FM/DAB car radio antenna

C

cybermaus

Member
Hi. I am new here, and a complete noob at radio.
But I have an antenna problem, and I suspect there are people here who can help


I have a small camper-van with flip-open top-sleeper roof and terrible FM/DAB reception.
My current theory is those two are related: The builder (proper quality brand Westfalia, but maybe they should stay away from electronics) has moved the antenna from the iron roof to the fiberglass roof.

I already checked, and indeed the FM coax and DAB coax do not have continuity with the frame of the car.
My noob theory is that a normal car antenna is designed to use the metal surface of the car as antenna base, and being deprived from that it suffers.
If I plug the cables into the radio, it does of course have continuity via the radio, but that is through 2 meter or so coax, probably not quite the same as mounting it on a metal surface

Unfortunately, it is not possible to reach or un-mount the antenna backside without breaking things. I am willing to break things, but would like to know if such is useful or would be in vain. Hence I would like some guidance before I grab the hacksaw.

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Some additional info:

It is a combined FM/DAB+ antenna, with a amplifier in its base, but not one of those sharkfin ones. A stubby 20cm one.
Fed by 12V phantom power. Separate Fakra plugs for FM and DAB+. The GPS antenna is a completely separate device

"breaking things" would mean removing inner roof liner, where I expect to find the former ~15mm mounting hole. Drilling that to ~80mm so I can read the current backside of the antenna mounted to the fiberglass cover. Remount antenna on iron strip that is also connected to the now mangled remainder of the iron roof. Redoing redoing roof liner.

I also considered placing an entire new antenna on a new location. Unfortunately, I have no more visible iron on the roof. And even if I do find a spot, it would also mean drilling into my camper.
 
Hi Cybermaus!

Welcome to the forum:)

Trying to remote diagnose problems such as these can be tricky, but I'll do my best!

I did have a quick look around the web before answering, and I noticed that poor reception of DAB and FM signals seems to be quite a common problem in a number of brands, not just Westfalia.

A number of sources suggest that poor reception was caused by a mixture of high electrical noise levels from the vehicles onboard electronics coupled with poor antenna mounting/siting.

In your case, because the antenna has been mounted on to fibreglass body work there is as you deduced yourself, a very good chance that this could be one of the causes of poor performance. Having a metal panel beneath the antenna would certainly help. In simplistic terms you could think of the antenna as a two-terminal device, like say a battery. With both 'terminals' connected you get good performance, with only one connected you get nothing!

So in the case of the antenna, one 'terminal' is the antenna itself, the other 'terminal' would be the metal bodywork of the vehicle. If you remove the body work (by substituting fibreglass), you loose that second connection and straight away signal strengths could be considerably weaker. Also, because the current cannot flow out across the body work it looks for the next easiest path to follow, which would be the outer surface of the coaxial cable connecting the antenna to the radio. Once at the radio the current would then flow across the outer cover of the radio and so on.

However, the wanted radio signal is not alone. There would be many other electrical currents flowing around the various wires and metal panels/framework of the vehicle, any of which could mix with with the wanted signal and find their way back to the receiver.

You might have heard the term "signal to noise ratio"? In an ideal world there would be only signals and no noise! But because there is electrical noise, depending upon the strength of that noise it can make an otherwise perfect signal from a radio transmitter sound really crappy. So although you might in reality have pretty good reception from the antenna, the vehicle's electrical 'hash' will degrade this.

So metal work beneath the antenna can very often help in lots of ways, both improving received signal strengths and helping to shield the antenna from the electro-smog within the vehicle.

Although the bodywork where the antenna is attached is fibreglass, it might be that the vehicle manufacturer has added some kind of conductive coating on the underside of the panel in question, or they may have added a self adhesive foil sheet. If not then you could do that yourself, but as you said, you would need to break things in order to gain access to the panel.

A couple of products that come to mind are self adhesive aluminium foil tape, about three inches wide, normally sold in rolls and used for sealing around gas fire back plates. Another product I use here is called "Rustins G-Shield". It is a paint, but instead of containing a pigment it uses graphite (a form of carbon). Once dry it acts like a conductive layer, and a few coats will give a really low resistance, almost as good as metal. I use it here to shield the 'electrics' in various guitars, it's pretty good stuff, it makes a piece of wood act like a piece of metal! The paint is not as cheap as the aluminium tape, but if you have a complex form to cover it is easier to just paint over the surface rather than trying to get the metal tape to follow complex contours.

