You’ll have worked out for yourself that the VM cable is only really useful if your TV has a Cable tuner (DVB-C/DVB-C2), and then there’s a bunch of detective work to do to get the correct combination of Frequency, Symbol Rate, Network ID and QAM. It’s definitely not plug-n-play LOL. Those who have managed to negotiate the labyrinth of requirements have then reported they got a small selection of BBC and ITV channels, but it’s not the equivalent selection that you’d get from Freeview or Freesat.
Tapping in to the existing aerial system (if it works) is probably your best option. Ideally though you could do with knowing if splitting one of the TV feeds is going to result in lost signal or not before you start clambering about outside running cables, drilling holes and making connections waterproof. Fortunately, there is a way.
Forget about using the TV’s signal strength and signal quality metering. Most are hopelessly optimistic and so give a false impression. A more reliable method is to use aerial signal attenuators. These are a way of reducing the signal by known amounts, and with a little bit of knowledge you can make some fairly accurate assessments not only of whether splitting will work, but also of the general health of your aerial system. Attenuators are cheap too, so you’ve gotta love that; accurate, insightful and cheap.
The amount of signal reduction that happens from splitting is fairly predictable. It’s 4-5dB. This is made up of 3.5-4.0dB for the splitter plus another 0.5-1.0dB for the connection ends. The closest single attenuator value to this is 6dB. That’ll simulate the amount of loss plus provide a 1-2dB safety margin.
- Go to the TV that is fed from the line you’re thinking of splitting and uplug the aerial feed from the back of the set. Connect the 6dB attenuator inline, then plug the whole lot back in to the TV
- The main transmitter for Belfast is Divis. Here’s a list of the channels and their muxes. Take a not of the power levels. The main muxes are 100,000W. The non-essential commercial channels are 50,000W. Then there are some lower power muxes too. Try a channel from each to test reception.
You can check whether or not Divis is your transmitter by using the postcode checker on this page:
https://www.freeview.co.uk/corporate/detailed-transmitter-information
Results time: If you get all the channels you’ve checked then swap out the attenuator for the 10dB version. Repeat the process. If all is well then you know you have a safety margin of at least 5dB, and that should be plenty to withstand the ups and downs in Freeview signal level as well as cope with any minor variation in the sensitivity of one TV’s tuner from another.
A word on fly leads. The shop-bought moulded type costing anything for £2 up to £10 for a couple of metres range from mediocre to utter junk.
They have three problems. First, inadequate shielding means they let in a lot of noise, and they’re quite prone to picking up RFI from poor HDMI cables and dodgy switchmode power supplies. This combination can be enough to kill some or all of your TV reception. Second, they can be very lossy. I’ve measured fly leads where they contribute almost as much signal loss in 2m as I’d expect to get with 20m of decent quality coax. (WF100 loses 1.5dB/10m at TV frequencies).
Third, the nail in the coffin with these moulded fly leads is they’re fragile. The pin to cable connection inside is soldered and it doesn’t take much to break it. The thing is with being encases in plastic you can’t tell unless the break is catastrophic. What happens instead is a dry joint is formed where the break still allows some capacitive coupling. The cable looks like it still works, but the signal level drops because the resistance is much higher.
If you have the opportunity, ditch any store-bought fly leads and make your own. That, or give me a shout about sending you some decent ones. They’re not expensive.
