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The doublet antenna is a form of dipole that uses a balanced feeder, often open wire feeder and an antenna tuning unit. As open wire feeder is able to operate with levels of standing waves and effectively becomes part of the antenna, it is able to operate over a wide band of frequencies.
As a result, the doublet antenna forms a very convenient multiband antenna and it is often used at HF where a number of different HF bands need to be covered, and it is relatively popular with radio amateurs where it enables several bands to be used with a single antenna.
Doublet antenna basics
The doublet antenna is essentially a balanced system and each half of the top plus each wire in the feed line must be equal in length. The antenna top is not cut to resonate at any particular frequency (unlike the half-wave dipole), and any length may be chosen to suit an individual location.
The key to the doublet antenna is the form of feeder used. Open wire or balanced feeders are able to operate as part of the actual antenna itself, not just feeding the power from an unbalanced source. The balanced feeder or open wire feeder is able to operate with standing waves along its length.
Note on Balanced Feeder:
Balanced feeder can come in one of a variety of forms: open wire, ladder line, or simple ‘ribbon feeder’. It provides the capability for very low loss, provided it does not pass near other objects that might cause imbalanced. Balanced feeder is often used on the HF bands.
Read more about Balanced Feeder.
Standing waves are a feature of radiating wires, but in the case of the open wire feeder, it consists of two equal length close spaced wires. As these carry equal and opposite currents their radiation cancels. However the feeder still remains part of the overall antenna itself.
For a doublet, the top section is typically a minimum of λ / 4 for each section (length L1), i.e. a total length across the top of λ /2.
However it is found that the top section, i.e. 2 x L1 can be reduced to about 3 λ / 8 without any major reduction in performance - it has about 98% of the efficiency of a half wave dipole, so the difference will not be noticed.
The doublet antenna can operate over a wide range of frequencies and as a result the radiation pattern will change according to its electrical length with respect to the number of wavelengths, or part of that it represents. As the electrical length increases, i.e. as the frequency increases the antenna increases in the number of wavelengths it represents although the physical length remains the same. As the electrical length increases, so the phasing of the fields around the radiating element mean that the radiation changes from a figure of eight pattern for a half wave top radiating element to a pattern that has lobes that increasingly move towards the axis of the doublet antenna.
Doublet feeder & operation
The feeder can be either open wire or what is termed ladder line. This can be either 300Ω or 450Ω. Also the length of the feeder can be cut meet the requirements of the installation.
When feeding an antenna of this type it is found that it can present an impedance over a wide range. Accordingly it is necessary to use an antenna tuning unit to ensure that the transmitter itself is presented with the required impedance. This can be measured using an SWR meter.
The antenna tuning unit used should be able to match impedances over a wide range, and it must also have a balanced output connection. If it does not, then an external balun is required so that the unbalanced to balanced transition is present.
Baluns can be bought or made. Essentially they are simple in their construction provided that the right components and tools are available. Particularly key is the former on which the balun is wound. Typically this is a torroid and it must have the required RF properties and RF power handling capability. Often a balun for a doublet antenna may be a 4:1 ratio providing a 300Ω output to the antenna for a 75Ω input. This provides an adequate match for many ATUs with only unbalanced output connections.
Direct connection to coax
Although it is always best to use an antenna tuning unit with a doublet antenna, it is possible to arrange the antenna so that this may not always be needed.
Although any length of balanced feeder can be used with the doublet, the impedance match is best if the total length of one leg of the antenna and feeder, i.e. L1 + L2 equals an odd multiple of electrical quarter wavelengths of the frequency to be used. Using this approach the impedance, around 50Ω is low and mainly resistive.
If the low impedance option is used to directly match to 50 Ω remember there will need to be a 1:1 balun in circuit to provide the required balanced to unbalanced transition.
However it is always good practice to incorporate an antenna tuning unit to ensure that the transmitter is presented with the right impedance, even if it is expected to provide a good match.
Doublet antenna designs
As might be expected there are many variants of doublet antennas that have different properties. Over the years many different antenna designs have been created that are based on the doublet antenna concept.
- G5RV antenna: The G5RV is a doublet antenna with specific lengths that makes it suitable for operation on a variety of amateur radio bands whilst maintaining a good match. The G5RV double antenna design was originally published in 1958 and was aimed at providing a good antenna for the bands of the day with the minimum of matching requirements. It can still be used today on a variety of amateur bands, although operating it on some of the newer bands, and also with solid state transmitter PAs it requires the use of an antenna tuning matching unit.
Read more about . . . . the G5RV antenna.
Doublet antenna advantages & disadvantages
When considering the use of a doublet antenna it is worth considering both the advantages and also the disadvantages.
Doublet antenna advantages
- Wide bandwidth
- Easy to make and install
Doublet antenna disadvantages
- Requires antenna tuning unit
- Balanced feeder does not like runs through buildings, etc
Doublet antennas provide an ideal form of wideband wire antenna that is low cost and can be erected very easily. As such doublet antennas can provide an ideal solution to many HF antenna requirements.
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