... 8 way splitters often have a loss of 11dB which is about right.
The lowest possible loss of an 8-way splitter is indeed 11dB and these are probably the most commonly seen.
An easy way to remember the losses of widely used splitters is to consider them as multiple 2-way splitters.
A 2-way splitter loses, nominally, half the power on each leg which, in an ideal world, would be 3dB.
In the real world, however, there are other physical losses to be taken into account and the loss should always be reckoned as 4dB, to be on the safe side. Some 2-way splitters are actually marked 3.7dB - although a detailed specification will almost certainly include a ±0.5dB tolerance!
It would be a tall order maintaining a response level to within 0.1dB from 5MHz to 865 or even 1000MHz, anyway, so sticking to round figures with a little room for error is much better - it also helps with mental calculations too!
If we now take another pair of splitters and put one on each output of the original we can create a 4-way splitter and it will be pretty obvious that the notional loss through them is 8dB.
By the time we move on to the next logical step of creating an 8-way splitter, the cumulative allowances for losses have reached a level whereby only a further 3dB allowance is needed for this extra stage and that is where the 11dB figure comes from.
Note that all the splitters we have considered are self-terminating: once all the input power has been shared between the outlets there is none left over to feed further splitters so we've literally reached the end of the line.
For just about anything that members of this forum will get involved with, whether it is just to distribute RF to a few sets in the workshop or a more grandiose affair with display areas, etc., self terminating splitters should be fine - even the much larger Dulwich Museum distribution system uses them! (As all the splitters that have so far been collected together for the project are of this type, that is a very good thing!)
If you consider a Cable TV network for a moment, however, things are not so simple. A single street cabinet in a standard UK network will typically supply up to 48 homes. The closest might be less than 5m away whilst the cable run to the most distant home could be over a 100m - a range of cable lengths and signal losses of 20:1 ...!
For this we need splitters with a range of losses and, most important of all, having an output port to feed additional splitters. This type of splitter is usually referred to as a tap because it 'taps off' a portion of the main signal.
Keeping to the 8 output variety, the range of taps starts at 14dB and goes up in 3dB steps to, typically, to 35dB but in modern systems which also communicate in the reverse direction, such a fixed attenuation range is excessive and 26dB taps are usually the highest value encountered.
A simple law of physics states that energy may neither be created, nor may it be destroyed. In the case of a tap, it will be obvious (I hope) that a tap with a high tap loss extracts far less energy from the input signal that one that has a low tap loss. This, in turn determines how much signal is left over for onward transmission to the next tap ...
In the days before computer spread sheets were widespread, this could make the task of designing the tap chain to get the right combination of signal levels a rather complex affair!
Back in the days of my apprenticeship, these Plessey manufactured sets were all around me. The 9a51 and 9a61, a similar circuit with revised layout to "improve" the panel burnups.
I would dearly love to get hold of one of these models.
