Low gain masthead amplifiers

Andy Wade Wrote:
zimbo000 wrote:
-
[...] why not dump the masthead and get a higher gain aerial?-

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.

--
Andy

Andy, thanks - the problem I'm trying to solve -might- be overload
causing interference. The Fringe WB1214 I currently have provides 14db
of gain. What I've been looking for - and let's assume I keep my Unix52
plus a masthead amp - is a new 1-way masthead amp with variable gain. I
found your proMHD11V.

Looks good I thought. But then I noticed the gain range is 7-22db. But
to solve my problem I might not need as much as 7db (I've put a 6db
fixed attenuator in circuit and I still get some interference).

I then saw the proMHD14V goes from 1-16db. Which is probably the gain
range I need, but I don't need a 4-way unit.

Why doesn't the 11V have a lower 'lower' limit?




--
zimbo000

zimbo000 wrote:

Andy, thanks - the problem I'm trying to solve -might- be overload
causing interference.

I wouldn't think so - you said that without the preamp the analogue
signal was noisy, so unless you've got a stupendously long downlead the
signal level at the aerial isn't going to be strong enough to overload
the preamp (unless the problem is out-of-band interference from nearby
non-TV transmitters).

First things first: have you got decent coax - benchmarked - one from
the list at
http://www.cai.org.uk/downloads/CAI%...g%20Scheme.pdf
and no unscreened outlet plate (particularly not an isolated plate) and
decent flyleads on the equipment (no crappy moulded ones with pigtailed
braids hiding in the plastic)?

[...]

Why doesn't the 11V have a lower 'lower' limit?

If you need less than 6-7 dB gain there's hardly any point in using a
preamp at all. OTOH the 4-way can serve as an 'active splitter' in the
sort of situation where you don't need any gain, but the loss of a
passive splitter wouldn't be acceptable.

Without any signal level readings from the aerial it's difficult to help
much further, but if you've got a normal length downlead, 10-15 m, say,
try a proMHD11L.

--
Andy

Andy Wade Wrote: [color=blue][i]
zimbo000 wrote:
-
Andy, thanks - the problem I'm trying to solve -might- be overload
causing interference.-

I wouldn't think so - you said that without the preamp the analogue
signal was noisy, so unless you've got a stupendously long downlead the

signal level at the aerial isn't going to be strong enough to overload

the preamp (unless the problem is out-of-band interference from nearby

non-TV transmitters).

First things first: have you got decent coax - benchmarked - one from
the list at
http://tinyurl.com/y5n4dq
and no unscreened outlet plate (particularly not an isolated plate) and

decent flyleads on the equipment (no crappy moulded ones with pigtailed

braids hiding in the plastic)?
-
[...]-


--
Andy

I replaced all external cable with Webro WF100 last year, which is on
the list. Internally, I made up my own flyleads using TC100 (like RG6
but the copper shield is tinned) and F-connectors. All outlet plates
were replaced with fully screened ones.

The downlead is 18m, so not overly long. I too am coming to the
conclusion that it might not be overload. I mentioned above inserting a
6bB attenuator in circuit; if I increase this to 9db, I lose the digital
signal totally and analogue goes -really- snowy. This seems to imply an
average-to-low signal i.e. I'm near reception threshold.

With 6db attenuation the signal quality reduces to 8-9 (from 10
without), and strength to 6-7 (from 7-8 without). Having watched about
3 hours TV last night, I still got the odd bit of blocking and a couple
of loud clicks on sound (one point here - I am a newbie on digital TV,
but I assume I should not get ANY blocking on screen or sound clicks...
correct?).

I'm thinking of getting a decent aerial company to do a proper signal
strength reading, plus check out the Unix52 aerial (to make sure the
balun is OK and in place), possibly replace the Unix52 WB with an A
version (all the channels I need are between 21-37, and the A will give
me about 3dB more gain in this channel range), and probably replace the
Fringe 14dB amp with a fully screened, lower power one - as you say,
the proMHD11L looks about right.

