2 cellular (gsm/gprs) antennas at the same time

[Johnny-B]

Hello! I am pretty new at RF so forgive me for newbie questions We are designing a security/tracking device triggered on gyro or accelerometer... So under assumption that somebody will try to destroy(disable) the device using a small accu. drill tool, we will shield PCB in steel casing and dislocate the gsm/gprs antenna an the other side of the plastic housing (since the metal would inhibit the signal strenght). Now, with that in mind, i have a few questions:
1. is it possible to have 2 internal cellular antennas at the same time, so that when one is destroyed the other one serves as a backup (to notify the owner with an sms..)?
2. This is probably "not smart" question but since it's a passive circuit i'll just ask -What is your oppinion on how much mechanical damage (like a trough hole) can a cellular antenna withstand to stay more or less functional (to send an SMS)? Is there a certain type of antenna, like SMT, PCB antenna.., that is more robust to that kind of damage?

I understand that to an expert those question will sound maybe even funny, but i will appreciate any answer with gratitude.

Thanks in advance!

 

[vfone]

Can use a coupler to build a VSWR meter and when the main antenna is damaged (high VSWR) automatically switch to backup antenna.

The antenna switch command can be used to send a SMS using the GSM AT commands.
**broken link removed**

 

[jiripolivka]

The problem you have is typically solved by "tamper" switch. Upon mechanical attack like shock or vibration it reports by a ON or OFF signal such attack to an inspection centre, or, triggers e.g. an acoustic alarm.

 

[Johnny-B]

Thank you vfone and jiripolivka! Your answers were what i needed to get a good google direction

In addition, i have another "brilliant" question: is it possible to use a PIFA antenna inside a mobile device with a metal (steel) casing (housing), and use the casing as a ground plane (or not, the important issue is that the device receives/transmits a solid signal)?

Now, i am aware that there is a lot of antenna theory behind the answer, but basically i would like to know if it is a feasible technology used today? (just a simple answer

 

[jiripolivka]

Thank you vfone and jiripolivka! Your answers were what i needed to get a good google direction

In addition, i have another "brilliant" question: is it possible to use a PIFA antenna inside a mobile device with a metal (steel) casing (housing), and use the casing as a ground plane (or not, the important issue is that the device receives/transmits a solid signal)?

Now, i am aware that there is a lot of antenna theory behind the answer, but basically i would like to know if it is a feasible technology used today? (just a simple answer

The antenna to radiate must be located OUTSIDE a conducting case. Some mobile phones with metal cases use a slot in it as an antenna. I pad had a problem with such antenna when the slot was shorted by user's hand.
A ground plane is good to be a part of the antenna but not enclosing it.

 

[E Kafeman]

What type of antenna to select do much depend on what modifications that is possible to make on the steel case and what problem an antenna can meet due to environment circumstances. Slits antenna is a good mechanical solution as it not can be removed without remove the case, but it can easily be made unfunctional, as jiripolivka says.

Dual radios systems are not much worth as an alarrm AFTER something already had happen. Assume an unwanted movement is sensed after that someone had covered it with an aluminum bag or turned on a jammer. Two antenna and two radio systems inside will add fault redundancy for the radio but from security reason is it probably a less effective solution.

If connection between two units is dropped, can that be a reason to trigger an alarm, but the other unit should then be located as long away as possible so that both units not can be tampered with at the same time. This is how many types of alarms operate, both wired and wireless.

How an effective antenna solution should be designed depends on how precise you can define the threat. Currently is the the casing a bit weak defined.
A car, bank vault, or a mailbox can all be metal casings with very different possible threats. If the threat is to protect a mailbox from kids throwing firecrackers inside it, is dual antennas not a solution.
For a briefcase containing papers with sensitive content, if jamming is sensed or if out of GSM coverage occur, can a self-destruction be a simple alternative that not need any backup for the antenna.
As you asked for a dual antenna solution, must the circumstances and threat be something else and there are situations where a such solution is of use.
One case is automatic accident alarm for cars. One antenna placed at the roof and one under the car. If the car then is upside down in a ditch can still one antenna be operational and by radio report car location. For the antenna placed under the car is then a PIFA a common solution which is placed in a watertight plastic box.