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Old 14th Mar 2017, 10:15 am   #1
stevehertz
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Default Switching a VHF signal between multiple receivers

I want to switch a VHF radio aerial downlead/plug between a number of receivers. Not being 100% sure of what hidden mysteries VHV hides from me, my idea is to have a multi way wafer switch mounted in a metal box and just switch the 'hot' inner wire of the coax feeder to the box's various output sockets that are in turn connected to each receiver, leaving the coax outer screen permanently made via the metal box. I'd go for a gold contacts, low current switch. Nowt wrong with that is there? Anything else to watch out for?
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Old 14th Mar 2017, 10:23 am   #2
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Default Re: Switching a VHF signal between multiple receivers

What about using an amplifier with the outputs you req no switching needed, just a thought , Mick.
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Old 14th Mar 2017, 10:39 am   #3
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Default Re: Switching a VHF signal between multiple receivers

Good point Mick, I can buy a 6-way mains powered UHF/VHF distribution amp for probably not much more than the cost of building a switch box, and I wouldn't have to operate a switch, the (amplified) VHF signal would always be present at every receiver..
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Old 14th Mar 2017, 11:00 am   #4
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Default Re: Switching a VHF signal between multiple receivers

Whenever I've had the need to re-direct VHF signals manually, I've always used a coaxial switch or coaxial relays. Just make sure that the characteristic impedance (Zo) is appropriate.
O.K., so you don't get the advantages that come with a multi-output dist. amp., but then you don't run the risk of possible overloads, intermods., etc.

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Old 14th Mar 2017, 11:03 am   #5
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Default Re: Switching a VHF signal between multiple receivers

as long as the length of conductor not in coax is very short compared to a wavelength (3 metres) you don't need to worry about impedance matching.

Gold isn't important. wiping contacts will do nicely

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Old 14th Mar 2017, 11:07 am   #6
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Default Re: Switching a VHF signal between multiple receivers

A distribution amp is the way to do that.
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Old 14th Mar 2017, 11:52 am   #7
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Default Re: Switching a VHF signal between multiple receivers

Hmm. I'd be inclined to give thought to the possibility of reflected signals degrading overall system performance if any of the outlets were left unterminated.
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Old 14th Mar 2017, 12:24 pm   #8
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Default Re: Switching a VHF signal between multiple receivers

Dist amps usually have around 10-20dB of attenuation at each output, so reflections shouldn't cause a big problem. I hope not, as I have 2 here (1 for TV, 1 for VHF) with several unused outlets around the house

I agree that DA rather than switch is the best way
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Old 14th Mar 2017, 12:27 pm   #9
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Default Re: Switching a VHF signal between multiple receivers

What's wrong with using a switch....

Lawrence.
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Old 14th Mar 2017, 12:41 pm   #10
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Default Re: Switching a VHF signal between multiple receivers

Well, for a start you've got to build it. Could you buy the parts (including a box and connectors) for less than this: https://www.amazon.co.uk/SLx-27820BM...dp_ob_title_ce - possibly, but what price time? Also, I don't see any advantage of switching away the signals from the unused receivers. A DA is a fit-and-forget solution.
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Old 14th Mar 2017, 12:47 pm   #11
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Default Re: Switching a VHF signal between multiple receivers

The one in the link looks like a TV booster.

Lawrence.
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Old 14th Mar 2017, 12:54 pm   #12
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Default Re: Switching a VHF signal between multiple receivers

According to the manual - https://www.manualslib.com/manual/74...-27820bmg.html - it does FM and DAB too... these things are generally broadband 40-700MHz
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Old 14th Mar 2017, 1:08 pm   #13
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Default Re: Switching a VHF signal between multiple receivers

Just seems more to go wrong to me.

2 pole X way switch etc, could also be used to switch an AM antenna as well.

Lawrence.
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Old 14th Mar 2017, 1:27 pm   #14
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Default Re: Switching a VHF signal between multiple receivers

The only place a distribution amp loses to a switch is in overload problems.

Are there any strong signal sources near you?

