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2019 No Nonsense General Class Study Guide: Antennas and Feed Lines, Part I

Posted: 19 Feb 2019 06:47 AM PST
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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 transmitters output power will
be reflected back towards the transmitter. This is not desirable. We
obviously want the antenna to radiate all of a transmitters 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 youre 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 cables 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 antennas 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 doesnt 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

The post 2019 No Nonsense General Class Study Guide: Antennas and Feed
Lines, Part I appeared first on KB6NUs Ham Radio Blog.