RFFEEDERS

RF Feeder Methods
City & Guild Telecommunication and Electronic Engineering (T1)
BY Wasantha Kumara (City & Guild London)
Introduction
With this you will have the whole picture, from antenna to receiver
/transmitter. I will briefly mention wave guides, but I will not give any
information since their use is a bit specialized.
Feeder Types
A feeder is simply a method of conveying the RF energy between an antenna
and a receiver or transmitter. The object of the feeder is to transport as much
power as possible to or from a transmitter/receiver to a antenna.
Feeders come in many forms, such as:
 Two parallel wires or "Open Line" feeder.
 A wire inside a metal tube or "Coaxial" feeder.
 Rectangular metal tube or "Wave guide".
Two parallel wires or "Open Line" feeder.

Open Line feeders are probably the most efficient feeders for HF work and
comprise two wires separated by an insulator at convenient intervals. This
type of feeder is balanced, that is to say, that each conductor carries an
equal and opposite current. They work on the principle the signal radiated by
each wire is 100% cancelled out by the other.
A wire inside a metal tube or "Coaxial" feeder.
Coaxial cable is the most common form of feeder used in radio

communication work since the losses are acceptable for HF/VHF and the
price (frequently) falls within a working man’s pocket. Radio energy
contained inside the outer metal tube which acts as an Earth or screen to
contain the signal. The RF power is as, summed to be conducted down the
center conductor. This type of feeder is unbalanced, as are the outputs from
most radio transmitters and receivers.
Velocity Factor
A feeder can be thought of as composed of a series of inductors, each with a
capacitance between the conductors. That is the way you could think of it,
but in reality is just one long inductor with a continuous capacitance to the
other conductor. But let us stick to the concept of loads of individual LC’s.
If you were to apply a logic voltage "step" to the end of the feeder then each
inductor will oppose the change. The following capacitors will take time to
charge up. Thankfully, this charging time is VERY fast, when compared to
the much longer rise-time of an RF waveform. In reality it would be
typically many thousands of times faster, but it is still there. The result of
this is that if you connected that logic level "step" to the cable and watched
the step at the other end, then you will observe two effects:
1. The step would arrive somewhat later.
2. The step would be rounded off a little.
In other words, the feeder cable imposes a time delay and has an attenuation
that is related to frequency. If the capacitance C in the above representation
were very low, then the signal could travel at typically a little less than
300,000,000 meters per second. This is about the speed of light, so the
"delay rate" or "VELOCITY FACTOR" would be 1. But if the feeder had
another material between the conductors other than air, then C would rise,
depending upon the dielectric constant of the material. This would increase
the "delay rate" or reduce the "VELOCITY FACTOR" (VF). If you know
the dielectric constant (K) if the insulating material then the VF=1/(Sqrt
(K)).
A typical VF for cheap 1/4" coaxial cable (URM76 or RG58) is 0.66 so that
a radio wave will only travel at 200,000,000 meters per second (300,000,000
x 0.66 = 200,000,000). 0.66 is also a typical VF for most coaxial cables. A
typical VF for Open Line feeder would be 1.
Losses
Real-live feeders have resistive losses, so the thicker the conductors, then the
smaller are the losses. If, however the inductance and capacitance per unit
meter can be reduced, then so can the losses. Cable manufacturers usually
give the loss of a cable type as the number of Decibels per unit length,
usually 10 meters. Microwave (Satellite TV cables) often quote a loss for
100 meters length. Here are a few examples of coaxial cable losses:
Coax Impedance Losses (dB) per 10m length (-) = no information Capac.
Type Z 10MHz 100MHz 200MHz 400MHz 1000MHz (pf/meter)
URM43 50 - 1.3dB - - 4.46dB 100
URM67 50 - 0.68dB - - 2.52dB 100
URM70 75 - 1.5dB - - 5.2dB 67
URM76 50 - 1.6dB - - 5.3dB 100
URM95 50 0.86dB 2.7dB - - 10.2dB 100
RG58 50 2dB - 3.1dB - 7.6dB 100
RG59 75 0.37dB 1.2dB - - - 51
RG174 50 1.1dB - 4.2dB 6dB - 100
RG178 50 1.8dB 4.4dB - - 14dB 96
RG179 75 1.9dB 3.2dB - - 8.2dB 100
RG213 50 0.18dB 0.62dB - - 2.63dB 100
RG214 50 0.22dB 0.76dB - - 2.9dB 96
RG223 50 0.39dB - 1.58dB 5.41dB - 96
RG316 50 - 36dB - - 190dB 102
URM203 75 - 0.75dB - - 2.9dB 56
URM203a 75 - 0.8dB - - 3.0dB 56
CT100 75 - 0.49dB 0.71dB 1.2dB 1.67dB ??
You will notice that some manufacturers quote different frequencies. This
should depend upon the intended use of the cable, but it varies almost as
much as ASCII (American Standard Code for Information Interchange)
varies from one computer to another.
Impedance
There are a lot of misconceptions about the impedance of a coaxial cable. It
is the inductance and capacitance per meter that determines the characteristic
impedance of a coaxial cable, but the impedance of the cable is totally
independent of the length as long as it is correctly terminated. This means
that if you were sending 150MHz down a 50-ohm cable of infinite
length, then the cable would indeed have a 50-ohm terminal impedance. If
you cut the cable to ANY other length and terminated it with a 50-ohm
resistor, then the impedance at the transmitting end would remain 50-ohms.
As long as the end load is a resistive load that absorbs all the power
presented to it, then the impedance at any point in the cable will remain a
constant along it’s length.
VSWR (Voltage Standing Wave Ratio)

