433 MHz antenna design for anti car theft system

[dm2954]

Hi i am building a anti theft system that will be installed in any car i am using a 10 watt radio modem to send alert . i am trying to find a design of an antenna that will be at least 15 DB gain , is any one can help me with any ideas including using the car chassis as an antenna

 

[JimB]

i am using a 10 watt radio modem to send alert

I cannot help but think that such a device would be illegal in most countries.
Where are you located and where are you obtaining tis radio modem?

i am trying to find a design of an antenna that will be at least 15 DB gain

Such an antenna would be BIG.
A 32 element yagi antenna would have about 15db gain.
It would also be 4 or 5 metres long.

is any one can help me with any ideas including using the car chassis as an antenna

You could try a slot antenna.
Thus would involve cutting a slot approximately 35cm long by 1cm wide in the car bodywork.
However this may not go down very well with the owner of a nice shiny new car.

You may want to reconsider some aspects of your design.

JimB

 

[dm2954]

the rang of freq is 430 to 580 so in each country you have to apply for a freq in most countries this freq are used for taxi company as you know they use a 25 watt radio system

i can not use a yagi antenna since i try to concealed the system
in regard to using the car chassis as an antenna it has been done before i need to have a special circuitry
but i think i i am going to try a suggestion by a member of this forum to check this link https://www.electro-tech-online.com/custompdfs/2012/04/ant-433-sp.pdf but if any one have other idea please let me know

 

[Nigel Goodwin]

the rang of freq is 430 to 580 so in each country you have to apply for a freq in most countries this freq are used for taxi company as you know they use a 25 watt radio system

And are very strictly regulated and licenced - you can't just randomly start broadcasting.

i can not use a yagi antenna since i try to concealed the system

To get gain from an aerial you make it directional - there's nothing that's going to be non-directional and invisible.

A FAR, FAR better idea would be use a GSM modem and send a text message to your phone.

 

[trash]

10W radio transmitter, with a modem, probably transmitting it's position. On a LIPD frequency inside the ham radio 70cm band.

WOW.... this is the radio equivalent of smearing yourself in seal blood and blubber and swimming at Margaret River or Capetown.

On top of this you're asking for an antenna which can be bought at most radio shops it is that common.
It's time to think this one through just a little more carefully. Not that I care much about legal issues but the lifespan of this project as it stands using 433MHz can be measured in minutes. Minutes - between you turning it on and somebody putting their boot print on your arse.

Now while a lot of people on this forum are not from a country where there are any good or bad rules, even in a country with no rules you're not going to last long.

As mentioned, mobile phones offer you a very simple, cheap and legal option with good coverage.
CB in it's various forms offers you another option.
CB & LIPD's like 433MHz are licensed as low power. There's a very good reason for it. Running high power is like shouting in a library. If everybody whispers, then lots of people can use the band without much interference to each other. But as soon as somebody starts shouting in a library, everybody else needs to start shouting. Like hundreds of people in a busy restaurant, everybody shouts and it's almost impossible to hear somebody even if they are close.

It helps to know exactly which country you are in when it comes to these dodgy radio questions. In some cases a quasi legal solution can be found for your country.



The simplest option of all. Get a ham radio licence and look into running APRS.
An example, one of my friends had their car stolen. They saw the guys steal it and immediately called the cops who said, "We'll send somebody around in a couple of hours."
When they told the operator that they knew where the car was and could track it's position to ~10 metres they sent two patrol cars after it while they gave the operator position updates over the phone.
The guys ditched the car and did the runner when the police cornered them. They recovered the car and the police knew who to arrest because the two idiots who stole the car drove to their mates' place to pick them up. This was logged. The cops just picked those guys up, and because they were regular customers, they knew who their buddies were.

And if you're not quite up to the task of passing a simple ham radio test, then there are mobile phone modules and apps which do the same thing.

 

[JimB]

WOW.... this is the radio equivalent of smearing yourself in seal blood and blubber and swimming at Margaret River or Capetown.

the lifespan of this project as it stands using 433MHz can be measured in minutes. Minutes - between you turning it on and somebody putting their boot print on your arse.

An excellent turn of phrase there Trash, I could not have put it better myself.

JimB

 

[dm2954]

well IN USA you get a privet freq in that rang and you can send have up to 6 watts radio in Latin America you can have a privet freq and you can us up to 25 watts
most taxi companies in Latin America the repeater that is been used is over 25 watts

 

[trash]

Just had a chat with DM on skype. In his country he will find that he will still have problems using the 70cm band for a number of reasons including legal reasons. In some countries there are no rules or rules enforced and so a little bit of careful planning can save a lot of hassles and money.

DM is on the right track, just needs a little help working out how to do it.
I'll put it in generic terms because I know there are a lot of places in the world that want such a system and don't know how to go about it.

In third world countries there is no insurance for cars. So if you car gets stolen, it's gone unless you do something yourself about getting it back.
Find the car and beat the **** out of the guys who stole it or pay the police to do it for you. That is if it isn't the police that stole it

A simple system would be a car alarm than instead of activating a siren, turns on a transmitter. Rather than a lojack style RDF system,
these days GPS modules are cheap and the transmitter can send out GPS data.
This can be done by just sending NMEA data, RTTY style or more complex AX.25 packets like APRS.

The problem is that their range is limited just like APRS. So digipeaters around town or on hills are a consideration.
Uncompressed APRS packets would be ideal because a base station monitoring the stolen cars could just see their positions with nothing more complex than a scanner and a computer with a sound card.

That's really simple and cheap to build and operate. It can be made better by feeding the data into mapping software.

So assuming that he has some tracking modules, GPS's and software all ready to go, the last question is.... What frequency and power to use.

We all know that he's focused on 433MHz because of LIPD's. Either cheap transmitters or receivers etc.
Of course this frequency is not good for this purpose because it's in the LIPD band, and interference is the first concern.
Next is ham radio operators who will notice the system running, and then their is military radar in the band.
It's just not suitable for the task in any part of the world.

So what else could be used ? If the country has radio law enforcement then you pay for a private (commercial) frequency.
In JM's case, he's planning on doing that. If he was in another part of the world, he would not bother paying for it.

So we can assume he's in a part of the world where commercial frequencies are cheap, and enforced.
What band to build his little network on ?

The one that jumps to mind is ~477MHz. In his part of the world, this is a commercial band. But he can get UHF CB's out of Australia dirt cheap.
Small hand held units start at $20 and it's not uncommon to find an old 40ch mobile radio for $50-$100 and radios like Philips PRM8010's are also
cheap and easily programmed to any UHF frequency.

Buying them over the counter mail order and having them shipped. And then build up the hardware and licence say 477.025MHz in his country.
Set all his radios to ch25 etc and he's done.

For his application 25W is overkill. 5 Watts would be more than enough and I know that I could make the system work with less than 1 watt for each tracking unit.