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 cable
As you can guess, signal is weak. If there are better specced bits to
use I'm all ears.
On Fri, 1 Feb 2008 11:14:20 -0800 (PST) someone who may be
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.
On 1 Feb,
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.
3. The Wanderer
more or less - its digital tv modulated to analogue rf, which is what
will be distribbed.
6. Andy Wade
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.
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?
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 :-)
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
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.
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.
... absolutely. Every dB of aerial gain is quite hard-won. Don't throw
it away with cable loss ahead of the LNA.