Antennas and feed lines

Antenna types

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.

T9A03
Which of the following describes a simple dipole oriented parallel to Earth’s surface?
A. A ground-wave antenna
B. A horizontally polarized antenna
C. A travelling-wave antenna
D. A vertically 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.

T9A10
In which direction does a half-wave dipole antenna radiate the strongest signal?
A. Equally in all directions
B. Off the ends of the antenna
C. In the direction of the feed line
D. 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.

T9A09
What is the approximate length, in inches, of a half-wavelength 6 meter dipole antenna?
A. 6
B. 50
C. 112
D. 236

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. To decrease the resonant frequency of an antenna, you lengthen it, because the wavelength is longer. Conversely, to increase the resonant frequency, you shorten the antenna.

T9A05
Which of the following increases the resonant frequency of a dipole antenna?
A. Lengthening it
B. Inserting coils in series with radiating wires
C. Shortening it
D. Adding capacitive loading to the ends of the radiating wires

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.

T9A08
What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
A. 112
B. 50
C. 19
D. 12

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 full-size, half-wave or quarter-wave 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.

T9A02
Which of the following describes a type of antenna loading?
A. Electrically lengthening by inserting inductors in radiating elements
B. Inserting a resistor in the radiating portion of the antenna to make it resonant
C. Installing a spring in the base of a mobile vertical antenna to make it more flexible
D. Strengthening the radiating elements of a beam antenna to better resist wind damage

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. Beam antennas concentrate the power that is applied to them in onedirection, thereby increasing the signal strength in that direction, when compared to a reference antenna, such as a dipole antenna. We call this increase in signal strength antenna gain. There are many different types of beam antennas, including the Yagi, the quad, and the dish antenna.

T9A01
What is a beam antenna?
A. An antenna built from aluminum I-beams
B. An omnidirectional antenna invented by Clarence Beam
C. An antenna that concentrates signals in one direction
D. An antenna that reverses the phase of received signals

T9A11
What is antenna gain?
A. The additional power that is added to the transmitter power
B. The additional power that is required in the antenna when transmitting on a higher frequency
C. The increase in signal strength in a specified direction compared to a reference antenna
D. The increase in impedance on receive or transmit compared to a reference antenna

T9A06
Which of the following types of antenna offers the greatest gain?
A. 5/8 wave vertical
B. Isotropic
C. J pole
D. Yagi

Most handheld VHF and UHF transceivers come with short, flexible antennas called “rubber duck” antennas. These 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.

T9A04
What is a disadvantage of the short, flexible antenna supplied with most handheld radio transceivers, compared to a full-sized quarter-wave antenna?
A. It has low efficiency
B. It transmits only circularly polarized signals
C. It is mechanically fragile
D. All these choices are correct

This disadvantage is compounded if you try to use a handheld VHF transceiver inside a vehicle. The shielding effect of the vehicle’s 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 inside the vehicle.

T9A07
What is a disadvantage of using a handheld VHF transceiver with a flexible antenna inside a vehicle?
A. Signal strength is reduced due to the shielding effect of the vehicle
B. The bandwidth of the antenna will decrease, increasing SWR
C. The SWR might decrease, decreasing the signal strength
D. All these choices are correct

A better option is to use an externally-mounted antenna. A popular choice for an externally-mounted mobile antenna is the 5/8-wavelength vertical antenna. One reason that it is a popular choice, is that it has more gain than a 1/4-wavelength antenna.

T9A12
What is an advantage of a 5/8 wavelength whip antenna for VHF or UHF mobile service?
A. It has more gain than a 1/4-wavelength antenna
B. It radiates at a very high angle
C. It eliminates distortion caused by reflected signals
D. It has 10 times the power gain of a 1/4 wavelength whip

 

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.

T9B03
Why is coaxial cable the most common feed line for amateur radio antenna systems?
A. It is easy to use and requires few special installation considerations
B. It has less loss than any other type of feed line
C. It can handle more power than any other type of feed line
D. It is less expensive than any other type of feed line

Perhaps the most important consideration when choosing a feed line is the impedance of the feed line. In general, you should match the impedance of the feed line to the output impedance of the transmitter and the input impedance of the antenna.

T5C12
What is impedance?
A. The opposition to AC current flow
B. The inverse of resistance
C. The Q or Quality Factor of a component
D. The power handling capability of a component

T5C05
What is the unit of impedance?
A. The volt
B. The ampere
C. The coulomb
D. The ohm

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.

T9B02
What is the most common impedance of coaxial cables used in amateur radio?
A. 8 ohms
B. 50 ohms
C. 600 ohms
D. 12 ohms

RG-58 and RG-213 are two types of coaxial cable often used in amateur radio stations. Both have an impedance of 50 ohms.

Despite being the most popular type of feedline for amateur radio stations, coaxial cable does have some disadvantages. One of them is that it may be lossy at high frequencies. Some coax cable types have more loss than others.

T9B05
What happens as the frequency of a signal in coaxial cable is increased?
A. The characteristic impedance decreases
B. The loss decreases
C. The characteristic impedance increases
D. The loss increases

T9B10
What is the electrical difference between RG-58 and RG-213 coaxial cable?
A. There is no significant difference between the two types
B. RG-58 cable has two shields
C. RG-213 cable has less loss at a given frequency
D. RG-58 cable can handle higher power levels

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-213 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.

