Hi again everyone
Just wanted to ask for a reality check here. Planning to install:
masthead amp 40dB gain 0.8dB noise (FTE maximal)
2 way splitter 4dB loss used as a combiner (Televes) (combine amplified ae signal with locally generated signal) 6 way splitter 10dB loss (Televes) (for distribution) CAI approved cableAs you can guess, signal is weak. If there are better specced bits to use I'm all ears.
cheers, NT
forgot to mention prices: amp: 25.99
2 way: 3.99 6 way: 7.99On Fri, 1 Feb 2008 11:14:20 -0800 (PST) someone who may be snipped-for-privacy@care2.com wrote this:-
Before doing anything with television aerials I would study and see if it has any useful information on the particular circumstances. It is the most imformative site I have found so far.
I would post this to uk.tech.digital-tv where some knowledgable aerial guys hang out.
FAR too much gain, even allowing for the fact you're splitting after the preamp. Use a standard high-gain type masthead with around 26-27 dB gain for your application.
I would suspect that 40dB gain figure. It would amplify an unusable
100microvolt signal to an overloading 10mV signal Somewhere nearer 26dB would be more realistic unless it is followed by a lot of passive splitting and/or an extremely lossy feeder run.
What is the 'locally generated' signal? Is it RF2 from your sky box?
more or less - its digital tv modulated to analogue rf, which is what will be distribbed.
David Hansen:
There's a 24dB amp there now, and its inadequate signal strength. Whats there is good, no ghosting etc, but signal strength isn't enough. Will be adding a 6 way splitter (10dB) and a 2 way combiner (4dB), which will knock that +40 back down to +26. Need to change the amp anyway, so will go for a much lower noise one. Its also adjustable to iirc 26/33/40dB in case the gain's ever too high.
Re inadequate signal level, the plan is to replace the ae with a high gain one too, but later. Only so much can be done in one time.
thanks everyone,
NT
I'm no expert, and I don't pretend to understand gain and dB etc, but if the signal is that weak, why start with amplification? Surely the real problem is the source of the signal, so a more sensitive / better aerial would be the place to start. Once that is maximised, then tune the amplification to get the best out of it?
Matt
Include the cable losses in your calculations - you can assume about 0.2 dB per metre at the upper end of the UHF band for decent '100' size cable. Then work out the net /overall/ gain between the antenna and the TV tuner. With typical preamp and tuner noise figures there will be almost no advantage to be had by increasing this overall gain beyond around 12 to 15 dB. By this point the overall system noise figure will be determined almost entirely by the preamp, so more gain simply will not help - in fact it will make matters worse by reducing the overall dynamic range, increasing the risk of overload/crossmod/intermod problems.
Remember that the sensitivity of the receiving system is ultimately set by the 'figure-of-merit' G/T, i.e. the ratio of antenna gain (G) to overall system equivalent noise temperature (T). T is the sum of the antenna noise temperature (Tant) and the receiver noise temperature (Trx). Tant you can do little about, Trx you've reduced to a practical minimum, so the only thing left is to increase G, which means getting more metal in the sky...
BTW be wary about variable or switchable gain preamps: some of them (not all) have some of the gain control implemented by input attenuation, which of course completely buggers the noise figure when you drop the gain. "Inter-stage gain control" is the buzzphrase required to avoid this.
As I said, more G :-)
Andy Wade wrote:
Well, yes... but one thing at a time. The distribution system needs replacing first, and that makes amp replacement necessary for practical reasons. So that'll get done first. Then the aerial can go.
I'll look into the variable gain question, thanks. The cable attenuation is not something I can do much about, all the CAI cables are specced about the same. Its only going to drop a few dB, or double that with the locally generated signal.
The main improvement expected from a new amp would be in noise figure. Not only is it better specced - depending on how gain adj is implemented - but it'll be at the aerial for another few dB improvement as well.
cheers,
NT
That was my point really, with the splitting losses you mentioned and only a few dB of cable attenuation I very much doubt that you'll need 40 dB gain in the preamp.
Of course...
... absolutely. Every dB of aerial gain is quite hard-won. Don't throw it away with cable loss ahead of the LNA.
just waiting for a reply from the mfr...
NT
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