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Perfect Feedline (ya, really)

A perfect feedline will have:

No radiation from the feedline itself

No loss of signal while passing along the line

Constant electrical characteristics throughout

Such a feedline will pass 100% of the RF energy
through it.

NOTE: This situation does not ever exist! (yet)!!

Feedline (transmission line)

Capacitance because of 2 parallel lines (plates) Inductance because

of the length of the lines and their proximity to each other.

Resistance in the metal itself slowing the flow

Therefore a feedline is a circuit which has reactance to the passage of

AC current and which varies inversely as the operating frequency which
means the value stays approximately the same over any given length.
This value is called the characteristic impedance of the circuit. (Zo)

At HF frequencies, the signal passes through the conductor while at

frequencies above 10 MHz, the signal passes along the surface, or skin
of the wire. This is known as skin effect where the losses increase with
the frequency
Balanced feedlines

Open wire feedlines

Characteristic impedance of 200 600 depending on the

diameter of the wire and the distance between them.

Zo= 276 log 2(S/D)

S=Distance between and
Unbalanced feedlines

One side to ground

Other side carries
RF to antenna
Coaxial cable is waterproof

Hardline or Heliax is best for

VHF/UHF and up (Heliax uses
copper not braid for the shield) Zo=138/e log D/d
e=dielectric constant
hard to bend - D=diameter of the outer conductor
D=diameter of the inner conductor
Feed Lines

Feedlines connect a radio to an antenna

They must be matched to the radio system - they should have

like impedence

Radios usually have a 50 ohm output

Antenna feedpoints have a very wide impedence range

Velocity factor .66 - .95

Feed Lines Cont

Feedlines can be easily made

The two favourite for amateur radio are the coaxial cable
and open wire feedlines
Feed Line Questions See Page 45

What connects your transceiver to your antenna?

Feed Line

What kind of feed line can be buried in the ground for

some distance without adverse effects? Coaxial Cable

A transmission line differ from an ordinary circuit or

network in communications or signal devices in one
important way. That important way is Propagation
Feed Line Questions Cont

The characteristics of a transmission line is determined by the

Physical dimensions and relative positions of the

The characteristics of a transmission line is equal to the Pure

Resistance which, if connected to the end of the line, will
absorb all the power arriving along it
Think of the paper towel absorbsion advertisment

The characteristic impedence of a coaxial antenna feed line is

determined by the Ratio of the diameter of the inner
conducter to the diameter of the braid
Feed Line Questions Cont

The characteristic impedance of a parallel wire transmission

line does not depend on the velocity of energy on the line

What factors determine the characteristic impedance of a

parallel-conductor antenna feed line? The distance between
the centres of the conductors and the radius of the

Any length of transmission line may be made to appear as an

infinitely long line by: Terminating the line in its
characteristic impedance

The characteristic impedance of a 20 metre piece of

transmission line is 52 ohms
Feed Line Questions Cont

The impedance of a coaxial line:

can be the same for different diameter line
Balanced & Unbalanced Feed Lines

A balanced transmission line: is made of two parallel wires

What is parallel-conductor feed line? Two wires side-by-side held apart by

insulating rods

What kind of antenna feed line is made of two conductors held apart by
insulated rods? Open-conductor ladder line

What kind of antenna feed line can be constructed using two conductors which
are maintained a uniform distance apart using insulated spreaders? 600 ohm
Balanced & Unbalanced Feed Lines - 2
What is an unbalanced line? Feed line with one conductor connected to ground

What is a coaxial cable? A center wire inside an insulating material which is

covered by a metal sleeve or shield

A flexible coaxial line contains: Braid and insulation around a central


What device can be installed to feed a balanced antenna with an unbalanced feed
line? A balun

What does the term "balun" mean? Balanced to unbalanced

Balanced & Unbalanced Feed Lines - 3
Where would you install a balun to feed a dipole antenna
with 50-ohm coaxial cable? Between the coaxial cable
and the antenna

A 75 ohm transmission line could be matched to the 300

ohm feedpoint of an antenna: by using a 4 to 1 balun
Popular Antenna Feed Lines Page 46

Why does coaxial cable make a good antenna feed line? It is

weatherproof, and its impedance is higher than that of most amateur

What is the best antenna feed line to use, if it must be put near grounded
metal objects? Coaxial cable

What commonly available antenna feed line can be buried directly in the
ground for some distance without adverse effects? Coaxial cable

If you install a 6 metre Yagi antenna on a tower 50 metres from your

transmitter, which of the following feed lines is best? RG-213

What are some reasons not to use parallel-conductor feed line? It does not
work well when tied down to metal objects, and you must use an
impedance-matching device with your transceiver

TV twin-lead feed line can be used for a feed line in an amateur station.
The impedance of this line is approximately: 300 ohms
What common connector usually joins RG-213 coaxial cable to an HF
transceiver? A PL-259 connector

What common connector usually joins a hand-held transceiver to its

antenna? A BNC connector

Which of these common connectors has the lowest loss at UHF? A type-N

Why should you regularly clean, tighten and re-solder all antenna
connectors? To help keep their resistance at a minimum
Line Losses Page 47

Why should you use only good quality coaxial cable and connectors for a
UHF antenna system? To keep RF loss low

In what values are RF feed line losses expressed?

dB per unit length

Losses occurring on a transmission line between transmitter and antenna

results in: less RF power being radiated

If the length of coaxial feed line is increased from 20 metres (65.6 ft) to 40
metres (131.2 ft), how would this affect the line loss? It would be
increased by 100%

What are some reasons to use parallel conductor feed line? It will
operate with a high SWR, and has less loss than coaxial cable
Line Losses Cont

If your transmitter and antenna are 15 metres apart, but are

connected by 65 metres of RG-58 coaxial cable, what should
be done to reduce feed line loss? Shorten the excess cable

The lowest loss feed line on HF is: 300 ohm twin- lead

As the length of a feed line is changed, what happens to signal

loss? Signal loss increases as length increases

As the frequency of a signal is changed, what happens to signal

loss in a feed line? Signal loss increases with increasing
Standing Waves Page 47

If the characteristic impedance of the feedline does not match the antenna input impedance
then: standing waves are produced in the feedline

The result of the presence of standing waves on a transmission line is: reduced transfer of
RF energy to the antenna

What does the standing wave ration means? ratio of maximum to minimum voltages on a
feed line

What does an SWR reading of 1:1 mean?

