It's funny how things always go round in circles and new ideas somehow aren't always so new reflects Richard Stallworthy. Log-periodic aerials seem to have followed that path. Back in the 60's when UHF transmissions first started with BBC2, signal ghosting was often a problem. Manufacturers solved the problem with the log-periodic design, which differs significantly to the traditional Yagi aerial.

A Yagi aerial consists of a boom upon which is mounted a chain of director elements that are positioned in the standing wave of the incoming signal. At the end of the director chain is a dipole to collect the signal. The dipole is normally isolated from the boom, which does little more than hold the directors and dipole in position. The dipole normally requires a balun transformer to ensure matching to the cable impedance across the entire band.

Adding director elements to make the aerial longer can increase the gain and has the effect of making the aerial more directional. The more directional the aerial, the less signal is received from the side of the array and consequently ghosting is reduced. However there is a practical limit as to how long an aerial can be.

Ghosting can be a problem for analogue signals. It is the result of the primary signal arriving at the aerial before the reflected signal, which takes a longer route as it is reflected from buildings or the terrain. Log periodics went a long way to solving this, with high Front/Back ratios and low side-lobe performance, but Digital Terrestrial Television (DTT) signals are largely unaffected by ghosting unless the reflected signals are very strong.

Design differences
Log-periodic aerials work in a different way to the Yagi design. The aerial consists of two parallel booms, about 10-20mm apart, joined together at the mast clamp end. At the other end the cable is connected with coaxial braid to one boom and the centre conductor to the other. Effectively the booms form an “open unbalanced dipole” which, if tuned correctly, has an impedance of around 75?? Normally then, log-periodic aerials do not need a balun for cable matching.

The booms are an active part of the aerial, and along the boom, tuned elements are mounted at logarithmic intervals- hence “logarithmic” in the name of the design. Each element pair is tuned to a required frequency, acting as the “dipole” at that frequency. The other tuned elements act as directors. A Ultra High Frequency (UHF) wideband log-periodic aerial has two elements at the rear (nearest to the mast clamp) tuned, for example, to Channel 21. Elements are tuned to periodically higher channels, mounted in order moving towards the front, giving the log-periodic aerial its very distinctive tapered arrowhead shape.

The other fundamental difference is that the rear end of the boom is attached to the mast and via this to the structure of the building. This forms a very effective ground plane, unlike a Yagi aerial, where the dipole is “floating” or isolated from the ground plane.

Impulse Noise
Digital signals are more affected than analogue signals by poor carrier to noise ratio and impulse noise. Of course, other forms of interference affect both analogue and digital signals, but impulse noise is by its very definition “impulsive”.
The susceptibility of DTT signals to impulse noise is an important factor when making a good digital installation, since it interrupts the data stream of picture information and corrupts that data. This can result in locked pictures, pixilation or total blanking. Unfortunately, impulse noise comes in many types and from many sources. The well-known ones are car ignition picked up on the aerial, or a home heating thermostat arcing causing spikes which rise up the coaxial cable. Both will eventually arrive at the digital tuner. Although increasing the signal level could help by improving the Carrier-to-Noise ratio, impulse noise is often greater in level than the wanted signal.

It is not difficult to protect the digital signal from impulse noise once it is received by using well-screened components. However, an aerial is designed to collect signals, so cannot be screened! What a good aerial does is to collect only the signals you want, and reject the ones you don't.

Why go log-periodic?
In tests carried out recently it was found that log-periodics exhibited an increased ability to reject impulse noise, and not by just a small margin, but by a whopping 15dB or more. This can be explained by the ground plane effect. Impulse noise affecting DTT signals tend to be a low frequency, high level (ie amplitude) form of interference.

Mains created impulse noise and spikes can get carried up the coaxial braid, reach the aerial and get transferred along the boom via the mast clamp of the log-periodic aerial to the building, (see diagram 1 and 2) which is an effective earth

or ground plane. On a Yagi aerial these spikes can be carried up the braid of the coaxial cable, through the balun transformer and down the centre conductor to the tuner. This is because the balun is isolated from the boom and clamp ofthe aerial. Using better cable can often make matters worse because the lower loss of the higher quality braid allows more of the spike or impulse noise to reach the balun.

Log periodic aerials also exhibit another major benefit over a Yagi design. The gain of a log-periodic array is usually flat across the desired band, with most commercially available log-periodic designs offering gain from channels 21-68 of around 7dBd or a little over 9dBi. This means that whereas a WideBand Yagi may have its gain sloped by as much as 6dB, a log-periodic will receive a similar level of signal at all channels in the desired band (see diagram 3). Many professional installers perceived that log-periodic aerials had low gain at 7dBi, and so changed to ten element Yagi aerials with gains of 5dBi at channel 21 and 8-9dBi at channel 68. In reality, log-periodics can give nearly as much gain as an eighteen element Yagi.

But in high to moderate signal areas, log-periodics display ample gain with the benefit of high impulse noise rejection. With higher impulse noise rejection, gain is less of an issue. In theory, 15dB more gain would be needed from a Yagi aerial to equal the log-periodic rejection capability. Because they do not have a reflector, log-periodics also often exhibit lower wind load and lower visual profile.

So there it is, log periodic aerials make a come back, proving that nothing is new!

Moving forward with Vision
Vision has launched three log-periodic models designed for the professional installer in the majority of DTT installations. They are;

The V10-LP Standard (Stock code A95524).
A low cost, lightweight design with twenty-eight elements, saddle and clamp connections, and a gain of 7.5dBd. The low wind load design is for DTT reception in high to moderate signal areas.

The V10-LP Super (Stock code A95541). A forty element log-periodic with
F-Connectors. offering 9.5dBd and a low wind profile. It is designed for use in moderate signal areas.

The V10-LP Pro (Stock code A94912). Chunky and robust, this log-periodic is designed where a rugged installation is needed, such as Master Antenna Television (MATV) and Integrated Reception Systems (IRS). The aerial offers over 8dBd gain, 18mm boom sections and F-Connections for reliable high connectivity. It is has a dark green, low visibility anodised finish to aid corrosion protection.