FM Interface

Beacon Components

Design - FM Interface

The military currently has a transmission standard in place called Frequency-Hopping, which allows for a more secure channel for data transmission and a huge reduction in interference. Frequency-Hopping works by changing the frequency used for data transmission at even intervals. These frequencies are set by predetermined sequences which are send out every 12 or so hours by a command point in the area. New sequences are determined so that in the event an enemy takes control of a device, that device will not receive the new sequence list, therefore preventing it from participating in any “friendly” communications. The FM interface is comprised of the antenna for transmission of data and reception of the Frequency-Hopping sequence, the Frequency-Hopping circuits, the timing circuit and an analog to digital converter for converting the sensor’s response to a digital signal (if necessary), which can be sent out.

Antenna
Since this device needs to remain stealth due to its nature, the antenna is built onto the top of the case so that it cannot be broken off. The antenna is formed in a U shape to allow the greatest surface area of the antenna exposed without having too much of a vulnerable structure.

Frequency-Hopping Circuits
Due to this device’s use in the military, we cannot acquire the details of the Frequency-Hopping component because it is classified. This component’s incorporation will be handled by the military when the beacon is adopted.

Timing Circuit
The timing circuit is a relatively simple circuit, which will trigger the Frequency-Hopping circuit which will then transmit the data. We build a sample circuit which will trigger an external device every hour. This was built using a 555 timer as the main IC, which operates in an ’astable’ mode as an oscillator, creating a pulse at an even interval. The supporting circuitry can be built onto a small printed circuit board, allowing for a more stealth design. Since we were not able to create a printed circuit board for this, we assembled the circuit on a breadboard (as seen below).

Additional Information
To power this test circuit, we used a power supply by Kepco (as seen in the second picture below). The 555 timer runs on +5v, and using a power supply we had on hand was the easiest method to attain this. The capacitor bank was built in anyway to show what would be optimally used in final production. These capacitors would ensure a perfectly smooth DC current supplied to the 555 timer, so that it would perform consistently. The variable resister (green) shown in the circuit below was used so that we could easily change the frequency of the transmission. In final production, a fixed resistor would be used. Also, all parts would be up to military specifications in the final product (the parts used here are available to civilians).

Housing

The housing for the beacon holds all the internal circuitry, as well as the ports for the external sensors.

	Power Source

The beacon’s power source is one of the most important parts, as it needs to provide enough power to supply the wireless interfaces, as well as some external sensors.

	GPS Interface

The GPS interface provides the precise location of the beacon through a military-grade GPS design.

	FM Interface

The FM interface provides the link for the vital information collected by the beacon back to a central command point.

	Sensors

External sensors will provide the versatility our beacon needs. These sensors are interchangeable, to provide the most possible usability of our beacon.