I'll also do a double-check on my flyleads this weekend. Any other
comments welcomed! :-)




--
zimbo000

zimbo000 wrote:

I replaced all external cable with Webro WF100 last year, which is on
the list. Internally, I made up my own flyleads using TC100 (like RG6
but the copper shield is tinned) and F-connectors. All outlet plates
were replaced with fully screened ones.

OK, you seem to have done the right things.

The downlead is 18m, so not overly long. I too am coming to the
conclusion that it might not be overload. I mentioned above inserting a
6bB attenuator in circuit; if I increase this to 9db, I lose the digital
signal totally and analogue goes -really- snowy. This seems to imply an
average-to-low signal i.e. I'm near reception threshold.

Is this attenuation before or after the amplifier? And if after the amp
can we just check that you're putting it in after the power unit - i.e.
you're not trying to put DC through the attenuator too? If 9 dB
attenuation after the amp & PSU in that system makes the analogue go
really noisy that's strongly suggestive of really quite weak signals off
the aerial, and that goes a long way to explain the digital drop-outs.

but I assume I should not get ANY blocking on screen or sound clicks...
correct?).

Ideally... In practice aim for "hardly ever".

[...] and probably replace the Fringe 14dB amp with a fully screened,
lower power one - as you say, the proMHD11L looks about right.

Yes, I see the Fringe product you're using has saddle and clamp
connectors, and the matching PSU might only have fairly basic screening
too (although that's pure conjecture on my part). A modern fully
screened F-connector preamp and PSU could well help. With the
suspicion of weak signals - where are you, BTW? - I'd be tempted to go
for the higher gain preamp ('11M) or the variable one ('11V ), or wait
until the aerial's been checked and you've got some reliable signal
level data to work on.

--
Andy

Andy Wade Wrote:


Is this attenuation before or after the amplifier?

where are you, BTW?

--
Andy

I am in Henley-on-Thames, viewing Crystal Palace, about 37 miles away.
Hannington is physically nearer but I prefer to be on BBC London rather
BBC South.

As the where the attenuator is... it's after the masthead amp on the
way 'down' to the TV, and it's only a temporary measure to assist my
problem solving. I did notice it was a bit warm! :-)

However, I did not give the whole story initially as I was trying to
keep it simple. My setup is:

Unix52 WB - Fringe WB1214 (on the aerial) - 5m WF100 cable - Global
LoftBox - 18m WF100 - DVD/HDD recorder (lounge) - RF OUT into Sky box -
RF1 OUT into idTV, RF2 OUT return to Loftbox - distribution to 3
analogue TVs around the house

The Loftbox TV ANT input (feed in from the aerial) provides 12V, 100mA
DC to drive the WB1214 so there is no separate PSU.

The Loftbox gives +6dB on the UHF downlink.

If I put the 6dB attenuator on the DVD/HDD RF IN, there is no
noticeable degradation of signal, nor a lessening in interference.

If I put the 6db attenuator directly on the TV ANT input of the
Loftbox, I see a lower signal as reported earlier (digital strength
down to 6-7 from 7-8, quality to 8 from 10). [And because the Loftbox
pumps out 12V, there's no choice other than this going through the
attenuator.]

If I put 9db on TV ANT, digital service disappears, analogue very snowy
(unwatchable).

In a previous post I mentioned I'd taken the masthead out of circuit to
see the effect: because of the DC from the Loftbox, I had to go on the
roof and physically disconnect it and join the cables temporarily
(using F-connectors). I kept the Loftbox in circuit, so it got the 6db
uplift on the downlink, so it was 'kind-of' like having a 6db masthead.
In this case, the digital signal strength went down to 2-3 (picture
visible but lots of breakup, unwatchable), analogue very poor.

Andy - not sure if this affects your previous (very welcome) advice,
but as per your last comment, I think I need to get the aerial checked
& signal professionally measured and see where that takes me. I might
replace the masthead before that however, as a fully screened one can
only help.




--
zimbo000

"charles" wrote in message
...
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.
Oh, well, if you're going to use cheapo aerials . . .



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.

