869MHz link problems - please help!!!

[1icarus1]

Hi there,

I need to get up to 1500 meters (in open field) comunication link, between ADEUNIS modules. My terminals are a master which is an 869.525 MHz ASK transmitter and a slaves (ASK receiver).

The complete set of modules I used is the ARF27 from ADEUNIS and it's datasheet can be found at:
**broken link removed**

The characteristics of the transmitter (ARF7243A) are:
Operating frequency 869.525 MHz
Conducted power from 25 mW (+14dBm) to 500mW (+27dBm) on 50 Ohm at 5V
Modulation ASK

The characteristics of the receiver (ARF7246A) are:
Frequency 869.525 MHz
Sensitivity 0.5μV (-113dBm) On 50Ω
Demodulation ASK
Bandwidth 100 kHz

The complete set characteristics are:
Link settling time 2.5ms
Range in open field 2000m
Binary rate from 500 to 2400 bps Manchester

ADEUNIS suggests whip antennas to be used, for high distanses.

Personally I don't need to get the maximum, but up to 1500 meters in open field. In practice the project is a transmitter that is always moving round the receiver and sending measurements from a sensor.

I have build the master and slave circuits, following Adeunis APNote's information (modules filters, protocols, e.t.c.) and the programs are working fine.

My first test was using simple cables 86mm (λ/4), at both transmitter's and receiver's ANTENNA pin, using them as antennas. The result was to get up to 150 meters at open field. So I realized that my problem, was my antennas.

It is impossible to use other than 86mm cable at the receiver (IP66), but it is possible to use any antenna you suggest me at the transmitter module.

I have no experience on RF theory. Could you please suggest me the antenna I need (a URL could be fine) and also the distanse of the coaxial cable I have to use (I neeb at least 5 meters of coaxial cable to connect the antenna to the transmitter).

Michael.

 

[vfone]

RX Sensitivity (-113dBm) – Lowest TX Power (+14dbm) = -99dBm
Free space Path Loss @ 869MHz for 1500m = -95dB
So, if you use RX and TX antennas with 0dB of gain will remain only 4dB margin that you can allocate for cable loss, and 5m of coax cable at this frequency could have more than 4dB attenuation.
You need more margin, because the real path loss is higher than ideal Free Space path loss. Probably you need more gain on both antennas.

 

[biff44]

I recommend that you hire a local RF Consultant to help you solve the problem. He is familiar with your local regulations, and will get your solution working with a minimum amount of redesign.

You do not provide enough information on your problem. We would have to know the country of use, data rate, message size (total bits), usage (fixed or mobile), battery or line powered, etc etc.

One might suggest a higher gain transmit antenna if you were fixed position AND your country regulations allow the higher eirp. One might suggest a different module using FSK modulation. One might play with the received bandwidths vs data rate to try to get a better signal to noise ratio. One might use a transmit power amplifier. etc.