Wiring a powered car aerial

[Arthur Marshall]

I want to instal a DAB aerial onto the boat roof, and most of these are powered. On a car, they just have one live connection and presumably the negative goes through the car body, which won't work on the boat. Should I just connect the negative wire to the bottom of the aerial? And could this be why the splitter I'm currently using doesn't seem to function very well - it's only got a single red wire connection.

[Paul C]

I want to instal a DAB aerial onto the boat roof, and most of these are powered. On a car, they just have one live connection and presumably the negative goes through the car body, which won't work on the boat. Should I just connect the negative wire to the bottom of the aerial? And could this be why the splitter I'm currently using doesn't seem to function very well - it's only got a single red wire connection.

 

They have 3 connections (plus the co-ax for the radio signal) typically:

 

+12V (ie live all the time)

"aerial up" signal - +12V to raise the aerial

negative.

 

The reason being, if the aerial is to retract, then the signal is turned off yet the aerial still needs power to motor down the mast. And they typically also have a negative wire, ie don't rely on having the negative for the motor connected through the metalwork of the aerial. If you saw a 1 wire aerial, I can't think how it could work twice - it might work once going up though!!

[WotEver]

Here comes the box of worms again...

 

A car aerial shouldn't make contact with the hull. If the aerial doesn't make contact with the hull then it doesn't have a ground plane and won't work very well. So... the usual method is to separate the screen from the aerial and reconnect it via a capacitor. But that won't work with a powered aerial...

 

I guess it would depend on the aerial as to how it's powered and what 'earth' connection it expects.

 

Can you point us at the aerial you have in mind?

 

Tony

[Paul C]

Also, if "most" of them are powered, it means not all of them are powered. Buy one that isn't powered? It has a valid use on a car, but on a boat....not so sure. We used one of those simple foot-long rubber aerials, angled so it would be knocked flat onto the roof with particularly low bridges etc.

[WotEver]

I used one of those long whippy things with a magmount, also angled back. It worked a treat.

[bottle]

Mines about 12 " straight up but is flexible, it is also mag mounted.

[MtB]

I want to instal a DAB aerial onto the boat roof, and most of these are powered. On a car, they just have one live connection and presumably the negative goes through the car body, which won't work on the boat. Should I just connect the negative wire to the bottom of the aerial? And could this be why the splitter I'm currently using doesn't seem to function very well - it's only got a single red wire connection.

 

 

I suspect you have confused several posters here into thinking by 'powered', you mean one with a motor that raises and lowers it as was trendy on one's Ford Capri back in the 70s.

 

I have a DAB car radio in the boat and it works pretty well with a non-powered aerial purchased from Halfords. Like Bottle's, mine is a rigid black metal rod about 12" high with a coily bit in the centre, rather like old carphone aerials. It has a strong magnet base so it is just plonked on the roof and the factory-fitted 5m cable is threaded through a pigeon box and plugs into the back of the radio. No earthing involved.

 

It was about £30 IIRC. I'll seef I can find you a link.

Here it is...

http://www.halfords.com/technology/car-audio/dab-car-stereos/sonichi-magnetic-roof-mount-dab-antenna

Edited September 9, 2016 by Mike the Boilerman

[Arthur Marshall]

Yes, sorry about the confusion, I mean an aerial with a built in signal amplifier, not one that whizzes up and down. I'll try a standard one and see how it goes.

BUT, I'm now confused about "A car aerial shouldn't make contact with the hull. If the aerial doesn't make contact with the hull then it doesn't have a ground plane and won't work very well. So... the usual method is to separate the screen from the aerial and reconnect it via a capacitor." The first two sentences seem to contradict each other. On my aerial, there's a serrated washer that fits underneath and grips into the bottom of the metal roof as you tighten it. Maybe, if this is wrong, this is why the bog standard VHF aerial isn't working well - advice?

ETA what I find is, that if the signal is a bit weak, moving about inside the boat can affect reception considerably on FM, slightly less so on DAB.

Looking back at old threads on this, should I have the radio itself earthed to the hull? I don't understand the technicalities behind it all, as you will have gathered.