Yes, but assuming you're starting with a clean sheet you should put a lot of
effort into the aerial before you reach for the masthead amp. Which is best:

1. Use a masthead amp with gain of 24dB
2. Find a location for the aerial that's 12dB better than the 'obvious' one
and use a masthead with 12dB gain?

No contest is it?

Working as I do on blocks of flats and other large rambling buildings I
often have a lot of scope for finding better aerial locations. Sometimes
it's possible to increase the signal by 12dB by merely relocating the aerial
from one wall to another. The improvement is a million times more than just
fitting a 12dB masthead.

Years ago I installed an aerial on the roof of the ITN building at 200 Grays
Inn Gr. The aerial they already had was behind a building and reception was
really poor. I simply put my aerial on the other lift motor room and the
signals were 20dB better. For a while Reuters had perfect reception and ITN
had a load of ghosting!

Many times I've moved an aerial from one block of flats to another,
achieving massive improvements in signal quality. It's a PITA running the
cable back but it's worth it. This is why it makes me laugh when people come
on here and say that the postcode checker says it should work so why doesn't
it?

Two weeks ago I went to a block of flats in Sheffield that was behind a
multistory. The aerial was on a chimney with dead easy access from a flat
roof. Reception was dreadful, with DTT dropping out all the time and
terrible analogue ghosting. I found out that an aerial on the other chimney,
that was only 20ft away but damn near impossible to get to due to the roof
being as steep as a steeple, was about 15dB better. It was still screened,
but not so badly. After a pretty scarey install the results were great.

You can't beat a determined rigger who's prepared to spend a bit of time
trying out different locations. Much better than just shoving a masthead on
the cable.

Bill

"Andy Wade" wrote in message
...
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.
Well yes, but the suggest seems to have been made that sticking a masthead
on a small aerial is OK.

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.
Fair enough, once the effort has been made to get the best from the aerial.
I don't advocate big high gain aerial though, except in extremis. I advocate
careful aerial location and positioning.


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).
I'm not saying 'don't use a masthead'. I'm saying that the results will be
far better if you get the best possible aerial signal and then use a
masthead if necessary. To emphasise the point:
Number of massive high gain aerials used by us in the last year: 0.
Number of mastheads used by us in the last year: probably 150.
Number of mastheads used by us in the last year without first thoroughly
investigating all possibilities for improving the aerial signal: 0.

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.

Yes, and increasing the signal level that enters the amplifier has a more
dramatic effect on your figures than anything you can do with an amp.

Bill

"Andy Wade" wrote in message
...
zimbo000 wrote:

Andy, thanks - the problem I'm trying to solve -might- be overload
causing interference.

I wouldn't think so - you said that without the preamp the analogue signal
was noisy, so unless you've got a stupendously long downlead the signal
level at the aerial isn't going to be strong enough to overload the preamp
(unless the problem is out-of-band interference from nearby non-TV
transmitters).
Or in-band overload from nearby TV transmitters (ie trying to receive
Belmont at a site 0.5 miles from Crosspool; the latter unusable due to
multipath. In that case you're up ****e creek basically, but if the strong
signals are not too close a bandpass filter can help).

Sometimes people think the patterning is caused by amplification when what's
actually happening is that the amp is reducing the snow and making the
patterning visible.


First things first: have you got decent coax - benchmarked - one from the
list at
http://www.cai.org.uk/downloads/CAI%...g%20Scheme.pdf
and no unscreened outlet plate (particularly not an isolated plate) and
decent flyleads on the equipment (no crappy moulded ones with pigtailed
braids hiding in the plastic)?

[...]

Why doesn't the 11V have a lower 'lower' limit?

If you need less than 6-7 dB gain there's hardly any point in using a
preamp at all. OTOH the 4-way can serve as an 'active splitter' in the
sort of situation where you don't need any gain, but the loss of a passive
splitter wouldn't be acceptable.

Without any signal level readings from the aerial it's difficult to help
much further, but if you've got a normal length downlead, 10-15 m, say,
try a proMHD11L.
If it doesn't work I'll give you a quid for it.