The best impedance match has been attained

What does an SWR reading of less than 1.5:1 mean? A fairly good impedance match

A resonant antenna having a feed point impedance of 200 ohms is connected to a feed
line and transmitter which have an impedance of 50 ohms. What will the standing wave
ratio of this system be? 4:1

What kind of SWR reading may mean poor electrical contact between parts of an antenna
system? A jumpy reading
Standing Waves Cont

What does a very high SWR mean? The antenna is the wrong length, or
there may be an open or shorted connection somewhere in the feed

If your antenna feed line gets hot when you are transmitting, what might this
mean? The SWR may be too high, or the feed line loss may be high

The type of feed line best suited to operating at a high standing wave ratio
is: 600 ohm open-wire

SWR meter measures the degree of match between transmission line and
antenna by: comparing forward and reflected voltage
Impedence Matching Page 48

What device might allow use of an antenna on a band it was not designed
for? An antenna tuner

What does an antenna matching unit do? It matches a transceiver to a

mismatched antenna system

What would you use to connect a coaxial cable of 50 ohms impedance to

an antenna of 35 ohms impedance? An impedance-matching device

When will a power source deliver maximum output to the load? When the
impedance of the load is equal to the impedance of the source
Impedence Matching Page 48

What happens when the impedance of an electrical load is equal to the internal
impedance of the power source? The source delivers maximum power to the

Why is impedance matching important? So the source can deliver maximum

power to the load

To obtain efficient power transmission from a transmitter to an antenna requires:

matching of impedances

If an antenna is correctly matched to a transmitter, the length of transmission line:

will have no effect on the matching

If the centre impedance of a folded dipole is approximately 300 ohms, and you are
using RG8U (50 ohms) coaxial lines, what is the ratio required to have the line and
the antenna matched? 6:1
Impedance A reading (for interest only)


scalar (one-dimensional) phenomena: resistance and reactance.

germanium, and gallium arsenide.

(when the net reactance is capacitive).

complex impedance is:

ZRL = R + jXL = 100.00 + j251.33


ZRC = R - jXC = 100.00 - j39.789


ZRLC = 100 + j251.33 - j39.789 = 100 + j211.5


and capacitive susceptance.Susceptance is the reciprocal of reactance.Susceptance combines with
of complex impedance.

Impedance matching
Levers do it.
Pulleys do it.
Ramps, transformers, gears, megaphones, and wheelbarrows do it.
Even screws do it.

Match impedance, that is.

Impedance is the opposition to the flow of energy.

If you try to lift your refrigerator, you will experience an opposition to the flow of energy.
The refrigerator will just sit there, and you will get tired. The ability of your muscles to lift
the weight is not matched to the weight.

There are a number of ways you can lift a 500 pound refrigerator by matching the
impedance of your muscles to the impedance of the load. You could push the load up a
ramp. You could use a lever, or a block and tackle, or a hydraulic jack, or a screw jack.
Each of these devices allows you to trade lifting the 500 pound load for lifting a smaller
load, say 50 pounds. You generally trade off time, pushing 50 pounds for ten seconds
instead of 500 pounds in one second. The same amount of energy is expended, but at a
much lower power level.
When impedances are mismatched, energy put into the system is reflected back. If you jump on a see-
saw with a refigerator on the other end, you will bounce back off as if you were on a diving board. But
if you move the fulcrum closer to the refrigerator, you can jump onto the see-saw, and your end will
move down, lifting the heavy load at the other end.

You can line up a row of billiard balls, and hit the row with the cue ball, and the last ball in the row will
shoot off down the table. But if one of the balls is made of steel, the cue ball will simply bounce off of it,
and most of the energy will be reflected.

We can match the impedances to get the steel ball to move. We put a row of balls in front of it, each
one made of a slightly lighter weight material than the last, until the ball nearest us is almost the same
mass as the cue ball. Now the speeding cue ball will stop dead when it hits the row of balls, and the
steel ball will slowly move off down the table, having absorbed all of the energy.

When you shout to a friend who is underwater in a swimming pool, the sound from your voice bounces
off the water, and very little sound energy gets to your friend's ears. But take a traffic cone and put the
narrow end of it into the water and shout into the large end. Now your friend can hear you, because
the low pressure sound waves over a large area are converted into high pressure waves over a small
area, and the water moves from the high power sound. Here we are not trading time. Instead, we are
trading a large area for a smaller one.

An electrical transformer also matches impedance. It takes high voltage, low current energy, and
matches it to a load the needs low voltage, high current. It also works the other way around. Without
the transformer, most of the energy is reflected back to the source, and little work gets done.

A water nozzle is an impedance matcher. So is cupping your hand behind your ear. A telescope is an
impedance matcher. So is a magnifying glass, or a winding mountain road, or the gears on your
bicycle. Now that you are aware of impedance matchers, you will start to see them everywhere.