5a.1 Identify and recall the use of coaxial and twin feeders. The
antenna is one of the most important parts of an amateur station as is
the feeder that connects the transmitter and receiver to the antenna.
The two most common types of feeder are coaxial and twin feeder. Coaxial feeder consists of a copper core, a dialectric and a copper braid. This is surrounded by a non-conducting PVC outer layer.
Coaxial
feeder used by amateurs has an impedance of 50 ohms. Coax
used in TVs and satellite receivers has a different impedance and will
not give a good match to amateur equipment.
Coaxial feeder is more expensive that twin feeder. Coax is an unbalanced system and needs, ideally, to be kept away from metal objects.
It has some shielding from electromagnetic interference.
It is important to protect the ends against water ingress as this will increase losses.
Coaxial feeder may be used in
long lengths to connect the antenna to the TX/RX, often coming down the
side of walls, or through roof spaces.
It has a higher loss value than
twin feeder, particularly at VHF and UHF. In the shack it is used in
short lengths to interconnect equipment.
Twin feeder consists
of two parallel wires with a separation of between 0.5cm and 30cm. The
wires are kept apart by a variety of methods. Plastic separators,
porcelain separators, wood dipped in wax, glass separators.
Twin feeder has an impedance of between 75ohms and 600ohms.
It is cheaper than coax
It is a balanced system, but still needs to be kept away from metal objects.
There is no shielding, but if balanced, induced interference will be cancelled out. PVC covered wire needs to be protected from water ingress.
Twin feeder can be used over long distances with little loss of signal
5a.2 Understand that equal and opposite currents flowing in a balanced
feeder cause equal and opposite fields around the two conductors.
Understand that these fields cancel out, but that nearby objects can
cause an imbalance that makes the feeder radiate RF energy. In
a properly balanced feeder equal and opposite currents flow causing
equal and opposite fields in each wire. These will cancel each other
out and prevent any radiation from the feeder. As stated above if the
feeder passes near to other objects this balance can be upset causing
radiation of RF energy from the feeder.
5a.3 Recall that in a correctly connected coaxial cable the RF field
only exists within the cable and is not affected by objects outside the
cable.
Note that correctly connected means screen continuity through any plug
and socket, and connected to a balun or unbalanced load, not necessarily
of 50Ω.
If a coax cable is correctly connected, i.e. there is an
unbalanced 50ohm connection, at both the antenna end and the TX/RX
end, there is only an RF field in the centre wire. This means that there
will be no feeder radiation.
It is important that connectors allow a
continuous screen to exist between the antenna and TX/RX and that
baluns are used to convert balanced to unbalanced systems.
In the top drawing opposite the dipole is balanced; the 1:1 balun at
the output of the antenna ensures a proper match to the unbalanced coax
and the 50 Ohm unbalanced coax matches the 50 Ohm antenna socket
on the radio. In this case there is no radiation from the coax.
In the bottom drawing there is no balun between the antenna and coax. This leads radiation from the coax.
5a.4 Recall that feeders cause loss of signal strength on both transmit and receive. The longer the cable, the greater the loss.
Recall that twin feeder usually has lower loss than coaxial cable.
5a.5 Recall that loss is measured in dB. Be able to calculate the power
delivered to an antenna for a given RF output and given feeder loss (in
multiples of 3 dB and 10dB).
Loss of power related to loss in dBs
The loss we mentioned above is measured in dBs.
A 3dB loss is equivalent to
losing half the power. So if a feeder has a 3dB loss at 14MHz and is
fed with 200Watts only half will be radiated by the antenna i.e. 200x1/2 =
100W.
A 10dB loss is equivalent to
losing 9/10 of the power. So, if a feeder has a 10dB loss at
440MHz and is fed with 50Watts, only50x1/10 = 5 Watts will be radiated by
the antenna. 50x9/10 = 45Watts will be lost.
There will be a similar loss on receive, The S meter measures received
signal strength in S units. One S point is equal to 6dBs. If a piece of
feeder has a loss of 3dBs then this will result in half an S point loss
in all signals picked up by the antenna.
We can work in multiples of these:
From a transmitter:
A 3dB feeder loss will result in the
antenna radiating half the power. 50 Watts from the transmitter =
25 Watts from the antenna
A 6dB feeder loss will
result in only a quarter of the power been radiated. 50 Watts from the
transmitter = 12.5 Watts from the antenna
A 9dB feeder loss will result in only an eighth of the
power been radiated. 50 Watts from the transmitter = 6.25 Watts from
the antenna.
A 10dB feeder loss will result in only a tenth of the power been radiated.
50 Watts from the transmitter = 5.00 Watts from the antenna.
From a receiver:
If an S9 signal is received by the antenna then 3dB of feeder loss will mean an S meter reading of 8.5 at the receiver.
If an S9 signal is received by the antenna then 6dB of feeder loss will mean an S meter reading of 8 at the receiver.
If an S9 signal is received by the antenna then 9dB of feeder loss will mean an S meter reading of 7.5 dB at the receiver.
Feeder characteristic impedance
5b.1 Recall that feeders have a characteristic impedance which depends upon the diameter and spacing of the conductors.
Recall that this impedance determines the ratio of the RF RMS potential
difference to the RF RMS current in a correctly terminated feeder.
Recall that for amateur use, 50 Ω coaxial feeder is normally used; that
coaxial cable for TV and satellite receivers has a different impedance,
and that balanced feeder is commonly available from 75 to 600Ω.
Note that correctly terminated means correctly connected with a resistive load equal to the cable characteristic impedance.
The impedance of a feeder is
determined by the diameter and spacing of the conductors. In ladder line feeder, the wider the
spacing the higher the impedance.
In coax cable the impedance is determined by the distance between the
inner core and outer sheath; the diameter of the central core and the
braid and also the type of dielectric used.
The impedance(Z) is calculated from:
Z = RF RMS Voltage / RF RMS current
For example if the RF RMS voltage on a line was 100Volts and the RF
current was 2Amp, the impedance of the line must be 100/2 = 50 Ohms.
This formula can only be used where there is no
reactance involved. I.e. in a situation where the load impedance is
equal to the line impedance with no capacitive or inductive reactance.
Different types of coax impedance are used in different situations.
In amateur radio transmitters and receivers the impedance of the antenna socket is 50 Ohms.
In terrestrial and satellite TV receivers the impedance of the socket and cable is 75 Ohms.
Balanced feeder used in amateur radio varies from 75 Ohms to 600 Ohms. Here the greater the spacing the higher the impedance.
