RF Tx/Rx Range

Archiver

Which of the RF transmitter/receiver sets that Parallax offers would
have the most range? The wire antenna set or the fixed antenna set?

How long is the antenna on the wire antenna set?

TIA
Hank

Archiver

Hank,

The fixed antenna (actually a loop antenna etched on the board) offer
the best range (typically 100m outdoors, 15m indoors). Those are the
Parallax 9600bps modules (although the data over the radio link is
actually 1200bps half duplex).

The other modules are of the same RF power level (approx 1mW), but do
not have the advantage of the error checking built in.

In general, the pulse-amplitude modulation used will limit the range
to around 100m anyway.

If you choose for a wire-antenna module, you can calculate the
antenna length in meters as follows: (3*10^8)/(433.9*10^6) which will
give you the WAVELENGTH the unit operates on at 433.9 MHz (approx
70cm). Change the 433.9 to 418 if using the 418MHz modules.
To calculate the wire length needed, you need an antenna of ONE
QUARTER of the wavelength.. ie, around 6 inches for 433.9MHz.

There is a fudge you can do on the Lynx modules used on the board to
increase the power very slightly. From the Lynx data sheet, there is
a power control resistor on the transmit module (fitted as a surface
mount on the Parralax board). You could try adding a resistor in
parallel to reduce the overall value to gain an extra bit of power
(about 1.5 dB).. again, this will not make such a huge difference to
the range (but will drain the battery faster!)

One word of advice concerning the receivers.. mount them WELL AWAY
from the BS2 (I used a BS2p24) because the stamp chip produces a LOT
of RF noise which interferes with reception. I spaced my rx module a
few inches away after putting an earth screen UNDER the stamp on my
PCB layout (my project was a 2-way RF remote control with LCD
display) and the space in the housing was tight.

The other consideration concerns the antenna itself - keep it well
away from metal, the processor and digital circuitry too.
The etched loop antenna is more tolerant to proximity of wood etc,
the wire antenna is more prone to detuning because of stray
capacitance (hands, wood surfaces etc..)

If you choose for the wire antenna, using a quarter wave, try putting
a ground plane at 90º under the antenna. That will increase your
range substantially. Otherwise, put another quarter wave in the
opposite direction, connected to ground to form a simple dipole
antenna.

Good luck,

Dare


--- In basicstamps@y..., "rcvehicles" <hankster@h...> wrote:
> Which of the RF transmitter/receiver sets that Parallax offers
would
> have the most range? The wire antenna set or the fixed antenna set?
>
> How long is the antenna on the wire antenna set?
>
> TIA
> Hank