I found many antenna advertised as "GSM/CDMA cellular antenna" over at sunsky-online. Looking at their specs, they seems to work in the XBee range. Might be a good piece to extend Xbee range if you could connect them together.
One particular thing I found is this "repeater".
Can someone chime in wether this is usable in the ground-end of the Xbee? I am interested in getting more range out of my serial modem but can't afford the XTend.
GSM900 Cellular Phone Signal Repeater Booster + Antenna
1) Frequency range: a) Up link: GSM 890-915/DCS 1710-1785(MHZ); b) Downlink: GSM 935~960/DCS 1805~1880(MHZ)
2) Gain(dB): GSM/DCS: >50/55/60/70
3) Output power (dBm):<10/17/20/27
4) Stray radiation: <=-36
5) Latency: <=5usec
6) Standing wave: <=1.5
7) Power supply: AC200V/DC9V
8) Interface Type: N-K
9) Noise factor: <=5dB
10) Impedance: 50ohm
11) Booster size: Approx. 197x105x32mm
12) Weight: 538g
13) Coverage: 100 meters
* Antenna speciation:
1) Frequency range: 900~1800MHZ
2) Product size: Approx. 39x20x2.5cm
3) Cable Length：10m
4) Weight: 811g
Replies