Edited September 9, 2016 by Arthur Marshall

[WotEver]

I'm now confused about "A car aerial shouldn't make contact with the hull. If the aerial doesn't make contact with the hull then it doesn't have a ground plane and won't work very well. So... the usual method is to separate the screen from the aerial and reconnect it via a capacitor." The first two sentences seem to contradict each other.

 

Yes, they do. An aerial designed for a car works very well on a car because the car wing/roof acts both as a ground plane and a -ve return. On a boat you should not have any negative connections to the hull other than the bonding lug close to the batteries. This is both for safety purposes and to avoid stray current corrosion. Hence the third sentence. which allows the aerial itself to connect to the boat's hull to give a ground plane, but disconnects the negative connection by virtue of a capacitor.

 

Your simplest way forward is with a mag mount which avoids all of these problems.

 

Tony

[Paul C]

Aaaah yes - apologies for the misunderstanding, if you mean an aerial with a built in amplifier (which is powered) then its a slightly different thing, and one I've not looked at in depth (had one a while back though). If it has only one red wire ie no separate negative wire and relies on the metal-metal contact with the bodywork to form the earth, then strictly it is not suitable for use on a boat. It might be possible to adapt it though, by finding a suitable place to solder/otherwise attach a negative wire (could be as simple as a screw hole or the mounting itself with a large round terminal).

[Keeping Up]

It probably relies on the cable screen to provide the negative connection back via the body of the radio. This is perfectly ok on a boat, provided that the isolating capacitor which has been mentioned above, is fitted at the aerial end of the cable rather than at the radio end.

[WotEver]

It probably relies on the cable screen to provide the negative connection back via the body of the radio. This is perfectly ok on a boat, provided that the isolating capacitor which has been mentioned above, is fitted at the aerial end of the cable rather than at the radio end.

 

I can't see how that would work. The aerial base must be connected to the roof to get the ground plane, so you fit it in that way and disconnect the screen, reconnecting it via a capacitor. That sorts out a non-powered aerial. If you do that with a powered one, how does it get the power? The -ve return is blocked by the cap.

 

Tony

[Onewheeler]

My DAB "preamp" (not sure if it does more than that, I have a feeling it is a receiver in its own right to act as a DAB adaptor) is powered by a dedicated connection to a socket on the back of the radio. I use a small DAB magmount (about fifteen quid from Halfords, outrageous but couldn't be bothered doing anything more homemade) with the cable run through a mushroom vent. Radio is something gaudy by Pie and Ear also from Halfords. The FM aerial is connected via an isolating capacitor as it's not a magmount. A bit of chocolate block is your friend for botching the isolating capacitor. 100 pF should do the job.

 

Martin/

[Keeping Up]

I can't see how that would work. The aerial base must be connected to the roof to get the ground plane, so you fit it in that way and disconnect the screen, reconnecting it via a capacitor. That sorts out a non-powered aerial. If you do that with a powered one, how does it get the power? The -ve return is blocked by the cap.

 

Tony

As long as you can isolate everything from the roof itself you can then connect a capacitor from the cable to the roof.

[Geo]

 

I can't see how that would work. The aerial base must be connected to the roof to get the ground plane, so you fit it in that way and disconnect the screen, reconnecting it via a capacitor. That sorts out a non-powered aerial. If you do that with a powered one, how does it get the power? The -ve return is blocked by the cap.

 

Tony

 

Surely an aerial mounted on a mag-mount is not connected to the boat's metal work, paint etc on the roof and possibly even a plastic film or coating on the magnet base.

 

Thus if I have understood you correctly it would not have a ground plane.

[WotEver]

As long as you can isolate everything from the roof itself...

 

Yes, as long as you can.

 

Surely an aerial mounted on a mag-mount is not connected to the boat's metal work...

 

That is correct, which is why I and others have recommended it. A mag-mount aerial is grounded through capacitive coupling between the magnet and the metal it's stuck to.

[pearley]

Yes, as long as you can.

 

That is correct, which is why I and others have recommended it. A mag-mount aerial is grounded through capacitive coupling between the magnet and the metal it's stuck to.

