What’s so intriguing about a folded dipole antenna? A folded dipole antenna is basically a dipole antenna with both ends connected by a rod or wire. It’s folded in a closed tubular shape.
One interesting fact is that the dipole’s length is basically the wavelength’s half, which is why it’s also known as a half-wave dipole antenna. Usually, these types of antennas run in the 3KHz – 300 GHz frequency range. And they’re mostly used as a feeder in other antennas, TVs, and radio receivers as well.
Hence, if you’re interested in using folded dipole antennas, keep on reading to learn all you need about them, including what advantages they have and disadvantages you may face as well as what makes them a good option.
What is Folded Dipole Antenna?
A folded dipole is a half-wave dipole connected at the ends with wires or rods. These connecting wires are equal to the dipole in cross-section and diameter.
It results in two radiations that are nearly identical. You get the same far-field radiation pattern as you’ll with a single wire dipole, but it gives 4 times the radiation impedance.
This high feed point impedance is what has made the use of folded dipole so widespread. You also get a broad bandwidth at an inexpensive operation cost.
Most folded dipole antennas have an operating frequency range of 3 kHz – 300 kHz, meaning they have a high central frequency. These antennas are best to use in situations where optimum transfer of power and a greater dipole impedance are necessary.
Structure of Folded Dipole Antenna
In a folded dipole antenna, two parallel dipoles are connected at their ends, forming a narrow wire loop. The length of the dipole here is almost always λ/2 but it may be slightly smaller in some cases.
One of its poles, the one with λ/2 length, runs continuously, while the other is divided into two in the middle. While transmitting signals, the center terminals of both poles are fed. Even when receiving input, the same spot is fed.
The width is much smaller than the length, which is what makes it cylindrical in shape. You will notice that the wires or rods connecting the two dipoles are spaced equally.
Why is a Folded Dipole Array Antenna Used? Their Common Applications
There are several advantages of this type of antenna that make them best for usage as a feeder element in the following types of antennas.
- Parabolic antenna
- Log periodic antenna
- Yagi antenna
- Turnstile antenna
- Reflector and phased antenna arrays
- TV and radio receiver’s antennas
Out of all these folded dipole array antenna applications, the most common one is its use as a TV receptors’ antenna.
They’re inexpensive to construct, provide optimum power transfer and offer reasonable quality, which is what makes them so popular for receiving broadcasting signals. These antennas aren’t the best fit for receiving narrow bandwidth.
Importance of Folded Dipole Antenna
As we mentioned before, the folded half-wave dipole antenna application is seen in situations requiring efficient power transfer and large impedances.
The most widespread use of this dipole is seen in TV signal reception. Folded dipole is used to construct a TV reception antenna that is called the Yagi-Uda antenna. So, basically this antenna is a type of folded dipole antenna used in TV signal reception.
Yagi-Uda antenna is known for its ability to get a high gain and wider bandwidth per unit area of the antenna. So, it’s also called a super derivative antenna or super gain antenna. The efficiency and performance of this antenna is mainly due to the folded dipole used.
So, folded dipole antennas are indispensable for watching television and broadcasting.
Operation of Folded Dipoles
It has two modes of operation, which we’ll look at in detail.
- Transmission Line Mode
In transmission line mode, the antenna current flows in the folded dipole array antenna in a loop that is created by closing the two dipoles with wire at the terminals. Therefore, the two dipoles have opposite current directions as they are situated parallel to one another.
No radiation occurs from the dipole elements in this mode of operation as the opposite direction of the current flowing in each electric dipole cancels out the electric field.
- Antenna Mode
A folded dipole in antenna mode has a current that flows in the same direction in both dipoles that are situated parallel to one another. So, the current is divided into two equal halves and there’s radiation.
Radiation Pattern of Folded Dipole Antenna
You will find the same radiation pattern for half-wave dipole antennas, whether standard or folded. They follow an omnidirectional pattern, which means the radiation flows in all 360 degrees.
So, you get connectivity in all possible directions from this resonant antenna.
Advantages of Folded Dipole Antenna
Several factors, like low power requirement from the source, a greater input impedance that ranges from 73 to even 300 ohms, and high middle frequencies, put a half-wave folded dipole antenna a cut above a standard half-wave dipole antenna. So, this type of antenna is best when an increase in feed impedance is needed.
Other advantages of folded dipole antenna include-
- Easier to match with transmission lines in terms of antenna impedance according to antenna theory.
- Serves a wider bandwidth than a regular single dipole that is similar in size
- Has an impressive signal reception, as well as outperforms standard half-dipole antenna in signal/noise ratio
- Can receive the targeted signal from a band of frequencies without a loss of signal quality.
Disadvantages of Folded Dipole Antenna
Folded dipole antenna’s disadvantages lie in their structure. When you get a larger size, it’s hard to manage these antennas in an outdoor environment. Relocating and adjusting the placement of these antennas is also difficult.
Tips For Using Folded Dipole Antennas
To make the best out of your folded dipole TV antenna, we have some tips for you.
Tip 1: Proper Mounting
Face your antenna to the station of interest. Mount it as high as you can as increased height will increase performance. Transmission lines and antennas should not be close to metals. Avoid coiling up the transmission lines. Instead, cut to size.
Tip 2: Add Reflectors or Directors
Using a reflector and even a director with your antenna can help with ghost rejection as well as increase signal strength.
To create the reflector, get a single insulated wire that is longer than the antenna by close to 5%. Place it parallel to and about 0.2 wavelengths distant from the folded antenna, in the opposite direction of the TV station. You don’t need to connect this wire to the antenna in any way.
For the director, get an insulated wire that is shorter than the antenna by 5%. Mount it facing the side of the station, at about 0.15 wavelength distance away from the antenna.
Folded dipole antennas are affordable and effective. Although best for attic installation in houses, you can also use them while living in a dorm or apartment. Handling the antenna while mounting can be a bit challenging, but it is worth the performance it offers.
The higher antenna feed point impedance and wide bandwidth of folded dipole antenna make it a superior option for TV receptors. You can enlist professional help if you don’t have much experience with TV receptors and antennas.