FM Aerial Wiring

[Thought Police]

Hello

This is a question for those that are knowledgeable about how FM aerials work. A while back I bought an Antiference aerial, the FM1083, which is a "Trumatch" three element aerial.

The coax is connected to this exactly as you would any other aerial. Here is an 800x 600 jpeg showing where the connections go:

is curious for me is that these connections are to two small parts of the total aerial structure. They connect to two rods one on the left and one on the right:

at neither side do these rods connect to the rest of the aerial:

The end is like this:

a view of a bit more of that side of the structure:

would have thought that most of the aerial would have connected to one terminal for coax and a small bit to the other, but here two small parts of the aerial connect to the terminals for coax, but most of the aerial does not connect at all.

Can anyone explain why it is so?

TIA

[Lurch]

On Fri, 9 Apr 2004 19:35:07 +0100, in uk.d-i-y Thought Police strung together this:

No, but that is perfectly normal. Any aerial you buy, TV or radio, will only have a couple of small parts of the unit connected. You seem to be an undue worrier!

[Dave Stanton]

Google on Yagi aerial theory and it should all become clear. Aerial theory of Yagis too much, too explain on here. The main point is the two rods are the main receiving part called ' the dipole', that is the bit that actually carries the signal.

HTH

Dave

[Andy Hall]

That's perfectly normal. These pieces form what is known as the dipole.

It is one piece of the antenna, the whole of which is referred to as a Yagi array - or a modified version thereof.

The dipole is basically the signal collector, the pieces behind are called the reflector and pieces in front of the pipole are called directors. Collectively these pieces are referred to as elements.

They are sized according to the frequency range that you want to receive, and the lengths of each element and teh spacing between them are important.

In your picture, the outer braid of the coax, which should be satellite grade CT100 cable, not TV co-ax is clamped under the clamp on the lower right and the core goes into the terminal on the top left.

The radio signals, in simple terms, are delivered from one element to the next. The number of directors an the structure of the reflector affect how directional the antenna is. Broadly speaking, it can be made to be more directional and provide more gain to the signal by having more elements.

However, for FM, it is sometimes required to be able to receive transmissions to the side or even the back (e.g. local radio stations from different transmitter sites. For this reason, and also because an antenna for the frequency range in question gets pretty big, most antennas are no more than 6 elements

- normally 4 directors dipole and reflector. Some are four or even two.

In some cases, omnidirectional antennas are used instead, and these have a different design entirely.

In short, connect it up, point it at the transmitter and enjoy Radio 3.

.andy

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[Thought Police]

On Fri, 9 Apr 2004 19:48:51 +0100, Lurch wrote (in message ):

Yes I am :)

[Thought Police]

On Fri, 9 Apr 2004 20:06:58 +0100, Andy Hall wrote (in message ):

Radio 3 is exactly the station I will be using it for :)

Thank you for your informative post which I've found most interesting.

[Thought Police]

On Fri, 9 Apr 2004 20:02:40 +0100, Dave Stanton wrote (in message ):

I will thanks :)

[Dave Plowman]

If you've ever moved round a room while listening to FM on a portable with its own aerial, you'll have noticed that the signal comes and goes. You're acting as a 'reflector' to the RF. Depending on the path lengths, this might increase the signal or cancel it. The same applies to the passive elements on a roof top aerial - they use this effect to increase the gain of the aerial.

[Huge]

There is no such thing.

Perhaps you mean "VHF"?

[The Natural Philosopher]

Course there is . Thats a wideband aerial innit?

:-)

[Dave Stanton]

They will be the same ' experts ' who sell Digital aerials !!

Dave

[Dave Plowman]

'Course there is. Mine's alongside my digital one.

[Huge]

*grin*

*Naughty* Dave.

[Soup]

Yes there is :-

[Huge]

There is, nonetheless, no such thing.

[Soup]

Mmm the beeb and Philips

Sony Yamaha a couple of pics to finish :- down to FM aerial . Always thought of an FM aerial as the plastic coated bit of wire but these are much more substantial

[Andy Hall]

Sorry, but Huge is quite correct.

FM is a modulation technique. In the case of FM radio as broadcast on 88 - 108MHz, the radio frequency carrier signal is maintained at a constant amplitude and the audio signal deviates the carrier frequency very slightly - in simple terms the amount of deviation corresponding to the amplitude of the audio and the rate of deviation according to the frequency of the audio.

This gives superior results compared with AM (amplitude modulation) where the RF carrier's amplitude is altered according to the amplitude of the audio at a rate determined by the audio frequency.

An FM receiver is able to clip away the amplitude peaks of the received RF signal and is thus relatively insensitive to signal fade and many types of interference.

With respect to reception, the antenna is always designed to be appropriate for the frequency of the carrier signal - the modulation technique is irrelevant to this and thus the term "FM aerial" is meaningless as well.

VHF is a term used to describe radio signals in a frequency range of

30MHz to 300MHz, so the 88 - 108MHz range is in this range. In terms of antenna description, one technically correct term is to refer to it as a Band 2 antenna to make clear the range being covered. Band 1 was the old VHF TV band used by the BBC, and Band 3 the old VHF band used by ITV, both in the days of 405 line TV. DAB radio uses part of the old Band 3. UHF TV uses Bands 4 & 5.

The manufacturers that you have listed have used the term "FM" quite incorrectly and should know better.

Arguably the term "FM" can be applied to a transmitter or to a receiver because there are elements of each that are dependent on the modulation technique. Equally, the term "VHF" can be applied to these.

However, with respect to antennas, the only correct term is frequency related (i.e. VHF). If radio were transmitted on the same frequencies using AM, the same antenna would be used.

.andy

To email, substitute .nospam with .gl

[Dave Plowman]

I'd say the reason they're known as 'FM aerials' instead of VHF is that when FM radio broadcasting started, TV was also on VHF. And AM.

So to avoid confusion, let's just call them Band II aerials?

[Soup]

Soup just had to say

Don't let the facts get in the way.

[Andy Hall]

I haven't........

.andy

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