RF range problem

[George]

I'm trying to build a transmitter - and had issues initially with some easy radio modules, I've now got some LINX modules which I'm having much more success with tho my range is somewhat limited (about 150mm) the guide says they are good for 3000ft! I was thinking my issue was in the antenna - but surely it can't make that much difference? (can it???) All i've got for an antenna is a 170mm length of wire stuck onto the ant output on the module. The module is 418MHz - any ideas? Should I double it to 340mm to make a 1/2 wave?

[mackrackit]

The short answer is YES. Antenna design makes all the difference.

Longer answer.
A ¼ wave only works against a ground plane, normally a wire “hanging out” does not have a ground plane to work against.

A ½ wave monopole or ½ wave dipole does not need a ground plane. The ½ monopole will have depending on the height above ground a lower radiation angle than the dipole.

Another problem to look out for. Both the transmitter antenna and receiver antenna need to be orientated the same way for maximum range. (vertical in relation to the earth or horizontal to the earth)

Longest answer.
Start here http://www.linxtechnologies.com/documents/AN-00500.pdf
then google antenna theory and design.

If you like electronics and programing you will have a blast with electromagnetic radiation (radio)

Dave

[Ron Marcus]

When these companies post range estimates, they are usually calculated as if the link were on the top of Mount Everest, and the other end is in a completely shielded helicopter at the range quoted. A space vacuum on a day where there is minimal sunspot activity, or solar radiation would also work. Other than that, treat these estimates as if they came from the lips of a politician.
In real life, we have all kinds of competing signals that will desensitize a receiver. Try to use an FM link as these are a little more immune to impulse noise than their AM or OOK counterparts. For ease of use, I use Radiotronix Wi.232 modules. They average about 300 meters with a 1/2" helical antenna @ 900 MHz.
Ron

[George]

I realise that incorrect antenna and less than ideal conditions will result in reduced range, but 1000m down to 0.1m? that's a 10,000 fold drop in range. I've tried 1/2 wave antenna on transmitter and receiver and tried a 1/2 wave dipole on the transmitter - note these are all made with cat5. I've turned off all the fluroescent lights to try minimise chance of stray noise in the workshop. Would a factory built aerial make this much difference and maybe give me 100m in less than ideal circumstances? Haven't yet tried a proper ground plane - any suggestios on size? Anything else you suggest I try? Surely LINX wouldn't be selling these modules if they can barely transmit over 0.1m

[languer]

my range is somewhat limited (about 150mm)

Do you mean 150m or 150mm?

->150m sounds about average for normal conditions (i.e. noise within the same band, limited LOS environment, fresnel limitations, etc ).

->150mm, I do not think even the best antenna will help you there.

[George]

Do you mean 150m or 150mm?

->150m sounds about average for normal conditions (i.e. noise within the same band, limited LOS environment, fresnel limitations, etc ).

->150mm, I do not think even the best antenna will help you there.

I mean 150mm (yes millimeters) - and that's at best! mostly i'm struggling to get reliable transmission at greater than 100mm I'm sending one byte and one bit of data , am sending $AA five times 1st do do what ever majic that does - if the modules are sitting right next to each other with 170mm wire hanging off each it works flawlessly - if i double that length so a 1/2 wave on both (doesn't need a ground plane to resonate with) it doesn't work at all.

[Ron Marcus]

Have you formatted your data for RF transmission? The Linx modules need to be conditioned to the incoming data. If you haven't done this, please search the archives for the proper conditioning of data to be transmitted.
Ron
P.S. I thought you were at 150 meters! My bad.

[George]

I would have thought that if it was transmitting flawlessly at close range and not at range then it's a transmission fault - not a comms fault

This is how I'm transmitting -

SEROUT PortA.0,0,[$AA,$AA,$AA,$AA,$AA,"A",timeset,1]

this is how i'm receiving:

serin PORTA.3,0,["A"],timeset,b1

[George]

Sorted the problem out finally, the problem is I had a 330 ohm resistor on the supply to the receiver chip, it says to do this in the datasheet if you are intending on using it on 5V - I removed the resistor and ran it on 3V and it works a treat, I might put a couple diodes on it to drop the voltage a little in future.

Thanks for helping out

[Ron Marcus]

What modules are you using?

[George]

LINX 418-LR modules - i ended up running three diodes to the supply of the RX module to drop the voltage - also have a 4.7 Tant to help level supply. The thing works great - I can get 300m now line of sight with just a couple bits of wire for an aerial. I have a 1/4 wave with a very small ground plane on the TX and a full wave with no ground plane on the RX