Anyone recomend a Digital TV aeriel?

Put a STB on the existing one and see if it works. Why buy a new one unless the old one doesn't do the job?

Reply to
dennis
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A passive reflector at the top will probably help or freesat/sky.

Reply to
dennis

Misleading as in true? There is no need for an expensive digital aerial and even less need after the digital switchover.

Reply to
dennis

I know, that means you might need a better aerial, but not a digital one.

Digital aerials are just an excuse to rip people off.

Reply to
dennis

Much will depend on the roof through which the ærial receives its signal. The roof will certainly absorb some signal dependant upon its material - is it slate, clay tile, concrete tile, man-made (plastic) tile? Absorbed or lying rain, snow or whatever will affect your signal's attenuation - indeed, it could possibly slightly affect ghosting (multipath reception). Having spent much of a working lifetime (it seems!) trying to resolve TV reception problems, I would really advise an outside ærial in virtually every problem case, even though it might involve borrowing a ladder. With Freeview you have to consider the frequency range of your local transmitters when selecting the appropriate ærial. ISTR that there are three or four multiplexes per area so the appropriate ærial has to be chosen. A wideband might be the most convenient, but you may get slightly better gain from the 'correct' ærial for your locality.

Personally I don't have a TV, on account of the poor quality of the vast majority of most programming.

Reply to
Frank Erskine

On Mon, 06 Oct 2008 22:44:57 +0100 someone who may be Andy Champ wrote this:-

The situation is more complicated than that. Local geography is everything, including trees. One house may get good reception while the next may need a tall pole to get the aerial over an obstruction. Moving an aerial by a metre or less may make all the difference. Television signals are not as good at going through/round things as FM radio signals are and getting good reception may be a matter of fiddling.

The deep valley may have a convenient repeater, near a transmitter it may be necessary to reduce signals to avoid swamping the television input by fitting an attenuator.

Reply to
David Hansen

Yes, it is, but is never as good as one out in the breeze up high and with a clear line of sight to the horizon.

Reply to
The Natural Philosopher

Actually, it fits rather better.

Given the complete inanity of the original post.

Anyway, I have probably only ever seen half a dozen episodes of Start-Wreck, by accident, through the years.

Reply to
The Natural Philosopher

dennis@home expressed precisely :

Absolutely!

Reply to
Harry Bloomfield

... Or reduce corruption of the bits, hopefully. We've done this one often enough Tony, both here and on uk.tech.digital-tv. If you connect a coax feeder to a symmetrical aerial (such as the centre-fed dipole of a typical TV aerial) without using any form of balun then the outer of the feeder becomes part of the aerial. If you're transmitting, the outer of the coax will be 'RF-hot' and will radiate, quite possibly causing EMC problems to low-level parts of the transmitting equipment as well as safety concerns if high RF power is involved. When receiving, unwanted signals picked up on this hot feeder will find their way into the receiver, however perfect the screening of the coax itself.

The relevance of this to DTT reception is that the house end of the feeder is likely to run near to mains wiring, and usually ends up connected to mains powered appliances (the DTT box and other TV equipment). There's likely to be significant RF coupling between mains-borne impulsive interference - from all those sparking switch and thermostat contacts, etc. - and the outer of the feeder. The balun-less aerial connection allows this interference into the signal path, as do any other 'leaks' in the integrity of the feeder system - notably old unscreened and isolated outlet plates and poorly screened receiver flyleads.

DVB-T is alas rather susceptible to impulsive interference. It causes those momentary drop-outs ('blocking') and sound clicks. The problem isn't so bad as in the early DTT days, but it's certainly still there. Good RF practice wrt screening and baluns to keep the coax cold is the best way of reducing it. The change to 8K FFT length that will come with ASO/DSO will help further, and DVB-T2 shouldn't suffer in this way since it includes a long time-interleave option (like DAB) to smear out impulse events in the time domain.

In the meantime, with the present interim transmission arrangements...

