussion with graphs of feed point impedance vs. frequency for various antennas in free space.
Then there is another discussion of impedance vs. height above ground on page 2-8 and 2-9, including figure 2.15. There is a sentence in there that I had based my original assumption of resonance invariance on:
“This is why few amateur 1/2-wave dipoles exhibit a center-fed feed point impedance of 75 ohms, even though they may be resonant.”
So, that section seems to infer that moving the antenna closer to or farther away from ground (a conductor) does not change the resonance, but does effect the feed point impedance?
Or maybe not, maybe they are assuming that the physical length is being changed to achieve resonance for every height above ground?
Dave New says
It is correct that you use inductors to make an antenna appear electrically longer. Capacitors are then used to tune out the inductive reactance that appears at resonance.
Dan KB6NU says
I’m thinking more like capacitance hats here. Maybe I should make that clearer.
Dave New says
“You usually connect the SWR meter between the transmitter and antenna, near the output of your transmitter because it is important to have a low SWR at that point.”
I know what you are getting at, but it is important to have a low SWR at the antenna end, to prevent excessive loss in the coaxial cable. In fact, you can have a low SWR at the transmitter end, and have a high SWR at the antenna end, with a lot of power being lost in the cable, and be scratching your head wondering how come no one can hear you.