Whats The Best TV Aerial?
One of the best TV Aerials to use is a Periodic Tv Aerial, One of the major drawbacks with many RF Aerials is that they have a relatively small bandwidth. This is particularly true of the Yagi beam Aerial.
One design named the log periodic TV Aerial is able to provide directivity and gain while being able to operate over a wide bandwidth. In particular, the log periodic dipole array is the most widely used version of this Aerial family.
The log periodic TV Aerial was initially developed by Dwight E. Isbell, Raymond Duhamel who published a paper in 1957 later additional variants were made by Paul Mayes. The concept of the log periodic TV Aerial was patented by the University of Illinois in the USA.
Types of log period TV Aerial
There are several formats in which the log periodic TV Aerial can be realised. The exact type that is most applicable for any given application will depend upon the requirements.
The main types of log periodic array include:
- Zig zag log periodic array
- Trapezoidal log periodic
- Slot log periodic
- V log periodic
- Log periodic dipole array, LPDA
- The type that is most widely used is the log periodic dipole array, LPDA, and that will be described here.
Log periodic dipole array basics
The most common is the log periodic dipole array basically consists of a number of dipole elements. These diminish in size from the back towards the front. The main beam of this RF TV Aerial coming from the smaller front.
Not all the TV Aerial is active at any given frequency. The active region, i.e. the sections of the TV Aerial that are contributing to the transmission or reception vary with frequency.
The element at the back of the array where the elements are the largest is a half wavelength at the lowest frequency of operation – the longest element acts as a half wave dipole at the lowest frequency. The element spacing also decreases towards the front of the array where the smallest elements are located. The upper frequency is a function of the length of the shortest element.
Basic log periodic dipole array
In operation, as the frequency changes, there is a smooth transition along the array of the elements that form the active region.
To ensure that the phasing of the different elements is correct, the feed phase is reversed from one element to the next.
There is also normally a sorted matching feeder stub attached to the end of the feeder furthest from the shortest element.
Log periodic performance
The main log-periodic TV Aerial performance differentiator is the wide bandwidth it possesses.
This type of RF TV Aerial design is normally capable of operating over a frequency range of about 2:1 while still being able to provide a usable level of forwarding gain over a dipole.
It has many similarities to the more familiar Yagi because it exhibits forward gain and has a significant front to back ratio. In addition to this, the radiation pattern of this RF TV Aerial design stays broadly the same over the whole of the operating band as do parameters like the radiation resistance and the standing wave ratio. However, it offers less gain for its size than does the more conventional Yagi.
In real terms a typical log-periodic TV Aerial might provide between 3 and 6 dB gain over a bandwidth of 2:1 while retaining an SWR level of better than 1.3:1. With this level of performance, it is ideal for many applications, although a log periodic TV Aerial will be much larger than a Yagi that will produce equivalent gain. However the Yagi is unable to operate over such a wide bandwidth.
Log periodic TV Aerial applications
The log periodic TV Aerial is used in a number of applications where a wide bandwidth is required along with directivity and a modest level of gain. There are several areas where the TV Aerial is used:
UHF Terrestrial TV: The TV Aerial is sometimes seen in the form of UHF terrestrial TV Aerial applications. The television spectrum extends over a wide bandwidth – more than normal Yagi TV Aerials can comfortably cover. Normally channels for a given area are located within a particular subset of the UHF television spectrum so that the channels are relatively close together. However, there can be situations where the UHF channels may be located over a wide portion of the television spectrum. This may be because channels from different transmitters located across the television spectrum may be needed, or that spectrum restrictions can mean that the channels are spaced wide apart..
HF communications: Log periodic TV Aerial arrays are often used in applications where HF communications for diplomatic traffic. These TV Aerials perform well because the diplomatic services may need to operate over a wide selection of frequencies in the HF bands, and it is often only feasible to have one TV Aerial, for example on an embassy building.
EMC measurements: EMC measurements require scans over a wide band of frequencies. Log periodic TV Aerials can be used in this application to enable operation and sensing of signals over the wide bandwidths needed. As such the log periodic TV Aerial is key to the operation of many EMC tests and test environments.
Other applications: There are many other applications where log-periodic TV Aerials can be used. Any applications where directivity and a wide bandwidth are needed are ideal applications for this form of RF TV Aerial design.
Whether to use a wideband / T group aerial or not is a source of much debate. The CAI and the BBC recommend the installation of wideband aerials, or they did, they`re not so sure anymore! For most installs, i.e. those in strong or medium signal strength areas (90% of homes, or more ? ), we agree with them and advise the installation of one or other of the Log Periodics, which are wideband or T group anyway. If your transmitter is a wideband we would (obviously) also advise fitting a wideband even if you live in a marginal signal area.
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