... there's little doubt that DTT is considerably more demanding of aerial performance than analogue, if you want acceptable results. Part of the problem is that (some) people will tolerate appallingly poor analogue reception without complaint, but will take the DTT box back to the shop if they get impulse interference break-up. DTT reception has to be pretty 'solid' to be acceptable. Analogue degrades gradually, if not gracefully, and with a modicum of technical knowledge you can see what the problem is: noise, multipath, temporary tropospheric interference, etc. are all quite easy to identify. With digital the margin gets invisibly used up and then you fall off the cliff not knowing why you fell. With a marginal aerial system this will happen more frequently.

Is there such a thing as a 'digital aerial'? Obviously a passive aerial doesn't care about the details of the signals' coding and modulation (at least for signals with similar RF bandwidths) - although an active one with a built-in preamp might. Isn't it equally valid to say that there's no such thing as an 'analogue aerial' either? More to the point though, some aerials are certainly more suitable than others for DTT reception and to that extent the term "digital aerial" does have some meaning, IMHO.

Wideband or grouped? (Wideband is Group W, so maybe that's grouped too!) An argument in favour of using wideband aerials where not initially necessary is future-proofing. Once the 6-multiplex DSO plan is complete (3-multiplex for most relay sites) there's a fair chance that we'll see additional DTT services coming on in what Ofcom call 'interleaved spectrum' and these are quite likely to be out-of-group relative to the original local analogue plan. It remains to be seen whether the TXs will be co-sited, but if they are then wideband aerials will be advantageous. Yes, the log-periodic (Benchmark standard 4) is a good solution.

Further reading:

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Reply to
Andy Wade

I don't think that is actually correct. It is as equally valid to say that the coax outer 'grounds' one limb of the dipole at its feed end, and it becomes a 'reflector' boosting the signal into the other half. Ie. the 'balanced' nature of the antenna is a myth. It doesn't care what the potential is of any limb: what it cares about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines. Like twisted pair.

Reply to
The Natural Philosopher

In article , Graham. scribeth thus

LOL!... Should've seen that one and theres me thinking he meant "shot noise"!.....

Course the analogue we see these days invariably started life as a digital signal in the studio and distribution systems but then it was put through the "mincer" with the gory results that we see all too often;(...

Reply to
tony sayer

Don't tell I, tell 'ee!.. As they say in Dorset;)..

Indeed as its been doing for all these years..

Yep such a wondrous system;!..

A simple signal to noise ratio problem..

Indeed, the cowboys have never had it so good;!..

Umm.. I've just bought a couple of Triax group A's recently I don't recall any certificates stuck thereon?..

Well seeing that the ERP's are so very relatively low then not too surprising really. Still come the allowed increases....

Yes I would have thought that the OP would have known that;!..

Indeed. A very good choice if existing levels will permit the usage which sometimes in the current intermediate climate they just don't have enough gain whereas a grouped Yagi will...

or throw up;!..

Reply to
tony sayer

I think that Andy's explanation, and these not a lot you can tell 'im about TV aerials and associated equipment's, is quite adequate for the layman...

In Transmission you just don't in practice have unbalanced aerials made, just -not the done thing-;!.....

Reply to
tony sayer

From the mists of my memory I recall a folded dipole has a characteristic impedance of 300 ohms - and an aerial balun changes this to the UK 75.

Reply to
Dave Plowman (News)

No, thats not the point.

The point is 'balanced with respect to what?'

I.e. a 'balanced' aerial feeding an 'unbalanced' feeder works just as well.

At the mathematical level, the balanced bit merely applies to the voltage difference between the antenna feed points. You can 'earth' one, the other, or neither and it changes nothing really.

As for putting a balun between the antenna and the cable, unless there is impedance matching to be done, its a waste of signal. The coax simply is a piece of coax terminated (hopefully) at 75ohms at each end. What it picks up as interference doesn't change between it having a balun/antenna or just an antenna stuck on it. The only advantage I can see is a slight reduction in likelihood of damage under high EMF situations, like lightning strikes nearby.

I did just enough antenna theory at college to realise I didn't want to do any more, and to realise that the equations governing it were independent of the actual reference voltage of any part.

I have done more than enough RF engineering to understand that at RF, there is no such thing as 'ground' or 'earth' either.

I spent enough time with a few people who really DID understand antenna theory to realize that in reality, you can probably count them on the fingers of one hand.

