There are three main types of aerial in use, two of which are designed to receive the signals required for Freeview HD.
VHF, UHF and Combination UHF/VHF.
UHF and combination aerials are both suitable for use with Freeview HD, though that in itself doesn’t guarantee reception. Various factors can contribute to the signal levels being too low, such as location, cable type, number of outlets (whether in use or not) and obstructions to the signal (trees, houses and hills).
As a general guide, if you can get a reasonable picture on Prime, then chances are you can get Freeview HD; Prime being broadcast on a similar frequency to Freeview.
In order to work out which kind of aerial you have, simply look at the length of the elements.
A VHF aerial will have a number of arms (elements) ranging from as much as 2 metres top to bottom, to around half a metre for the higher frequencies. Some or all of these long elements may loop back on themselves. No short elements (200mm or so) are present.
A UHF aerial generally has a number of short elements about 250mm in length, though in high signal areas there may only be one or two elements, a grid type reflector may or may not be installed on the back of it. There are no long elements. Possibly installed when an occupant of the house had Sky UHF.
Combination UHF / VHF aerials (often called a ‘combo’ aerial) will have long elements at the back, tapering down to a number of short UHF elements at the front. An exception to this are the Lincrad ‘Gizmo’ aerials, and their copies. These aerials have one long VHF element, a shorter VHF, and a looped VHF element, as well as two short UHF elements.
A quick lesson in the frequencies in use, and the channels they are used for
VHF – Very High Frequency. Early channels were launched on the VHF band as it travels long distances well, however the number of spaces available in this range were limited, so it has been largely superceded by UHF. Used for channels One, Two, 3 and C4. Requires long arms (elements) on the antenna due to the width of the wavelength (getting longer as the frequency goes lower).
UHF – Ultra High Frequency. Once the VHF band has filled up, the UHF band has been used for transmitting TV channels. The higher the frequency, the more loss there is over distance, and through obstacles, however many more spaces are available for use. Better quality coaxial cable is required as the high frequencies do not travel well down low grade cables. Now used for digital terrestrial transmissions as well as analogue. Used for Maori, Trackside, Prime, Sky UHF (encrypted) and Digital channels. Smaller elements are used as the higher frequency wavelengths are much shorter.
A typical VHF aerial
A typical UHF aerial
Lincrad’s ‘Gizmo’ combo aerial