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900hp range improvement

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Continuing from this answered question:
<http://www.digi.com/support/forum/49126/what-should-we-do-to-improve-xbee-pro-900hp-outdoor-range>

I am in a somewhat similar situation.
I have two 900hp-10k modules communicating with each other. One is inside of a moving farming vehicle, and one is acting as a base station.

I am trying to setup a low latency, low bandwidth, but reliable, communication network between the base station and vehicle.

The vehicles and base station always have visual LOS with each other. The maximum height at which I can mount the antenna on the vehicle is only 2 meters above the ground. As for the base station, I have the ability to mount the antenna higher, however less height is better as: (a) less height = less equipment = less cost, (b) more height = more environment problems since this is an outdoor application.

I am looking for an effective communication range of about 1 mile.

The guide <http://knowledge.digi.com/articles/Knowledge_Base_Article/Maximizing-range/?q=maximizing+range&l=en_US&fs=Search&pn=1> describes the Fresnel diameter at this distance/frequency as 32ft.

My current range test indicates that with both antennas mounted at <1 meter height, I can barely get 1000ft of reliable communication.

my questions:
(1) Given my absolute limit on vehicle antenna height will I be able to accomplish my 1 mile goal with these radios?

(2) The base station has a very good sense of where the vehicle will be when it needs to communicate. What types of gains will a directional antenna give us in this situation.

(3) Do we have a formula that describes the relationship between effective transmission distance and antenna height? For example, is this relationship governed by some inverse square law?

Thanks,
asked Sep 27, 2015 in IEEE 802.15.4 by collin New to the Community (1 point)

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2 Answers

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1: Only if you are able to get sufficient height on your Base to provide RF LOS conditions.

2: Directional antennas are only good if you are going in a straight line from Point A to point B with one of the two points being your base and the other being the remote NOT moving.

3: Yes there is but I don't recall what it is. You may want to do some basic searches in Google for antenna height.
answered Sep 29, 2015 by mvut Veteran of the Digi Community (11,302 points)
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Thanks for the response mvut.

As for (2) I have the ability to track my remote device such that I could mechanically move an antenna so that it always points towards the remote device. The only potential issue here is that the remote device could appear, to the tracking base-station antenna, to be moving towards or away at up to 40mph.

For(3), I did do some research and have found 'link budget' calculations, but have not found anything along the lines of "fresnelObstructionLoss_db = %Obstruction + ..."

Tonight I tried placing one of the modules, with a 2 dbi antenna, on the roof of my house (~20ft) and running a range and round trip timing program I created with libxbee.
The program sends, from the 'client' a timestamp. The 'server' device simply echos the timestamp and upon receiving the response from the server, I can find RTT. This program also sends an AT command 'DB' to the local XBee every time I receive a packet in order to retrieve the latest RSSI.

Sadly, there was no difference in range when the antenna was inside the house vs on the roof. This test was, however, in a neighborhood and I'm guessing the trees and other rooftops created enough obstructions.

Within a few days, I will try this same ground level antenna vs high antenna experiment in an open field to see if there is a difference.

One thing that was disappointing to me was that the largest RSSI reading ever got was 80. If I'm not mistaken what's advertised for the 10K 900hp module is -110db receiver sensitivity. Anyone know where the 30db are going?
answered Sep 30, 2015 by collin New to the Community (1 point)
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