Balanced Feeder: antenna twin feeder
Balanced antenna feeder can provide some distinct benefits when it is used with balanced antenna for HF radio communications systems and the like.
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Balanced antenna feeder is also know under several other names including twin feeder, and ribbon feeder. The term open wire feeder or open wire transmission line is often used to describe a particular form of balanced feeder consisting of two separate wires with spacers.
Balanced feeder is a form of antenna feeder that can be used for feeding balanced antennas (i.e. antennas that do not have one connection taken to ground).
Balanced feeder is mainly used on frequencies below 30 MHz, but in theory it can be used at any frequency. It also tends to be used for HF radio communications, often with medium or relatively high powered stations where its special requirements can be accommodated.
As the name indicates, balanced feeder offers the advantage of being balanced, i.e. not having one side grounded. In addition to this it can provide a form of antenna feeder that provides low levels of loss, provided that it does not pass close to other objects.
Balanced feeder is used less than coaxial feeder or coax, one of the main reasons for this is that it is affected by close objects and as a result it is not as convenient to use. It does not perform well if it is passed through a house in the same way that can be achieved with coaxial feeder.
Balanced feeder basics
A balanced or twin feeder consists of two parallel conductors unlike coax that consists of two concentric conductors. These two wires are kept spaced apart at a constant spacing either by a continuous or nearly continuous spacing plastic between the two wires, or by spacers placed along the two wires at regular intervals.
When carrying radio frequency signals, the currents flowing in both wires run in opposite directions but are equal in magnitude. As a result uniform structure of the feed line, the fields generated by the currents flowing generate equal and opposite electromagnetic fields which tend to cancel each other out. The result is that there is minimal radiation or pickup and minimal loss along the balanced feeder (if the resistance of the wires is low).
In order for balanced feeder to operate satisfactorily, the balance between the two wires must be maintained. Any imbalance will cause it to radiate, or pick up signals. Even nearby objects can cause an imbalance.
It is for this reason that running this type of feeder around buildings, or even into them gives rise to a significant drop in performance.
Also, to ensure efficient operation the spacing of the conductors is normally kept to within about 0.01 wavelengths.
As a result of the spacing required, balanced feeds tends to be used more widely for HF applications and it is rarely used for VHF, UHF etc radio communications applications. Another reason is that it tends to be used for HF radio communications is that for VHF and UHF, most applications tend to need the flexibility of coax where installations need the feeder to be run in close proximity to other objects.
Balanced feeder impedance
Like coaxial cable, the impedance of twin feeder is governed by the dimensions of the conductors, the conductor spacing and the dielectric constant of the material between them. The impedance can be calculated from the formula given below.
D is the distance between the two conductors
d is the outer diameter of the conductors
ε is the dielectric constant of the material between the two conductors
As an example, when using 18 SWG wire which has a diameter of 0.125mm, i.e. radius of 0.625mm, and a spacing between the conductors of 27mm, this gives an impedance of 451Ω, i.e. virtually 450Ω
Types of balanced feeder
Balanced feeder can take a variety of forms. Different formats are used for different applications, as each type has different physical characteristics.
Open wire feeder: An "open wire" feeder can be made by having two wires running parallel to one another. Spacers are used every fifteen to thirty centimetres to maintain the wire spacing. Usually these are made from plastic or other insulating material. Typically this feeder may have an impedance of around 600 ohms, although it is very dependent upon the wire, and the spacing used.
Open wire feeder is often used for HF antennas for radio communications systems. The spacing is typically a few centimetres (two inches or so), but this is very much dependent upon the spacers used. Open wire feeder is typically not bought, but made from the wire and spacers which can be bought at specialist ham radio or antenna outlets.
Normally the wire and spacing are selected to give the required impedance.
Ribbon balanced feeder: The feeder may also be bought as flat 300 ohm ribbon feeder consisting of two wires spaced with a clear plastic. This is the most common form and is the type that is used for manufacturing temporary VHF FM antennas. If used outside this type absorbs water into the plastic dielectric. Not only does this significantly increase the loss on damp days, but the moisture absorbed causes the wire to oxidise which in turn leads to increased losses over the longer term.
This form of balanced feeder is rarely used for performance applications. It is most widely used for domestic FM radio and a very few other applications. A few VHF FM tuners have a 300Ω balanced input for this type of antenna. Often it can be fed using the ribbon feeder.
