433MHz transmission

[thekman]

I use a simple contact sensor with an RF transmitter to fire a relay output on the receiver up to 40' away. This receiver is enclosed in a steel box with little or no air gaps, usually a NEMA 4. It seems to work ok with an antenna wire run out through a small hole in the box. I've been looking for an alternate transceiver, and have tested 3 (completely different vendors/models).
The original used 318MHz, and will barely if at all communicate w/o an antenna. Same for the one that uses 915MHz. The one using 433MHz works as if the box isn't there, up to over 100'.

I understand there are a lot of factors to consider which frequencies will penetrate which materials under certain conditions, but is it just dumb luck that this 433MHz transceiver blows the others away on distance? The receiver doesn't even have an antenna wired into it. I have a SS enclosure on my desk that I put the receiver in, and walked various distances (same path) away from it inside the building for my test. In real life, the transmitter will be outdoors mounted to metal moving between 10-40' away from the receiver, mounted inside said metal enclosure. I would like to understand if the frequency has something to do with it, and why. I don't have access to additional models by the other manufacturers using 433MHz, so I need to understand why the one works so much better. Incidentally, it is the smallest, lightest, cheapest one of the 3.

 

[IRstuff]

As a general rule of thumb, the lower the frequency, the less likely it is to make it through a hole of a given size.

This is the basic reason behind the metallic grid on the windows of microwave ovens. You've said nothing about transmitter power, so you've not presented much that can be processed.

TTFN (ta ta for now)
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[Comcokid]

It gets into other possible complex issues. Internal dimensions of the box may resonate near 433 MHz, and/or conduit running to the box may be integer multiple of the wavelength making the box and/or the conduit and/or wires to the box function as part of the antenna. It could also be that the length of the gap around the box front cover is near-resonance with 433 MHz - not only will a wire function as an antenna (E-field), but a slot in a metal plate (the cover gap) can be an antenna (H-Field).

 

[VEBill]

In terms of free-space path loss, higher frequencies are worse exactly as longer distances are worse.

FSPL(dB) = 20 log[sub]10[/sub](d) + 20 log[sub]10[/sub](f) + 32.45; where d is km and f is MHz

So the difference you're seeing is caused by something else of the sort mentioned by the others.

RF modules (and their modulation schemes) can vary widely in performance. Some are amazing, others are amazingly bad.

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In the real world, higher frequencies allows physically smaller antennas, and that can enable improvements.

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It's quite common to deal with RF equipment inside sealed metal boxes by installing a bulkhead BNC receptacle, wiring it up, and plugging in a little 'rubber duck' antenna on the outside. For these sorts of projects over very short distances, nothing is critical. This approach keeps the box weather-sealed, while allowing the RF in/out.

 

[thekman]

IRStuff - my transmitters are powered by either 2 AA's or 1 9v battery.

The only 'hole' in the box I'm testing with is the compressed gasket and whatever gap is created by me squashing the wires coming out....3 or more 18awg or smaller....when I close the hinged lid.

Thanks for the replies...