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dB, dBuV - whats the difference?


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Old 20-07-2004, 12:06
ŽEmm
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How do I calculate the theoretical DTT signal level at the ‘outlet plate’?


Using hypothetical figures:

Aerial gain = 16dB
Masthead amp gain = 23dB
Distribution system = 7dB (per outlet)
Lead / connector loss = 6dB

So,
16dB + 23dB + 7dB - 6dB = 40dB (is this the ‘outlet plate’ signal level?)
But, a general figure quoted as being a minimum signal level for DDT is 45dBuV

The part I don’t get is the ‘uV’

How does the 45dBuV relate to the calculated 40dB?

The reason for all this is that I require a new aerial and I’d like to determine a suitable replacement.
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Old 20-07-2004, 13:45
Roxof
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Hi ŽEmm

The 40 dB calculation you have shown illustrates the power gain from signal in the air to signal at the outlet.

dBuV (the u is really meant to be the Greek "mu" character) is the power relative to one microvolt (uV), so 45 dBuV is 178 microvolts. The final crucial thing needed is the input field strength at the aerial in uV/m (microvolts per metre) times roughly the length of the dipole element in metres. For an idea of field strengths try a reception predictor, e.g.:

http://www.wolfbane.com/cgi-bin/tvd.exe?

HTH

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Old 20-07-2004, 15:47
ŽEmm
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Originally Posted by Roxof
.........The final crucial thing needed is the input field strength at the aerial in uV/m (microvolts per metre) times roughly the length of the dipole element in metres........

Thanks for replying Roxof.

According to the predictor link the field dBuV/m is 47

So, with a dipole on my existing aerial having a total length of approx 0.3 metres gives me a figure of about 14.1dBuV

Armed with this figure, what can I do with it
How does this relate to my initial calculation?
Does it simply mean I chose an aerial with a greater figure?
(Apologies for being dense - I feel I'm missing something here....... )
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Old 20-07-2004, 16:12
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Hi ŽEmm

47 dBuV/m is 224 uV/m or 67 uV in the 0.3 m dipole. Bumping that up 40 dB would give 6.7 mV at the output, probably too high! Unless you're in a poor reception spot locally you probably won't need nearly so much amplification, which can't be a bad thing!

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Old 20-07-2004, 17:43
ŽEmm
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My house sits in a dip surrounded by other properties and large trees (the transmitter is 14 miles away, but obviously not visible...)

A picture can only be obtained using a masthead amp.
My current arrangement is a 10.5dB gain maxview 18 element group C/D contract aerial using a 23dB gain group C/D Fringe masthead amp (all in place when I moved in 7 years ago). This setup feeds a maxview 6 way distribution amp boosting each output another 7.5dB. This weekend just gone, I've replace the downlead from aerial via masthead to distributor amp with CT100.

So 10.5+23+7.5-6=35dB


Originally Posted by Roxof
Hi ŽEmm

47 dBuV/m is 224 uV/m or 67 uV in the 0.3 m dipole. Bumping that up 40 dB would give 6.7 mV at the output.........
I understand perfectly the first sentence, but how did you arrive at 6.7mV after 'applying' 40dB?

Thanks in advance,

ŽEmm
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Old 20-07-2004, 17:58
JefUK
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For calculation of signal at TV aerial socket see:-
http://www.megalithia.com/elect/fieldstr.html
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Old 20-07-2004, 18:00
Roxof
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Hi ŽEmm

Sorry, i should have explained that. dB is a logarithmic measure of power ratio, i.e. 10 * log ( a / b ) so 40 dB means that a / b = 10,000. The voltage ratio is the square root of the power ratio, so it is 100.

It's clear from what you say that local conditions leave you with a much lower field strength than given by the predictor. It should be possible to guesstimate how much to change the gain on the basis of the quality of your present reception - which transmitter are you using, and what is the DTT like at the moment?

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Old 20-07-2004, 18:11
ŽEmm
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Tacolneston

DTT - variable. Prone to impulse interference, hence the downlead replacement. New cable has made an improvement as has the slight aerial direction tweak. ITV CH4 History channel tend to pixellate more. Currently after the 'improvements' the signal strength on my Netgem is averaging 42 on strength and 70% (ITV etc) to 85% quality for BBC. BER on the whole is 0 but on the ITV CH4 this can rise.....
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Old 20-07-2004, 18:20
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Hi ŽEmm

You're doing all the right things. Tacolneston has a somewhat feeble DTT output compared with analogue, although C/D is spot on. Because the aerial size was of course chosen for analogue, one of those serious monster aerials might well solve the problem. Any extra height might well help too.

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Old 21-07-2004, 08:41
ŽEmm
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Thanks for the advice Roxof.

