Whether you're planning a new distribution from scratch, trying to work out why an existing one isn't performing as well as you hoped or just contemplating an extension to the existing set-up, sooner or later you are going to need to know how much of your precious signal is being lost in all that cabling.
For the answer, go to the Tech Tools section of the Technical Library and download the spreadsheet titled VRAT_Cable_Calc.xls.
This should work in any version of Excel from 97 on and has also been checked in both Open Office and Libre Office.
With this simple tool you can calculate the loss of any cable at any frequency over any distance you like.
Most of the cells in the table are locked to prevent accidental overwriting of the formulae. The exceptions are coloured green and point to the areas you can edit to customise the tool for your own particular purpose.
To use the tool you will need to know what type of cable you want to measure. If you are in the planning stage I would suggest that you specify CT100 or similar but if the cable already exists you will need to know what it is. If you don't know and it is not marked on the outer sheath (usually once per metre), get a micrometer or vernier gauge and measure the diameter of the centre conductor and the dielectric. You should then be able to identify the cable from data tables that you either have or find on the internet to a reasonable degree of accuracy.
Now look up the manufacturer's data and find the worst case attenuation per 100m at 862 or 865MHz. Beware of American manufacturers who state attenuation per 100 feet rather than metres! The 100m value is usually also there if you look! NB: I've included data for most popular cable types in the box on the right of the graph.
862MHz is the top edge of Band V and it will not be unreasonable for you to ask why as you have no intention of running anything at such a high frequency - you may not be interested in UHF at all! The answer is that this should be the design frequency of any UHF/VHF network, irrespective of the channels you actually want to use. However, with this one item of information, we can calculate the loss at any frequency we want.
On the spreadsheet, enter the cable type and the loss at 862MHz in the appropriate cells. If you want to use this on more than one type of cable, right-click on the sheet tab and select Move/Copy (sheet) to create as many duplicate sheets as you need. (In Excel, remember to tick the 'Make a copy' box!)
A number of frequencies are highlighted in green. Some of these have been chosen to point to channels B1, B9 and E33 - the Crystal Palace/Croydon allocations - but for the purposes of this example I've moved up country a little to Anglia land and included the B3, B11 & E55 Talconeston/Mendlesham allocations. Feel free to use any frequencies that you want.
The original spreadsheet uses cable length increments of 5m but you might prefer smaller units. No problem - I've changed them to 2 metre intervals here.
Let us say that your overall cable length is 20 metres. You can see immediately that BBC2 will be 4.6dB down, ITV 2.4dB and BBC1 1.3dB.
This assumes, of course, that all three signals were at the same level in the first place. In a practical network the individual channel levels can be adjusted to average out the imbalance across the band for both short and long distances.
This can be done by reducing the lower channels by half the difference between them and the highest channel. In this example, B11 should be 1.1dB below E55 and B3 1.7dB below the E55 level. However, this does get more than a little hypothetical as even a good quality Level Measuring set is unlikly to have better than ±0.5dB accuracy!