Planning a simple RF Distribution System

[ppppenguin]

I think you've misunderstood. Terry said "Don't" as an answer to your question 1. Then he said use inductive splitters. Always use inductive splitters. All decent commercial ones are inductive, you'll rarely if ever see resistive ones.

Unless you're using 10s of metres of cables then cable lengths are irrelevant. Certainly in Band I.

[Terry]

Terry said don't use inductive splitters , I thought they were all resistive.

Oh, isn't the panto season over yet?

Oh no I didn't!

The 'No' referred to your query about resistive splitters. If you re-read what I wrote, you will see that I specifically told you to use inductive splitters!

On post 7 terry has put some links to ebay sellers selling splitters , are these resistive or inductive ?

If I ever post a link to a resistive splitter, feel free to have me taken out and shot!

You say your cable is double screened - do you mean two woven screens or one woven and one aluminium foil? If it is RG59 it it most likely the latter but, like RG6, it is a type. The cable type should be clearly marked or embossed on the outer sheath - tell us what it says.

Is the inner dielectric semi air-spaced or foam? If it is solid it is (a) not RG59 and (b) useless in your application. How accurate is your 6.5mm measurement? It is worthless, anyway, because a cable with two woven screens will be fatter than one with only a single one.

The only real way requires a micrometer or similar to get accurate measurements of (a) the centre conductor diameter and (b) the outside diameter of the inner dielectric. Then compare them with a range of cable specifications. Something else which it important to note is that the ratio between these two measurements determine the characteristic impedance and will be different for 50Ω & 75Ω cables or any other impedance cable.

If you can conclusively prove that it is RG59, the loss per 100m is 5.6dB @45MHz (Channel B1) and 12.3dB @ 214.75MHz (channel B13). You can use these figure to calculate the actual loss of whatever length of cable you actually use.

Comparable figures for CT100 types are 4.5dB & 9.9dB. per 100m

[shortcircuit]

Yes I did misunderstand terry`s reply , sorry about that.
I thought all splitters were resistive , as I said I don't know much about this subject but I`m learning now.
I have some of my cable here and it has written on it " belden 43187 75ohm double braid broadcast video coax ".
I have measured the centre conductor and outside diameter of the dielectric with a digital caliper and they are 0.6mm & 3.75mm.
The centre conductor dielectric is solid transparent plastic.
Is this cable good enough to use ?

Robin

[Terry]

The centre conductor dielectric is solid transparent plastic.
Is this cable good enough to use?

No, Robin, as both Jeffrey and I have already said, a solid dielectric is useless at RF. It is, as it says, intended for carrying video.

As your cable lengths are short, you say, a 100m drum of WF/PF100 is a bit of overkill so you might want to look for someone who sells it in smaller quantities better suited to you but price up a full drum first as the short lengths will command a premium price per metre, so be careful! I haven't looked for some time but I'm sure that B&Q used to sell smaller amounts or you might have local hardware shop like one we have here who sells cable by the metre!

[shortcircuit]

Thank you terry , I value your opinion.

Robin

[ppppenguin]

Terry, can you please confirm that this is OK:

Available in 25, 50 and 100m lengths.
https://www.screwfix.com/p/nexans-nx100-...lack/647fk

[Terry]

Robin, something you may find useful, depending on how you display your collection is that it might make things easier by doing the splitting in a different way. For example, if you have half on one wall and the other half on a different wall or shelf, a different arrangement to a single 8-way tap might be better.

Taps and Splitters

All splitters can be used as taps but not all taps can be used as splitters! However, this latter distinction only applies to large networks such as operated by Virgin Media and there is normally no need for them in the preservation world. Even the complex system at the Dulwich Museum is built entirely using splitters. A tap simply 'taps off' the small signal used to feed one receiver and it is not normal to document the wiring at this level, so a simple symbol can be used but the wiring between splitters normally needs to be accurately documented for planning, installation, maintenance, modification and expansion.

So, the pairs of symbols on the left of the drawing below are physically the same unit.

Options

On the right are three ways of achieving the 8-way split and the second one would be better for the scenario I mentioned above.  The last one shows how a number of feeds can be connected over a large physical area and is unlikely to appeal to you.

The figures in the symbol are the maximum loss of each unit, rounded up to the next dB and these figures should be added together. Typical specified maximum losses are 3.7dB, 7.5dB and 10.5dB. In the examples, the second and third have losses totalling 12dB whilst the first is 1db better at 11dB but, essentially, all three arrangements achieve the same result.

[Terry]

Terry, can you please confirm that this is OK:

I haven't a clue, Jeffrey.

The only real way would be to compare specifications - something that Screwfix are very bad for but when I googled it, I came up with a Turkish company that describe NX100 as: "Fire alarm systems emergency lighting power safety circuits when installed in conduits or protected trunking."!

Now, on their home page they do have a link to a section on coax cables but you have to register to get the catalogue, so I've left it there for the moment.

It does seem a little odd, though!

http://www.nexans.com.tr/eservice/Turkey...bles_.html

You can expand the Screwfix picture, but not by very much but it does look like a foamed inner dielectric and a copper foil screen

[Terry]

B&Q say it has a CAI Benchmark - might they know a bit more?

https://www.diy.com/departments/nexans-n...932_BQ.prd

Ah just found this:

https://www.cai.org.uk/index.php/contact...ables/file

[ppppenguin]

Terry, thanks for researching the cables. Screwfix is very convenient for me and may well be for others. The prices on their xx100 cable are reasonable.