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Old 14th Mar 2017, 5:07 pm   #21
stevehertz
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Default Re: Switching a VHF signal between multiple receivers

Yes I've used them many times. They are a no-nonsense type of company who tell it like it is and - as far as I can see - don't try to upsell you to buy stuff that you don't need. On the other hand they do point out how and when cheap is 'nasty'. I like them. I'm probably going to buy this 6-way mains amp: http://www.aerialsandtv.com/onlinesp...#4WayMainsAmps
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Old 14th Mar 2017, 5:11 pm   #22
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Default Re: Switching a VHF signal between multiple receivers

That is one of the better ones, in the domestic range.

We tend to use Wolsey, which is an arm of Triax, for D/As. Other than that, the Televes amps, like the 7+1 are high spec devices suitable for domestic and commercial. We also install many 8084 Televes mashead D/As, never had a bad one.
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Old 14th Mar 2017, 5:26 pm   #23
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Default Re: Switching a VHF signal between multiple receivers

Quote:
Originally Posted by stevehertz View Post
I'm probably going to buy this 6-way mains amp: http://www.aerialsandtv.com/onlinesp...#4WayMainsAmps
These amplifiers have a 12v DC supply for a mast head amp, do they sense no mast head amp and turn the supply off or current limit it? Thinking if it is connected to an aerial with a folded dipole.

Appreciate this only affects the UHF connector.

Frank
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Old 14th Mar 2017, 5:36 pm   #24
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Default Re: Switching a VHF signal between multiple receivers

I don't know? In any case what problem is an open circuited PS? I'm not using the UHF feed/input socket, it will be fed from one output of a similar loft mounted distribution amp.
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Old 14th Mar 2017, 5:41 pm   #25
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Default Re: Switching a VHF signal between multiple receivers

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I'm not using the UHF feed/input socket, it will be fed from one output of a similar loft mounted distribution amp.
Not sure if I am understanding that. Are you saying that you will put the two amps in series? If so that is a bad idea. Just disconnect the FM aerial from the existing D/A and extend it to the FM input of the second D/A.

Either that, or passive split the UHF aerial to feed both amps, likewise with the FM input.
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Old 14th Mar 2017, 5:55 pm   #26
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Default Re: Switching a VHF signal between multiple receivers

Hi Steve,
Appreciate you only want VHF and the UHF will be unused, it was a generally question on how the 12V PSU output would behave connected to a UHF dipole, i.e short circuit.

Frank
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Old 14th Mar 2017, 6:13 pm   #27
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Default Re: Switching a VHF signal between multiple receivers

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Not sure if I am understanding that. Are you saying that you will put the two amps in series? If so that is a bad idea. Just disconnect the FM aerial from the existing D/A and extend it to the FM input of the second D/A.

Either that, or passive split the UHF aerial to feed both amps, likewise with the FM input.
Right. My existing loft mounted DA feeds UHF, DAB and VHF signals down to 'many' rooms for use thereof. Where relevant, combined DAB and VHF signals are split passively in each room, but UHF feeds are brought into each room separately, from the DA. In one of those rooms I want to also feed a free standing, multiple shelving unit bearing very large, very heavy hifi receivers that will be used one at a time. This laden shelving unit will weigh hundreds of pounds and getting around the back to swap or mess around with cables and wires is out of the question. Hence the need to switch or distribute the VHF signal to each receiver in a simple, hassle free way. They can't touch me for that surely?
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Old 14th Mar 2017, 6:27 pm   #28
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Default Re: Switching a VHF signal between multiple receivers

Most DA's fold back the current limit to the UHF masthead, i.e. they still try to supply current but only a weeny bit into a dead short. They can, and do, sit doing that for years as they are designed to. It may be worth terminating the UHF port if unused to minimise any noise/instabilities (highly unlikely).

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Old 14th Mar 2017, 7:09 pm   #29
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Default Re: Switching a VHF signal between multiple receivers

If you're worried about DC/power-supply interaction between a "power-to-masthead-feed" DA and your existing setup, it's dead-easy to build a power-break: - a couple of coaxial-sockets-of-your-preference, a box, and a 1nF capacitor.
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Old 14th Mar 2017, 7:18 pm   #30
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Default Re: Switching a VHF signal between multiple receivers

As is usually the case, this one only provides 12V on the UHF input. The VHF input can be connected to anything upstream

Scrolling down to the next item, the 6-way version has the "FULL" output that I mentioned in post #18. Just a thought: it might be worth replacing the previous amplifier with this one, feeding the full output to the room in question, and then use passive "taps" to split the signal out to the receivers. This approach has 2 advantages: no powered device in the room, and it's very easy to add more outputs if the collection grows - just buy another tap.

Here is a 4 way example: http://cpc.farnell.com/vision/v26-4-...oss/dp/AP02609 - note the "through" connections on the left hand side. The last one of these needs to have the 75 terminator (supplied with the 6-way amp) fitted. To add another, remove terminator, add new cable and tap, put terminator on end. Or, you can get taps that include the terminator - intended to be on the end of the run - these are often just called splitters (but they have higher losses than regular splitters, by design, so that's an unfortunate use of the terminology).

