antenna-theory.com

[David M]

I had a thread almost a year ago about the same application i am asking about now but i want to get thoughts on a different approach or ways to improve what i have, any thoughts would be appreciated.

Here is the application:
Center Freq is 380 MHz
Radio sits directly on the ground
The Antenna will be 8" off the ground sitting flat
Radio system is a mesh network communicating with other ground mounted radios just as described
Think of a flat rectangle 8" x 6" x 1" which is the space reserved for the antenna to mount in the radio enclosure (made of plastic) that is 8" off the ground in this box with a clear view at the horizon but with heat sinks (grounded) and other circuitry underneath the area reserved for an antenna.
It needs to radiate in all directions parallel to the ground to communicate with other mesh netwrok radios sitting up to 500' away.
RF output power is +24 dBm.
Rx Sensitivity is -105 dBm

Here is what has been tried so far:

Horizontally polarized dipole is currently implemented (I have a picture of the antenna i could submit but not sure how in this forum)
The two diploe antenna elements sit at a right angles to eachother to fit in the space and form two sides of the imaginary rectangle forming 8" x 6" sides
Fed with a double shielded Coax cable with the center conductor soldered to the 8" element and coax shield soldered to the 6" element.
The elements are printed on FR4 substrate
It is fairly well matched at our frequency and radiates reasonably well ( between 0 and -5 dBi along the horizon in all directions) in free space in the desired enclosure
It has been measured in a large anechoeic chamber with no ground plane under it to get these numbers

However, in the actual application due to being so close to the ground (i think) I am seeing large signal nulls as two radios are spaced along the ground radomly out to 500' in any direction
I think this is due to the ground being in the Fresnel zone and causing multipath interference big time.
After we saw issues as described, it was measured again with a ground plane in the anechoeic chamber and the radiation pattern changes significantly as we are seeing in real opration.

Any ideas on different structures or ways to do this? I have done the PL equations and thought we would be fine with the output power and senstivity described (the power and sensitivty has been measured and verified) but in actual operation the ground is causing huge problems. Any help is appreciated.