FM Aeriel from a TV antenna?

[b0redom]

Hi All,
I've recently got an A/V amp with a built in radio tuner. Does anyone know if there's some sort of splitter from a standard TV aeriel or something similar?

[Stoo]

yeah, if the signal is good enough you should just be able to use an RF splitter (they sell them in B&Q):



If it's dodgy, then you may need a combined tv/fm antenna (most new ones already are though - tv and FM are pretty close together)

[b0redom]

Hmm, the back of my Amp looks like this:

http://www.usa.denon.com/AVR1910_Large_Back.jpg

That doesn't look like TV coax, is there some sort of converter/cable I need to look at?

[Stoo]

Nope, that's fine, the one you want is the one labelled "tuner coax 75ohm" in the middle - it's a standard aerial coax cable

you plug the splitter into the source, then you need a standard aerial cable , one with two ends like this:



although, depending on everything you have, you might need a gender bender in there too

[peterb]

UK FM transmits from about 97MhZ to 108MHz, TV is from around 450MHz to 800MHz. Ideally you should use a separate antennas for the FM and TV signals for best results.

You can use the same cable though by using a diplexer at the antenna end and a similar diplexer at the receiver end. These are apassive filters that provide signal separation from the two antennas so the FM signal doesn't interfere with the TV, and vice versa.

You may be able to get an FM signal from the TV arial, if you are reasonably close to the transmitter, the only way to find out is to try it.

[Saracen]

You can also get active systems. Labgear do a range from basic duplexer/diplexer pairs, right up to multi-distribution systems with amplified FM, DAB, TV, Twin Sky (Sky Plus) and even CCTV all via a single cable drop.

So I think what Peter is getting at is that while you can use a TV cable, the widely differing frequencies of TV and FM radio means a TV aerial will be very inefficient (if it works at all) at receiving FM radio signals. For a start, the antenna elements are far too close together (because TV is higher frequency) for TV antenna to be good at FM radio. Secondly, TV antennas are highly directional, so unless the radio station is at the same place, or at least in the same direction, as the TV station, a directional antenna is designed to not pick up extraneous signals.

For FM radio, you either want :-

- an omnidirectional antenna, or
- a directional antenna and an aerial rotator system, or
- you'll only get a very limited range of stations from a directional FM antenna.

[Stoo]

Yep, you might as well give it a go, as what ever you do it's not going to be a waste - it's all usable whatever you do.

Although most of the time it's simply the fact they you're letting it use a whacking great length of cable that actually acts as the aerial - normally what you get is a tiny thin bit of wire with a plug on it with most AV receivers.

But yeah, if you're in a duff signal area, you'll need a dedicated FM aerial - they're usually two element round things:



I can't imagine that you would though..

simple rule of thumb, if a portable radio gets a decent signal, just plugging into the tv aerial will probably be good enough

[Saracen]

....

simple rule of thumb, if a portable radio gets a decent signal, just plugging into the tv aerial will probably be good enough

A bit of a simplification, because portable radios usually have either extendable telescopic aerials, or built-in ferrite rods, or both. If a radio signal is strong enough, you can pick it up all on sorts of weird things, from a copper saucepan to grannies false teeth. Well, maybe I exaggerate about the false teeth.

Also, the "tiny thin bit of wire" you get with receivers are a bit misleading too - they're actually carefully designed. They're what's called a half-wave dipole, and the lengths of the bits of wire are important. Those lengths determine the frequency the antenna is tuned for. Get it wrong and you substantially decrease the effectiveness of the antenna. They're not quite as simple as they seem, and they are actually pretty damned effective, providing you can get them high enough and sited correctly .... and there's no radio opaque obstacles between them and the transmitter.

That's probably the single biggest advantage of the 'proper' circular omni-type - you can get them high up, i.e. in the loft or better yet, outside the roof, and minimise obstructions.

Slapping any old length of wire in the back might or might not work. If, in a given location it works for a given desired radio station, then fine. What works, after all, works. But it often won't. You usually need to be a bit more scientific than that.