At present my aerial is some 9m off the deck at the gable end on a long pole (it seems plenty high enough when up a ladder ).

Positional wise I'm restricted to mounting it on the front of the property to avoid pointing it at a group of trees. The road is some 6m away, more or less running at right angles behind the aerial. I have a very slight tilt upward on the aerial to aim it at the horizon.

One of the replacements I'm considering is the Antiference XG10C/D (balun, grouped 16.4dBi / 14.2dBd fwd gain, 25dB front/back ratio, 14deg acceptance angle) with all my other existing gear.

As the existing Maxview A1003/C aerial is something like 10.5dB gain, would I be running the risk of overcooking the signal despite living in a lousy signal area? Is the 10.5dB to 14.2dBd / 16.4 dBi gain a huge jump? I don't want to kill the analogue signal particularly either.

Any advice / opinions gratefully received.

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Old 21-07-2004, 12:59
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Hi ŽEmm

The Antiference XG range are very good. Whatever you choose, make certain that it has a balun to avoid interference pick-up and standing waves in the otherwise mismatched feeder. An increase from 10.5 to 14.2 dB (assuming that the Maxview figure is dBd) corresponds to a 50% increase in signal voltage - it's most unlikely that overloading would occur since things are at present marginal for DTT, although it's true that the stronger analogue signals usually hit the top of the headroom first. You might consider pushing the boat out and going for an even larger model, since the marginal cost isn't so much and it would be exasperating if the gain still isn't enough (in the unlikely event of going too far, reducing the amplification would actually lead to a better outcome anyway, since the metal is "noise-free"!)


Last edited by Roxof : 21-07-2004 at 13:02. Reason: Spilling mistak
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Old 21-07-2004, 13:14
ŽEmm
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Originally Posted by Roxof
........You might consider pushing the boat out and going for an even larger model...........

Roxof,


I assume you mean the Antiference XG16CD over the XG10CD

That'll be fun wrestling that up 9m of ladder

Cheers

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Old 21-07-2004, 13:24
Roxof
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Hi ŽEmm

Yes, have had one or two near - Rod Hull experiences doing things like that - take care!!!

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Old 22-07-2004, 09:16
efo
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Hi,
What an interesting conversation, its a pity more postings are not as good and informative. Yes and what happened to poor Rod is a good example as to why people shouldn't do the job without the right equipment and experience.
cheers all and thanks
henry
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Old 23-07-2004, 12:47
ŽEmm
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Roxof,

If I decide to go down the XG16CD route, what are the requirements for mounting one of these beasts?

At present the existing aerial sits on top of an 8' x 1" dia pole supported by a 'T & K' bracket.

I'd imagine that the wind loading on the Antiference model is considerably more than my existing.

(Incidently, scanning round the rooftops of nearby properties, there is not one aerial of XG16 proportions - only a handful of XG10 type size plus one DAT45 looky-likey)

ŽEmm
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Old 23-07-2004, 13:10
Roxof
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Hi ŽEmm

A 'T & K' is usually a sound thing, provided that the bolts go into solid brick (which is not cracking - some expanding fixings break the bricks!)

Would suggest replacing the mast with 2-inch diameter (rather more than 1-inch, at least), or it's likely to flex a lot with the wind.

It well might be that you don't need a beast as large as the XG16, but signal strength in "cluttered" locations can vary hugely from one spot to another.

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Old 23-07-2004, 15:25
ŽEmm
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Thanks again Roxof for the advice.

As an aside, have you had any dealings/experience of the Televes DAT45 and DAT75 including their MRDs? (I notice that these are only available in wideband).

Cheers

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Old 23-07-2004, 16:04
Roxof
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Hi ŽEmm

Sorry, haven't tried the DATs out yet - but they are very well spoken of and are serious contenders. The MRD might well be a boon, since you're struggling for signal. The performance of a wideband isn't much below a C/D at high UHF channel numbers.


Last edited by Roxof : 23-07-2004 at 16:06. Reason: {bulb!} afterthought
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Old 24-07-2004, 08:15
efo
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Originally Posted by ŽEmm
Thanks again Roxof for the advice.

As an aside, have you had any dealings/experience of the Televes DAT45 and DAT75 including their MRDs? (I notice that these are only available in wideband).

Cheers

Hi,
Just out of interest I have used both of these aerials at various locations and they work very well ( including the mrd ). If however you use them in the vertical mode on a vertical transmitter it can be prone to picking up transmitters that are not in the wanted direction, because of the aerial construction. This only presented a problem when I was sending a video signal and sky around a system and realised that there were loads of small blips on the analyser making it difficult to find a clear channel to re modulate on.
cheers Henry
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Old 25-07-2004, 14:08
ŽEmm
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Thanks for the input, efo.

