Antenna feed lines: characteristic impedance, attenuation, SWR calculation, measurement and effects, matching networks
Feed lines are the cables used to connect antennas to receivers and transmitters. The most important characteristic of a feedline is its characteristic impedance. Many different factors affect the characteristic impedance of a feedline, including the distance between the center of the conductors and the radius of the conductors.
QUESTION: Which of the following factors determine the characteristic impedance of a parallel conductor antenna feed line? (G9A01)
ANSWER: The distance between the centers of the conductors and the radius of the conductors
Coaxial cable is the mostly commonly used feedline in amateur radio. Most amateur radio stations use coaxial cables with a characteristic impedance of 50 ohms, although sometimes 75-ohm coax is used. The reason we use cables with these impedances is that most amateur radio transmitters have an output impedance of 50 ohms and commonly used amateur radio antennas have input impedances close to these values. Half-wave dipoles, for example, have a feedpoint impedance of 72 ohms, while quarter-wave verticals have a feedpoint impedance of 35 ohms.
QUESTION: What are the typical characteristic impedances of coaxial cables used for antenna feed lines at amateur stations? (G9A02)
ANSWER: 50 and 75 ohms
If there is a difference between the feed line impedance and the antenna feed point impedance, then a portion of the transmitter’s output power will be reflected back towards the transmitter. This is not desirable. We obviously want the antenna to radiate all of a transmitter’s output power.
QUESTION: What might cause reflected power at the point where a feed line connects to an antenna? (G9A04)
ANSWER: A difference between feed-line impedance and antenna feed-point impedance
When there is a difference between the feed line impedance and the antenna feed point impedance, we say that there is a “mismatch” between antenna and the feedline. A measure of this mismatch is the voltage standing-wave ratio, or simply SWR. The SWR is equal to the ratio of the impedances.
QUESTION: What standing wave ratio will result when connecting a 50 ohm feed line to a non-reactive load having 50 ohm impedance? (G9A11)
ANSWER: 1:1
This is the best possible case. When the SWR is 1:1, we say that the feedline is “matched” to the load, and there are no standing waves on the antenna feedline.
QUESTION: What must be done to prevent standing waves on an antenna feed line? (G9A07)
ANSWER: The antenna feed-point impedance must be matched to the characteristic impedance of the feed line
When the characteristic impedance of a feed line does not equal the feed point impedance of an antenna, an SWR greater than 1:1 will result, and the SWR will be equal to the ratio between the two impedances.
QUESTION: What standing wave ratio will result when connecting a 50 ohm feed line to a non-reactive load having 200 ohm impedance? (G9A09)
ANSWER: 4:1
QUESTION: What standing wave ratio will result when connecting a 50 ohm feed line to a non-reactive load having 10 ohm impedance? (G9A10)
ANSWER: 5:1
In order not to damage your transmitter, it’s important that the impedance its output “sees” is 50 ohms. To accomplish this, we often use devices called antenna tuners, and when adjusted properly, they transform the impedance at the end of the feedline to 50 ohms. That makes the transmitter happy, but the SWR on the feedline is unchanged.
QUESTION: If the SWR on an antenna feed line is 5 to 1, and a matching network at the transmitter end of the feed line is adjusted to 1 to 1 SWR, what is the resulting SWR on the feed line? (G9A08)
ANSWER: 5 to 1
To transfer the greatest amount of power from the transmitter to the receiver, the SWR on the feedline should be 1:1. When the SWR on a coaxial cable feedline is greater than 1:1, it will attenuate the signal because the loss on a coaxial cable feedline increases as the SWR increases.
QUESTION: What is the interaction between high standing wave ratio (SWR) and transmission line loss? (G9A12)
ANSWER: If a transmission line is lossy, high SWR will increase the loss
SWR measurements can, however, be misleading. Transmission line losses can cause the SWR readings made at the transmitter end of a feedline to be low. So, you might think you have a good match, and that you’re transferring maximum power to the antenna, but the reality is that the feedline is dissipating the power.
QUESTION: What is the effect of transmission line loss on SWR measured at the input to the line? (G9A13)
ANSWER: The higher the transmission line loss, the more the SWR will read artificially low
Even when perfectly matched, a coaxial cable will attenuate the signal somewhat, and the attenuation increases as the length of a cable and the signal frequency increases. On a coaxial cable’s data sheet, this loss will be given in decibels (dB) per 100 feet.
QUESTION: How does the attenuation of coaxial cable change as the frequency of the signal it is carrying increases? (G9A05)
ANSWER: Attenuation increases
QUESTION: In what units is RF feed line loss usually expressed? (G9A06)
ANSWER: Decibels per 100 feet
It is better to feed some antennas with parallel transmission line. There are several types of parallel transmission line that are used in amateur radio stations:
- TV twinlead has a characteristic impedance of 300 ohms.
- Ladder line or window line has a characteristic impedance of 450 ohms.
- Open wire feedline has a characteristic impedance of 600 ohms.
