Home analogue RF network: splitters and combiners

[kan_turk]

Originally Posted by backintime

J, do you think this would be a suitable selective combiner?

Yes but you will want one with 3 single channel inputs - that is no problem - it's just a matter of messaging the seller and he'll tune the unit to your spec - the pic is just a representative example

Rgds

J

[AndiiT]

Hi, whist I would agree with the majority of advice here, speaking from my own experince of doing similar in the past with a Security camera distribution system and multiple VCR's which were "exploited" as modulators, I found using RF pass through it could cause problems with raising the noise floor so I preferred to terminate the rf inputs of the modulators/vcrs and combine the outputs using a multi - way splitter/combiner then amplifying the output of the combiner if required before splitting the signal(s) to be fed to various television recievers.

Regards

Andrew

[terrykc]

Originally Posted by Nuvistor

UHF LMS? I have searched but they come back with various information?

Sorry, Frank, if I've confused you. LMS is Level Measuring Set - Signal Level Meter, if you prefer.

I bet your 'splitter' was one of those 'Y' shaped plastic things. They come in three different flavours: Inductive (but rare, I think), Resistive and Cowboy. Guess which one you bought?

A decent splitter comes in a chunky metal housing like Will's picture in Posr #20.

Originally Posted by backintime

I found a manual online (Though I think we're not allowed to link to manuals?) - it states the RF Output is 75 OHM Unbalanced.

Will, that is the standard output impedance for TV work NOT the output level!

Originally Posted by backintime

J, do you think this would be a suitable selective combiner?

No! If you look slosely at the channel frequencies, they are for 6MHz wide US channels. If you found that on Amazon, the seller only has one and is unlikely to have the skills or equipment to adjust it. It is also expensive and will incur punitive custom charges (mainly due to the greed of the Royal Mail who collect them).

However, I've found another source which is much better at a much lower price and 'made to measure'

"Combine 2 UHF antenna feeds into a SINGLE feed suitable for connection to your TV. This antenna combiner merges the signals from a 1st UHF antenna with a 2nd UHF antenna on the same down lead. It is ideal for those who need to combine new UHF antenna with an existing UHF antenna. TV channels you can define yourself, and they will be tuned according to your needs. Frequencies you can chose: Standard ATSC with channel bandwidth 6MHz (CH 14...CH69), or standard DVB-T (DVB-T2) with channel bandwidth 8MHz (CH 21...CH60). VHF band can be done also. Pictures is only for an example."

However, the website doesn't make it clear if they sell one offs or only in quantity. Could be worth an email?

I came across a large box a few years ago, filled with filters tuned to the Crystal Palace analogue channels 23/36/30/33. These had come from ntl: SMATV sites that had been upgraded to digital. They would be of no use in the CP area today as those channrls now carry digital mixes but would be suitable for use in many other parts of the country. Alas, they will have been disposed of a long time ago but I'm sure a few must remain in some parts of the country, so it could be worth putting out a 'Wanted' post to see if anybody has one, remembering to ask what channels they are tuned to.

They can, in fact, be retuned - but opnly if you have the right equoment, which I doubt.

[Nuvistor]

Yes it was one of those Y shaped plastic things, in the 1970’s I occasionally used them for customers that required an extra set but we had a choice of resistive or inductive. I always used the inductive, we were in a strong signal area and we always good aerials and cable.

I never thought that someone would market the “cowboy” type.

One of the last jobs I had before I left the trade around 1980 was a call to a house that reported poor reception. My new boss used rigger that was to put it bluntly was not very good, I was used to having someone who knew what they were doing.

The house had I think 4 sets fed from a loft aerial, an aerial in the loft was not that bad a problem were we were. It was obviously an aerial problem though, so up in the loft and found 5 coax wires taped together, at least he kept the screen and centres apart. I was not a happy chap at that point.

[kan_turk]

Originally Posted by terrykc

No! If you look slosely at the channel frequencies, they are for 6MHz wide US channels. If you found that on Amazon, the seller only has one and is unlikely to have the skills or equipment to adjust it. It is also expensive and will incur punitive custom charges (mainly due to the greed of the Royal Mail who collect them).

However, I've found another source which is much better at a much lower price and 'made to measure'

"Combine 2 UHF antenna feeds into a SINGLE feed suitable for connection to your TV. This antenna combiner merges the signals from a 1st UHF antenna with a 2nd UHF antenna on the same down lead. It is ideal for those who need to combine new UHF antenna with an existing UHF antenna. TV channels you can define yourself, and they will be tuned according to your needs. Frequencies you can chose: Standard ATSC with channel bandwidth 6MHz (CH 14...CH69), or standard DVB-T (DVB-T2) with channel bandwidth 8MHz (CH 21...CH60). VHF band can be done also. Pictures is only for an example."