When the magnet bit came off the bottom of my Halfords mag mount thus exposing the cable connection, there was no sign of any coupling capacitor. Edited September 9, 2016 by pearley

[Geo]

 

Yes, as long as you can.

 

That is correct, which is why I and others have recommended it. A mag-mount aerial is grounded through capacitive coupling between the magnet and the metal it's stuck to.

 

So the mag-mount Ae is connected electrical to the boat,albeit not DC wise. Now that makes me wonder why not just stand the OP's Ae off by using a plastic film between the roof and the Ae. Although I would personally use a mag-mount easier and less hassle.

When the magnet bit came off the bottom of my Halfords mag mount thus exposing the cable connection, there was no sign of any coupling capacitor.

 

There is no capacitor he is talking about the capacitive effect between the roof and the base of the mag-mount

Edited September 9, 2016 by Geo

[WotEver]

 

There is no capacitor he is talking about the capacitive effect between the roof and the base of the mag-mount

Zactky.

 

Or even Zackly

Now that makes me wonder why not just stand the OP's Ae off by using a plastic film between the roof and the Ae.

Because it wouldn't work. If you've ever seen the instructions for a car aerial they state to vigorously remove any dirt, paint or underseal to ensure good electrical contact. That's how they're designed, unlike a mag mount.

 

Edited for tryping

Edited September 9, 2016 by WotEver

[Bugsworth Tippler]

I've just had this problem. Having bought a powered DAB aerial I discovered that it needed a larger hole in the boat roof than I currently have, and I was struggling to get the wires/cables through the insulation to the radio.

 

I chickened out and replaced the old rusted aerial with a similar/traditional car aerial (black, about 12" long) and then used a DAB/FM amplified splitter just behind the radio. Just did this yesterday but all seemed to be working and I got a whole gamut of DAB stations.

 

If anyone wants an amplified DAB aerial (unused condition) I paid £30 for it but would happily accept half that. Otherwise it can go in the garage with my collection of things I bought but never used.

 

The short magnetic based aerial that came with the DAB radio sometimes works inside a cupboard in the boat. In some areas I've stuck it just outside the front door but this isn't very good when it's raining.

 

ETA : The splitter had a red wire and a black wire which I wired as you would expect. The DAB aerial has only a red wire.

Edited September 9, 2016 by Bugsworth Tippler

[Bugsworth Tippler]

I'm glad I've got my radio working as much of the postings above, though very helpful I'm sure, just befuddle my remaining brain cells.

[Geo]

Zactky.

 

Or even Zackly

Because it wouldn't work. If you've ever seen the instructions for a car aerial they state to vigorously remove any dirt, paint or underseal to ensure good electrical contact. That's how they're designed, unlike a mag mount.

 

Edited for tryping

 

Sorry electrically you say add a capacitor in the earthy side of the cable up to the Ae. Now that capacitor has to be effectively 0 ohms at the operational frequencies. Surely by judicious choice the same level of capacitance should be achievable by suitable selection of the correct size of film.

 

Mind you a mag-mount is easier

[Arthur Marshall]

Bugsworth's is how mine currently works, but as said in some areas I get no reception on FM or DAB, and in weak areas I get interference as I movre round in the boat. Is this normal?

[WotEver]

 

Sorry electrically you say add a capacitor in the earthy side of the cable up to the Ae. Now that capacitor has to be effectively 0 ohms at the operational frequencies. Surely by judicious choice the same level of capacitance should be achievable by suitable selection of the correct size of film.

We had a member here a while back who stated that this was the case. Another member tried it and it didn't work for him. My opinion is that the area of 'contact' of a conventional car aerial is too small to get the capacitance anything like big enough.

 

Tony

[Jess--]

When the magnet bit came off the bottom of my Halfords mag mount thus exposing the cable connection, there was no sign of any coupling capacitor.

in it's simplest form a capacitor is just 2 pieces of metal separated by an insulator.... so if it's designed correctly the plastic film on the bottom of the magmount can act as a capacitor to couple the antenna