How to set aerial distribution amp gain?

[Mathew Newton]

Hi all,

I've recently moved into a relatively new (2007) house which has a loft aerial and a Philex 2-way aerial distribution amplifier, the latter which I suspect was fitted by the previous owner given that whilst it is mounted inside a cupboard the power isn't so lead has to come out of the door hence stopping it closing. There are also 3 aerial outlets throughout the house so that's probably another giveaway!

So, to neaten this up I've installed power inside the cupboard and moved the loft aerial to a less in-your-way location inside the loft - all correctly aligned and working hunkydory.

The distribution amp is a Philex 2-way

with adjustable gain and I was wondering how one goes about setting up the correct amount of gain to suit? Of course, with analogue it might've been more obvious given the varying quality of picture however in the works-or-it-doesn't digital world it doesn't seem quite so obvious.

With the gain set to the low and high extremes my Humax PVR (furthest from the amp) signal detection page varies between 78% and 90% respectively, both cases providing 100% signal quality and a perfect picture. My el-cheapo signal meter similarly varies between ~60dBuV and ~80dBuV (max'd out - could be higher). With no amp in the loop I get 62% on the Humax, still a good picture, and ~50dBuV (lowest figure) on the signal meter.

Any suggestions? Whilst I'm getting a perfect picture during testing even on the lowest gain want to set it as 'properly' I can. The fact the gain is adjustable suggests to me just setting it at the max is not really what you're meant to do!

For what it's worth I am 35 miles away from the transmitter (Mendip) and judging by the varying arrangement of aerials in the surrounding area Freeview reception is seemingly somewhat of a challenge here (admittedly perhaps the aerial situation is a consequence of the pre- switchover situation).

Mathew

[The Natural Philosopher]

at 35 miles you might as well set it to max and be done with it.

Unless you overload the TV input, the gain wont make a huge amount of difference, as the signal and the noise will both get boosted, and DTB is all about signal to noise.

In short, it probably makes sod all difference what its set to, in your case! :-)

[John Rumm]

Even 50dBuV is adequate for DTV (assuming that is measured at the TV and is the true level across all muxes).

One way to set the gain is to insert a 6dB attenuator between the aerial lead and the TV. Adjust the gain down as low as you can while you still get a 100% reliable signal. With the attenuator removed you then have a minimum of 6dB margin to allow for atmospheric variation and the effects of ageing on the cables etc.

You may get away with setting it to max, however the thing you need to watch is what happens when you overdo it. That can cause some amps to throw a wobbly and start sticking intermodulation products into the output which can mess things up. This is sometimes a problem at distance from a transmitter when the analogue level is quite high with respect to the digital one. Using enough gain to get the digital to acceptable levels can lift the analogue too much[1]. Also at max gain you may compound any impulse noise interference the aerial picks up.

Personally I would leave it set near the minimum and tweak it up a bit if you notice picture breakup etc.

(Also bear in mind that post switchover, you may well get another 9dB of signal from the transmitter anyway).

I am surprised you are getting that much signal from a loft aerial at that range!

[1] I don't know what the actual max output on that amp is - you may find its got plenty of headroom anyway. Perhaps Mr Wade will be along shortly to tell us! ;-)

[Mathew Newton]

Ah okay; makes sense...

I forgot to mention that the other outlet fed from this amp is only a couple of meters away so I guess I need to be conscious of what effects unnecessary extra gain might have on the receiver plugged into that.

I might give that a go.

The circumstances are probably very favourable - it points through the roof (not gable walls etc), doesn't have anything else near it, and I think we're fairly high up here too. Of course, I dare say my installation skills with regards to positioning and cable terminations also helped... ;-)

0 - 18dB from the specs (if that's what you mean) with a noise figure of 3.5dB if that's relevant.

Mathew

[Dave Plowman (News)]

A DA should simply be unity gain to all the outlets. If you need gain, that should be from a head amp at the aerial.

[Spamlet]

We have a 6-way amp (don't recall the details) in the loft and sockets in most rooms, fed out via the eaves. It's fine for digital, but analogue has been getting weaker for some time: even the BBC stuff has lines on the screen. I suspect they have been reducing output: but I can't be b'd to tinker with it, while we have freeview and cable.

S

[The Natural Philosopher]

no..its normal to have 6-10dB sort of gain, to allow for possible bad cabling I guess.

And the use of passive splitters.

