I am interested in aircraft altitude measurement during approach. Off the shelf sonar doesn't provide enough distance, and inexpensive radio sensors don't provide a short enough distance. I have seen other radio wave projects attempt distance measure with "time of flight" and believe this is the wrong approach - a difference of phase angle would allow slower electronics and less critical rf circuits.
A search of this forum shows a number of discussion how impractical "time of flight" calculations are for simple electronics. I suspect the signs in many cities showing car speeds are using phase angle and not "time of flight", but they seem to quit measuring at 30-50ft.
I am trying to figure out what the lowest frequency will be needed for measuring 5ft to 200-300ft. Of course measuring down to 5ft is the critical frequency limitation and generally several cycles are needed for a the transmitted wave to measure accuratly. The table here is partially reproduced below.
Freq length 1/4 wave ................................ 50 MHZ 6m 1.5m 100 MHz 3m 28cm 300 mhz 1m 25cm 500 mhz .6m 15cm
Due to a less than perfect world, it looks like measuring down to 5ft (10ft round trip) will need at least 300mhz (1m full wave), but the 1/4 wave column got me wondering if some how less than a full wave can be measured accurately? I understand the 1/4 wave data is supplied for antenna purposes.
My project would be much simpler if I could use a 100mhz transmitter instead of 300mhz. I have found an off the shelf sensor (AD8302 board) for less than $10 with 1 degree phase resolution.
Does anyone know if there is a simple theory or whether it is possible to measure phase difference with less than a full wave?