Dulwich Museum RF Distribution System

I raised this subject in this post and felt it might be of interest to add further details of the system.

For reference :-

My brief was to design a system that will last 20 - 25 years and I hope I've achieved this. The rules governing expansion are simple - there are only two for the different ends of the network - so it should be possible for it to be successfully expanded long after I've gone - in fact, having now moved 150 miles away, I doubt I'll ever see it again!

As with any such project, money (or, rather, the lack of it) was a primary consideration. I had a quantity of suitable splitters and someone managed to produce a pair of high quality Teleste RF amplifiers capable of driving at much higher levels than most of the stuff normally available so there is plenty of gain to permit one amplifier to drive the entire network. Having a second almost identical amplifier ensures that, in the event of a failure, the service can be restored very quickly.

The amplifiers are designed to accept a reverse path module - a second amplifier capable of carrying signals up to, usually, 30MHz, to enable set top boxes to communicate with headend equipment but these amplifiers were not so equipped and, most important of all, were not fitted with diplex filters to split the two streams as, normally with such systems, the forward path starts at Ch. E2 - 47.25MHz - and the all important UK ch. B1 would have been blocked!

This is the Mark II design. The first was centred on one of the racks in Gerry's bedroom that housed the original BBC and IBA Standards Converters, now rendered obsolete by the Aurora because, when they were acquired it was the only bit of space available!

Since Gerry's death, however, the headend has been relocated to the Shed, with any feeds still required in the House being back fed along the cable installed under the edge of the Garden.

Some of the legends on the schematic may be a bit puzzling to anybody who doesn't know the museum layout. HOUSE is self explanatory, the Bedroom being Gerry's former ground floor bedroom and the Front Room being just that.

If SHED, however, conjures up a vision of something about 8' x 10', you couldn't be further from the truth! Think Swiss Chalet and you will get a much better idea! It is sometime since I measured it but it is of the order of 10 metres square.

An important consideration was the square wave RF output of the Auroras. Not normally a problem, even for those folk who use two together, but the thought of what might happen when all those harmonics hit the input of a high gain amplifier wasn't nice! However, I was able to source a diplex filter intended for a more recent cable network with a 65/85MHz split with a very sharp roll off which took care of the harmonics of 45MHz and a decent VHF/UHF diplex filter which would deal with the harmonics of 194.75MHz sufficiently to keep everything out of the UHF band, so problem solved!

The design levels at any outlet are a maximum of 3mV - more than that is likely to cause overloading - and a minimum of 1mV below which low gain sets will suffer whilst sets with reserves of gain a likely to display increasingly noisier pictures as the gain increases. By setting the absolute limits at 1mV and 3mV  (0dBmV to +10dBmV) with a mid point of 2mV (+6dBmV) every set in the museum should be happy. 

Amplifier tilt is set, nominally, at 3dB between 40 and 550MHz. This is the inverse of the response of ~20m of CT1oo type cable. In other words, after 20m, the levels will be flat across the band. Beyond this, the higher frequencies will be increasingly attenuated so that after ~40m the initial 3dB tilt will become a 3dB slope. Due to the relatively short cable runs, all the sets in the shed should see a slightly higher level at UHF which will benefit those early dual standard receivers with valve UHF tuners.

The same tilt is equivalent to ~30m of RG11 which results in the signals entering the house being flat across the band, as was intended. Actual levels, after commissioning, are shown on the schematic.

The range of overall cable lengths in the Shed does not vary by much. There is a greater variation in the house but  it isn't particularly significant, so the rest of the signal losses are flat loss in the passive devices. There will be some increase in losses at UHF but not sufficient to be of any concern and, in any case, the loss figures I've used are worst case, so actual losses will be lower than calculated. All of the taps and splitters are self terminating - there are no high level outputs - so it is a simply matter to add up all the flat loss figures for any required additions and, provided they add up to the specified figures: 40 ±1db in the House or 43 ±1dB in the Shed. the output levels are guaranteed to be correct!

The loss figures used are 4dB for a two way splitter, 8dB for a four way tap or splitter and 11dB for an 8-way tap or splitter. All of the taps at present are 8-way because it happens to suit the density of feeds required at those locations but the same 8 feeds could be distributed over a much larger area if required. By replacing the 8-way with a 2-way splitter feeding another two 2-way splitters each, in turn, feeding a pair of 2-way splitters would still give 8 outputs but the cabling could be arranged to cover a much larger area or a number of small rooms. The flat loss is nominally 12dB so very close to the 11dB figure for the 8-way this arrangement replaces.

I must admit to one reservation I have about the future of this network and it will affect others too. It seems to be virtually impossible to buy 865MHz or 1GHz splitters these days. This is because a lot of domestic installations send FM, DAB and Satellite signals down a single cable to wall outlets equipped with triplex-ed outputs. Unfortunately, designing a tap or splitter to handle signals up to 2.4GHz isn't easy and losses increase. Moreover, these increased losses also affect the lower frequencies and the spec of some of them that I've looked at are horrendous! If someone finds a job lot of these lossy devices and attempts to use them at Dulwich, I can see a lot of problems arising in the future.

That doesn't mean that the other types are not available - the cable TV industry uses them in large numbers but they are not easy to find in small quantities. The museum has been lucky so far as, apart from one small order that I know of, all of the taps and splitter have been salvaged or scrounged from either cable TV companies or, in the case of the two amplifiers, they and a number of passive devices came from a distribution system in a large showroom that was being stripped out.