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Out of sight (cont.) suit the lashing bracket if you need it. Even better, you don't need to buy any special tools to fit the Vision VC Chimney Lashing. Got a pair of tin snips or cutters? Got a screwdriver? That's all you'll need for professional, solid and practical fixing to a wide range of chimney stacks. Chimney lashing needn't be secondbest with the right kit. Vision VCB1 Single lashing kit (94555) Vision VCB2 Double lashing kit (94572) Vision VB25 25 metre banding roll (94640) No leaks. No fuss. Ask a professional installer what causes most problems on a through-the-roof mast and the chances are “weatherproofing” will come near the top of the list. There isn't much that upsets a homeowner more than a leaky roof during the next downpour. So what do you do to keep the water out of their home and off your reputation?

Mast Pole

Rubber Collar

Zinc Flashing

Log Periodic Why are log-periodic aerials making a comeback?

Log periodic boom (diagram 1)

Mast

Mast Clamp

Impulse Noise

Signal

It's funny how things always go round in circles and new ideas somehow aren't always so new reflects Richard Stallworthy. Logperiodic 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.

impedance of around 75?? Normally then, logperiodic 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.

Lead Tile

The answer might be to try the Vision V15-100 Weatherproofed Roof Seal. Designed to replace the tile you have taken away, the Roof Seal sheds rain like a duck's back. And you know that it will carry on protecting as the tile itself is high quality lead, the flashing is malleable but hard wearing zinc, and it's topped off with a custom fit rubber collar to seal the mast as it passes from inside the loft to outside the roof. You don't need to worry about wind-damage, as the same design that lets a palm tree withstand a hurricane when concrete buildings are reduced to rubble is built into the Roof Seal. By floating free from the tile and flashing, which are fixed securely to the roof, the rubber collar and mast flex in the wind without ripping the waterproof seal apart. The end result is a seal that stays sealed even under extreme rooftop conditions. Vision V15-100 Weatherproof Roof Seal (91954)

22 | issue 8

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

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 wellknown 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 wellscreened 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 logperiodics 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


feature

Out of sight (cont.) suit the lashing bracket if you need it. Even better, you don't need to buy any special tools to fit the Vision VC Chimney Lashing. Got a pair of tin snips or cutters? Got a screwdriver? That's all you'll need for professional, solid and practical fixing to a wide range of chimney stacks. Chimney lashing needn't be secondbest with the right kit. Vision VCB1 Single lashing kit (94555) Vision VCB2 Double lashing kit (94572) Vision VB25 25 metre banding roll (94640) No leaks. No fuss. Ask a professional installer what causes most problems on a through-the-roof mast and the chances are “weatherproofing” will come near the top of the list. There isn't much that upsets a homeowner more than a leaky roof during the next downpour. So what do you do to keep the water out of their home and off your reputation?

Mast Pole

Rubber Collar

Zinc Flashing

Log Periodic Why are log-periodic aerials making a comeback?

Log periodic boom (diagram 1)

Mast

Mast Clamp

Impulse Noise

Signal

It's funny how things always go round in circles and new ideas somehow aren't always so new reflects Richard Stallworthy. Logperiodic 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.

impedance of around 75?? Normally then, logperiodic 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.

Lead Tile

The answer might be to try the Vision V15-100 Weatherproofed Roof Seal. Designed to replace the tile you have taken away, the Roof Seal sheds rain like a duck's back. And you know that it will carry on protecting as the tile itself is high quality lead, the flashing is malleable but hard wearing zinc, and it's topped off with a custom fit rubber collar to seal the mast as it passes from inside the loft to outside the roof. You don't need to worry about wind-damage, as the same design that lets a palm tree withstand a hurricane when concrete buildings are reduced to rubble is built into the Roof Seal. By floating free from the tile and flashing, which are fixed securely to the roof, the rubber collar and mast flex in the wind without ripping the waterproof seal apart. The end result is a seal that stays sealed even under extreme rooftop conditions. Vision V15-100 Weatherproof Roof Seal (91954)

22 | issue 8

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

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 wellknown 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 wellscreened 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 logperiodics 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


Archive Rigger's Digest Issue8 April 2003  

This an archived copy of a previous edition of Rigger's Digest. Prices quoted will no longer be valid and some products featured or advertis...

Archive Rigger's Digest Issue8 April 2003  

This an archived copy of a previous edition of Rigger's Digest. Prices quoted will no longer be valid and some products featured or advertis...

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