Question for Roxof....

Considering the DAT75 +MRD option, am I in reality going to make an improvement on my current setup?

Existing:
10.5dB - aerial gain
23.0 dB - masthead amp
33.5 dB - total

Option:
19.0dB - DAT75
13.0dB - MRD
32.0 dB - total

In view of the above would a better option (and cheaper) be to change the existing contract aerial for an XG10 (or 16) CD and retain the original 23dB (unscreened) masthead amp?

14.2 dBd - XG10CD
23.0 dB - masthead amp
37.2 dB - tota

15.0dBd - XG16CD
23.0dB - masthead amp
38.0 dB - total


I realise that I've simplified the maths / comparison, but your opinion would be appreciated.
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Old 25-07-2004, 22:21
Roxof
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Hi ŽEmm

The more gain that you can get via metal up top before using any electronics, the better. This is because aerial gain is essentially "noise-free" whereas when using any amplification another crucial consideration enters - the Noise Figure of the input stage of the amplification chain. Costs permitting, a 19 dB aerial will win hands down over a 15 dB model. This noise-free advantage would only be offset by seriously bad amplification further down - and the MRD units are specifically designed for the purpose in question, so are unlikely to let you down. 6 dB less overall - well, a "cheapo booster" at the bottom of the chain could easily restore that if it should prove necessary.

HTH

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Old 26-07-2004, 09:36
ŽEmm
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Thanks yet again Roxof

Based on you reply above.....

Existing:
10.5dB - aerial gain
23.0 dB - masthead amp
33.5 dB - total

Replacement option 1:
19.0dB - DAT75
13.0dB - MRD
32.0 dB - total

Replacement option 2
17.0 dB - DAT45
13.0 dB - MRD
30.0dB - total

.....The replacement options would be better even though their dB is slightly down overall compared to existing?

My current arrangement suffers from some impulse interference (no balun on current aerial) and inconsistent 'quality' (according to the Netgem diagnostics) resulting in pixellation and sound corruption.

As viewing in the early hours (don't ask) shows a far more 'stable' picture, would I be right in assuming that a hike in overall gain might not be necessary?

If that is the case, I presume a DAT75 or DAT45 aerial with high impulse immunity and high F/B ratio would be the answer? - Could I go as 'low' as a DAT45?

Many thanks

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Old 26-07-2004, 10:07
Roxof
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Hi ŽEmm

As you note, it's the quality of the signal which is paramount . Difficult to say whether the extra 2 dB from choosing a DAT75 over a DAT45 would be worth the extra expense, but either is bound to give an enormous improvement.

It is also possible that all of your problems are arising from the fact that the current aerial doesn't have a balun, and although that clearly needs to be replaced it's difficult to be certain whether more gain is required as part of the solution.

23 dB of amplification following a 10.5 dB aerial would inevitably be inferior to 13 dB following 17-19 dB, but if there are times when you can achieve perfect reception with the current set-up the finger might point to external interference rather than amplifier noise, although it is also quite likely that the signal is subject to fading. A better, more directional aerial is bound to be a step in the right direction.

It is a difficult situation when uncertain how large a sledgehammer is required to crack a nut and ideally one would like to get a survey done to establish what the signal levels actually are.

HTH, to some extent at least.

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Old 26-07-2004, 15:01
ŽEmm
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Thanks yet again Roxof for a comprehensive reply

Originally Posted by Roxof
...........it is also quite likely that the signal is subject to fading..........
What would cause this to happen?

I know I've 'aimed' my current aerial as best I can to avoid pointing at one of the several trees in the vicinity. The signal 'drop' doesn't seem to relate to trees moving - the picture's become unwatchable before now when it's dead calm....
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Old 26-07-2004, 15:59
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Hi ŽEmm

Propagation over any distance tends to become increasingly variable because of various factors - some are obvious, such as the weather, trees, bodies of water, traffic etc. In addition, signals received anywhere near the ground actually consist of many reflections and diffracted patterns. Where the signal strength is locally low it's often particularly multi-pathed in this way and the relative strengths of the reflections and patterns tend to be restless. Because the multi-pathed components are all at different phases, they will add and cancel each other to varying extents. Changes can occur over any timescale, and often seem bafflingly unrelated to the wind in the trees. On analogue, of course, the worst of the multi-pathing shows up as ghosting or a general "smearing" of the image, although the receiver's automatic gain control usually hides the fading completely. With digital, apart from possibly "falling off the digital cliff", the bit error rate will deteriorate where multi-pathing is severe; on the plus side, our system is very robust; on the minus side, where things are really dire, the remedy might not always be as simple as a highly directional aerial...

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