406MHz/ 121.5MHz Design advice for student project (PLB)
I am currently working on a Personal Locator Beacon redesign for a student project. from my research so far, PLB's use a vertically polarizing antenna to send both a 406HMz emergency signal (at 5watts)l to satellites and a 121.5MHz homing signal for rescue services to track on the ground.
I am a real novice when it comes to antenna design and electronics, I was wondering if you could help share some of your wisdom and answer some rookie questions?
1. For the two signals, what advantage does vertical polarization have? does it help the signal reach the satellites more clearly, does it increase the range?
2. Would a simple dipole antenna design be appropriate for this type of transmittion? are there alternative form factors for the antenna that could increase signal range or portability?
3. The environment of use for my design will be mountains and plains, the device will likely be close to the users body when the signals are sent, what kinds of facotrs will affect the absorbtion of the signal? (e.g rock faces, water, body contact)
Thank you for your time! any advice is appreciated :)
JH.
what advantage does vertical polarization have?
None. The signal will be subject to farady rotation as it passes through the ionosphere to the satellite. The use of a vertical whip type antenna is about the best compromise for this application. The direction to the satellite or rescue aircraft is not known so using an antenna that has an omni-directional radiation pattern is teh best choice. The radiation null will be along the axis of the antenna, so unless the receiver is directly in line with that axis it will receive something, even thouhg it wil not be as strong as it could be. Unless the antenna is in the open with little surrounding it, like on the open ocean ther will probably be enough reflections from local objects to fill in the null. For the 121.5MHz signal if the rescue craft is even close to the null the signal will be strong enough anyway. an aircraft flying at 1000 feet will be at an angle of 45 degrees 1000 feet (300m) away.
Would a simple dipole antenna design be appropriate for this type of transmission?
Yes, but not very practical for the application. You need something simple to deploy, a whip using the electronics and pcb as a groundplane is about as simple as you can get. You need a larger antenna to increase the radiated signal strength but going beyond a quaterwave whip is counter productive, the increase in signal strenght in one direction is at the expense of another. Conservation of energy applies, ideally you want to raditate the same signal density in all directions.
You have no control over the user environment ideally the antenna would be kept away from any absorbing objects. Rock will likely reflect more than it absorbs, anything damp, muddy bank, human body etc will absorb the signal. Make the output circuits robust enough to survive any vswr, in real life it will see it.
A word of warning, be very careful with any expreiments you do at these frequencies, and don't connect an antenna to your transmiter, the receivers on aircraft and satellites are very sensitive, you could end up with a heicopter hovering over your lab. It has happened. Lookup Friis equation and see how far a 1 milliwatt signal will propagate in free space.
Thank you very much for the fast and clear reply!
I've been looking into whip antennas for transmitting the signal so you've helped to clarify the situation for me. Many thanks!
Cheers, The experiments I will be required to do will relate more to ease of use and the mechanism needed to unfurl the antenna, so hopefully I wont be scrambling the rescue services anytime soon :)