Here's an 8 way tap: http://cpc.farnell.com/vision/v26-8-...oss/dp/AP02614

I didn't realise you already had an amp in the loft. Knowing that, I'd say that using the "high level plus tap system" should result in a better noise figure as there's only 1 amplifier in the system. Whether that makes any material difference is obviously hard to predict from a distance.
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Old 14th Mar 2017, 8:01 pm   #31
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Default Re: Switching a VHF signal between multiple receivers

Ah! Knew I had it somewhere.

Attached is a picture of the schematic of a DA - the Labgear CM7073. This was reverse-engineered some 25 years back (E&OE), and apologies for the quick-and-dirty picture rather than a proper scan...

You can see the amp followed by all the attenuators for each outlet. You can also see the "FULL" output. A "tap" basically contains those resistors (indeed, there's one just out of shot on the same page - a Labgear CM9102/20). Overall, this one is nominally unity gain from input to outputs. I assume the numbers were written on the unit - I can't check because I left it at my last house. I took the "full" output to my workshop and used those taps to give me 4 outlets at my bench - still have those up in the attic...

Note this one allows you to switch the masthead power. Although shown as a switch, I'm pretty sure this was a jumper wire on the PCB that you could pull off and relocate as required. There's no current limit, so a short at the input would pull down the 7812 regulator.

This one is old and basic, but hopefully gives an idea of how they work. Modern ones are similar, but might offer "pass-through" for Sky remote extenders, and better filtering from TETRA and mobiles... I also assume that they use switched-mode power supplies these days, but haven't taken one apart recently to find out.

Hope that's of interest,

Mark
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Old 14th Mar 2017, 8:29 pm   #32
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Default Re: Switching a VHF signal between multiple receivers

Thanks Mark, lots of interesting stuff there to mull over. One thing though, you say:"...it might be worth replacing the previous amplifier with this one, feeding the full output to the room in question, and then use passive "taps" to split the signal out to the receivers. This approach has 2 advantages: no powered device in the room, and it's very easy to add more outputs if the collection grows - just buy another tap."

Without getting my head fully around that; swapping the new DA for old one, what effect it will have on the existing outputs already use, how compatible they are, in number, whatever, I'm not sure exactly what's in the loft.. But anyway, if I did that I would still be stuck with my original problem, how to distribute one feed to say five or six receivers! You mention "taps", physically what form do these take? Can you provide a link? As for there being "no powered device in the room", what benefit is that? My house is full of powered devices in every room.

Anyway, I'll have to dig out my paperwork and see what I've got in the loft as clearly that's part of the equation. I think it's a Labgear unit.
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Old 14th Mar 2017, 10:45 pm   #33
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Default Re: Switching a VHF signal between multiple receivers

We'd need to know a bit more about your existing loft amp to be sure, but assuming it's a standard DA with modest gain, then it'll be a straight swap - all other outputs should work the same as they did before. The only difference being that the cable going to your "showroom" () will plug into the "FULL" output rather than one of the regular outputs. All existing feeds to other rooms will plug into regular outputs.

This means that the cable arriving at your showroom will have a higher signal level than the other rooms. But the "taps" will bring that level back down. If you ever wish to return your "showroom" to normal in future, you'd simply need to pop up in the attic and move the cable from "FULL" to one of the regular outputs. Simple

About the taps, I did provide a couple of links in post #30:

4 output tap: http://cpc.farnell.com/vision/v26-4-...oss/dp/AP02609

8 output tap: http://cpc.farnell.com/vision/v26-8-...oss/dp/AP02614

Easy to hide away on the back of your rack, and no power needed. These ones use F-type connectors rather than Belling Lee, and that's to be recommended, IMHO.

A key spec of a tap is the loss. In the examples above, the 4 way is 12dB and the 8 way is 15dB. Remembering that the "FULL" output from the DA will be higher than the regular outputs, that's good; the outputs of the tap should be at about the same level as the regular outputs from the DA. The exact numbers don't matter hugely here, but in a more "precise" installation, the designer would take into account cable lengths and all the other factors.
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Old 14th Mar 2017, 11:01 pm   #34
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Default Re: Switching a VHF signal between multiple receivers

You can also use a splitter at the end of the full output line. We do a six way that we use with the Televes 7+1 amplifiers. The +1 signifies the full output of 20dB. Add a bit of cable loss and the splitter loss and you are about right. Six way splitters are easier to get than taps and terminators.
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Old 14th Mar 2017, 11:13 pm   #35
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Default Re: Switching a VHF signal between multiple receivers

Is the isolation between outputs as good with a splitter? If a splitter is losing, say, 10dB, whereas a tap is dropping by 20dB or thereabouts...

But yeah - lots of approaches. It's easy to overlook the passive methods, but with the right planning, it can work really well
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