T9B11
Which of the following types of feed line has the lowest loss at VHF and UHF?
A. 50-ohm flexible coax
B. Multi-conductor unbalanced cable
C. Air-insulated hardline
D. 75-ohm flexible coax

Many other factors, including water intrusion, high SWR, and multiple connectors in line can cause losses in a coaxial feed line.

T9B08
Which of the following is a source of loss in coaxial feed line?
A. Water intrusion into coaxial connectors
B. High SWR
C. Multiple connectors in the line
D. All these choices are correct

Moisture contamination not only causes higher feed line losses. If not addressed, it can also cause coaxial cables to fail.

T7C09
Which of the following causes failure of coaxial cables?
A. Moisture contamination
B. Solder flux contamination
C. Rapid fluctuation in transmitter output power
D. Operation at 100% duty cycle for an extended period

One way that moisture enters a cable is via cracks in the cable’s outer jacket caused by ultraviolet light.

T7C10
Why should the outer jacket of coaxial cable be resistant to ultraviolet light?
A. Ultraviolet resistant jackets prevent harmonic radiation
B. Ultraviolet light can increase losses in the cable’s jacket
C. Ultraviolet and RF signals can mix, causing interference
D. Ultraviolet light can damage the jacket and allow water to enter the cable

Air-insulated hard line is also susceptible to moisture problems.

T7C11
What is a disadvantage of air core coaxial cable when compared to foam or solid dielectric types?
A. It has more loss per foot
B. It cannot be used for VHF or UHF antennas
C. It requires special techniques to prevent moisture in the cable
D. It cannot be used at below freezing temperatures

PL-259 connectors are the most common type of connectors used on coaxial cables in amateur radio stations.

T9B07
Which of the following is true of PL-259 type coax connectors?
A. They are preferred for microwave operation
B. They are watertight
C. They are commonly used at HF and VHF frequencies
D. They are a bayonet-type connector

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.

T9B06
Which of the following RF connector types is most suitable for frequencies above 400 MHz?
A. UHF (PL-259/SO-239)
B. Type N
C. RS-213
D. DB-25

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 crimped or soldered properly and that they make solid connections with one another.

T9B09
What can cause erratic changes in SWR?
A. Local thunderstorm
B. Loose connection in the antenna or feed line
C. Over-modulation
D. Overload from a strong local station

 

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 a load, such as an antenna, is matched to a transmission line. 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. If you measure the SWR of a perfectly-matched antenna system, the SWR meter will read 1:1. The higher the SWR reading, the greater the mismatch between the antenna and the transmission line.

T9B12
What is standing wave ratio (SWR)?
A. A measure of how well a load is matched to a transmission line
B. The ratio of amplifier power output to input
C. The transmitter efficiency ratio
D. An indication of the quality of your station’s ground connection

T7C04
What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?
A. 50:50
B. Zero
C. 1:1
D. Full Scale

T7C06
What does an SWR reading of 4:1 indicate?
A. Loss of -4 dB
B. Good impedance match
C. Gain of +4 dB
D. Impedance mismatch

When the feed line impedance matches the antenna input impedance, the SWR will be low. Low SWR on a coaxial cable feed line is a good thing because when the feed line impedance matches the antenna input impedance, signal losses are low, meaning that the antenna will radiate more power.

T9B01
What is a benefit of low SWR?
A. Reduced television interference
B. Reduced signal loss
C. Less antenna wear
D. All these choices are correct

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 is lost in the feed line. Power converted into heat is not radiated by the antenna, meaning your radiated signal will be weaker.

T7C07
What happens to power lost in a feed line?
A. It increases the SWR
B. It is radiated as harmonics
C. It is converted into heat
D. It distorts the signal

Most amateur radio transceivers today will reduce output power if the antenna system SWR is high. The reason for this is to protect the radio from being damaged by reflected power.

T7C05
Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?
A. To protect the output amplifier transistors
B. To comply with FCC rules on spectral purity
C. Because power supplies cannot supply enough current at high SWR
D. To lower the SWR on the transmission line

An SWR meter is not the only way to measure SWR. You can also measure SWR with a directional wattmeter.

T7C08
Which instrument can be used to determine SWR?
A. Voltmeter
B. Ohmmeter
C. Iambic pentameter
D. 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. The SWR of an antenna system will be 1:1 at the resonant frequency.

T7C02
Which of the following is used to determine if an antenna is resonant at the desired operating frequency?
A. A VTVM
B. An antenna analyzer
C. A Q meter
D. A frequency counter

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.

T9B04
What is the major function of an antenna tuner (antenna coupler)?
A. It matches the antenna system impedance to the transceiver’s output impedance
B. It helps a receiver automatically tune in weak stations
C. It allows an antenna to be used on both transmit and receive
D. It automatically selects the proper antenna for the frequency band being used

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.

T7C01
What is the primary purpose of a dummy load?
A. To prevent transmitting signals over the air when making tests
B. To prevent over-modulation of a transmitter
C. To improve the efficiency of an antenna
D. To improve the signal-to-noise ratio of a receiver

T7C03 (B)
What does a dummy load consist of?
A. A high-gain amplifier and a TR switch
B. A non-inductive resistor mounted on a heat sink
C. A low-voltage power supply and a DC relay
D. A 50-ohm reactance used to terminate a transmission line