Ubique
Member
- Messages
- 2,587
- Location
- East Midlands
Nothing out of the ordinary up until today - an L405 with DAB reception issues, not the usual 'it drops out every time I'm in a valley or up in the hills - which is a 'feature' of DAB across all vehicles to some extent.
This made me curious....
A bit of background. DAB in the UK is broadcast in band III, which is 174-250MHz, there are several 'multiplexes' (MUX) that combine several stations into one 'channel', these channels are broadcast at varying output powers, essentially governed by co-channel interference and how much the media companies wish to pay...
The strongest being 11D, 12A (operated by Arquiva) and 12B operated by the BBC. The lower power ones tend to be in the 10's - which is where the problem lies.
The 'Absolute' range of channels are run by a cheapskate media company, so all their output which extends far beyond 'Absolute' is all problematic in areas with lots of contour lines on the maps, except this tended to fade in and out quite quickly, every few seconds, sometimes it'd be OK in a strong signal area, but even then you'd think 'did it go quiet then?'
So time to do a bit of digging....
Here's the antenna setup
Ignore the crayoning for now, the antenna & amplifier we're interested in is 'B' which is DAB band III & FM, the RG174 co-ax connects to a DAB tuner module, which then sends it's output on MOST (optical fibre), the FM component connects directly to the audio head unit.
Time to get a baseline for received signals, the spikes on the RF Spectrum Analyser are DAB MUX channels, this was in 'free air' away from the vehicle.
This was with the RF SA antenna jammed up under the DAB antenna
So far, normal.
This is with the ignition switched on - every few seconds, there is a spike at ~250MHz, at a level higher than the DAB reception, especially the lower MUX channel
There is also an increase in background noise floor from approx 200MHz upwards - but not always....
So what's causing it?
Well, there's a rear camera for the dashcam installation, the cable is fed into the liftgate using the standard cable routes, the blue crayoning shows the DAB & FM antenna cable route, the green shows the dashcam cable route, dashed where it disappears inside the liftgate.
Disconnecting the camera causes the 250MHz spikes to cease, reception return to usual and no further problems.
I'm reasonably certain the screening on the camera cable is poor / incomplete / damaged, causing 'leakage' which is flooding the antenna - so the plan is to cover it in copper foil tape with a drain wire connected to a ground point on the body.
This made me curious....
A bit of background. DAB in the UK is broadcast in band III, which is 174-250MHz, there are several 'multiplexes' (MUX) that combine several stations into one 'channel', these channels are broadcast at varying output powers, essentially governed by co-channel interference and how much the media companies wish to pay...
The strongest being 11D, 12A (operated by Arquiva) and 12B operated by the BBC. The lower power ones tend to be in the 10's - which is where the problem lies.
The 'Absolute' range of channels are run by a cheapskate media company, so all their output which extends far beyond 'Absolute' is all problematic in areas with lots of contour lines on the maps, except this tended to fade in and out quite quickly, every few seconds, sometimes it'd be OK in a strong signal area, but even then you'd think 'did it go quiet then?'
So time to do a bit of digging....
Here's the antenna setup
Ignore the crayoning for now, the antenna & amplifier we're interested in is 'B' which is DAB band III & FM, the RG174 co-ax connects to a DAB tuner module, which then sends it's output on MOST (optical fibre), the FM component connects directly to the audio head unit.
Time to get a baseline for received signals, the spikes on the RF Spectrum Analyser are DAB MUX channels, this was in 'free air' away from the vehicle.
This was with the RF SA antenna jammed up under the DAB antenna
So far, normal.
This is with the ignition switched on - every few seconds, there is a spike at ~250MHz, at a level higher than the DAB reception, especially the lower MUX channel
There is also an increase in background noise floor from approx 200MHz upwards - but not always....
So what's causing it?
Well, there's a rear camera for the dashcam installation, the cable is fed into the liftgate using the standard cable routes, the blue crayoning shows the DAB & FM antenna cable route, the green shows the dashcam cable route, dashed where it disappears inside the liftgate.
Disconnecting the camera causes the 250MHz spikes to cease, reception return to usual and no further problems.
I'm reasonably certain the screening on the camera cable is poor / incomplete / damaged, causing 'leakage' which is flooding the antenna - so the plan is to cover it in copper foil tape with a drain wire connected to a ground point on the body.
