What sort of aerial should i chose

[Lower]

I want to replace an ancient analogue aerial that currently supplies 2 tv's with a modern digital aerial as the analogue aerial that is currently fitted is not up to the job.

Physically fitting the aerial itself i can do. What i don't know is what sort of aerial i should have for my area and i want to select one that can supply two Tv's without needing an amplifier. I really want to split the signal at the aerial itself and then run one cable down the front of the house and the other down the back of the house.

I live in Market Harborough, LE16 area.

Any guidance would be gratefully received.

 

[Stivino]

Have a look at the link, it might be helpful.
http://tinyurl.com/2b2m6ul

 

[Lower]

Thank you for the link.

Unless i've missed it on that site, i'm looking for more specific advice of what type of aerial to buy and fit. The BBC site refers you to professional installers.

 

[Stivino]

I think there's no such thing as a "digital aeriel". But, I stand to be proved wrong.

 

[Lower]

You're probably right. I mean an aerial suitable for picking up the tv signal for digital TV.

 

[gman76]

Have a look at this site: http://www.wolfbane.com/cgi-bin/tvd.exe?DX=L&HT=10&OS=le16

Says you'll need an amplified extra hi-gain wideband horizontally polarised at 10 degrees from North.

At 22 miles from a repeater I'd agree about the amplifier - as you can't just run two feeds from the aerial directly (you just can't), I'd suggest a two way masthead amp.

 

[ericmark]

Transmission lines is an art. Some where I have a book that explains it all with pages of calculations. But the basic problem is to split a 75 ohms line needs a network of some sort to keep it the correct impedance. The normal way is to use resistors which although does match the two outputs also attenuates the signal. So the aerial needs to have over double the required gain in order to split the signal. As the aerial gets bigger the holding of it in the air against wind resistance becomes harder. Although one can stack aerials for ham radio with TV the frequencies are so far apart this will not really work. So options are two independent aerials or an active rather than passive splitter which normally also boost the signal. There are two basic types. Those with built in power supplies and those with external power supplies. The latter normally referred to as mast head amplifiers use the coax to deliver the power and either one can be used. With a mast head amplifier likely you will not need another aerial.

 

[plugwash]

So the aerial needs to have over double the required gain in order to split the signal.

I would note that we normally measure antenna gain in dB (technically dB relative to some reference antenna). dB is a lorithmic scale so iIf you double the actual gain you add 3dB to the DB value and if you halve the actual gain you subtract 3dB from the dB value.

With a reissitive splitter the output will be 6dB (four times) lower than the inputs, other types of splitter can do better.

The latter normally referred to as mast head amplifiers use the coax to deliver the power and either one can be used.

The advantage of a masthead amplifier is that as the name suggests it is designed to be mounted on the masthead. This means that the length of cable between the aerial and the amplifier is minimised. This improves the signal to noise ratio (an amplifier boosts both signal and noise and it adds noise of it's own, so the more signal you have at the input of the amplifier the better the overall signal to noise ratio of the system).

 

[Lower]

So if i want to split the signal in two, what sort of db rating should i be looking for for the aerial or is not that simple?

Would it be more straightforward to put up two aerials as i'm going to be up on the roof anyway?

 

[ericmark]

From what Pugwash has written you need an aerial with 6db more gain than needed for reception of same signals feeding one set when using a resistor splitter.

Yes you can use two aerials but considering wind resistance and problems handling large aerials I would opt for mast head amp method.

There are times where mast head amps don't work too well. For example where there is a taxi, police, ambulance or other transmitter close to your house but this is rare and for most cases the mast head amp is good option.

I don't use a mast head as I also use sky so I combine the terrestrial and Sky signals before I boost and split the signals into each room.

I transmit at 5W without any problem. Even at 25W it is rare to have problems so I would say unless there is something very special about your location mast head amp is way to go.

 

[JohnD]

presumably he'll need to fit new co-ax as well?

I live in a weak analogue area esp for C4 and C5 is very bad, but my digital reception is fine