However, the website doesn't make it clear if they sell one offs or only in quantity. Could be worth an email?

I came across a large box a few years ago, filled with filters tuned to the Crystal Palace analogue channels 23/36/30/33. These had come from ntl: SMATV sites that had been upgraded to digital. They would be of no use in the CP area today as those channrls now carry digital mixes but would be suitable for use in many other parts of the country. Alas, they will have been disposed of a long time ago but I'm sure a few must remain in some parts of the country, so it could be worth putting out a 'Wanted' post to see if anybody has one, remembering to ask what channels they are tuned to.

They can, in fact, be retuned - but opnly if you have the right equoment, which I doubt.

Hi - that is the same product that I recommended & from the same source - how can one be 'much better' when they are the same - they are happy to sell in one off quantities and I know from personal experience that they are excellent? YES they are suitable

J

[winston_1]

Originally Posted by backintime

Terry - I've just ordered 20 metres of CT100 coax!

Where from? CT100 has been obsolete for over a decade. It was made by one company that is no longer in business. Unless it is old stock (doubtful) you have almost certainly ordered a fake copy.

[terrykc]

Originally Posted by kan_turk

Hi - that is the same product that I recommended & from the same source - how can one be 'much better' when they are the same - they are happy to sell in one off quantities and I know from personal experience that they are excellent? YES they are suitable

J

Why you chose to quote the entire post, I don't know.

As there was no link to the source, I could only respond to what I had in front of me. A search took me to Amazon where a US seller was offering one for ~$35.

The evidence in the photograph clearly shows that it is tuned to American 'A' channels - net European 'E' channels.

The picture says Ch2 ... 69
54 - 806MHz

This is channels A2 - A69 - European channels E2 -E69 cover 47 - 862MHz - quite a difference!

Ch27 548 - 554MHz is Ch. A27. E27 covers 518 - 526MHz, - completely different! In fact, A27 encompasses the top 2MHz of Ch. E30 and the lower 4MHz of E31 so, apart from the reduction in channel width from 8 to 6MHz, completely useless with a modulator pre-tuned to the E channels.

Ch53 644 - 650MHz. E53 covers 726 - 734MHz whilst A53 similarly overlaps E44 and E43 so, again, totally useless in Europe.

As I said, I could only comment on the image as posted but I have no idea where Will found it and you wouldn't have known either.

If the source you used is the Hungarian supplier I linked to, then YES, they are suitable and your recommendation will be useful not only to Will but to anybody else with the same problem.

One other observation: whilst I agree in principle to your suggestion of a filter with three tuned inputs, the type shown in the original post could help Will if he is reluctant to buy a third identical modulator as there is a through loss difference of 3dB between the wideband and single channel inputs. If the VCR uses the broadband input, the additional 3dB could help towards correcting the imbalance between the VCR and modulator outputs.

However, unless we can get a definitive figure for the VCR's output, this is still just guesswork.

[crusher19860138]

In my locality, band 4 (21-38) is fully occupied, so i use 48 upwards which is clear.

[kan_turk]

A couple of observations re post #27

1. My quoting of entire post 23 was due to ineptitude, either on my part or that of my 'smart' phone

2. My focus was solely on addressing OPs question - hence the lack of detail - in hindsight it might have been more useful to others if I had expanded a bit more.

3. Device in post #20 pic was a representative example only as I stated in post #21 - all products from this source are custom tuned to customers requirements

4. Supplier is Slovak and not Hungarian

Anyway we seem to have reached a concensus even if we came 'the long way round'

Hope this clarifies - if any further guidance is required please just ask as I have significant experience with these systems and am glad to be of assistance

rgds to all

J

[terrykc]

1. I sympathise I hate texting at the best of times and no way would I ever attempt to write a post for a forum on a phone - if that makes me a coward, so be it!

2. OK

3. I plead guilty to not spotting your reference to the supplier in post #17 but the picture in post #20 came from the OP with no indication of where it came from. The first copy of that picture that my search found was a one-off on offer from someone in the US.

4. Yes - just a mental aberration on my part as I did, by then, know that the vendor was in Slovakia.

We still need some more info from Will, regarding the VCR and whether he is going to obtain a third modulator to ensure the RF level is the same for all 3 channels.

Something that I meant to do earlier but which got overlooked is to issue a warning about 3-way splitters which could affect Will and anybody else carrying out an exercise like this, so now is as good a time as ever.

Three-way splitters come in two flavours: equal way, as you would expect, but also unequal.

The loss of an equal splitter is 6dB in round figures but an unequal splitter can be thought of as two 2-way splitters in one box, one feeding the other. So one output comes from the first splitter and has a loss of 4dB but the other two come from the second splitter, which is fed from the other output of the first splitter and, therefore, both outputs have a loss of 8dB.

So, if you come across a supply of 3-way splitters, make sure they are right type for your design or you may get some unexpected problems!

