Interesting remote control unit - antenna inside alu box!

[Blueteeth]

Hi there,

I got a small job modifying remote relay controllers (recievers) for continuity testing (just adding a transistor, couple of diodes, LED etc..). They use those small generic 433Mhz AM radio modules, along with a small micro for decoding - typical china type thing. (not my design)

Anyways, whilst working on them I noticed, the antenna they use is a helical one - just thin multiicore wire poorly formed into a coil, definitely not tuned, just haphazardly done. But here's the thing, it is completely enclosed inside the aluminum enclosure! There are only mounting holes for LED bezels, XLR connector and a power switch (which admittedly is plastic). The enclosure isn't grounded, or connected to any signal, but is pretty much solid. One thing of interest though is... the dimensions of the enclosure - the long side is 17.5cm.... pretty close to 1/4 wavelength for 433Mhz. Surely it can't use the enclosure itself as an antenna?

There is vast variation in range of these units, I'm surprised they work at all, especially at 30m+. So whilst I have them open, I was thinking of either replacing the antenna with a proper helical one (I don't have the facility to tune them though..) either hand wound, potted, or bought from Farnell, or working out a better way.

I guess the best would be coax from the RF module to an SMA/M4 then to an external ducky antenna, but these units will be thrown around and I fear it will snap off. If external is the only way to guarantee consistency, would a horizontal 1/4 antenna do? enclosed in a plastic box, mounted on the main enclosure? I'm just thinking half the power will be lost as it'll be laying flat, maybe 2-3cm off the enclosure.

Sorry for all the questions, but I'm just surprised it works at all... RF must be getting in there somewhere/somehow. So any idea's, or even just why it works would be appreciated. I'm not much of an RF guy.

Blueteeth

 

[JimB]

Radio module and antenna, completely enclosed in a metal box.
Mmmm... that sounds like a case of "Oooh, I never thought about that!"

My thoughts:
Antennas
There are so many other compromises going on here, that attempting to replace a random coil of wire with another one seems an exercise in futility.

How does it work in a metal box?
Do the edges of the metal box make GOOD contact with each other? Probably not, and the gaps are probably working as "slot antennas", especially if one side it about a quarter wevelength long. A half wavelength would be far better, but quarterwave will do the business.

You mention an XLR connector.
Is there a cable connected to that connector in normal operation?
If there is, and assuming that the wires to the connector are not filtered for RF, then there is a means of coupling RF in/out from the screened enclosure.

How to proceed?
If they work adequately as they are, don't worry and be glad that RF screening is not as easy as it sounds.
If you want to improve the range, or ensure reliable operation, then by some means make an antenna connection external to the metal box.
Your idea of an SMA connector and bought antenna is a proper engineering solution, but if it is physically prone to damage in normal use, something simpler and cheaper could be used.
How about a simple wire, about a quater wavelength long passing through the box on a simple robust connector. 4mm Banana plug/socket ??

JimB

 

[alec_t]

the antenna they use is a helical one - just thin multiicore wire poorly formed into a coil

Does that come into contact with the metal enclosure? If so, could it act as a base loading coil, the enclosure being the antenna 'proper'?

 

[Blueteeth]

Jim, alec, thanks for the replies!

Ok, the box is extruded aluminium with two halves that slot together, so, aside from the end plates, they make very good contact with each other. I couldn't find the exact model, but here's a couple that are similar (save dimensions):
https://www.electrostock.com/images/Bop/build_a_box.gif
**broken link removed**

The grooves that allow the two halves to slot together aren't just a rectangular cross-section, more of a teardrop, so it has to slide together, holding it in place - pretty damn good physical, and electrical contact - its a stupidly tight fit. The end plates are just flat alu with 4 screws, but there is no discernible gap.

In 'normal' operation, there is indeed a cable attached to the XLR connector, they use this as 'relay out'. The wires to this XLR inside the box are not screened, one is the negative terminal of the SLA battery, the other is the relay NO output (open). I think you are right about that XLR cable... I haven't seen how long it is (or even what they attach it to.. could be a bulb or something) but it looks like that is the *only* real way to get RF into this case. As to length of this cable, or what the load its attached to, I do not know - that could explain the wild inconsistencies in range... some cables 'just so happen' to have a reasonable length, along with loading, to allow some amount of signal to get in.

I'm always dubious about an external antenna of any kind, even just a wire, as this can be pulled off, lost, snapped, or even intentionally cut of (I have designed for 'public hire' and factories before.. if something can be broken, it will be in days). But of course, it *needs* a half decent antenna.

alec, the antenna does not come into contact with the enclosure, at all. It is soldered the radio module, which iself is mounted on a small PCB, that is bolted about 2mm off of the inside of one of the end plates (what a mouthful..).

As I seem to be typing lots here, I'll knock up a diagram later on, just with dimensions, and where the antenna is. As I said in my first post, these things work, and I wasn't asked to 'increase range' it really is just a curiosity.. I seriously doubt the manufacturer of these things has used some clever RF coupling techniques (ala apple..) because the tolerances of machining is quite slack.

Cheers!