It is necessary not to confuse this "ribbon" cable with the multi-wire ribbon cable which is often multicoloured or grey/blue and used for connecting digital circuits using insulation displacement type connectors.
Ladder line: The feeder can also be bought with a black plastic dielectric with oval or rectangular holes in the dielectric spacing. These give the appearance of the feeder looking a little like ladder.. This type gives far better performance than the clear plastic varieties which absorb water if used outside. Accordingly it can be used in many dipole and other antenna systems for HF radio communications.
Ladder line can be bought with a variety of different impedance values. Typical values are 300 Ω and 450Ω.
75Ω flex: This type of balanced feeder looks a little like ordinary low current twin mains flex. It has each wire insulated, and the overall cable has an additional insulated layer. In fact low current cable can often be used in this application because it has an RF impedance characteristic quite close to 75Ω.
This 75Ω feeder can be used to feed a half wave dipole and then transition to coax using a 1:1 balun at a suitable point. This enables the low loss of the balanced feeder to be used in the open and combined with the convenience of coax for runs within the buildings. That said the great advantage of open wire feeder is that the dielectric is air and losses exceedingly low. This cable is encapsulated in plastic and often the wires are insulated, so this will introduce some additional losses into the system.
Making Open Wire balanced feeder
It is very easy to make a length of open wire feeder provided that the right components are available. In fact all that is needed are the two equal lengths of wire, and sufficient spacers.
The wire can be suitable wire: hard drawn copper antenna wire is good as it does not stretch: it is possible that the feeder will hang slightly unevenly and this could stretch one wire more than the other giving rise to unequal lengths and hence an imbalance.
The spacers can be almost any insulating dielectric that can be fashioned to attach to both wires. It is possible to buy specially made plastic spacers from ham radio antenna specialist, or alternatively other plastic material with holes drilled for the wires to pass through. One person even used old plastic ballpoint pen cases, but it would require a lot of writing to be done to use up sufficient pens for this.
There needs to be a method of securing the spacers in position - bought ones generally "snap" over the wire, but home made ones will need fixing - a blob epoxy glue on the wire either side of the spacer should work.
At least with home made spacers, it is possible to select the spacing to obtain the required spacing and hence feeder characteristic impedance for the wire used.
Spacers should be spaced, generally equally spaced along the length of the feeder.
Using balanced feeder
There are a number of points to note when using balanced feeder if the best performance and flexibility is to be obtained.
Keep away from nearby objects & especially buildings: In order that balanced feeder is able to operate effectively, the fields from the two wires must balance each other out. This can only happen when the feeder is used in open spaces. If they are taped to a tower, or in proximity to it, if they are brought into a building, etc, then the balance is compromised and the feeder will not operate correctly. Under these circumstances it can radiate and pick up signals along the length of the feeder to a degree dependent upon the imbalance. Also the level of loss will increase.
As a result, balanced feeder tends to be used to connect to an antenna to take account of the low loss and high VSWR capability, and then there may be a transition to coaxial cable - this must be done correctly.
Use a balun to convert to coax: For many radio communications systems, it is necessary to transition from balanced feeder to coaxial cable. This is achieved using an item called a balun. It is a balanced to unbalanced transition. There are many ways of implementing a balun, from using a transformer, to chokes, etc.
Note on antenna baluns:
Baluns are used with many antennas and their feed systems to change from a balanced to an unbalanced system, or vice versa. They can take many forms and used in many antenna systems.
Read more about Antenna baluns.
Use an ATU: One of the advantages of using balanced feeder is that the loss levels are low. Accordingly they can operate very satisfactorily with high levels of VSWR, and in fact many antennas are designed to operate in this fashion. Whilst there is the need for a transition from balanced to unbalanced, it is also necessary to ensure that a good match is present, especially for any transmitter that is used.
Low loss: One of the benefits of using balanced feeder is that it has a low loss when compared to coaxial cable - this really applies to the open wire format. Here small spacers are used to keep the wires apart, but the majority of the dielectric is air, and this provides exceedingly low levels of loss. If long runs of the feeder can be kept in the open, then it can be used to good effect, and a transition to coax can be made when it nears a building, etc.
Although balanced feeder is less widely used than coaxial cable, it can be sued to very good effect in a number of different applications, especially for various forms of antenna used for HF radio communications.
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