Strange Happenings on Retuning a Digi Box

[Bill Wright]

Ian Jackson wrote:

Yes agreed, but see below.

I was really thinking, I suppose, about the situation where the attenuator is on the input to the distribution amplifier. In that case it could very likely reduce 4G problems. That would be the best place for it as long as there is a largish ratio between the wanted and unwanted TV signals. That is the most common situation after all. Given the extreme sensitivity of many modern receivers it is quite common to have channels appear in the 800s that are from muxes 40dB or more below the wanted signals. Bearing in mind the directional properties of the aerial, if the said aerial is properly installed and is not a turd on a stick there should almost always be a large ratio between the wanted and unwanted signals. Consider the case where the wanted and unwanted signals are of the same field strength. The directional properties of the aerial should be able to provide a decent ratio between them. The exception would be when they come from the same direction and have the same polarisation. Another exception is when reception is from a distant transmitter (most likely a main station) but there is a local relay close by. The relay cannot be used because it is Freeview Lite or because signals are hoplessly garbled by reflections. This happens in some of the urban areas near the Crosspool mast, where trees and topography conspire to deliver to some locations massively strong muxes that are mangled to death. Even if they are decodable they are unreliable. Aerials look towards Belmont, so passive filters are no help. At one recent job at a sheltered scheme we found the aerial producing signals from Crosspool that were dancing up and down and all over the place, but were around

10dB stronger than the (wanted) Belmont signals. This was only 2km from Crosspool but screened by trees on a high bank. The aerial was one of the popular huge wideband high gain things (it was a Superbo Antennisimo Fantastico Gereralisimo Grandioso with 164 elements and a built-in microvolt maximiser, as I remember). We installed two log periodics phased together, and achieved far better results. This sort of thing: but the spacing was arbitrary because the s**te (Crosspool) was coming at us from all directions.

Bill

[tony sayer]

Though sometimes if the level of the interferer can be taken down enough so that it doesn't drive the RX frond end into distress then that can be useful to permit reception of the weaker signal..

Bill W might know isn't there a new box out that you can prog your postcode into and it only tunes the allocated TX for that area?..

[Ian Jackson]

In message , tony sayer writes

Where something is being driven into nonlinearity by a strong signal, it's nonlinear for all signals, and the addition of a front-end attenuator is a well-established possible cure for a multitude of problems.

That could lead to problems when the signal from your assigned TX is poor where you live, and you have to receive another. You would have to fiddle things by using a false postcode - one for the area definitely covered by TX you actually wanted to tune to (like you can do with Freesat if you want a specific ITV area local content). Maybe every TX should have had a 'tune to me' code for you to enter. On the other hand, it's probably a darn sight easier for every TV receiver and STB to be obliged to have manual tuning.

[Bill Wright]

Most of them now ask for your country and region. Unfortunately they often then go on to ignore the information. I don't know whether this is the fault of the receivers or the givers.

Bill

[The Natural Philosopher]

In case of strong wanted and unwanted the trick is to align the antenna

- a good directional one, - so the unwanted coincides with one of the nulls in the lobe pattern. Getting the main signal 'on beam' is far less important than getting the unwanted signal 'off beam'

with luck you can also attenuate the main beam enough not to need reduction either.

using a laptop with a tuner dongle in it up in the loft or even on the roof is a way to do all this.

Though you would have to be very close to a transmitter to get into front end overloading.

I've had no problems a couple of miles from e.g sandy heath..

[Ian Jackson]

While what you say is true, I doubt if many people who buy a digital TV receiver will want to play at realigning their aerial - especially when they can certainly avoid the problem by manual tuning, or have a good chance of avoiding it by the use of a £3.15 attenuator.

[The Natural Philosopher]

I thought we were

(a) a technical group and not 'most people.' (b) trying to avoid a problem that was almost certainly NOT caused by overloading (c) using a set that we couldn't manually retune.