The reality is that there are some standard designs, that have been dreamed up by a few people years ago, and everything is a simple reinvention of the yagi, dipole, quarter wave whip, log periodic etc.etc etc.

Putting a few turns of wire round a ferrite lump sounds like pure brand differentiation to me.

Reply to
The Natural Philosopher

Thats a different issue: there a balun has validity - as impedance matching.

I don't refute that, merely the bollocks abut what is stuck on the end of a piece of coax (as long as its a 75 ohm load at the frequencies in question) being of any relevance to the interference pickup of the cable itself.

Cables only act as antennae when they are mismatched. If the antenna is

75ohms, then it matters not one jot if its fed via a balun, or not.
Reply to
The Natural Philosopher

Sorry, but your perceptions about this are almost completely skewed. Andy has it just right.

There is no kind of "RF ground" or "RF earth" up there at the aerial - it's fallacy that invariably leads to the wrong conclusions. The only things that exist up there are potential *differences* and some concepts of "balance".

(Yes, you did mention potential differences, but immediately let the "earth" fallacy creep back in).

The stuff about one limb of a dipole being "a 'reflector' boosting the signal into the other half" is also complete nons\\\\\\\\ a completely unhelpful way of thinking about it.

Likewise the final paragraph about baluns being generally associated with twisted pair. A balun makes a transition between a "balanced" (ie physically and electrically symmetrical) system, and an "unbalanced" system like coax. Andy is talking about the transition between the dipole (balanced) and the top of the coax feedline (unbalanced), which is a classic application for a balun.

A balun may also include an impedance transformation, but that is categorically *not* its primary function.

(PS: Just read your other two postings, and the same comments apply.)

Reply to
Ian White

Exactly waht I said.

Well it is and it isn't. I was merely trying to make the point that connecting one side of a presumably screened cable to one side of a dipole does not 'unbalance' it.

No, you have just refuted your own argument.

The cable can ot be considered unbalanced just because one side of it is earthed at one end, that is exactly what you said, and I agree.. and it make bugger all difference if you actually have no cable at all, and simply earth one side of the dipole - in which case its a quarter wave whip and a ground plane.

No, and yet in his case its its ONLY valid function.

I suggest you go and actually study the theory then.

your two statements are contradictory.

If you want the ultimate in RF screening - or audio, you use a balun in a TWO core cable that is shielded.

A piece of coax terminate with a 75om load, does no change its interference reaction whether or not its that 75ohms is a balun and antenna, or indeed simply a 75ohm antenna.

At best, all you have done is DC isolated (and to an extent out of band RF isolated) the ANTENNA from the CABLE. But in terms of actual RF interference pickup, that is generally very small beer, and in theory, it won't make a hoot of difference. For exactly the reasons you described. The antenna doesn't care whether its floating, 'earthed' at one end, or 'earthed' in the middle. The transfer of energy from the EM waves is not with respect to any particular volatage on any particular part of it. It appears as simply a 75ohm impedance generated EMF across the two output terminals.

All that is in question here is where isolating that from the unbalance cable has any effect wghatsoever. I can see no mechanism other than dimly grasped and misunderstood 'don't connect balanced to unbalanced' to account for your view. And yet you have already admitted, nay seemingly CORRECTED me, to say that you cant realy consider a dipole to be balanced or unbalanced!

Let's just repalace that dipole with something that it really represents. A signal generator, that is completelty unearthed in any sense at all frequencies. Generating e.g. TV signals. Now, why would anyone introduce a lossy balun into its feed if not for impedance matching or marketing?

Reply to
The Natural Philosopher
[Ian White had written}

What I just gave is the normal textbook description of what a balun is for. If you find that in any way self-contradictory, the problem is all yours.

In standard engineering usage, coaxial cable is called "unbalanced" because it doesn't have the same left-right symmetry that a "balanced" dipole does.

Again, it's only wrong according to your own misunderstanding of the subject.

Oh I have, I have... and also designed baluns, built them, measured them, used them, written about them and had more discussions about them than you could possibly imagine.

That's why I can say with confidence that Andy and I are right in the mainstream on this subject, and you are in a minority of one.

Reply to
Ian White

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