Yea, its that new guy again....
Posted 19 February 2009 - 07:28 AM
After flying for a few days and being absolutely addicted, and having limited idea on how this RF stuff really works, I've come to the conclusion that most of my problems are multipathing interference.
I started doing some google searches, and stumbled upon this antenna design, the choke ring antenna.
Has anyone tried one? It looks easy enough to fabricate. Well, easy if I had dimensions to start scaling one to my 910mhz stuff.
BTW, when describing multipathing, most images make it look like a game of billiards. How accurate is that depiction for my frequency? Would a chain link fence be a source of reflections in my frequency band? Moving away from these reflectors should rapidly improve performance, yea?
Also, while I'm going to try this, does lowering the output power of my "500mw" transmitter make sense with regards to reducing multipathing problems? According to the 'documentation' if I reduce DC input power, the Tx will reduce output power. I'm currently running off a 3s lipo, and could easily use a 2s setup.
Posted 20 February 2009 - 12:05 AM
Absolutely, it's a multipathing nightmare. Multipathing will also occur from cars, buildings, hard earth, etc.
Would a chain link fence be a source of reflections in my frequency band?
I recall seeing a popular FPV vendor advertise that the 900Mhz 500mW system would reduce RF power with lower voltage. But it's not an accurate claim (it applies to the 200mW, but not the 500mW). There's a 7808 8V Vreg inside that powers everything, including the RF circuitry. It ensure that RF power remains the same as the 12V battery discharges. Once you get below about 9V, the video quality begins to degrade, then goes away completely on voltages a bit lower than that. So, by the time you've lowered the voltage enough to substantially reduce the RF power, the video image will be toast.
According to the 'documentation' if I reduce DC input power, the Tx will reduce output power.
If you are experiencing too much multipathing on your 900Mhz system then you need to move to another flying site. It needs to be wide open area, away from everything. Flying in a field with metal fences, cars, man made structures, and other RF reflective surfaces will just cause grief.