Something else to be wary of: a splitter can be used as a tap (and usually is in circumstances such as we are discussing) but a tap cannot be used as a splitter!

A tap, on the other hand, is designed to tap off a small signal from a high level feed, - along a street of houses, for example - so it has a high side loss and a small through loss. At the next tap, the previous through loss added to the cable loss from the previous tap has to be subtracted from the original input level and a value of tap selected to provide the required levels. Fortunately, it is unlikely that anyone here will need to go through this design process which takes a lot more effort than the simple distribution system we are discussing here but beware if you come across a box of what look like splitters at a swap meet or some such. They may be splitters but the could also be taps - or a mixture, so watch out!

[terrykc]

Originally Posted by winston_1

Where from? CT100 has been obsolete for over a decade. It was made by one company that is no longer in business. Unless it is old stock (doubtful) you have almost certainly ordered a fake copy.

It will be interesting to see what arrives! Lots of people refer to CT100 when, in fact, they mean PF100 or WF100.

I thought WF100 was Webro but according to their website, their foam dielectric cable is still CT100!

And here's another vendor who list their WF100 under the heading 'CT 100 Coax Cable' so I think we have to say that it looks as if CT100 has just become another generic name.

[AC/HL]

You need to get friendly with a cable TV Engineer. We used to share a service centre with the Leeds crowd, and they regularly emptied their vans in the skip. It was all F type, and unfortunately I didn't have any interest then, but it's pretty universal now. Every configuration of splitters, attenuators, fly leads, you name it. Anyone who has cancelled their service might just have the odd bit gathering dust round the back of their TV.

[Scimitar]

All of the WF100 we buy comes from Webro. They do not own the CT100 brand to my knowledge. PF100 is Philex junk, how it got certified I will never know, unless it involved plain brown envelopes.

[kan_turk]

There seems to be a proliferation of cables all with the '100' numeral but prefixed by various pairs of letters - I have found performance to vary widely - some OK but some utter rubbish - I can't be bothered trying to sort them out and now only use Webro WF100

J

[terrykc]

My opening comment in post #7 was that is is easier to work in dB than voltage. If anybody doubts this, then try this 'simple' experiment.

Take a look at the attached schematic. It's a lot more complex than what is under discussion but that doesn't matter - the principle is the same.

Ignoring the House, calculate the output voltage at any of the 8-way taps.

Take the amplifier output as 330mV @ 560MHz and use the following loss figures.

8dB pad: 60%; Directional Coupler: 5%; 4-way splitter: 60%; 2-way splitter: 38%; 8-way splitter and taps: 73%.

As these are conservative worst-case figures and good quality components were used, the actual losses compensated for the loss of the relatively short cable runs and can be ignored.

Oh, and did I say that this is a mental arithmetic test?

Give up? Why not try it using dB instead? All the information you require is on the schematic and all you have do is add the figures up (in your head, of course!)

Easy, isn't it?

I was asked to design a new RF distribution system for the museum that would last for 20 years. As I'm now 74, I think it will outlast me by a considerable margin! In any case, I've now moved 150 miles north of the museum so it's unlikely I'll be back there anytime soon.

At the museum, everything in it should ideally cost nothingpence! I had a number of 4-way and 8-way splitters, a smaller number of two-way splitters, a couple of directional couplers and a range of fixed pads. Someone else provided two good quality Teleste amplifiers (it's always nice to have a spare!) and some more splitters which had been rescued from a large former store which had had lots of TV's on display. The only thing the museum had to buy was the cable and connectors, plus the UHF modulator and UHF/VHF diplexor for the headend.

A lot can happen in 20 years and, no matter how well a system is designed, an idiot can destroy it in a day, so I had to keep in mind that this was never going to be the final design, so needed to be easy for anybody to upgrade or expand. I can't think of anything easier than 2 + 2 = 4, so I think I've achieved that.

This is not the original design. Originally the headend was in Gerry Well's bedroom! However, after Gerry's death it had to be moved to the Shed and the House is no longer part of the museum but it is continuing to expand and when I get the information, I'll update the schematic. By the way, if you've never visited the museum, you may be puzzled by references to 'The Shed'. For 'shed' think Swiss Chalet and you'll have a better idea. From memory, when I did the original survey I measured it a 10m deep and 11m wide.

As the forum shrinks large files such as this, I suggest you open it in Paint (or another program of your choice) and increase the size to 200% (or more, if you prefer) - that will make it much easier to read.

[terrykc]

It was a right pain converting all those losses from dB to percentages but, if you tried it, you should have ended up with something like 330*0.4*0.95*0.4*0.62*0.27*0.27=2.27mV which is equal to +7.12dBmV.

This compares very favourably with the figure on the schematic of +7.9dBmV which is the actual level measured during commissioning.