The reason to use a parallel transmission line is that it has much less loss than coaxial cable when the antenna’s feedpoint impedance is not close to the characteristic impedance of the feedline. Untuned doublets are an example of this type of antenna. You typically need to use an antenna tuner to match the antenna system, including the feedline, to an amateur radio transmitter.
QUESTION: What is the typical characteristic impedance of “window line” parallel transmission line? (G9A03)
ANSWER: 450 ohms
Basic antennas
There are many different types of antennas, including:
- random-wire antennas,
- dipole antennas, and
- vertical antennas, including ground plane antennas.
As the name implies, random-wire antennas are a random-length. To match the antenna to the transmitter, you’ll need an antenna tuner, which is normally located in the shack. Because of this, there may be high RF levels in the shack when you are transmitting. This can cause all sorts of problems.
QUESTION: What is one disadvantage of a directly fed random-wire HF antenna? (G9B01)
ANSWER: You may experience RF burns when touching metal objects in your station
The half-wavelength dipole antenna is perhaps the most common amateur radio antenna because it is simple to build and operate. In practice, the half-wave dipole antenna is a bit shorter than a half wavelength. This is due to the effect of the ground and nearby objects on the antenna.
The formula most often used by radio amateurs to calculate the length of a dipole antenna is Length (feet) = 468 / f (MHz). Here are two examples of how to use this equation:
QUESTION: What is the approximate length for a 1/2 wave dipole antenna cut for 3.550 MHz? (G9B11)
ANSWER: 131 feet
L = 468 / 3.55 ≈ 131 feet
QUESTION: What is the approximate length for a 1/2 wave dipole antenna cut for 14.250 MHz? (G9B10)
ANSWER: 32 feet
L = 468 / 14.250 ≈ 32 feet
When the feedpoint is at the center of a half-wave dipole antenna, the impedance is approximately 72 ohms, making it a good match for 75-ohm coax and 50-ohm coax. As you move the feedpoint away from the center, the impedance increases. At the end of a half-wave antenna, the feedpoint impedance will be approximately 5,000 ohms.
QUESTION: How does the feed point impedance of a 1/2 wave dipole change as the feed point is moved from the center toward the ends? (G9B08)
ANSWER: It steadily increases
Dipole antennas are usually mounted horizontally. One advantage of mounting them horizontally, rather than vertically, is that there are lower ground reflection losses, when installed high in the air.
Which of the following is an advantage of a horizontally polarized as compared to a vertically polarized HF antenna? (G9B09)
ANSWER: Lower ground reflection losses
Ideally, a dipole antenna should be mounted a half-wavelength off the ground. This preserves the figure-eight radiation pattern of the antenna and makes its behavior more predictable.
QUESTION: What is the radiation pattern of a dipole antenna in free space in a plane containing the conductor? (G9B04)
ANSWER: It is a figure-eight at right angles to the antenna
QUESTION: How does antenna height affect the horizontal (azimuthal) radiation pattern of a horizontal dipole HF antenna? (G9B05)
ANSWER: If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional
Antenna height also affects the feed-point impedance.
QUESTION: How does the feed-point impedance of a 1/2 wave dipole antenna change as the antenna is lowered below 1/4 wave above ground? (G9B07)
ANSWER: It steadily decreases.
The quarter-wave vertical antenna is arguably the second-most popular amateur radio antenna. It doesn’t require a lot of space, so can be installed on small city lots, or even on the roof of a building. Other advantages include an omnidirectional radiation pattern and a low angle of radiation, which makes it a good antenna for making long distance contacts.
QUESTION: Which of the following best describes the radiation pattern of a quarter-wave, ground-plane vertical antenna? (G9B03)
ANSWER: Omnidirectional in azimuth
To calculate the approximate length of a quarter wave vertical antenna, you use the equation Length (feet) = 234 / f (MHz).
QUESTION: What is the approximate length for a 1/4 wave vertical antenna cut for 28.5 MHz? (G9B12)
ANSWER: 8 feet
L = 234 / 28.5 ≈ 8 feet
When mounted above ground and used with radials, the vertical antenna is called a ground plane antenna. The vertical, or driven element, works against the radials to generate the radio wave. One problem with vertical antenna is that if buried too deeply, the radials can cause ground losses. To reduce ground losses, you should simply place them on the ground or bury them just below the surface.
QUESTION: Where should the radial wires of a ground-mounted vertical antenna system be placed? (G9B06)
ANSWER: On the surface of the Earth or buried a few inches below the ground
The natural feed-point impedance of a quarter-wave vertical is 35 ohms. To increase the feedpoint impedance to 50 ohms, which would make it a better match to 50-ohm coax, you could slope the radials downward.
QUESTION: Which of the following is a common way to adjust the feed-point impedance of a quarter wave ground-plane vertical antenna to be approximately 50 ohms? (G9B02)
ANSWER: Slope the radials downward
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Thanks, Jim! I’ve subscribed you to my blog. You should get an email from Feedburner verifying this. Thanks!