433MHz RF Modules not yielding expected range / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

433MHz RF Modules not yielding expected range

Move Thread

LAN_403

The Zander

Sat Nov 11 2006, 01:35AM

Registered Member #295 Joined: Tue Mar 07 2006, 06:59PM
Location:
Posts: 23

I recently purchased several of the following 433MHz modules off eBay

Transmitter
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=020&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=300008280968&rd=1&rd=1

Reciever
http://cgi.ebay.com/RF-ASK-Superheterodyne-Receiver-Module_W0QQitemZ300008281339QQihZ020QQcategoryZ1200QQcmdZViewItem

I'm building an RF PIC - PIC serial link (i know they are commercially available, but thats no fun) and i honestly haven't the slightest idea how these modules work, but i really only need to know how to use them. so heres my question, right now the superhet reciever modules outputs between 0.5-1.0 volts when the 433MHz carrier from the transmitter is not present (the voltage just bounces around in this range w/o rhyme or reason). When the transmitter is powered (12VDC) and the TX line on the transmitter is pulled high the voltage @ the output of the reciever module rises to 1.5V +/- 0.05V (very stable). So while i can verify the two (RX/TX) modules are interacting i can neither achieve any decent range (< 10m) nor can i get the high output of the RX module anywhere near engough to power a single LED. as i side note i don't acutally want to power any LED, there just isn't enough between 1.0V and 1.5V to assure completely reliable, medium distance communication. more bandwidth would make my life easier

so what is common practise when integrating this types of devices into projects. pleae note, i have connected a simple 1/4 wave dipole to the antenna connection (just a straight piece of 20AWG magnet wire), and what can be done to to improve there performance? could my workshop environment be the problem (picture lots of CRT monitors, large speakers, power supplies... you know)?

Thanx
-TheZander

...

Sat Nov 11 2006, 02:12AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The output from the module should be ttl, ie 0v or 5v, not any of this junk in the middle....

Are you sure you are powering the modules with a good clean power supply at their rated voltage?

I would be inclined to think you have a RX problem.

But I should also point that that the little RF modules always have their range overstated. -100dbm will help, but still if you only have like 5mw of transmit power you will have problems. I should point out that my rocket telemetry link (using a maxstream x-tend modem) has a 0.5w (27dbm) 900mhz tx and -100db rx, and it gets ~3k feet. I should also point that my xecom modules (same specs) got ~300ft.

On that note, I don't need the xecom modules anymore. If you want them I will sell you them for $100 for the pair. The data sheet can be found at xecom.com (it is under catalog->wireless, then on the left XE900S-500 ). They are a complete system, you just connect with a terminal at 9600-8-n-1, type atd001, and viola it works. I have had the link work a few hundred feet, but really I haven't got anything close to the rated range

I should also point out that the rocket born transceiver came down pretty hard once and ripped out the 4 useful pins, but I have since repaired them (just be gentle with them)

Dave Marshall

Sat Nov 11 2006, 03:34AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Unfortunately, as is sometimes the case with older computer serial ports, a 0 isn't always 0v. You frequently find poorly constructed circuits that have a lot of noise in the off state. The solution here would be a zener diode to give you a cleaner output, and a transistor to give you a little bit of amplification on the output that might make it more useful. Once you clean it up a bit and remove the noise in the off state, 1.5v in the on state should be MORE than sufficient to turn a small signal transistor like a 2n2222 or 2n3904 on.

These little modules suck, plain and simple. However, you can make them suck a lot less with a decent antenna. The antenna connection on the board isn't a suggestion, its a requirement. Without one, they are pretty much deaf, and the only reception you'll get is what little signal the traces on the PCB might pick up. The 1/4 wave vertical (dipole describes an antenna with a grounded counterpoise as well) will help a bit, but its far from ideal.

Something as simple as a 5/8 wavelength (41cm) piece of wire from the antenna port will make a difference. Somewhat less practical but even more effective is a half wave (32cm) dipole. 16cm tied to the antenna output, 16cm to the circuit ground.

If your modules will be stationary and you're looking for some real range, a pair of well designed Cubical Quad antennas can add substantial gain, and give you excellent range. I believe those modules output something around 5mW. A 7 element 440Mhz quad isn't a tiny antenna, but its not prohibitively large either. Such an antenna can reasonably be expected to provide 7-8dBi of forward gain, and improve reliability with a substantial noise rejection behind and broadside to the antenna. Just on the gain in the forward lobe alone, your measly 5mW becomes 35-40mW ERP. Some of the LEO ham satellites have 440Mhz downlinks that aren't quite 4 times that much power, and they can be heard for thousands of kilometers. With one of these antennas on each end, at 6 feet above the ground, I'd expect range to be atleast 2km if there isn't anything large and solid in the way (mountains, big buildings, etc). If you could get the antennas above the trees, it could be a good deal farther than that.

All of the antennas I described above would be plenty for the receiver, as SWR (impedance matching) is less critical on a receiver. On the transmitter, matching the antenna impedance to the output impedance of the module will be critical, and can mean the difference between great reception, and none at all. If the output impedance were 50 ohms, and with a perfect ground (you dont have anything close to a perfect ground) that 1/4 wave bit of wire has a feedpoint impedance of ~20 ohms. This is an impedance mismatch of >2:1, and all other things being ideal, gives you less than 2.5mW out of the antenna. Take into consideration the 1/4 wave probably has 0 gain at best, and you aren't radiating much.

For a 50-75 ohm impedance a half wave dipole will be plenty sufficient, and should be fairly well matched to the module impedance without too much tinkering. If its some other impedance, an antenna match could be constructed, or you could tinker with an Off Center Fed dipole or an inverted Vee that provides a way to adjust the antenna impedance.

If the data sheet doesn't specify an impedance, constructing a Pi type antenna match, a half wave inverted Vee, and using a UHF receiver to determine at what tuner settings the signal is loudest could give you a general idea. Alternately, you could borrow a UHF SWR meter from a local ham, a 50 ohm UHF dummy load, and just figure out what tuner settings match your transmitter to the 50 ohm dummy load. Knowing the inductive and capacitive values could let you work backwards to figure the transmitter impedance.

Dave

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob