DAB in car. Combination or dedicated antenna or splitter?

JonathanEllis

Standard Member
I recently fitted a DAB receiver in my car. It came with an internal antenna which is basically a wire dipole stuck to the inside of the windscreen with an RF amplifier built into the body of the antenna. The amplifier is powered by the receiver. This works but the reception is patchy in London and drops out completely when I leave London.

I am considering installing an external whip antenna. I see you can get combination FM and DAB antennae. Do these have two elements inside the single whip, both properly matched to their bands or is it one element that is a good match to one service but a poor match to the other? Are these combined antennae worth buying? I would prefer not to drill another hole for a second antenna.

I have also seen a splitter (Autoleads 06-536) which plugs into the existing FM antenna and has two cables going to the receiver, one for FM and another for DAB. I presume this box is a diplexer with an RF amplifier built in. This is probably the cheapest option but are these boxes any good? I imagine this will not give as good a signal as a dedicated external DAB antenna but will it be better than the amplified stick-on antenna?

Thank you for your consideration.
 

Sonic67

Banned
Try and avoid the aerials stuck to the glass. The guy who did mine hid the DAB aerial inside my cars rear bumper.
 

Sonic67

Banned
I guess this would only work for a plastic bumper.:) The fact that I remember cars with metal bumpers dates me!

Mobile aerials (such as those on cars) should be oriented as near vertical as possible so they receive equally well from all directions. I am guessing your aerial hidden in the bumper is horizontal? I can imagine a few problems with this:

  • Reception from the sides of the car might be poor as a whip antenna is not good at receiving signals end-on.
  • I believe DAB transmissions are vertically polarised to match the vertical orientation of the car antenna. A horizontal antenna will not receive a vertically polarised signal very well.
  • The vertical metal surface of the boot or tailgate could act as a reflector, blocking out signal from the front of the car. The metal sides of the car might block the signal further. Essentially the antenna's 'view' to the transmitter is restricted by any metal objects in the way.
Whip aerials are designed to go on the roof and use the roof as a ground-plane. The ground plane acts like a mirror so the whip forms half of a dipole and the other half of the dipole is formed by the image of the whip in the mirror (roof). Basically a quarter-wave whip mounted on a ground-plane behaves like a half-wave dipole mounted in free space - although probably a little less efficiently.

I would welcome any thoughts on this. Although I have a degree in electronic engineering it has been many years and I don't remember a whole lot of antenna theory very well.

My car is a Mini (BMW) and the rear bumper is plastic. It is also horizontal but it does also work very well. Probably a lot better than the stick on mini ones that go on the glass. It is also bigger than those. The only time I've ever had problems picking anything up was driving through some back roads of Wales where the only thing around was hills and sheep.

Your points are correct in that the aerial should be vertical but then I've found DAB to do its best with whatever you give it. Years ago I first got a DAB signal with a TV aerial before I got a proper DAB aerial (that was with a DAB PCI card for a PC).

I didn't want another aerial on the roof as I thought it would look odd, create wind noise, and I wanted to keep it as a 'stealth' install.
 

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