"Hidden" radio antenna / aerial

PostPost by: crypto » Sun Feb 01, 2015 2:30 pm

Yeah, I'm one of them who sometimes listens to the radio instead to the engine :lol:

As I'm preparing the car for new paint I thought about closing the antenna hole on the left rear wing. Because I have a coupe It should be easy to install an antenna underneath the roof without drilling a hole ? (radio waves should easily go through fiberglass). Has anybody done this and how does the solution looks like ?

Thanks, Peter
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PostPost by: stugilmour » Sun Feb 01, 2015 4:41 pm

Peter, I mounted a Sirius Satellite Radio antenna between the roof panel and headliner on my Plus 2 and it works fine. I tried a hidden AM FM antenna and it didn't work very well.
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PostPost by: bob_rich » Sun Feb 01, 2015 8:06 pm

Hi Peter

I have a hidden Radio antenna-- But at present no radio ! but hop to find a suitable one soon. I gave details in a post when I got my +2S running and have attached a link to the post. You should find the info down the post a bit when someone els asked about the antenna It is certainly worth a try if the car is stripped down

Hop it helps best of luck

Bob

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PostPost by: Andy8421 » Sun Feb 01, 2015 8:34 pm

It has been a while since I looked at vehicle antennas, but I recall that broadcast radio transmitters were generally vertically polarised and required a vertical aerial to be used to receive radio stations efficiently. While a horizontal aerial will still pick up something, it will be far less effective than a vertical aerial.
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PostPost by: bob_rich » Sun Feb 01, 2015 8:47 pm

Hi All

On a vehicle a vertical antenna is better especially for FM. For Long, Medium and Short wave it matter less. I tried the set up and it worked OK on a radio I quickly hooked in. But I could not install it as it was too large. The arrangement I used has both horizontal and vertical foil and while it is not at all "designed" it seemed worth a try and nothing was lost as it is now hidden under the roof lining and rear trim.

Best of luck

Bob
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PostPost by: crypto » Mon Feb 02, 2015 7:01 pm

Thank you all for the good input !
I will try and test some configurations and if successful, close the antenna hole in the wing. As proposed, I'll try to route as much as possible of the antenna in vertical position.

Thanks, Peter
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PostPost by: markcs » Wed Feb 11, 2015 10:08 pm

Crypto,

I have installed a number of hidden tape antennas in fibreglass aircraft and was planning on doing the same during my current Plus 2 restoration.

I am sure that you know this but here goes anyway!

The 'basic antenna' is a dipole. If you imagine two pieces of wire, one vertically above the other, but not touching. Then take your coax cable from the radio and take it to the middle of the two wires and connect the centre of the coax to one wire and the outer braid to the other, you have your centre fed dipole. This is also what is known as vertically polarised, in the same way that VHF radio transmissions are in the UK (TV can be either vertically or horizontally polarised depending on where you are in the UK).

The trouble with this arrangement is that it produces a long and unwieldy antenna; in order to cut down on the length of antenna we can remove the bottom half of the antenna and voila - well not quite! We have to replace the bottom half with something and that is known as a ground plane.

On a metal bodied car, this is simple, the body acts as a ground plane (the centre feed from the coax is connected to the vertical aerial and the coax braid is connected to the car body). In a fibreglass car we do not have a metal body to act as this ground plane; as per the photo of the Plus 2 with the copper strip antenna it looks as if the coax braid is just cut off and the centre of the coax is connected to the copper strip - this will work but not very well and very poorly in areas with low signal strength.

So what to do in a fibreglass car - well I assume that Lotus took a braid from the bottom of the aerial (connected to the coax outer braid) and connected it to the largest piece of metal in the same area - the fuel tank (acts as the ground plane) - that is what I found in my Plus 2 anyway!

The answer is to use your imagination to achieve as vertical an aerial as you can with a ground plane, or fit a strip full dipole. It really doesn't matter if some of the aerial is vertical and some horizontal - yes you will loose some performance but the ground plane is really very important to excellent reception. The size of the ground plane is not critical but you should aim for it to be of a diameter similar to the length of your antenna and it does not need to be solid.