Turning to the antenna itself, is it definitely OK? Could it have been damaged by an electrical storm say? Also, is the phantom power feed to it working? Is it the correct type for the country you are using it in? Although FM broadcasting is 'generally' between about 70-108MHz, the DAB transmission frequencies vary enormously from country to country. Here in the UK DAB is around 220Mhz, but I seem to recall that in Europe and USA the transmissions are on much higher frequencies. It's worth thinking about just in case some of the problems are being caused by lack of compatibility rather than electrical or installation issues.

Another obvious question would be, is the radio receiver itself OK? Is there any way you could try it on a different antenna, even a temporary one?

You mentioned about not wishing to have to drill your vehicle (I get that!), and I was wondering if you had heard about the various glass mounted antennas? Sometimes they are known as "on-glass" antennas or "glass mounting" antennas. The idea is they simply stick to the inside of one of the vehicles glass windows/sky lights. Many of them are "active" antennas, so they contain an electrically short antenna which is matched using an in-built buffer/preamplifier circuit.

"On-glass universal antenna" is another search phrase to try.

Hopefully some of the above might be useful. I tried not to get too technical. If you did decide to try an on-glass antenna then have a good look through all of the reviews. Like anything there are good ones and some really poor ones (it's the same with ham radio antennas). Also you would need to make sure the frequency coverage was correct for the country you intend to use it in. As mentioned above, DAB frequencies vary wildly from country to country.

Mark...
 
Ham4CW said:
Hi Cybermaus!

Welcome to the forum:)
Click to expand...
Thanks.

Many of the things you mention already crossed my mind, but I do not always have the ability to know if they are true or useful or not.
Let me mention what I already did, or which reasonings I already made:

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You suggest maybe the antenna is broken, or the wrong one, or the phantom power is not working.
Well, it is a French car build for European market, so I am pretty sure it originally fit the 87-108MHz FM and 174-240MHz DAB+ ranges in Europe (see, I did my homework). Also, when testing, I disconnected the phantom power insert, to observe the meagre signals drop completely.

However, I am indeed unsure if the 2cm stubby is the original whip, or if Westfalia or previous owner replaced it for vehicle height reasons. And if they replaced it, I have not too much confidence they replaced it with a correctly spec'ed one. I randomly tried various other antenna's of course, right now it is running with a 20cm stubby, seems a little better then the 2cm stubby, but the difference is so small, I am not even sure.

Also, correct me if I am wrong, but I was under the impression the very reason car manufacturers moved the antenna amplifier to the antenna base is to deal with the SNR and cable cross-talk. By amplifying a clean signal at the source rather then at the radio, you amplify only the signal, not the cross-talk noise, making it mostly immune to bad SNR due to cable cross-talk?

---

You asked about the radio itself. I did spend some time on that. Even the OEM radio is pretty weak in both FM and DAB+. It seems my village is at a low signal level for the specific stations I want to hear. And on top of that, I am trying to install an aftermarket radio. One of those ubiquitous Chinese Android radio's. Why? Android Auto navigation of course. But I think the initial push was because even the OEM radio annoyed me with it's frequent drops. If I look back, I think the thought process was: "What a crappy radio, should get a better one. Might as well get one of those fancy ones"

I did put the original radio back, but I am still annoyed with it's drops. The setup is just shitty. Could it be the radio? Strictly speaking yes, but with the antenna moved from metal roof to fiberglass roof cover, it seems such a obvious clue, I really do not think I should fault the radio.

Also, I did the following: I had the aftermarket on my bench, with an antenna salvaged from a car from the salvage yard. I mounted that antenna outside the window on a strip of iron. After some messing with cables to ensure proper connections, especially power, USB and coax. I got it into my head the HU itself may be radiating noise and indeed adding a ferrite on power, the USB and internal LCD cable definitely cleared up the reception to the point I had a good signal. With "definitely" I mean, I cold recreate it: remove ferrites, many DAB drops, include ferrites, no more DAB drops. repeatedly, and with long test periods. But when moved into the car, it was pretty bad again.

Using that salvaged antenna as temporary one at the car seems to work if I set it on its metal strip stand next to the camper. Of course, I have not driven it around like that, nor have I drilled a "temporary" mounting hole. And sticky-taping it against something without any metal base... well what would that prove? That both antenna's won't work without a base? That is sort of the point I was at when I decided to seek help before doing something that leaves a mark.