The wider the bandwidth of an amplifier, the wider the bandwidth it could be susceptible to. Wideband is convenient, but sometimes has problems

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Old 14th Mar 2017, 1:28 pm   #15
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Default Re: Switching a VHF signal between multiple receivers

Quote:
Originally Posted by Nymrod121 View Post
Hmm. I'd be inclined to give thought to the possibility of reflected signals degrading overall system performance if any of the outlets were left unterminated.
Hmm. Due to the complexities of that, and me not understanding a jot of it -
or more importantly, how to do anything about it! - I'd be inclined to NOT think about reflected signals But I know from years of practical experience that this type of thing (wrt aerials) is often more an academic issue 'for discussion and debate thereof' rather than a real practical problem.

Anyway, I've pretty much decided on getting a powered VHF distribution amp, the price of which is not much more than building a switching unit with the associated components for a six-way unit. I can get one for £20. And as I have already said, no need to switch anything, signals are all there, ready to go.
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Old 14th Mar 2017, 1:42 pm   #16
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Default Re: Switching a VHF signal between multiple receivers

Quote:
Originally Posted by mhennessy View Post
Dist amps usually have around 10-20dB of attenuation at each output, so reflections shouldn't cause a big problem. I hope not, as I have 2 here (1 for TV, 1 for VHF) with several unused outlets around the house
Mark, I realise you know your stuff on this subject, but I've never owned or seen a domestic distribution amp that claimed to provide attenuation at its outputs. The one I'm looking at quotes "The SLx range of aerial amplifiers from Philex is designed to improve picture and sound quality by amplifying weak UHF, VHF, DAB and FM radio signals and distributing the signal to multiple locations around the home." It quotes a figure of 12dB gain per split: http://www.philex.com/product/slx-go...-compatible-2/
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Old 14th Mar 2017, 1:53 pm   #17
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Default Re: Switching a VHF signal between multiple receivers

I've been using a six way distribution amp for years. It's fed from a six element yagi pointed at a transmitter about 15 miles away. There are generally at least two FM tuners connected to it.

Until I read this thread I hadn't realised that I should be having lots of problems with it!
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Old 14th Mar 2017, 2:22 pm   #18
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Default Re: Switching a VHF signal between multiple receivers

Quote:
Originally Posted by stevehertz View Post
Mark, I realise you know your stuff on this subject, but I've never owned or seen a domestic distribution amp that claimed to provide attenuation at its outputs. The one I'm looking at quotes "The SLx range of aerial amplifiers from Philex is designed to improve picture and sound quality by amplifying weak UHF, VHF, DAB and FM radio signals and distributing the signal to multiple locations around the home." It quotes a figure of 12dB gain per split: http://www.philex.com/product/slx-go...-compatible-2/
The signal is first amplified, and then attenuated by an appropriate amount.

Overall, the net gain though the whole system is what you're seeing on the spec sheet. But in practice, the amp has more gain, and there is attenuation between the output of the amplifier and each output socket. This attenuation gives good isolation between the outputs, minimising the risk of problems from unterminated (unused) outputs and other such problems.

Overall, there is normally at least 3dB net gain to help with connector loss. More is common. Some are adjustable.

Some DAs give you access to the "raw" amplifier output, and the idea there is you can distribute the signal at high level to wherever you want, and get at it via "taps" like this: https://www.blake-uk.com/218-thickbo...proception.jpg - note that the high-level signal passes through with minimal loss on its way to the next tap (or a 75 ohm terminator if it's the last one), and there are 2 outputs that are -8dB compared to the input in this example.

This is commonly used for larger buildings, where it saves cable because you don't need to run all the wires back to a central location. You choose different levels of attenuation (more attenuation nearer the amp, less at the end of the run) so the outlets have nominally identical signal levels. Works well.
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Old 14th Mar 2017, 2:26 pm   #19
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Default Re: Switching a VHF signal between multiple receivers

The other thing to bear in mind is that bottom of the range tat will have much higher noise figures and other issues, than something of reasonable manufacture.
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Old 14th Mar 2017, 4:10 pm   #20
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Default Re: Switching a VHF signal between multiple receivers

Here http://aerialsandtv.com/ampsandsplitters.html they even spotted that I ordered the 'wrong' diplexer with respect to what I ordered a few years before, I hadn't, it was an odd change to my system. Perfect service and advice.
 
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