Which Direction for FM Aerial?

[patrick j]

Hello again

Thanks to Dave Plowman, Dave Stanton, Andy Hall, Lurch and others for illuminating me about the way my FM aerial works. I was actually "Thought Police" for that post, now I am "patrick j" which I'm hoping to settle on. I used to be "PJ" but there's another PJ so I can't be that.

Anyway now to my next question which for those knowledgeable about FM (VHF) aerials will no doubt be screaminingly obvious :)

I know that I am to point the aerial at the transmitter, but I want to be sure I have it the right way around.

I've "borrowed" a 296x173 pixel jpeg from the Antiference web-site which shows the aerial along with an omnidirectional:

is a three element aerial and the widest element is at one end, the narrowest at the other. The middle element is a bit more towards the narrowest element than it is the widest one. The dipole is immediately below the middle element.

My feeling is that I should point the end with the narrowest element towards the transmitter. Is that right?

The next issue is getting it actually pointing accurately at the transmitter. Swivelling it around until I get the best signal makes sense, but the tuner it will be plugged into is in my living room and the aerial is on the roof so it isn't the easiest of things to do.

What I plan to do is to work out the where the transmitter is on the compass from my house using an ordinance survey map and then use compass to angle the aerial. Has anyone got a better way? I can't align it with other FM (VHF) aerials because there aren't any :)

Today I have been up on the roof putting the big cables around the chimney to grasp the brackets to which the pole is attached. Great fun :)

[Andy Hall]

I think that's better.....

THe middle, unconnected element is part and parcel of the dipole since it is in line with it.

Generally there is a larger space from dipole to reflector than dipole to first director as you describe.

Yes it is.

For a three element antenna, pointing is not going to be at all critical, so the compass will be good enough. Even if you are 30 degrees off beam it won't make a huge difference and you ought to be able to do better than that. Another way is to work from a landmark on the map...

Is the VHF transmiiter that you want to use cosited with TV for your area? If so then just aim in the same general direction

With FM transmission, unless the signal is weak the tuner will not give any or much hiss as you turn the antenna away from the transmitter, so unless you have a signal meter in the tuner, it wouldn't be much use anyway.

A professional installer would use a signal strength meter, although these are mainly for UHF TV where antennas are much more directional normally and settings critical.

I hope that the chimney pointing is sound:-)

Do be careful. Personally I don't like going above eaves level and I'm not wildly enthusiastic about that.....

.andy

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[Set Square]

That's fine - but don't forget that the map will give you a direction based on true north, whereas the compass needle will point to magnetic north which is a little (I think about 5 degrees) different. It probably isn't critical, but you might as well get it right by making the necessary correction.

[Dave Plowman]

Yup.

[tony sayer]

Wherever do you find these pro installers?. Let me know;-)

Make sure your lashings are tight. Loose lashings will cause the thing to work loose and fall off the roof and damage the chimney. Fer xhrists sakes do use a roof, crawler, ladder. As Andy pointed out the shortest side is the bit that needs to go closest the transmitter. You would be quite well advised also to have the rods in horizontal polarisation.

And also use a decent aerial downlead cable. CT100 the one used for satellite installs is one of the best for domestic use. Ignore that junk sold in the DIY sheds as "low loss". Also solder the aerial plug on at the tuner unless it has wrap round connectors.

Sit back and enjoy!...

This may be useful to see how it shouldn't be done!!..

[Lurch]

On Sun, 11 Apr 2004 21:18:35 +0100, in uk.d-i-y tony sayer strung together this:

Although CT100 is a perfectly good cable for the use the OP wants it is actually for TV signals, CT125 is for satellite.

Excellent advice, it's crap that cheap stuff!

[Andy Luckman]

Have I not been posting long enough for you to know? We use analysers and proper meters for all aspects of our work and provide a certificate with the readings thereon.

[Andy Luckman]

Quite correct.

If your TV aerial is fed from a main station, then point your VHF aerial in the same direction. The beamwidth is wider than your TV aerial generally, so you shouldn't have too much of a problem with getting correctly aligned.

[Andy Hall]

I found that the best way is to find a firm who does commercial CATV and SMATV installations. I then gave them the spec. of the job that I wanted and asked them to quote and indicate what equipment that they would use for setup and why.

.andy

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[Andy Hall]

That depends on the distances involved and frequency ranges in use.

.andy

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[Lurch]

On Sun, 11 Apr 2004 22:52:28 +0100, in uk.d-i-y Andy Hall strung together this:

Maybe the words 'commonly' and 'usually' should have been inserted somewhere! And a note saying that CT125 is perfectly good for most TV\radio distribution systems, if not somewhat excessive for a majority. For the OP: anything CT100 or better.

[Dave Stanton]

And can I make a plea, that if he's using aircore coax, he seal both ends to stop moisture ingress. just a little thing, but if the job is worth doing etc, etc.

Dave

[patrick j]

On Sun, 11 Apr 2004 16:08:39 +0100, Andy Hall wrote (in message ):

Hello

I don't think it is. In fact the VHF transmitter is very close, about 4 miles, so my roof aerial would be considered by some to be overkill. I know from experience that the sound quality will improve considerably with a roof aerial even if the transmitter is as close as this one.

Once I'm up there it's very nice actually. The weather is very pleasant at the moment.

[patrick j]

On Sun, 11 Apr 2004 17:29:05 +0100, Set Square wrote (in message ):

I'd forgotten about that. I notice that ordinance survey do mention it. They have it as 4 and a bit degrees where I am in Hove. I will account for it as I might as well get it as right as I can.

[patrick j]

On Sun, 11 Apr 2004 21:48:38 +0100, Andy Luckman wrote (in message ):

Thanks for that, but I don't have a TV :)

[patrick j]

On Sun, 11 Apr 2004 21:18:35 +0100, tony sayer wrote (in message ):

Yes they are tight. The only thing about the lashings which strikes me as questionable is the way the cable is secured. At one end the cable is already fixed to an "eye bolt" which is bolted into the bracket, that is fine. At the other end the cable goes around a "hook bolt" and then I wrap the cable around itself a bit. It is this part which strikes me as being a bit questionable. I've wrapped the cable as tightly as I can in the circumstances and it seems to be fine. I am using two brackets so there are two cables, which some would consider over-kill, but it is creating a very secure mount and the winds here are extremely high indeed. I'm going to check it in a few months time to make sure all is well, I think it will be.

I am using one of my own creation. Nothing I could buy would suit this roof so I've had to make my own, which works very well so far (touch wood). It hooks around the chimney.

The polarisation is mixed from the transmitter, but I'm putting the aerial in the horizontal position.

Yes I'm using CT100.

around where I live are obviously quite insecure. The most common issue being many aerials crammed onto one small mast.

[Andy Luckman]

That's because it is incorrect. What you must do is seperate the individual strands and wrap each one individually after the loop. Wrapping them as a bundle will be quicker but is more likely to give as you tighten the bolts.

[Dave Plowman]

That's rather a simplification. Once you have an adequate signal, increasing it won't make any difference. Multi-path reception is usually the thing which makes FM radio sound nasty, and a decent roof aerial correctly aligned can help this. But this tends to be independent from signal strength.

[Set Square]

It varies a bit over time - in some sort of cyclic pattern taking a number of years. So make sure you've got an up-to-date OS map!

[tony sayer]

In article , Andy Luckman writes

Humm...we don't have 'em up here:-((