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You also mentioned copper tape and even paint. Well I do not want copper tape, nor paint, on the visible outside. Also, how would I connect the tape/paint to the antenna base, if I cannot dismount the base. And should it not also be bonded to the car itself? It will not be if I paint the fibreglass. Some questions though:

Would alu-foil work, for testing, as I have that handy. Or too thin? And still the question about whether connecting to antenna base and/or car ground is needed.
And if a connection is not needed, well then I wonder about the orignal antenna again. It is not connected to the roof, but the roof is still there, sitting there as some sort of radiation mirror about 7 cm lower. Would that still work as a base, meaning my theory about the problem is wrong

---

And to do these things on the inside of the fibreglass, well, that is the "breaking things" decision I am facing. Remember, even if I remove the liner, I would be looking at the inside of the original metal roof, not the fibreglass. Right now, the dome light is my access, and I feel the orignal antenna cables dissappear through what I suspect is the orignal mounting hole.

After removing the liner completely, I would have to take a oscillating multitool and make a large hole in the metal roof to see the fiberglass roof, and then ground/bond the base, and then foam up or something to soundproof it, before putting the liner back.

In fact, I had already searched for conductive foam. It would be ideal if you could buy a spray-can builders PU foam, or whatever it is called in English, but conducting. I am not expecting any wires to exists in the cavity between the original roof and fibreglass roof, so should be fairly safe. But alas, conductive foam seems to not exists. Maybe a good thing. We would not want people using conductive foam in their houses, with 230V everywhere, insurance would go through the roof.

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You suggested on-glass antenna's. I did try the included on-glass DAB+. It was slightly better but not good enough. The salvaged antenna is clearly better and also works for FM.

But I suspect you refer to a better class of on-glass antenna than what you get with the Android Radio. And I had already picked up on that, sometimes also call NGP or no-ground-plane antenna's. Yes, if I cannot get the orignal antenna, or original antenna postion, to work, I would go for a new location and these may be an option.


Thanks for your responses,
Maurits

 
Having the amplifier at the antenna end is better in one respect as that is where the strongest signal is to be found (for any given antenna), but it is a double edged sword in that any noise present will also be amplified by the same factor as the wanted signal. Also, electrical noise present on the braid or shield of the connecting coaxial lead will be transferred to the antenna and that too gets amplified.

Even the amplifier itself will add more noise. With too short a whip length the internal noise from the preamp will form a significant proportion of the RF energy reaching the receiver front end. Conversely, beyond a certain length length there will be no more improvement since you would have reached the true signal to noise ratio for any given reception location. So antenna length is important if you want to get the maximum out of he system, especially if the antenna is on the short side of the original, optimal, desired design length. Is there no info obtainable from Westfalia? Maybe even a photograph of the same vehicle might give a clue to the original design. I did notice that many websites carry comments to the effect that Westflia uses off the shelf antenna assemblies from third party companies.

Revisiting the whip antenna itself, there are a number of ways it may have been constructed, so just because a substitute whip will fit, there is no guarantee it is the correct type for optimal performance.

The simplest method of construction would use a single conductive element. But there is a chance that the designer might have adopted to use a helical wound element (like a spring) in order to make the antenna physically shorter but still resonant within the desired band. Another method is to use coaxial sleeves with say an inner rod in use for one band and then an outer concentric sleeve being used for another. You can also have 'trap' designs, where one long element is broken in to sections by the use of resonant 'traps' which only allow some frequencies to pass while others are blocked. Some of the small 'rubber duck' antennas used in Amateur Radio can be built to be resonant on multiple bands by the clever use of traps and tuned loading coils. One could only guess that the original whip used on the vehicle would have been the optimum one for the preamp used and the mounting method.

When I mentioned using either self adhesive aluminium tape or conductive paint I was of course referring to applying it to the inside (hidden) of the fibreglass body panel, not the outside!

Aluminium foil is fine! It does not need to be thick at all since the RF currents involved in radio flow over the surface of the conductor, NOT the interior. It's only down at either DC current or else extremely low radio frequencies that the current uses the entire cross section of the conductor it is flowing along. It's somewhere around a few tens of kilo Hertz where the current begins to flow near the surface of a conductor. The phenomena is called the 'skin effect'. So even thin conductive sheets of metal such as aluminium foil will work just as well as a sheet of metal many millimetres thick.

You asked about ground the foil to the rest of the bodywork, well short answer is no, there is no need, and it could even make the electrical noise level higher due to an earth loop being formed. You would normally need to connect the foil (in Amateur Radio it would be referred to as a 'ground plane'), to the end of the coax/antenna mount. This would of course depend upon how the antenna base was fitted. Maybe a single locking collar, maybe a few screws, who knows?

I haven't come across conductive foam spray, only the paint on graphite paint I mentioned before.

Looking at your comments about temporary mounting ideas and not wishing to drill holes all over the vehicle, well, maybe some of the antenna mounts that are used in Amateur Radio might help. One type that springs to mind is a 'gutter mount' (intended to clamp on to the gutter rail used on some vehicles to guide rain water away), and its slightly bigger cousin a 'boot lip mount' (same idea as a gutter mount but capable of being clamped on to the edges of the vehicles doors or boot hatch). Another type of temporary mount is the 'mirror mount' used by CB Radio operators. This again is another clamp type mount that is designed to fit around the metal supporting arm used to hold mirrors on lorries and trucks.

