RF Transmitter/Reciver

[Fredrick]

Have just placed an order for 2 433.9 MHz transmitters and 2 recivers..

The parallax manual says upp to 500+ feets and LINX says upp to 3000 Feets, what is the actul range?
Can i make parallax 433.9 MHz transmitter to send whit more power?

[Fredrick]

Have looked at the datasheet for the TXM-433-LR and find an intressting point.

USING LADJ
The Level Adjust (LADJ) line allows the transmitter’s output power to be easily
adjusted for range control, lower power consumption, or to meet legal
requirements. This is done by placing a resistor between VCC and LADJ. The
value of the resistor determines the output power level. When LADJ is connected
to VCC, the output power and current consumption will be at its maximum. Figure
4 on Page 3 shows a graph of the output power vs. LADJ resistance.
This line is very useful during FCC testing to compensate for antenna gain or
other product-specific issues that may cause the output power to exceed legal
limits. A variable resistor can be temporarily used so that the test lab can
precisely adjust the output power to the maximum level allowed by law. The
variable resistor’s value can be noted and a fixed resistor substituted for final
testing. Even in designs where attenuation is not anticipated, it is a good idea to
place a resistor pad connected to LADJ and VCC so that it can be used if needed.
For more sophisticated designs, LADJ can be also controlled by a DAC or digital
potentiometer to allow precise and digitally variable output power control.

It´s look like i can easy modifie my RF module to work whit the range of 3000 feet or (900 meters) by removing or replace the resitor R5 (se Picture), and i think it´s still legal i sweden to transmit at this ranges on the private 433.92 MHz band.

http://www.parallax.com/Portals/0/Im...79/27980-L.jpg

[mackrackit]

As others have said check your local laws.

You are reading the data sheet, did you see the part about 3 volts. Do not make the mistake of using 5 volts like you normally would with a PIC.

[Fredrick]

As others have said check your local laws.

You are reading the data sheet, did you see the part about 3 volts. Do not make the mistake of using 5 volts like you normally would with a PIC.

The Typical voltage for the LINX module is 3 volts and the parallax module need 5 volts so i assume that there is an voltage regulator on board.

So if i measure the voltage between GND and infront of the resistor R5 and have 3 volts it shold not be an problem to just bypass the restistor to get maximun power or?

I just check the local law and there is no problem to run at maximun power on the 433.92 MHz band.

[dhouston]

The legal limits are on the radiated power not distance. Distance is heavily influenced by environment. It's impossible to say whether you are within the legal limits without some sophisticated and expensive test equipment. The tests required by the FCC for the USA cost about $US3000.

You can see the limits here...

• http://www.radiometrix.co.uk/apps/apnt102.htm

Of course, whether the authorities will catch you and punish you if you exceed the limits is another matter.

[Fredrick]

The legal limits are on the radiated power not distance. Distance is heavily influenced by environment. It's impossible to say whether you are within the legal limits without some sophisticated and expensive test equipment. The tests required by the FCC for the USA cost about $US3000.

You can see the limits here...

• http://www.radiometrix.co.uk/apps/apnt102.htm

Of course, whether the authorities will catch you and punish you if you exceed the limits is another matter.

Ok... but in teori (legel or not) i only have to bypass or change the resistor R5 to another value.... or?

In the doc i find the limit is Max 25 mW in sweden not 10mW.
http://www.scanner.nu/pdf/riks.pdf (it´s i swedish)

[mackrackit]

Ok... but in teori (legel or not) i only have to bypass or change the resistor R5 to another value.... or?

In the doc i find the limit is Max 25 mW in sweden not 10mW.
http://www.scanner.nu/pdf/riks.pdf (it´s i swedish)

If R5 is connected to LADJ then you may be right. Just remember the max voltage of 3 any place on the LINX.

Another way to get maximum distance from any radio is to use a good antenna. I doubt the , as we call them here "duckie" that is on the Parallax is very good. Most duckies are not very good for for distance.

A quarter wave with ground plane or a half wave will work the best for distance at any power.

I think dhouston has antenna info on his web site. Or google for a ham radio site for more info. The antenna is half of the radio in terms of performance.

[dhouston]

I've never used the Parallax receiver and transmitter so there may be reasons why the things I've learned with the simpler (and order of magnitude cheaper) TWS transmitters and receivers may not all be applicable.

From the LADJ excerpt that was quoted, shorting the resistor so that Vcc is applied to LADJ will give maximum output power from the transmitter.

The distance above the ground will also have a big effect on range. Reflections/refractions from the ground, buildings, trees, etc. degrade the signal. Most of the range estimates that accompany these devices are outdoor, line-of-sight figures. Indoors, my rule of thumb estimate is 20-25% of the outdoor figure.

An efficient, directional antenna (e.g. a yagi) on both the transmitter and receiver can give you large increases in range by concentrating the power in one direction rather than spreading it equally in all directions. The stubby antennas on the Parallax modules are not very efficient and are omni-directional. There are some yagi antennas for this frequency that are printed on circuit boards (or even mylar). Here's an example...

• http://www.wa5vjb.com/products1.html

A wideband preamp between antenna and receiver can also give a big increase in range but this may not work with all receiver designs. In some cases, the increased signal can overwhelm a receiver but I've used them successfully with the TWS receivers and have even tested with 2-3 of them cascaded. This has one major advantage - it does not violate any of the legal limits on transmitted power levels.