[Stoo]

Okay, what I was getting at was that it's a standard aerial connection on the back of the receiver, and it won't hurt to split off from the tv aerial - because anything would be better than nothing

I'd be highly surprised if there was nothing there at all

[Saracen]

I'd be a bit surprised if there was nothing, but it might well not be what you want. It may well also be subject to all sorts of harmonics, and interference. In fact, what you pick up may well be a harmonic, not the main signal. It's certainly likely to be far less effective that that simple wire T you usually get given with a receiver.

If you think about it, it;s obvious why. Firstly, generally, we know where the TV transmitters will be, and even those in an area where two regions overlap generally choose one or the other, and point the TV antenna at it. Why point the antenna? Because the "Yagi" design for TV antennas are designed to be directional.

I don't know how many people reading this might understand how a TV antenna works, but I'm going to take a few liberties and simplify it. Look at the typical TV antenna (and I don't mean the very simple set-top type). It's going to be some variation on a long boom with elements crossing it at regular intervals. At the very back, you might have a single element, or a double, or something more substantial. That's a reflector. The next element in, where the cables attach, is the actual aerial element (the "dipole" I referred to earlier), and is the but where the antenna cable actually connects, because its the only bit that actually receives the signal. The rest of the elements, and there could be a lot of them, are directors. The size of each element, and the gap between them, is dependent on the antenna design (accounting for log-periodics and the like) and, mainly, the frequency you want the antenna tuned for. The antenna is only actually optimally tuned for ONE frequency, so you set that in the centre of the range you want, and Peter mentioned them earlier. The size of the elements, and the gap between them gets smaller the higher the frequency is, because the optimum size depends on the wavelength, which is the speed of light (a constant, at last for these purposes) divided by the frequency (which is why they're optimised at one specific frequency).

If you look at the pattern such an antenna operates in (perhaps by using it to transmit, and measuring field strength around it) you'll find a "directional" antenna has one main lobe (a sort of stretched oval shape) pointed out in the direction the antenna points, and a few much smaller side lobes. And, within the limits of practicality and subject to the constraints of the specific design, the longer and narrower than main lobe will be, and similarly, the smaller the side lobes will be.

The point of all this is that those larger antenna with more elements are better at :-

- firstly, being sensitive to weak signals in the direction you point them, and
- have better rejection of signals coming from directions other than where you point them.

Both are important for good TV reception, but note the bit about rejection. Part of the design is to improve signal quality of what you do want, by deliberately reducing the ability to receive signals from where you don't want. But, for FM radio reception, unlike TV, the station could be almost anywhere. Sure, mainstream BBC stations are limited in location, but there's a load of other stuff.

Therefore, if the station(s) you want happen to be more or less in the direction in which a TV antenna is pointed, you might be lucky enough to get a usable reception even though FM radio frequencies are way outside the design parameters fora TV antenna. Which, by the way, is why you find FM antenna elements are about 4 times the size of TV ones, and why the gap between them will be in about the same proportions. Which, of course, is why a basic TV aerial may well be 10 to 12 elements but FM ones will be more like 3 or 4. A 12-element FM antenna would be too big to be practical for most domestic purposes, especially if indoors. It'll be pretty big.

So .... while just splitting a feed from a TV antenna might give someone the stations they want, it's a gamble that they will. They'll certainly be sub-optimal because the antenna design is totally wrong for that frequency range and there's a decent chance some stations you want are being effectively eliminated by the rejection characteristics of the TV antenna.

So, better than nothing? Probably. But good enough? Maybe, if you're limited in the stations you want and lucky with the direction. But unless you're in unfortunate physical surroundings, like maybe a flat where there's too many obstructions for the simple wire T to work, you'd generally be better off with the simple wire T.

A FAR better solution, assuming you can get access to do it (which, in a flat, you might not) is a proper FM antenna in the loft or on the roof, and the duplexer/diplexer arrangement (or a distribution amp) piggybacking on a single cable drop. Better yet, of course, is dual cable drops and not using the duplexer/diplexer. And as FM antennas, and the diplexers, are pretty cheap, it isn't expensive to to it. Running dedicated cables is preferable, but actually doing it might be a right pain, or might simply not be possible, or not possible without major works. I'm lucky - it's a right pain, but doable.