Personally I am going for a centre fed dipole with the upper half as vertical as possible and then bent over in the roof. Try a few options with throw away wire to get the best reception before hard wiring it into your car. The length of the top wire of the dipole should be around the same length as a fully extended commercially available telescopic aerial as this is tuned to the centre frequency of FM reception.
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PostPost by: Andy8421 » Thu Feb 12, 2015 5:30 am

The lack of ground plane is a common problem with non conductive attachment points for aerials.

I guess to try to overcome this, the Elise has an 'active antenna' with a small 12v powered amplifier built into the base of the aerial to boost the signal. It doesn't help that the aerial is mounted just above the plug leads on the engine cover.

Radio reception on my Elise was poor at best.
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PostPost by: Quart Meg Miles » Sat Feb 14, 2015 10:25 pm

I was under the impression that all FM stations broadcast using "circular" polarisation which works equally well with vertical and horizontal aerials in order to allow reception in cars which usually have vertical aerials. Home receiver aerials are usually mounted horizontally which was the original polarisation.

Possibly modern car radios don't bother with a matching-impedance aerial and just pick up the electric field but I have no data to back that up, only the short length of most car aerials.
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PostPost by: Andy8421 » Sun Feb 15, 2015 10:05 am

I stand corrected.

A bit of googling unearthed that in the UK the BBC moved to 'mixed polarisation' in the 70s. Apparently FM transmissions had been horizontal until then as most home listeners stretched out a horizontal wire. With the emergence of car radios capable of FM reception and the use of vertical car aerials, the decision was made to transmit both horizontal and vertical polarised signals (not strictly circular, but close enough).
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PostPost by: elanfan1 » Sun Feb 15, 2015 12:14 pm

Interesting subject and replies so far but I wonder if anyone minds me extending the subject a little?

What about DAB signals -I understand that an approx metre long antenna is best and ideally should be mounted vertically ( I did download a guide to building your own which could be googled?). So far my intention is to fit a windscreen antenna which is an L shaped stick on jobbie. Anyone know of a better solution?
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PostPost by: ricarbo » Sun Mar 01, 2015 3:18 pm

I've just fitted a dab radio on my son's Suzuki Swift (sshh). Apparently they don't work well from FM aerials, so the suggestion was to fit an active booster. Also dab car radios are fitted with a different style of connector, so you need a splitter with the right terminals at the very least. So I fitted a Kinetic DAA -7001 active converter from dab on wheels at ?19.99 and it is giving excellent results on FM/AM and dab. It requires a 12 volt supply and an earth, connects to the existing FM aerial connector and has flyleads giving the separate FM and Dab signals. I've heard the stick on aerials aren't too good and personally I don't like that they advertise you have an expensive radio, I wanted something that shouldn't attract attention. the dab on wheels site has useful info., i thought.
My Lotus has a 20 year old 'BiFi' active aerial which uses the heated rear screen element and works well on my FM/AM radio. No use for a drophead of course and 'BiFi' went out of business years ago. Some manufacturers, e.g. Ford fit or used to fit something similar, so you might find something.
regards
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PostPost by: crypto » Sun Mar 15, 2015 8:46 pm

I made some trials with different "antenna" configurations and made signal measurements in the FM radio band. The length of a quarter wavelength antenna is about 76cm for the center of the FM band (98Mhz). The DAB band is around 200Mhz, in other words the same type of antenna is only about 38cm long. As I was only interested in a rough comparison of these configurations, I ignored the effects of correct matching to the antenna impedance. The measurement was done in an underground garage (and not in free field), so reflections and change of polarization are likely.
1) trial with original car antenna
2) trial with vertical 1/2 wave dipole inside the car
3) trial with 1/4 wave wire antenna (half vertical, half horizontal). the shield is connected to aluminum sheet of the parcel shelf. see pics.
The signal differences for all configurations are +/- 1dbm, whereas 1) and 3) are almost equal and 2) is worst especially at the ends of the FM band.
Cheers Peter

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PostPost by: AHM » Sun Mar 15, 2015 11:13 pm

Or connect by Bluetooth to your smartphone
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PostPost by: h20hamelan » Tue Mar 24, 2015 3:14 am

The sound of the engine is about the best sound out there for me. :lol:
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