TV aerials: where does the sleeving connect?

[John]

Apologies if this is a stupid question.

I'm re-jigging the TV aerial which is in our loft. I put it up myself some years ago.

At that time, I very neatly stripped away the outside braided copper sleeving (at the aerial end), twisted it, and connected it to one of the two terminals on the aerial; I connected the aerial cable itself to the other terminal. The aerial has always worked fine.

Looking at this arrangement this weekend, I wondered why I'd done it: in effect I am connecting the two together, since the two terminals are at opposite ends of the same cross-piece on the aerial (i.e. the rear-most cross-piece).

But any fule kno that you shouldn't allow the braid to come into contact with the aerial core: I don't see how it can work!

So what are you actually *supposed* to do with the earthing braid at the aerial end of things? (My old Reader's Digest DIY book doesn't mention the 'top' end of the aerial installation.)

Cheers John

[T i m]

For DC you are infact shorting out the end of the feeder but at the radio frequencies you are talking about it's not a short cct.

All the best ..

T i m

[Dave Plowman (News)]

RF doesn't behave like DC. You've got a folded dipole there which will have a supposed impedance of 75 ohms at the frequency it's working at.

However, better quality aerials will have a 'balun' transformer that gives a more exact match.

You've done it the correct way.

[Andy Luckman (AJL Electronics)]

Because resistance =! Impedance, at the resonant frequency of the aerial.

You are supposed to clamp it under the saddle provided. The type of aerial you describe is just a "contractor" type with no balun. I heartily disapprove of those, due to the mismatch. You can get away with a great deal when you have analogue TV, but you will find that once it is shut down, your whole installation will need doing properly.

[google]

Not neccessarily. Our old aerial and installation gives us DTV just fine.

It does of course depend upon reception quality and other factors in your area which are impossible to gauge from the OPs question.

Andrew

[bigcat]

what would you recommend instead? Im about to put a new aerial system in, and am in a poor reception area.

NT

[Andy Luckman (AJL Electronics)]

Depends on your definition of "fine" I can guarantee that if an analyzer was put on your aerial, I would be able to point to a deficiency. Most likely multiplex flatness.

[Dave Plowman (News)]

A decent make - Televes - for example. But expect to pay three times that of a shed type. TLC sell them.

[Andy Luckman (AJL Electronics)]

It does depend on many factors. Although others may disagree, I would ALWAYS inssit that it is set up on an analyser that reads all the relevant parameters. You can have too much signal as well as too little. You can also have lots of signal and lots of noise. For that reason, see if you can find a professional installer in your area and ask for the levels to be recorded. We issue a "certificate of conformity" at the end of our jobs.

Aerials differ greatly. Avoid anything called a "contractor" aerial. See what the range of channel frequencies are and order the aerial for that range only. Some areas require a wideband response. There are a range of aerials that have been benchmarked and found satisfactory. These include the Triax Unix 52. Ideally get one so marked.

Despite the propaganda put about by the once respected "Antiference" company, a balun IS REQUIRED. So is the correct grade of cable which should be certified to EN50117. Many quality satellite cables are to this spec, but it will be marked as such if it complies.

One of the drawbacks of digital, is that you can no longer say "that's a good picture, it suits me". Unless the signal parameters are within the correct range, you get nothing, or constant breakup, with digital.

Hope that helps?

[Chipmunk]

[snip]

[more snip]

Thanks for clearing that up for me, I always thought it was odd to have to terminate 75 ohm cable to what appears to be a nice set of 300 ohm terminals. Would one of the 'cable co special' 300 ohm to 75 ohm F connector type tubular baluns work in this application, if you cut the fork spades down to just the crimped part and put that in the terminals?

[google]

No visible or audible problems.

What difference would "fixing" that make to my experience of digital TV? It's already better than analogue.

Andrew

[T i m]

And I suppose a virus is the last thing Mr Hawking needs ..

Virus: Voice mode: Homer Simpson / On .... ;-)

All the best ..

T i m

[bigcat]

thanks, will start by checing out TLC site for aerials. Do teh bteer aerials have the balun in them, or is it fitted somewhere else?

You wouldnt think tronics is my home ground, but there ya go.

NT

[Dave Plowman (News)]

They all should have for best results. It's in the cable connection box. And it's a pretty simple device - only a few turns of wire for each winding. Can't cost more than a few pennies.

IIRC, the actual impedance of a folded dipole is 300 ohms, and many countries use a 300 ohm balanced feeder system. But this has to be spaced off the wall to work properly, so costs more to install.

But as with all aerials, what actually works ok in individual circumstances is somewhat of a black art.

I've got a DAB and Freeview tuner (used only for radio) both connected to a horizontal FM aerial and both work ok - or as well as either do at best. ;-) Mind, I can see Crystal Palace mast from this window.

RF is a minefield. ;-)

[Bill]

In message , "Andy Luckman (AJL Electronics)" writes

If you put an analyzer such as a 141T on the aerial it would definitely show flatness, unless it is a very robust aerial.

>

[bigcat]

The cable is ultracheap, being little more than speaker wire, but here labour costs rule.

NT

[Andy Luckman (AJL Electronics)]

It's only 300 ohm in free space. When you start adding reflectors and directors, the impedance drops. The point Antiference were trying to make is that the feedpoint is 75r. Conveniently missing the fact that there is a balance to unbalance mismatch remaining of course.

It depends. I personally would just get a "proper aerial".

[Andy Luckman (AJL Electronics)]

Compared to what?

Depends on the answer above, but reliability of service mostly.

[Andy Luckman (AJL Electronics)]

The balun is in the feed box and often is a stripline PCB.

BTW. I can supply approved kit as well as the free advice. :-)

[bigcat]

will be checking your site out too then. You dont sell round tuits in bulk do you?

NT