Many of the above mentioned mounts can be purchased as a bare clamp assembly ready to take whatever antenna you want to experiment with. You might have to make a small adaptor plate or use washers etc, depending upon the size of the car aerial you want to try, but other than that it might make for a quick and easy way to try out various antennas in assorted locations on the vehicle.

In the end though your reception will be at the mercy of the transmitter site itself. In towns the signal strengths tend to be pretty good, but 'out in the sticks' it is another story. Unless the transmitter is particularly powerful AND is located on a high point (hill top, mountain top, TV tower etc.), then range will be limited to a few tens of miles at best. Also while driving along there will be Doppler shift on the signal AND constructive & destructive interference (caused by the many numerous reflections from surrounding objects). Effects like that wreak havoc with digital signals!

It is possible to buy antennas known as 'diversity antennas', where the unit has not one but two or more separate antennas in one housing, connected in such a way that as one antenna looses the signal the other/s will peak up. The idea is to give a continuous best strength, but like any design, some are good and some are bad, and nearly all are expensive!

Mark..
 
Thanks, if I dare to summarise:
Yes, it can be the missing ground plane (original premise), but there are many other factors, so no guarantees.


Based on your suggestion the radio itself was at fault, I did an extra tests:
- OEM radio on workbench with salvaged antenna: good DAB. OK on FM
- OEM radio in car, salvaged antenna beside car, OK-ish FM, good DAB

Based with what I already had done before
- OEM radio in car, OK FM, bad DAB (original car setup)
- Aftermarket in workbench, OK on FM, good DAB
- Aftermarket in car, car antenna, OK-ish FM, Close to non-existent DAB
- Aftermarket in car, salvaged antenna beside car, OK-ish on FM, pretty OK on DAB

So to me, it means the issue definitely is not the OEM radio, and not even the Aftermarket radio. Because the salvaged antenna was beside the car, using a different cable run, it could still be the local EMI from the car or cable crosstalk rather then the car antenna itself, but in my view, the issue is definitely with the car's "overall antenna environment" and not with either radio.

So, here is what I will do, probably this weekend when I have some time:

I will remove the liner, and cut the now hidden remainder of the metal roof, in order to gain access. I will probably try *both* rebuilding the ground plane and replacing the antenna-base (and its wire) completely. It does not really matter as for both I need access.

If/when that works, I can still opt to keep either the original OEM radio or the aftermarket radio.

Lets hope, antenna success or not, I will at least get the liner back nicely.
 
Hi Maurits.
So, here is what I will do, probably this weekend when I have some time:

I will remove the liner, and cut the now hidden remainder of the metal roof, in order to gain access. I will probably try *both* rebuilding the ground plane and replacing the antenna-base (and its wire) completely. It does not really matter as for both I need access.

If/when that works, I can still opt to keep either the original OEM radio or the aftermarket radio.

Lets hope, antenna success or not, I will at least get the liner back nicely.
Click to expand...
Best of luck with your project!

I know it can be a real pain when apparently simple systems fail to work. I've had loads of head scratching moments over the years only to find that the problem was caused by something straightforward (hindsight is a wonderful thing!).

Let us know your results, it will be interesting to see what 'fix' works in the end. Feel free to upload any photographs as well.

Mark...
 
Ok, so not all done yet, but for as far as I got, full success:

First, as @Ham4CW suggested, Westfalia did stuck a large sheet of sticky alu-foil to the inner side of the fiberglass roof, and the antenna base did make contact with this. See photo 1. So it had a ground plane, even if that ground plane was not connected to the car.
And while dismounting the antenna, I broke that. A piece of foil tore away with the ample amount of sealant they used.
But I fixed it again, see photo 2.

With the other, salvaged, antenna mounted, I now have very good reception on DAB and also good reception on FM

But alas, not quite done, as the replacement salvaged antenna slants to far backward, and it prevents me from opening my camper-roof.
So I need to find some more antenna's. But it seems the result is worth some more effort.

Thanks.

PS: The round-ish whole was made with a sheet metal nibbler
 

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Hi Maurits.
Sounds like you cracked the problem. I wonder if there was a fault with the original antenna or if it was just a poor design.

That nibbler did a good job! From the photo it looks like a very clean cut edge. At least now the hole is there if ever you need to work on the antenna again it will be much easier to gain access to the base.

Best of luck, Mark...
 
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