Bill

zimbo000 wrote:

As the where the attenuator is... I did notice it was a bit warm!
:-)

Ah!

Unix52 WB - Fringe WB1214 (on the aerial) - 5m WF100 cable - Global
LoftBox - 18m WF100 - DVD/HDD recorder (lounge) - RF OUT into Sky box -
RF1 OUT into idTV, RF2 OUT return to Loftbox - distribution to 3
analogue TVs around the house

OK that puts a slightly different complexion on things. As it stands
you've got something like 16 dB overall gain between the antenna and the
living room (and even more to the remote TVs).

If I put the 6dB attenuator on the DVD/HDD RF IN, there is no
noticeable degradation of signal, nor a lessening in interference.

AS expected.

If I put the 6db attenuator directly on the TV ANT input of the
Loftbox,

Yebbut that's a totally invalid thing to do. As well as cooking and
quite possibly damaging the attenuator you'll reduce the DC supply
voltage to the preamp which will reduce both its gain and signal
handling capability, the latter possibly leading to all sorts of
nasties, as well as attenuating the signal.

[And because the Loftbox pumps out 12V, there's no choice other than
this going through the attenuator.]

The choice is to use a power-passing attenuator (aka satellite attenuator).

If I put 9db on TV ANT, digital service disappears, analogue very snowy
(unwatchable).

Yes, mainly because the preamp is now close to being switched off and is
probably acting as a (pure guess) 20 dB non-linear attenuator.

In a previous post I mentioned I'd taken the masthead out of circuit to
see the effect: because of the DC from the Loftbox, I had to go on the
roof and physically disconnect it and join the cables temporarily
(using F-connectors). I kept the Loftbox in circuit, so it got the 6db
uplift on the downlink, so it was 'kind-of' like having a 6db masthead.
In this case, the digital signal strength went down to 2-3 (picture
visible but lots of breakup, unwatchable), analogue very poor.

Yes it is like having a masthead, although 5 dB gain and 5 dB noise
figure, allowing for the 5 m of cable between the antenna and the Global
box.

In the system you've described I reckon as follows:

(a) with no preamp (other than the loftbox) a 60 dBuV analogue signal
off the aerial should give perfectly good reception (almost 50 dB RF C/N
at a receiver with a 7 dB NF), ditto for DTT (43 dBuV giving over 30 dB
RF C/N);

(b) adding the '11L masthead (9 dB gain, 2 dB NF) will improve the
overall noise figure by about 5 dB and the 'boost' to the digital
signals will help mask any impulsive interference getting in after the
aerial;

(c) increasing the masthead gain to 14 dB would only improve the system
NF by a further ~1 dB.

but as per your last comment, I think I need to get the aerial checked
& signal professionally measured and see where that takes me. I might
replace the masthead before that however, as a fully screened one can
only help.

Yes, and as Bill has rightly pointed out, there may be scope to improve
the signal significantly be re-siting the aerial. In the meantime I
think we're back to the 9 dB preamp being the appropriate choice.

--
Andy

Bill Wright wrote:

Fair enough, once the effort has been made to get the best from the aerial.
I don't advocate big high gain aerial though, except in extremis. I advocate
careful aerial location and positioning.

Yes of course. I was writing from the point of view of having a certain
field strength available and having to make the most of it, but
you're quite right to remind us that the possibility to sniff out more
signal often exists.

Yes, and increasing the signal level that enters the amplifier has a more
dramatic effect on your figures than anything you can do with an amp.

Well in the figures I was mainly addressing the improvement in overall
noise figure you can get by adding a preamplifier in a given system, and
that isn't affected by the incident field strength (at least while the
system remains reasonably linear). Of course after a certain point this
'improvement' just isn't required, or won't be realised due to receiver
back-end noise effects and AGC operation, etc. and then there's no point
at all in fitting an amplifier. OTOH in the sort of situation you
described recently with a good field strength but a 70 m downlead a
preamp is surely a good way of avoiding having to put up a
mind-bogglingly large antenna.

--
Andy