Low gain masthead amplifiers

Andy Wade wrote:
Mark Carver wrote:

After DSO we'll have all 6 DTTs between 41 and 51, but the two extra
TBCs at 39 (good) and 66 (Bad !) , a neat solution might be required
for 2012.

Although if ch. 66 ends up as DVB-H you might be looking for selective
attenuation, rather than gain...

:-) I'll grow some more trees then.

I've travelled the world with my GSM mobile phone, yet guess the one
place where it can't get a signal :-)

On Wed, 15 Nov 2006 00:39:38 -0000, "Bill Wright"
wrote:

It often happens that a masthead amplifier is needed, but one with very low
gain.

Is a low gain amplifier better than a variable gain turned down.

Reason I ask is that I have used two proMHD14V - four-way amplifier
which are variable gain.

My own one is set to 1/3rd of the available adjustment with reliable
reception, my daughter's one shows good quality signal on 5 of the 6
channels but one channel can be eratic.

Geoff Lane

Bill Wright Wrote:
It often happens that a masthead amplifier is needed, but one with very
low
gain.

Can I resurrect this thread... as I'm attempting to resolve some
interference problems on a newly purchased digital TV and I am
investigating numerous avenues as to the cause. One possible issue goes
back to the original question: -why- do you need a masthead amp?

If I can explain: I have a Unix 52 wideband aerial with a Fringe WB1214
masthead amp, and my analogue reception was excellent.

I have now an idTV (Panasonic plasma). Without the masthead, my
analogue reception is poor (snowy), and digital borderline (signal
strength 2-3 on the TV {out of 10}, sound and picture are there, but
lots of breakup and not "viewable"). With the amp in I get 'perfect'
analogue, and good digital (signal strength 7-8) but unfortunately
frequent interference (blocking, sound clicks, sometimes 'no signal'
for 1/2 secs).

I think one possible cause could be analogue overload, i.e. too much
signal. If this is case - and let's talk "theory" here - why not dump
the masthead and get a higher gain aerial? I fully appreciate there are
lots of other factors that could be relevant (and I have replaced all
the cable with double-screened etc, and read this very helpful article:
http://www.wrightsaerials.tv/digitalterrtvrecep.htm ) but what's the
reason why "it often happens that a masthead amplifier is needed"? Why
not just put up a higher gain aerial?

I considered replacing the WB1214 with a proMHD11V (so I could adjust
the gain to suit if it was an overload problem), but then wondered
whether I need a masthead at all...?




--
zimbo000

"zimbo000" wrote in message
...

Bill Wright Wrote:
It often happens that a masthead amplifier is needed, but one with very
low
gain.

Can I resurrect this thread... as I'm attempting to resolve some
interference problems on a newly purchased digital TV and I am
investigating numerous avenues as to the cause. One possible issue goes
back to the original question: -why- do you need a masthead amp?

If I can explain: I have a Unix 52 wideband aerial with a Fringe WB1214
masthead amp, and my analogue reception was excellent.

I have now an idTV (Panasonic plasma). Without the masthead, my
analogue reception is poor (snowy), and digital borderline (signal
strength 2-3 on the TV {out of 10}, sound and picture are there, but
lots of breakup and not "viewable"). With the amp in I get 'perfect'
analogue, and good digital (signal strength 7-8) but unfortunately
frequent interference (blocking, sound clicks, sometimes 'no signal'
for 1/2 secs).

I think one possible cause could be analogue overload, i.e. too much
signal. If this is case - and let's talk "theory" here - why not dump
the masthead and get a higher gain aerial? I fully appreciate there are
lots of other factors that could be relevant (and I have replaced all
the cable with double-screened etc, and read this very helpful article:
http://www.wrightsaerials.tv/digitalterrtvrecep.htm ) but what's the
reason why "it often happens that a masthead amplifier is needed"? Why
not just put up a higher gain aerial?

I considered replacing the WB1214 with a proMHD11V (so I could adjust
the gain to suit if it was an overload problem), but then wondered
whether I need a masthead at all...?

I've always said that masthead amplifiers should only be used when every
possibility for increasing the signal from the aerial has been exhausted.
There's no substitute for aerial gain and good aerial positioning. No amount
of amplification can ameliorate the effects of poor carrier/noise ratio from
the aerial. Masthead amps should be thought of primarily as a means of
overcoming the effects of loss on cable by amplifying the signal at the
point where it is strongest, thus maintaining the best possible
carrier/noise ratio. In reality, and contrary to theory, mastheads can help
a little bit more than this would suggest, but only a little bit.

The trade tends to fit mastheads because it's quick and easy. What they
should be doing is a bit of heroic aerial rigging. Not that most of the
young buggers at it these days have any idea how to sniff out a good signal.
Some of them don't know one end of an aerial from the other.