If our feeder is perfectly matched, then the impedance along it’s length is a
constant. Let us stuff a 100MHz 50-watt transmitter into the infinite length
feeder. The transmitter is 50-ohms output, the feeder is 50-ohms, so the each
cycle will run down the feeder (or up the feeder), away from the transmitter
at the speed of light (ignoring VF). If you were to take out your sine-wave
surfboard and ride the wave, you too would travel at the speed of light. Now,
let us cut the feeder and put a 50-ohm resistor on the end of the coaxial
cable. Now you can still ride the wave on your sine-wave surfboard, but the
wave would suddenly stop when it was all burned up in the resistor. If you
were to monitor the current and voltage at any point in the coaxial cable then
both would be a constant since the wave is still traveling and without
impediment.
Now let us change the 50-ohm resistor for a 25-ohm resistor. The
transmission line is no longer matched. The 25-ohm resistor would no-
longer burn 50 watts, but would have only 1/2 the voltage across it. Since
power cannot be destroyed and cannot be 100% burned up, there is only one
way for it to go - back down the line where it came from. If you were now to
ride the forward wave with your sine-wave surfboard then you would meet
more waves coming at you at the speed of light. Since you are also traveling
at the speed of light, the reflected wave would be hitting you at twice the
speed of light.
A stationary observer would see you are being hit every 1/2 wavelength, and
if the observer could see the waves, then he would see the waves adding and
subtracting at 1/2-wave intervals. He would see 50-volts RMS moving away
from the transmitter and 25-volts leaving the resistor towards the transmitter.
He would see a pattern where the forward 50V would have 25V added (75V
peaks) and he would also see 50V being partially cancelled by 25V (25V
troughs). The ratio of 75V to 25V is 3:1 and is known as the Voltage
Standing Wave Ratio (VSWR). Since the mismatch (the 25-ohm resistor) is
stationary, then the forward and reflected additions and subtractions would
always be stationary and occur at the same place in the coaxial cable.
Although I have over-simplified this a little, I hope that some form of

clarification will be given to those who find it hard to understand. But let us
delve a little deeper into our now mismatched coaxial transmission line.
What has happened to the impedance?
These additions and subtractions of our transmission line will also affect the
impedance since the current and voltage are now dependant upon the
location in the transmission line. 1/2-wavelength back from the 25-ohm
resistor, if we were to measure the impedance then it too would be 25-ohms.
In fact, the impedance would be 25-ohms 1/2-wavelength and every 1/2-
wavelength thereafter. Between these 25-ohm impedance points there would
be higher impedance - 100-ohms.
VSWR – How can we correct it

