UHF or Ultra High Frequency Band is a portion of Electromagnetic spectrum and is defined as Radio Waves having high bandwidth requiring smaller antennas. This post will discuss in detail about UHF (Ultra High Frequency), various types of antennas for Ultrahigh Frequency transmission, applications, advantages and disadvantages.
What is UHF (Ultra High Frequency)
Ultra High Frequency (UHF) is also known as Decimeter Band as it’s wavelength ranges from 1 meter to one tenth of a meter, whose frequency ranges between 300 MHz and 3 GHz. They propagate along LOS (Line Of Sight) path. Radio Waves in UHF has little or no reflection from the ionosphere; they travel entirely by Line of Sight propagation (LOS) and ground reflection. On channels 14 through 83, UHF waves usually bear television signals.
Fig. 1 – Introduction to UHF (Ultra High Frequency)
UHF are used in Cell Phones, Television Broadcasting, GPS, Wi-Fi, Bluetooth and numerous applications. UHF antennas are ideally stubby and small because of the short wavelengths. Quarter-Wave Monopole, the most common omnidirectional antenna, is between 2.5 and 25 cm long at UHF frequencies.
Types of Antennas for UHF Transmission
There are several types of antenna designs used for Ultra High Frequency Transmission like:
- Log periodic Antenna
- UHF Yagi Antenna
- Conical Array Antenna
- Fan Dipole Antenna
Log Periodic Antenna
In 1952, John Dunlavy invented the Log Periodic Antenna. The design and working principle of Log Periodic Antenna is very much like Yagi antenna. Log Periodic Antenna operates over a wide band of frequencies.
There are different types of Log Periodic Antennas such as Planar, Trapezoidal, Zigzag, V-Type, Slot and the Dipole. The commonly used one is the Log Periodic Dipole Array also called as LPDA. Basic components of LPDA is as shown in the Fig. 2.
Fig. 2 – Components of UHF Log Periodic Dipole Antenna
- The LPDA consists of several Half-Wave Dipole driven elements, whose length increases gradually. Each Dipole consists of pair of metal rods. In a line, the dipoles are placed close together, connected with an alternating step parallel to the Feedline.
- Even though the LPDA looks like Yagi antennas, their working principle is different. When elements are added to Yagi antennas, it increases the directionality whereas adding elements to LPDA increases bandwidth, or frequency response.
- Frequency ranges between 30MHz – 3GHz, which belong to both UHF and VHF Bands.
- The array consists of dipoles that are fed from a two-wire transmission line of varying lengths and spacings. Between each adjacent pair of dipoles, the line is transposed.
- The physical and electrical characteristics are repetitive in nature in LPDA design.
- The radiation patterns are Unidirectional or Bi-directional.
The relation between the Dipole lengths and distance of separation is given by the equation:
- T is the design ratio and T<1
- R is the distance between the feed and the dipole
- l is the length of the dipole.
UHF YAGI Antenna
This antenna is also known as Yagi-Uda Antenna. The name is derived from Hidetsugu Yagi and Shintaro Uda, two Japanese inventors. The Yagi antenna design has a dipole. The amplitude and phase of the current induced is dependent on the length and the spacing between the dipole.
There are three types of elements in the Yagi antenna.
- Driven Element
Driven Element is usually a half wave dipole to which the power is applied.
The Reflector is about 5% longer than the driven element. The Yagi Antenna always has one reflector and behind the driven element. Usually, a Reflector in the forward direction would add around 4 or 5 dB of lift.
The Directors are shorter than the driven element and are placed in front of the driven element as shown in the Fig. 3. There can be none, one or more Directors in Yagi antenna. It adds 1dB of gain in forward direction.
Fig. 3 – Elements in UHF Yagi Antenna
Conical Array Antenna
Conical Array Antenna consists of a conical structure with cross slot radiators fed by a Feed network that provides controllable amplitude and phase excitations of each arm of the crossed slots. For ease of implementation, S-band was selected as the frequency of operation. The Cone has a half angle of 10.25 degrees.
Fig. 4 – Conical Array Antenna
Fan Dipole Antenna
Fan Dipole Antenna operates in such a way that each dipole presents a low impedance at the Feed point at its resonant frequency. As the frequency travels away from one dipole’s resonant frequency, its impedance increases, and power is not absorbed and the impedance falls. However, at the Resonant frequency of another dipole, it will take power from the Feeder.
Fig. 5 – (a) Schematic Representation (b) Physical View of Fan Dipole
The Fan Dipole has number of resonant frequencies, each corresponds to frequency of different Dipoles. In any Fan Dipole construction, all the dipoles should be connected in parallel. If the UHF antenna is set up as a set of parallel dipoles, then the dipole appears to bear the weight of all the other dipoles at the lowest frequency, and this can cause the entire antenna to sag. Hence, the parallel dipoles are implemented as an inverted V as this can reduce the sag considerably.
Applications of Ultra High Frequency
The applications include:
- UHF antennas are used in television broadcasting. Even though the bandwidths are reallocated to mobile telephones, land mobile radio system etc., UHF channels are still used in digital television (Transmission of audio-visual signals using digital encoding).
- Used in military for voice communication as two–way Radios.
- The GSM and UMTS mobile networks use UHF cellular frequencies. This facilitates the connectivity of cell phones and mobile computing devices to a publicly switched telephone network and to the Internet.
- UHF Radars are used in tracking enemy fighter planes or bombers.
Advantages of UHF (Ultra High Frequency)
The advantages include:
- UHF has short bandwidth with high frequency.
- The size of the antennas for transmission and reception is correlated with the size of the radio wave.
- UHF antennas are stubby and short.
Disadvantages of UHF (Ultra High Frequency)
The disadvantages are:
- The major drawback of UHF is its restricted broadcast range and reception, sometimes referred to as a line of sight between the transmitting antenna of the TV station and the reception antenna of the customer.
- For high gain levels, the UHF antenna becomes very long.
- Interference with the signal occurs due to weather conditions such as rain.