Ooops, I've gone all grumpy. Sorry.

Bill

In article ,
Bill Wright wrote:

The trade tends to fit mastheads because it's quick and easy. What they
should be doing is a bit of heroic aerial rigging.

Some 25 years ago, I was told that in very windy areas a small aerial and a
Masthead amp was far more reliable than a giant array. I tend to believe
that.

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

Bill Wright Wrote:


The trade tends to fit mastheads because it's quick and easy. What they

should be doing is a bit of heroic aerial rigging. Not that most of the

young buggers at it these days have any idea how to sniff out a good
signal.
Some of them don't know one end of an aerial from the other.


Indeed... last week I called the 'old' b***** (as it happens) who,
about 2 years ago, fitted my aerial and masthead amp to find out his
thoughts etc on my problem now that I'd gone digital: I asked him to
come back to see what might be the cause and please could he make sure
he had his digital signal strength meter with him. "Oh I haven't got
one of those", he said, "Not sure they make them, I think they only
make analogue ones.... I'll bring a DTT set-top box and we'll stick
that on and see what we can find." Needless to say, I cancelled the
appointment and decided a bit of self-analysis would be the initial way
to go (as well as being cheaper!).

I guess what he's done for the past few years pre-mass-digital usage is
just stick Unix52s everywhere, and if the analogue reception wasn't good
enough (like mine), put a masthead amp on. It worked OK then, but now,
when folks go digital, this might not work anymore and needs a more
rigorous approach.

From my limited investigations to date I think I might be borderline
for getting a good digital signal, so with a more powerful aerial I
might be OK: I'll dig around some more.

NB. As part of my quest, I intend turning off the masthead amp and
calculating signal stength, cable length loss etc. and try to work out
the gain needed. Anyone know what dB loss I will get through a
switched-off masthead amp? Is it significant or not worth bothering
about for the fag-packet calculation I'm attempting? [PS. I'm doing
this so that if/when I call another aerial installer, I'm
-half-informed- and he doesn't try and bulls*** me into ripping
everything out and starting again. :-) ]




--
zimbo000

"charles" wrote in message
...
In article ,
Bill Wright wrote:

The trade tends to fit mastheads because it's quick and easy. What they
should be doing is a bit of heroic aerial rigging.

Some 25 years ago, I was told that in very windy areas a small aerial and
a
Masthead amp was far more reliable than a giant array. I tend to believe
that.

I don't advocate the use of massive high gain arrays except in extreme
circumstances because they are not efficient. Better to use a decent 18
element and use some skill to find the optimum location. All things being
equal, where there are constraints such as the strength of the chimney, a
decent 18 element can be mounted higher than a multi element whizzbang.

But a giant array erected properly will far outlive a small cheap
installation. The reason big aerials fall down is because they are put up by
idiots. You know full well Charles that if you amplify crap all you get is
more crap.

Bill

"zimbo000" wrote in message
...
Anyone know what dB loss I will get through a
switched-off masthead amp?

Totally unpredictable. It depends heavily on the type and the channel.

Bill

In article ,
Bill Wright wrote:


But a giant array erected properly will far outlive a small cheap
installation. The reason big aerials fall down is because they are put up
by idiots.

Not entirely, sometimes they just come apart becasue they aren't made to
stand up to constant high winds.


You know full well Charles that if you amplify crap all you get is more
crap.

Yes, true, but a mast head amp close to the aerial can give a better noise
performance than the set on its own.

I've been to amny places where, without an amp, there'd be no tele.

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

zimbo000 wrote:

[...] why not dump the masthead and get a higher gain aerial?

Aerial gain becomes very hard-won after a certain point, more so when a
wide bandwidth is desired. If you already have a "high-gain" aerial
set-up (whatever that might mean) then increasing the antenna gain by 1
dB can imply a considerable amount more metal in the sky - often an
impracticable amount more.

With no preamp, you are taking your hard-won signal and immediately
attenuating it by a few dB (in the downlead), and then connecting it to
a fairly noisy receiver (TV tuner). With no preamplification the feeder
loss and receiver noise figure detract directly, dB for dB, from the RF
signal-to-noise ratio (C/N).

As I showed in an example earlier in this thread a masthead preamp of
quite modest gain can dramatically improve the overall system noise
figure. By amplifying the signal at the masthead the effect of the
downlead loss and receiver noise on the C/N is very much reduced. The
overall result can be equivalent (C/N-wise) to having a _vast_ amount
more metal in the sky.

But note that much TV reception is not limited by thermal noise
considerations; it's interference limited, i.e. co-channel and adjacent
channel signals from other TXs set the minimum usable field strength. A
preamp doesn't help in this case, but a more directional aerial, in
general, does.

--
Andy