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The importance of a good car antenna for DAB

By Art Grainger
Posted 31 March 2015, 12.24pm edt
Robert Basic
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For the past few days I've been driving a Mokka for my work.

The stereo is factory fitted and includes DAB, which in itself works quite well. Unfortunately, those clever people at Vauxhall have neglected the antenna again, leaving a spikey thing that sits at the rear of the roof at an angle of about 40 degrees.

Needless to say FM works as good as can be expected (some would say it works brilliantly but after years of listening to DAB in the car, I get annoyed when FM signals hiss and flutter because it's really noticeable).

DAB on the other hand works for more than 99% of the time but in the areas that I know are black-spots for FM, DAB also takes a dip. The experience, though, is a wee bit different to what I've had before with factory fitted DAB stereos in Vauxhalls. Instead of a total loss of signal for a few moments, I notice that the audio gets quieter, which is making me think that the aerial is getting some signal amplification. Whilst this does help, if the signal is too weak to be picked up by the non-DAB aerial, the stereo gives up and searches for a new signal. This takes 2 or 3 seconds until the signal comes back. It doesn't happen too often (I'm glad of that) but it still falls short of what I expect to get from a proper DAB antenna, sitting totally upright (not at a angle), for which I do get rock-solid reception, even in those black-spots.

So, compared to other Vauxhall cars that have had a DAB stereo fitted, with a non-DAB antenna, this is an improvement - but it still needs to be better. I have fitted a Pure stereo, bought form Halfords for £100, with a proper DAB antenna - and I don't get any signal problems with DAB. The car manufacturers need to wise-up.

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Comments

PRO2 years, 7 months ago

With DAB+, there is some nicer handling of error correction, and it appears that some of that learning might have made It over to this unit, so that it ducks the volume where otherwise you'd get the familiar - and highly annoying - crunch and squawk of a dying signal.

I would entirely agree with you that the antenna is the most important thing. I installed a glass-mounted external antenna in my last car, and it worked flawlessly if pointed straight up. If raked back to the angle of the FM antenna, it lost signal quite alarmingly.

A senior DAB radio engineer told me that although DAB is broadcast with vertical polarisation only, this didn't matter because the reality was that it was bouncing around all over the place and your radio got a mixed polarised signal anyway. My experience, while nowhere near as long as this senior engineer, leads me to believe, on an evidence-based criteria, that this is bullshit.

2 years, 7 months ago

Slanted directional aerials, stubby aerials near the rear of a car roof & back screen indoor antenna are hopeless for DAB reception with its vertical polarised signal characteristics (signals falling like rain to the ground).
A sexless looking vertical aerial on the car roof will give robust signal well beyond the official DAB reception boundaries with signal pickup/robustness superior to FM reception in some hilly areas. ie driving through the Southern Uplands in Scotland & through the Sperrin Mountains in Northern Ireland with very little ''bubblin' purridge'' to endure!

Actually, DAB reception in cars is not as bad as reported in the press! There will be a lot of complainers using inadequate aerials like they did in the past, listening to FM stereo while enduring the hiss & now listening to DAB, experiencing a muting tuner every time DAB reception drops below its minimum signal threshold.
The antenna is the cheapest & most important piece of a sound system, yet some manufacturers & customers underrate its value....

2 years, 7 months ago

Indeed James.

DAB signals do bounce around off buildings - and even the mountains and hills that Willie refers to, which helps to "build" the little bits of signal needed for robust reception. This comes especially useful when Single Frequency Networks are in use - but even in places where there is just one DAB transmitter for the multiplex (e.g. Inverness), you still get a considerable improvement over FM because the DAB signal is bouncing around off objects - and with DAB being DIGITAL, it overcomes the problems of multipath signals that can be detrimental to FM reception. This is why DAB is working so well in the highlands of Scotland. Mountains hamper FM - but they do wonders for DAB.

FM stations try to overcome the problems of drop-out, multipath etc by having a mix of horizontal and vertical polarity. In days of old, FM was horizontal only, on the basis that listeners would probably use a roof aerial to pick up the station(s), as they did for TV. However, the advent of portable FM radios and more especially car stereos meant that vertical polarity was also necessary.

As the years progressed it became the norm for stations to broadcast with horizontal polarity at a certain power level necessary to cover their intended service area (on the basis of a roof aerial being used), as well as vertical polarity at a higher power level (typically 3 or 4 times more than horizontal), so that the signal could be received by portable radios and car stereos, with their near-vertical antennas.

As the years progressed again, it was discovered that when having the car antenna positioned at an angle, the signal on FM was more robust because it was picking up both the H & V components.

DAB doesn't (normally) broadcast with a mix of the two - because being a DIGITAL platform, it is not really necessary, especially with error correction, single frequency networks and the receiver's ability to collect bits of signals that come from all directions and build up the signal to eventually make audio. There would be instances where V becomes H if bounced off an object (and vice versa) but since DAB is not (normally) broadcast with horizontal polarity, there simply wouldn't be enough signal on the H plane for the antenna to pick up, even if it was at an angle.

I have experimented with positioning the rod antenna on a DAB portable radio so that it was horizontal. Only multiplexes with the very strongest signals (i.e., within pissing distance of the DAB TX) could be received to give a good quality signal through the speakers.

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