Improving Radio Reception and/or Fitting Elise Hard Top

[PV4EV]

Looks like they opted for the Lotus Elise hardtop rather than the Roadster top too!

Err, that would be me !

I specifically wanted to improve the FM radio reception as the stock aerial is next to useless, but do it without drilling holes in the existing bodywork. I also didn't like the appearance of the stock Tesla solid roof and wanted to keep the car looking low and sleek. Plus the Kevlar version is nearly £5k here in the UK ..


One reason why FM is so bad is because the car body is carbon fiber and not metal which on most cars acts as the essential "ground-plane" side of the antenna - something that seems to have been overlooked by Tesla / Lotus engineers! Another reason is the electrical noise emanating from the cars electronics, but this is reasonably well contained.

Ideally a single wire aerial should be about 85cm long to act as a quarter wave monopole antenna for FM frqs of 85-100Mhz. I made up an 85cm one to test and it works exceptionally well when vertical, but obviously it looks odd on such a small car whether vertical or tilted over at 45' or more. So I fitted a 1/8th wavelength instead. This works almost as well and is fine with moderately strong stations but really needs a balun coil at the base to improve it a bit more. Tilting is a compromise which gets worse the further over you go.

In order to improve the signal reception with the 1/8th wavelength aerial, I needed to fit a ground plane at the base of the aerial to make it more like a 1/4 wavelength dipole antenna. To do this I bought an Elise double-skinned roof from ebay, repainted it to match the bodywork, added two layers of sound absorbing material and put a copper mesh ground plane sandwiched in between and connected to one side of the FM aerial feed. I used a fine grade mesh covering an area of about 0.7m2, the bigger the better.

I've now got *significantly* improved FM reception and the car is noticeably quieter at highway speeds of 70-90mph and I'm very happy with the result. Total cost of everything was under £500.


(Mods:- This ought to be split out into separate thread for improving Radio reception and / or fitting Elise hard top .. )

 

[wiztecy]

Very Cool and thanks for the pics and documenting this. The Lotus Elises I believe ran their antenna under the front hood and / or behind the headlights which was then shielded by the Carbon Fiber. I heard the CF filters out the signal as well and the reception sucked on those cars as well.

As for not having a ground plane, why do portable Radio's work and pick up FM reception pretty well? They're made of plastic so there shouldn't be a ground plane either.

Beautiful Roadster and color, I always liked the look of the Elise tops over the Roadster, it shows the lines better on the two rear triangle/fins on the back trunk.

By the way, what rear view mirror is that? Looks it has a function or two.

 

[PV4EV]

Very Cool and thanks for the pics and documenting this. The Lotus Elises I believe ran their antenna under the front hood and / or behind the headlights which was then shielded by the Carbon Fiber. I heard the CF filters out the signal as well and the reception sucked on those cars as well.

As for not having a ground plane, why do portable Radio's work and pick up FM reception pretty well? They're made of plastic so there shouldn't be a ground plane either.

Interesting question !! Antenna theory really is a black art worthy of at least 10,000 pages of information encompassing a very wide range of knowledge about impedance matching, modulation types, EM strength fall-off with distance VS use of the ionosphere for certain bands, polarisation, bandwidths and loads of other stuff


For an FM radio, if the signal strength is good enough then a single bit of damp string will work.

More realistically, the best reception of marginal signals in a car (as it moves around relative to the fixed transmitter) will be achieved with a vertically mounted monopole aerial roughly equal to at least a 1/4 wavelength of the band being used. Being vertical it will be perpendicular to the plane of the energy radiated from the point-source transmitter and therefore will gather the most RF energy from it.

Better still would be a full wavelength aerial, but that would need to be approx 3m / 10ft for FM and therefore impractical.


The idea behind using a ground plane in the roof of the car (or a cars metal bodywork) is that it effectively makes a half-wave dipole aerial that's twice as effective than a 1/4 wave monopole for any given field strength.

At least, thats how I understand it !

Beautiful Roadster and color, I always liked the look of the Elise tops over the Roadster, it shows the lines better on the two rear triangle/fins on the back trunk.

By the way, what rear view mirror is that? Looks it has a function or two.

Thanks ! The colour used was a custom colour from the Alfa Romeo range used on certain EU cars in 2010.

The rear view mirror was the normal clip-on one that was supplied with the car. Its only function is to have a dim-mode for night time driving. I've been toying with the idea of making up a frankenmirror with a 4" 720p TFT screen behind semi-silvered mirror glass, just to hide the camera.

I have front and rear facing dashcam's, both being "Mobius 1080P's" with 32Gb giving about 4 hours of 30FPS loop-overwrite recording, and includes a lipo 1hr battery. I've replaced that with a 5F ultra-capacitor to avoid any potential problems with unprotected Chinese lipo's !! The camera is remarkably effective night or day for the modest price of about $70.

I'll post up some example stills from the camera if anyone is interested - they really are amazing little cameras.

 

[dhrivnak]

Very cool PV4EV; Very good job. Was the Elise hard top just a drop in? I assume with the Antenna wire you can just pop off the top when the weather permits.

 

[PV4EV]

Very cool PV4EV; Very good job. Was the Elise hard top just a drop in? I assume with the Antenna wire you can just pop off the top when the weather permits.

Thanks !



ELISE ROOF

The Elise S2 roof is 100% identical/compatible with all lugs/mounts/holes on the Roadster. There are quite a few varieties from lightweight single skin roofs, double skinned, double skinned and lined (and full length "long tail" ones which will not fit a Roadster). I bought a 2nd hand double skinned one on ebay for £300 and lined it with about 15mm of sound absorbing material to eliminate all wind rush over the roof and deadens a lot more of the interior noise as well. Principle source of cabin noise is now through doors/windows and general cabin 'hiss', but its much better than it ever was before.


Antenna Wire / plug

As for the antenna wire .. I ran a small coax cable from the back of the Alpine head unit to a location just behind the drivers seat head position terminating in an inline plug/socket. Its easy to hook it up to the removable solid roof OR a small 15inch dipole antenna suckered onto the roll bar behind my headrest. However, this small antenna isn't any better than the stock windscreen aerial.

*But* with the roof off its fairly pointless trying to listen to the radio and 99.5% of the time I have the roof on anyway !!


Sound deadening inside doors

I'm currently experimenting with adding sheets of sound deadening material to the inside of the door shell. An interesting side effect of the door lining material is that when you shut the door, it closes with a nice solid "thunk" with no cheap sounding rattles and clacks !


Door sill water/grit problems


I've also been experimenting with ways to reduce water/road debris ingress due to the flawed design of the door base / sill area. I'll probably end up having a bespoke rubber extrusion made that fits under the doors and presses up onto the door sill without scratching the sill paint. This also cuts down noise from all the stuff chucked up onto the underside of the doors in bad weather. If this works well, I'll make up a kit for sale to others.