Not without correcting the impedance mismatch. We can place an "Antenna
System Tuning Unit" ( ATU ) between the coaxial cable and the 25-ohm
resistor. This would just be an impedance transformer to convert 50-ohms
down to 25-ohms. A 2:1 balun ( Balance to Unbalance Transformer )
would do exactly the same job.
It is common practice to place the ATU between the transmitter and the
coaxial cable. In this event, it does NOT cure the problem, but makes it look
as if the problem has been cured. It creates another bad VSWR so that the
reflected power is re-reflected back towards the mismatch. The load will
absorb a load of that re-reflected power, but the even smaller portion that is
re- reflected will go back to the ATU yet-again. In this way, it will keep
bouncing back and forth until eventually it will the mismatched load absorb
all.
Now you can probably understand that the instantaneous power in a
mismatched transmission line is somewhat higher than in a matched line. It
is for this reason that it is often under a (corrected?) mismatch that baluns
and antenna/feeder insulators most often fail, and occasionally with quite
spectacular visual effects. You will also realize that RF power is bouncing
back and forth through the same feeder losses, time and time again. The
feeder will therefore dissipate more power, and the load will dissipate less
power.
It is my personal opinion that a transmission line should ALWAYS be

matched. An ATU itself is a passive device and therefore has a signal loss. It
is far better to have a resistive antenna that is correctly matched to the
feeder. Having said that, there are occasions where a mismatched
transmission line can be used.
VSWR – How can we USE it

If you had an antenna that was the wrong length, then you can add a bit of
feeder to it to make it the right length. The remainder of the feeder would
then have to be an EXACT multiple of 1/2-wavelengths to reach the
transmitter.
This technique is knows as "Resonant Feeder". All resonant feeder systems
therefore begin at the load as a mismatch. In this way one can make an
antenna appear (at the transmitter) to resonate at more than one frequency
that are NOT harmonically related, e.g. 7MHz and 10.1MHz. The most well
known such antenna is the G5RV antenna.
Open Wire Transmission Line

The stuff is very easy to construct, as far less loss than coaxial cable and it is
cheaper. The only factors that affect the characteristic impedance of an
open-wire feeder system is the diameter of the wires used and the distance
between them. The formula is:
Z = 276 x Log(2 x Sp / D)
Where:
 Z = Impedance in Ohms.
 Sp = spacing between the two wires.
 D = Diameters of the two wires.
The spacers that hold the two wires apart can be anything that insulates;
plastic pipes, rings, book-binders, polythene, even a 1/4-wave aluminum
stub will work for a single frequency.
Coaxial Cable Transmission Line

This stuff is not very easy to construct, but in essence it is just a conductor
inside a tube. If you are making VSWR bridges or any "through line" RF
meter then you will need to make a bit of coaxial transmission line. The only
factors that affect impedance are, the Inside-Diameter of the outer
conductive surface, and the outside diameter of the conductor. The formula
is:
Z = 138 / (Log(Dt / D))
Where:
 Z = Impedance in Ohms.
 Dt = Inside diameter of the outer Tube.
 D = Outside diameter of Inner conductor.

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RFFEEDERS

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RF Feeder Methods

City & Guild Telecommunication and Electronic Engineering (T1)

BY Wasantha Kumara (City & Guild London)

Two parallel wires or "Open Line" feeder.

A wire inside a metal tube or "Coaxial" feeder.

Coaxial cable is the most common form of feeder used in radio

VSWR (Voltage Standing Wave Ratio)

Although I have over-simplified this a little, I hope that some form of

VSWR – How can we correct it

It is my personal opinion that a transmission line should ALWAYS be

VSWR – How can we USE it

Open Wire Transmission Line

Coaxial Cable Transmission Line

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