Using the body of the car as a receiving antenna !!!

[eltonjohn]

I have this crazy IDEA!

I have a project and i will like to approach it differently .
It turns out that that i have already designed this device that goes in a car .It hooks up to the cigar lighter .It mesures something . Now ,i will like to RECEIVE that information in a very unintrusive way . i will like to USE the BODY of the CAR as a RECEIVING ANTENNA . the signal that i'm supposed to receive is 1 yard away from the CAR !

I need to design l TRANSMITTER and RECEIVER .The data received is binary , but very little (couple of bytes ) So the speed is not a big COCERN!
So any ideas on what band to design THIS ! and what PITFALLS to avoid?

 

[sda]

Re: I have this crazy IDEA!

Hmm,
Shure you are not stoned mate?
If you are thinking of using the car's body for reception (receiver), and the signal you are supposed to receive (again) is a yard away from the car, then where the heck does the transmitter fit into the story?

cheers
sda

 

[roli]

Re: I have this crazy IDEA!

I'm not sure that this is physically duable.
The Receiving antena [in this case - the car body] MUST have an impedance matched to the transmitting Source. Think about it as a transmission line - with reflection and losses for non-matched.....

 

[eltonjohn]

Re: I have this crazy IDEA!

Mr ROLI :

Tell me why i can't match the car's body ... to my receiving circuit

If the body beheaves INDUCTIVLY i will add a CAP if is CAPACITIVE i will add a COIL ..

IS PERFECTLY DUABLE ... I'm not VIOLATING any PHYSICAL LAW .!

the FREQUENCY chosen will be a FUNCTION of the CIRCUIT DETERMINATION ... That's where i need SOME INSIGHT ..!

i olny drink CHAMPAGNE! !

 

[flatulent]

other ways

This car body idea has several alternatives.

1. The RF signal can enter through the glass windows and you have the antenna inside the passenger area.

2. Separations in the metal like doors or hood/bonnet areas can be used as slot antennas.

3. Antennas can be hidden as window stickers with copper antennas on the side facing the interior of the car.

4. Put a diplexer in the lead in to the car broadcast radio to divert your signal to another receiver.

With this short range, you should not need any special matching networks. A few mW of RF will work even with inefficiencies.

 

[klug]

Re: I have this crazy IDEA!

Hi, eltonjohn!

This idea is not new for me. I was asked to make anti-thief global system for cars near 10 years ago. They asked me to make hidden beacon on car with 1-5 Watts on frequencies 100-200 MHz. There was question about antenna - It had to be unremovable and hard to find/break. I was thinking about using car's body as antenna. There is no problem to take 2 points on chassis and match transmitter with impedance of this construction. But, matching is only half a problem - next you have to achive efficiency and diagram. It is difficult task. If you are taking points on inner area of body, then you get shielding by this machine body and bad efficiency. You can imagine the can containing transmitter in it, and this can used as antenna feeded on inner side. Close this can and it will transmit nothing. But if you will feed on outer side of can it will radiate well.

I have not made that anti-thief system - they asked to make it very quickly. But few years later I have found some description about anti-thief device with using car's body as antenna - It was made on Russia.

I think there is no problem to use car body as rx antenna on distance of 1 yard.

It is not new trick to use any metallic construction as antenna - At August 1991 on USSR was revolution and TV programs were showing only ballet and limited news. Few men on Moscow have made TV broadcasting by using metallic fair-escape on side of house as antenna. They were transmitting news from sieged goverment.

 

[eltonjohn]

Re: I have this crazy IDEA!

Thanks for your replay .KLUG .. i'm gathering MOMENTUM now .
I know probably i have to go and connect my RF source to some kind of antenna and make an RF kind of METER ..in my CAR's LIGHTER ..
and see how much energy i can get and in what frequency ..
Probably later move to another car an see if is CONSISTENT .
I just wat to see what is your take on this . As to save me some agravation ..!

The difference bewteen your project and mine is that i'm only concerned in receiving .So matching a transmitter is not the problem . I think that FLATULENT has also good ideas ....as always
Unfortunatly i can't give more information without given away the application .. but i would like to solve it as i stated it in the beggining.

 

[klug]

Re: I have this crazy IDEA!

There is no difference between receiving or transmitting for matching and diagram. This, you can connect transmiter to chassis and use your RF-meter to find signal level outside a car - it is much easy to find diagram by this way. Pay additional attention on QRMs from ignition system - this problem is big if you gonna use chassis as RX antenna.

 

[Element7k]

Re: I have this crazy IDEA!

Depending on what frequency you are operating on, I have seen previously air vents of the car being used as slot antennas (e.g. like in aircrafts) . If you want to hide it, I will probably put a seperate antenna somewhere hidden under the dash board or put several on the bumbers or even the sunroof. Look at
https://www.multiband-antennas.com/
for some covert hidden antennas. I have made some police band antennas which sticks on (like what flatulent has described) to non-metellic parts of the vehicle but you have to tune them slightly for different vehicle models. Also one other problem is that the car body is used as ground for most vehicle electronics so I think you may encounter EMC problems.

Element7k

 

[FANT]

Re: I have this crazy IDEA!

Until some years ago in Italy there was a tax if you wanted to install a car radio. For this reason ( and also to avoid to make holes on the roof ) some people used the rear glass anti freeze wires ( buried in the glass ) as an antenna, installing a sorta of diplexer.
The gain was very poor but it was working !.
If you need only 100 meters, do't take care about impedence, just drop about 100 mW and you will not have any problem !.
Mandi