ariel fitting

[wickedteen]

today i put my new 52 point high gain rather large ariel up in my loft, but it still picks up poor, so i pressume that the ariel is still out of focus.
i called a ariel fitter and he said that my ariel was to big and will cause overloading is this true or just a load of crap so i buy his smaller ariel, thanks all

[punky]

to the forum.

I have moved your post out of the articles section (which is reserved for reference articles) into the main forum, which is for queries and help.

Originally Posted by wickedteen

today i put my new 52 point high gain rather large ariel up in my loft, but it still picks up poor, so i pressume that the ariel is still out of focus.
i called a ariel fitter and he said that my ariel was to big and will cause overloading is this true or just a load of crap so i buy his smaller ariel, thanks all

Use these predictors to establish what size aerial you need for analogue...

http://www.wolfbane.com/cgi-bin/tva.exe?

...and digital...

http://www.wolfbane.com/cgi-bin/tvd.exe?

...TV reception.

It is entirely possible for you to have an aerial that is too big, and for it to overload your telly.

[bw41101]

Having not seen your aerial, I will assume that it is of (what they call) a Yagi design, I.e. load of element arranged in a rows, something like this?:

Attachment 14966

Now if you look at the elements, you should see (I've drawn) an elipse around them. This indicates the bandwidth of the aerial, the more elements you have the more gain (recieve power) it has. However the down side, is the narrower the bandwidth - this concept is referred to as the Capture Effect. To compare, it's like looking at an object through a telescope, I.e. you see the object close up but within a very narrow field of vision, whereas binoculars you see a smaller object but with a wider field of vision - make sense?

Taking this into account, you have a very powerful recieve capability but at the expence of a very narrow field of view. In order to get the best out of your aerial you're going to have to adjust it's position very carefully in order that it can see the signal coming in.

Unfortunately (being loft mounted) you will (no doubt) recieve some signal degredation (attenuation) due to the roofing materials within the close proximity.

Apologies for the techno speak, but (I hope) I've been able to clarify the problems you're having. The proffesionals use a field strength meter in order to read the signal strength directly from the transmitter. As you've (probably) not got access to one of these. The only way (I can recommend) is that you take a portable TV set up into the loft with you, fit a plug on to the cable (from the aerial) then connect the aerial to the TV. You should then (standing behind the aerial) gently adjust the position of same in order to get the best picture.

Channels 4/5 are usually the weakest signals, so if you can get these, then all of the others should be okay too. Look for pictures without any shadows (ghosting) and a good clean audio without any hiss.

Once you get what you're looking for, you can then lock the aerial in place and attach the main cable via a socket (should be okay for a loft installation) and (hopefully) you're in business

Good luck and Si thee

[wickedteen]

the ariel i have is this

http://www.maplin.co.uk/Module.aspx?...17681&doy=25m9

i thought it should be fine what i need

[bw41101]

Originally Posted by wickedteen

the ariel i have is this

http://www.maplin.co.uk/Module.aspx?...17681&doy=25m9

i thought it should be fine what i need

This type of aerial is (actually) two aerials mounted (back to back), as one unit. This gives you an increase in bandwidth without unduly affectig the gain - so "should" be easier to set up.

However, exactly the same principle applies. as described before with regard to setting up.

Cheers and Si thee

Have you used the predictor to establish what sort of aerial it thinks you should have?

Please post your results.

[v0id]

If the signal's too strong, get an attenuator :/

[wickedteen]

whats one of those

These

http://www.maplin.co.uk/Search.aspx?...source=15&SD=Y

Although it helps to know which one you may need.

Have you used the predictor yet?!?!

[wickedteen]

sorry but dont quite understand all of that jargon

[Taf]

This indicates the bandwidth of the aerial

Nope that's the beamwidth, bandwidth is determined (theoretically) by the thickness of the dipole. The small elements (directors) multiply the signal in a given direction... more directors, more gain in a certain direction.Their length and spacing are fairly critical.

Bigger aerials do not always mean better final signal, their choice is quite complicated and technical if you want close to perfection.

If you are receiving too much signal, an inline plug/socket attenuator between the receiver (TV or whatever) and the aerial feed will reduce the power, but to get it just right you will need a Rectum Paralyser (Spectrum Analyser) or similar to measure the levels you're getting at the receiver... quite expensive!!!

I'm a radio guy, but I know it works out cheaper in the long run to call in an aerial rigger who will have all the tools , and expertise, necessary for a one-off job.

[bw41101]

Originally Posted by Taf

Nope that's the beamwidth,
.

You're right, silly mistake on my part - I stand corrected.

Originally Posted by Taf

I'm a radio guy,

Would that be professionally (telecommunications), military (fairey) or Radio Amateur?

Please not a CB'er

73's and Si thee

[Matth]

The more gain an aerial has (the more elements), the more critical the aiming due to the lower beamwidth.

Unless it is a seriously strong signal area, I'd be surprised if overloading was an issue. That would be easier to see on analog TV, as the difference between weak signal "snow", reflections "ghosting" and possible overload - distortions, double images, grit rather than snow is visble.

On digital, any signal defect will cause loss of the weaker channels, dropouts, freezing, squeaks in the sound. Digital receivers may give a level & quality indication, but the calibration can be pretty rudimentary - and no help at all until the signal is giving something.

Originally Posted by wickedteen

sorry but dont quite understand all of that jargon

Fair enough - just post the results and let us interpret it for you.