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getting over-lap The V10-34E3, a great Log Periodic problem solver
If Digital Switch Over (DSO) has taken place nearby, you may have already come across the dreaded transmitter channel Overlap problem. If you’re looking for practical advice or need to prepare in a pre-DSO area - read on!
Vision Products are flying the flag for UK installers and specifiers! Working with new UK electronics partner, Precision Manufacturing Ltd, Vision are helping to boost the British economy with products built and designed for use by UK installers - and creating more UK jobs. But why is having PML as a manufacturing and design partner here in the UK important? When you look at what’s involved in modern electronic manufacturing, the advantages become clear. Precision Assembly Whilst some manufacturers still use old-fashioned components that stand up on legs, the best RF signal components use Surface Mounted Technology. In fact, many leading edge RF products come from the demanding mobile phone industry. SMT components satisfy the increasingly smaller and ultra-low power requirements in next generation hand-sets. Intricate SMT devices often measure less than 1mm across, so the delicate task of placing them on the PCB is best handled by automated machines with microscopic optical inspection systems. But this is just part of the story. For long service life, great care must be taken when soldering. The quality of a soldered joint is dramatically affected by the time spent soldering. Performance and service life can be reduced if a component overheats during assembly. PML uses automated “pick and place” precision machinery that flow solders to achieve accurate placement and ...continues on page 3
Precision manufacture in the UK
“Hand-built” somewhere overseas
A National Problem Recent information from DigitalUK warns that an increase in the power of DTT transmissions postDSO will result in 92% of the 26M homes in the UK facing some form of Overlap with a number of transmissions competing with each other. Problems in Granada, Wales and West have produced lurid headlines, and indications are that it will be worse in landlocked regions like Central. Typically, post-DSO Overlap could result in a digital ready TV or Freeview box tuning into channels from a non-local region, perhaps showing “No Signal” or with intermittent break-up on some channels. All bad enough by themselves, but as DSO rolls out countrywide it will get worse. Southern coastal regions are anticipated to be particularly badly hit by foreign European signals swamping us as they follow the UK lead in adopting a common broadcast standard. Understanding Overlap Freeview receivers are usually programmed to regularly auto-scan or after an automated instruction, often following channel lineup or transmitter frequency changes. By tuning “upwards” from Ch21 to Ch68, lower end UHF signals will be found first. The channel
found may not necessarily be the right one, and may even be in a language you don’t want! In some cases lower frequency channels are from the right region but are from a weaker distant transmitter that can’t provide consistent quality signals, or overlapping signals can overload amplifiers in tuner causing distortion and signal break-up. What To Do DigitalUK recommend a few workarounds. Possible solutions are:- Manual Freeview box tuning - Remove the aerial during tuning and replacing it during auto-tune - Channel Sort or Favourites. Unfortunately, Overlap “sticky plasters” will need re-applying every time a new channel is added or the Freeview receiver is moved. With every single receiver in a household at risk this could become a very time consuming and costly problem. Prevention is always far better than cure so when changes do occur, call-backs won’t be generated. We recommend:- In extreme cases, add a grouped band pass channel filter to eliminate out-of-band signals. - Grouped Log Periodic, rather than wideband Yagi aerials, reducing levels of unwanted signals below tuner threshold, maximising signal reception and impulse noise reduction. - An attenuator to ignore weaker channels. A Log Periodic Solution The Log Periodic design is ideal to help combat Overlap. With each
dipole tuned to a different wavelength, each element works as:- A dipole, collecting signals from a narrow frequency range - A director, focusing lower frequency signals onto dipoles behind it - A reflector, protecting the dipole(s) in front of it from unwanted signals. With a “natural impedance” of 75Ω, signals are transferred to the 75Ω coax cable with minimal loss and reflection, just what’s needed to keep signal integrity and minimize impulse noise. This means smooth performance across the desired frequency range and very high directivity. Impulse noise is dramatically reduced as the design is grounded to the mast. The range of Log Periodic aerials best suited to solving overlap problems are three of the models in the Vision range. Installers can choose from the Group A 40 element V10-40A1 or V10-20A2 and for Group E the 34 element V10-34E3. For weak signal areas or applications requiring exceptional directivity and noise immunity, use the wideband STEALTH. It outperforms every other benchmarked wideband aerial on the market and is the only log aerial to meet or exceed all 4 wideband Aerial Benchmark categories.