What range will we get with AM receiver/transmitter with no antenna?

[treez]

Hello,

We wish to put an RF receiver and transmitter inside LED warning beacons on trucks. They will be used to receive data pertaining to which type of flash pattern they should be showing.
There will be another receiver and transmitter in the truck cab, for the driver to communicate to the beacon with.

We will use this receiver and transmitter….

AM-HRR30-433 (£4.97) miniature receiver module datasheet:
https://www.rfsolutions.co.uk/acatalog/info_AM_HRR30_433.html

AM-RT5-433 (£5.71) Transmitter datasheet…
https://www.rfsolutions.co.uk/acatalog/DS013-9 AM-RTx.pdf

..these are said to be good for 50 metres when used with the lambda/4 antenna…

FLEXI-M4-433 antenna datasheet (1/4 wave) (£3.95)
https://www.rfsolutions.co.uk/acatalog/DS016-13_AM-HRR.pdf


We do not want to use the antenna, what range do you think we will get without the antenna?
In fact, instead of the proper antenna, we may just solder a 17cm bit of 7/0.2mm wire to the antenna pin.
We will just leave the wire coiled into whatever position it ends up in, as it wont be perfect, but surely better than nothing? What range do you think we will get? (Its only 15 metres from truck cab to the warning beacon.)

 

[FvM]

The RF modules are designed to interface a 50 ohms antenna. A straight 17 cm wire is a perfect λ/4 antenna at 430 MHz with near 50 ohms impedance, similar to the "rubber duck" antennas you can buy. Coiling it up considerably reduces the effectivity and demands for a matching network to re-improve it.

 

[treez]

Thanks, the size of the antenna is a problem to us, I therefore think we should look to 2.4GHz..... but when we look at 2.4GHz rx/tx devices, they always have the packet encoder included with it, eg

http://www.hoperf.co.uk/shop/info-RFM70P-Transceiver-Module-2.4GHz.html#SID=261

...we do not want this, we just want a 2.4GHz receiver which works by literally ASK modulation, as in the rx/tx in post #1...we do not want any encodeing as we can't manage the software for that...we will do our own bit bashing protocol. We just want to send a "1" or "0" from the cab and receive it as a "1" or zero in the beacon, but none of the 2.4GHz modules have this simplicity.....do you know of any 2.4GHz transmitters and receivers which have the same modus operandi as the AM tx/rx in post #1?

in other words, we effectively want the rx/tx pair to act like a digital isolator (in effect)...do you know of a 2.4ghz tx/rx pair that allow this?

 

[FvM]

I believe that using 434 MHz band is the simple and reliable solution, provided the product is only distributed in a region allowing 434 MHz small radio devices. 2.4 GHz is much more prone to multipath and dead zone problems.

You can check with an electrical small helix antenna without matching network. Or review the TI application notes about 434 MHz antennas, e.g. www.ti.com/lit/an/swra161b/swra161b.pdf‎

 

[mtwieg]

I've never seen any simple RF link modules at 2.4GHz, which makes sense given that 2.4GHz only worthwhile if you need high datarates, in which case having the encoding and packetization integrated is very useful.

Going to 2.4GHz just for the sake of size doesn't really make sense though, since higher frequency inherently will have poorer sensitivity over large distances. I'd bet that a 430MHz module with a little chip antenna would get comparable performance with a 2.4GHz module with a nice whip antenna. Also look at helical and splatch antennas for 430MHz.

 

[FvM]

The only reason to use 2.4 GHz for a simple low data rate application would be international license free operation.

 

[treez]

Going to 2.4GHz just for the sake of size doesn't really make sense though, since higher frequency inherently will have poorer sensitivity over large distances

thanks, of course, our distance is max of 15metres, to the beacon from the cab.....some of the 2.4ghz transceivers I have seen on RF solutions website are really cheap.....£2.70.....its a shame they don't do simple "digital isolator"-like ones as we only need 15metres......and after all our mobile phones use 2.4ghz and they do ok.

 

[betwixt]

I use AM-RT5-433 transmitter modules with AM-HRR3 receivers (from RF solutions) here. I use the 'rubber duck' on the receiver but the coiled antenna specified on the data sheet for the transmitter. It works fine over a range of about 20m but that includes two brick walls so in free space I would expect better results. I strongly recommend you use data encoding, it isn't difficult and it makes a huge difference to reliability. I did one way tests over about 50m with a coiled antenna at each end and it still seemed to work OK.

As for 433 or 2400MHz, bear in mind that most 2.4GHz units are transceivers and contain the antenna switching circuits, when you use 433MHz with those modules you have to make sure you can't transmit and receive simultaneously and for simplicity you need an antenna for each. Your other option is to use PIN diode switching which pushes the cost and complexity up.

Brian.

 

[treez]

thanks, I hadn't realised both tx and rx need different antennas....thankyou....also, we will use encoding , but it will be our own protocol. its so much easier when you've done it yourself...ive never seen a good explanation os i2c , spi etc


think we will have just tx in the cab, and just rx in the beacon....save on antennas

 

[Audioguru]

Super-regen receivers are used on very cheapo kids toy remote controlled cars. What will prevent yours from receiving nearby interference? Nearby interference can also overload a super-regen input and cause it to stop working.

 

[treez]

thanks, it would juts mean the flash pattern wouldn't get changed.....so maybe we could have , er, double send of data, and only do it if both the same.
Or indeed we may just have to have 4 antennas and do acknowledges so we know data got sent ok.
Datasheet doesn't say about overloading...thanks for pointing it out

 

[Dan Mills]

Just a thought, but are not truck cabs basically big metal boxes at least as far as radiation towards the rear is concerned, are there not often other big steel boxes between the cab and the warning lights?

Generally placing a large metal plate between the TX & RX aerials is not a recipe for reliable communications......

Regards, Dan.

 

[treez]

Just a thought, but are not truck cabs basically big metal boxes at least as far as radiation towards the rear is concerned, are there not often other big steel boxes between the cab and the warning lights?

Generally placing a large metal plate between the TX & RX aerials is not a recipe for reliable communications......

thanks but I think 433mhz is low enough frequency to swerve round the metal?...also....our mob phones work in similar places?....my mobile worked inside a factory which was all metal (of course maybe they had base station inside I admit I dont know

 

[betwixt]

My approach to this, and I have considerable experience, would be a one-way radio link with the lights themselves being used to confirm reception of the data.

ive never seen a good explanation os i2c , spi etc

these are data protocols, the kind you need to get reliable RF communication are different. The most common system is "Manchester" encoding but there are others. It is all to do with the symetry of the data and spaces between consecutive bits. I use a modified bi-phase algorithm of my own coding but it is very easy to do. I can share the algorithm, or at least the principles of operation if you like but not publicly.

Brian.