[John Rumm]

Chances are if it works on the longer run it will be ok on the shorter one. If an amp is going to start clipping a analogue channel due to to high an input level, the resulting spurious mess on the output would affect all sets. TVs are usually fairly tolerant of a wide range of input levels.

No, that's the max gain - but does not tell you what the max output is.

Say it were 90 dBuV, and you wanted to use max gain to add 18db to a digital mux sat at 30dBuV, then the resulting output at 48 dBuV would be well within its output capability. However say there was also an analogue signal present at the input at 80 dBuV, you potentially now have a problem. Even though you may have no interest in the analogue channel, its presence will try to drive the amps output to 98 dBuV and that could cause it to start clipping on that channel. The resulting broadband noise created from the clipped signal could then mess up the decoding of your otherwise OK digital muxes.

(in real life this is not usually encountered that often on simple domestic installs - plenty of amps have pretty high max outputs, and the disparity between signal levels on input are not usually too great)

[Dave Plowman (News)]

That amount of gain would produce severe overloading round here.

They're usually a better bet unless you have a very low signal.

[The Natural Philosopher]

really? you must be getting several VOLTS off your aerial then.

Its a very modest gain level.

IIRC mine has +8db on most ports and +18db to feed a three way passive splitter on the last two ports.

am 12 miles from the transmitter with a decent loft antenna. No issues. Signals appear in the sort of good to very good strength: occasionally quality is crap in certain weather conditions on some muxes.Sporadic E probably.

Not if you are feeding 9 cables :-)

Which I am :-)

I got some 6db pads in case, but their use made no difference.

[Frank Erskine]

The problem at the moment (in most (?) areas) is that digital transmissions are currently being sent at reduced power until the analogue signals are switched off, so that analogue signals can cause overloading on distribution systems optimised for digital reception..

I would suggest that, unless/until your local transmitter has had its analogue transmissions turned off, you just more or less live with it until you can achieve a signal at each outlet plate of perhaps 54 to

70 dBuV with nothing in excess of ~76 dBuV.

The real problem is that many distribution amplifiers are pretty broadband (for cheapness) and they quite willingly respond to strong local signals on all sorts of other frequencies, such as Band II radio broadcasts, or amateur, CB, PMR (taxis etc). Any particularly strong such transmissions are quite likely to overload your amplifier.

A distribution amplifier is normally designed to receive the acceptable range of input signal as mentioned above and to distribute it to the various outlets at similar levels. Better ones have individually adjustable outputs. Since they aren't really designed as low-noise amplifiers it's expected that they're fed with a reasonable signal from either a decent ærial or an indecent one and a low-noise masthead amplifier (again not broadband).

[Mathew Newton]

Thanks Frank, and everyone else, for your input.

As suggested I will just set it and see. It all seems to be working fine whatever I do so the question really was borne out of aiming for the theoretical optimum, however it is clear from the discussion that I've already started off on the wrong foot with the amp I'm using but as it was already here I don't see any real benefit in replacing it.

Thanks again,

Mathew

[Dave Plowman (News)]

Since most FreeView devices show the signal level I'd just set it up so they are ok. If one run shows a much higher level than the rest you could attenuate that with an inline attenuator.

[The Natural Philosopher]

There is no optimum really. just a broad range in which reception is as 'perfect[' as it can ever be ad an upper and lower limit of signal strength beyond which Bad Things May Happen.

One of the features of transistorised TVs and STBS is that gain is never a problem, but noise always is. neither is gain control via AGC. reception quality is more about signal quality, than strength.

[tony sayer]

Hasn't digital switch over happened now in the OP's area Mendip?...

[tony sayer]

If you know where Dave lives then thats about right;!...

[tony sayer]

Not quite that simple tho;)..

[The Natural Philosopher]

Tony..we would leave 60% behind if we told it like it really is..

In terms of average TV reception, I reckon thats near enough the troof.

*I* remember getting cheap tranny radios and retuning the IF strips eand all the RF sections entirely by ear...Heck I used to do the timing on my cvan by ear. Someone hired me and decided not to trust that in case it burned extra fuel. Paid £25 to find I was spot on.

All the OP has to do is set it to something, and see if he gets decent reception. If not, well we can discuss the niceties. Otherwise,. sod it. When running well, leave well alone.

Too many people trust instruments more than results, and argue theory when its irrelevant.

[John Rumm]

Good point... according to the details:

- completed in April.

[tony sayer]

Loosen up NP .. that was a play on the strength quality issue which if you think about it go very much hand in hand in most all reception environs;)...