Field Strength Meters for 433 Mhz RF Modules

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clockwork

Member
As PICAXE projects sometimes use low power 433 Mhz RF modules I wonder if any forum member has a circuit of a Field Strength Meter that will work at this frequency and more importantly with such low power. Preferably aperiodic, simple and use commonly available components

Whilst I can find lots of circuits for Field Strength Meters most appear to require much higher levels of radiated power before they can give any useful indication.

I am trying to develop a couple of projects that use PICAXE's and 433 Mhz RF modules that require some experimentation with the aerials.


Many thanks

Clockwork
 
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srnet

Senior Member
Does the RF receiver module have a RSSI\AGC output ?

I was testing Video 2.4ghz antenna performance using this method.

With a 10mw video transmitter, in an open space (transmitter on a tall stool in the middle of a large park) I was able to get some reasonable receiver antenna comparisions just by hooking up a multimeter to the RSSI\AGC line of the RF module.

By plotting how the signal reading dropped off with distance with one antenna I could then draw assumptions as the the relative dB gain of various antennas since field strengthshould drop off with the square of the distance.
 
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srnet

Senior Member
I also have an analogue meter with a digital output.

The analogue needle makes it easy to point the antenna for max reception, then read off the level on the digital bit.
 
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manuka

Senior Member
That park setup is also much what I've often used, but enhanced with binoculars for readings from greater distances. Ranges are typically several 100m LOS of 433 MHz devices, & even at 30 metres antenna benefits can be redaily noted this way.

You can of course set up a simple transmitter, & conveniently note receiver performance as the antenna is adjusted/rotated.

UHF CB sets are typically ~450-470 MHz of course, & their high power (500mW at least) will tickle a regular FSM along nicely. Use one of these sets hence while trialling the antenna - it'll be close enough in freq. to 433 MHz for valid insights.
 
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manuka

Senior Member
You're quoting a full wavelength- most vertical whips are just a quarter wave. Hence at 433 MHz a mere ~16- 17cm (160-170mm) is the norm.
 
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Pauldesign

Senior Member
Hey Clockwork,

Just bear in mine (except in a system) using only the receiver RSSI feature might not be a very accurate method to measure received 433MHz signal strength; as the RSSI (depending on the vendor chip internal design) might give indication of any RF signal/interference present and not necessarily those emitted by 433MHz transmitters.

Using some ingenuity as mentioned by Manuka might be more worthy.
 
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Dippy

Moderator
A) Read the module manufacturer's Data Sheet. They will provide a Data Sheet if they are any good.

B) Don't forget to allow for connection between module and antenna and/or tracks - unless you are using a RF PCB techniques and/or screened cable an then it may be different.


Paul makes a good point, though this can be tested for by siwthing the carrier on and off.
Most little problems can be surmounted by a little imagination, common sense and having a play ;)
 
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srnet

Senior Member
Pauldesign said:
Just bear in mine (except in a system) using only the receiver RSSI feature might not be a very accurate method to measure received 433MHz signal strength; as the RSSI (depending on the vendor chip internal design) might give indication of any RF signal/interference present and not necessarily those emitted by 433MHz transmitters
Click to expand...
True, and the nearer you are to lots of people and buildings (particularly industrial types) the more this can be a problem.

With the transmitter you are measuring off, you should get an idea if this is a problem.
 
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clockwork

Member
I have one set of Tx/Rx modules that have a RSSI output, thts OK but I also have another set (The ones sold by Rev Ed) that do not appear to have any method of determining received signal strength. Also both sets work on differing frequencies.

To date I have used the "S" Meter of a VHf/UHF Communications Reciever to give a rough indication of Transmitted Power. I would like to have a bit of kit that I can use without having to have my Communications Receiver to hand.
 
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