Thread: SI Disco: Poor Range for 315MHz Security Remote

Thanks for the feed-back guys. I'll take some photos of the alarm position and connector and post them in a day or two to clarify the mod for you.

Incidentally, my issue with the alarm was not so much the lack of outright range, but because of the poor sensitivity, there were a number of locations where it refused to work altogether. Notably, it would not work in my local supermarket car park which is near a water tower that houses a number of communication services. I presume that amongst them is a service around 300MHz which creates an effect called "blocking" in the receiver (the receiver is de-sensitised by large signals even though they are not on the frequency the receiver is tuned to receive). With the modified antenna the remote does now work there but I do have to be within a metre or two of the vehicle.

I also work at the Adelaide MF broadcast transmitter site which has two 50KW AM services - the remote doesn't stand a chance of working within about 1Km of the 160m mast there! As 4bee noted this is also a problem with public car-parks sited near the high power VHF / UHF broadcast transmitter sites like Mt Lofty, Black Mountain Tower (Canberra), Gore Hill (Sydney), Mt Coot-tha (Brizzy) and so on. It's sensible, near these sites, to use your key rather than the remote in case it locks successfully but won't unlock due to the high interference levels (and it seems the key method of disabling the immobilser isn't entirely reliable).

If the vehicle is parked in a clear area with no interfering services, it looks like I've effectively improved the receiver sensitivity enough to get around 6m range, which also gives me sufficient sensitivity for the remote to work at around 1m to 2m in "difficult" rf environments. I accept that it's a big ask to work around 100KW broadcast sites so I'm pretty happy with the performance now.

As far as the chin-enhancement technique goes, it appears that height is irrelevant; the improvement is due to my head modifying the radiation pattern of the remote's antenna by its close proximity, somehow. It doesn't seem to matter how the remote is oriented as long as it's against my chin. It might not matter which part of my skull is involved, unless it's a matter of density (enough said) so further experimentation is required. I'm still waiting for a real explanation on this - I'll let you know when I have an answer.

If that radiation can kill a car remote system , what is it doing to the driver?
Just a thought.

Trev.

If moving the remote to the chin or something similar makes it work better, it would suggest to me the problem is the remote.

Try this as an experement: put a small wire to the remote's antenna to see if that improves range.

Or take keys off remote ie use remote only no keys no key rings.

Yes guys, the Electro-Magnetic Energy levels are quite high near broadcast transmit sites, but it's non-ionising radiation and there's no need to wear al-foil hats when you step out of your cars. Honest. I've worked in high power transmitter sites for years and it's never done me any harm. Oh - but I have a thing for LandRovers ... I see your point!

Cruiser, you're quite right that the real issue is the antenna in the key fob remote - but it's compounded by having a poor antenna in the vehicle. The key-fob antenna is way too short to be a significant portion of a wavelength at 315MHz (a quarter wavelength is about 220mm). There just isn't enough room inside the remote to fit that much wire. In fact the antenna track on the PC board is only about 50mm long. That's the main reason why UHF remotes work better - at 500MHz a quarter wavelength is only around 140mm - it's still a squeeze but you're closer to the mark and it helps enormously. If you could fit 5/8 of a wavelength in there it would be much better.

Now, I've discussed this with my work colleague who is an expert in this field, and we considered the following points...

The two ideals are to get the antenna to develop a high level of current on its wire and to give the antenna a characteristic impedance which is similar to the air around it to ensure that most of the rf energy generated by the transmitter is coupled to the air and radiates in all directions, but especially towards the LandRover. (I'm trying to keep this simple but it is a fairly complex subject.)

In fact anything in close proximity to the antenna modifies it's characteristic impedance and generally that means your hand and arm becomes a significant part of the antenna system. When you hold the remote to your chin you tend to hold the remote between your finger tips rather than wrapped between your fingers and thumb as you would normally. So some grips will better match the antenna to the ether than others. Also your hand provides somehing like a ground plane which sort of turns the monpole antenna into a half-wavelength dipole but now we're getting complicated.

Considering the remote's antenna is effectivley a monopole antenna (albeit a very short one), it provides maximum field strength for the reciever if it is oriented in the same plane as the receive antenna. Running the receive antenna up the windscreen pillar and then across the windscreen is a good thing because it gives it elements of both vertical and horizontal polarisations - this makes it care less about how the remote is oriented. If you've got a Disco with the short vertical wire dangling down the passenger-side kick panel it should help to hold the remote on its edge.

Holding the remote to your chin couples it's antenna closely to your skull and probably effectivley lengthens the antenna changing both its match and gain pattern. It seems likely that the match is improved and that the gain pattern is enhanced directly in front of the remote. Don't panic about being irradiated thoug - I'm only talking flea power here and the lenght of time you do this is very short.

Further, the body can be considered a reflective screen (rather than a container of saline liquid absorbing some of the transmitter's energy) which may improve the forward gain of the antenna by a small but significant amount. It might actually improve the signal even more to hold the remote closer to a quarter wavelength in front of your chin - maybe you can select the optimum distance for maximum coverage remembering that as you move it away from your chin you probably reduce the coupling to you skull and may worsen the impedance match. It's the old "tune for maximum smoke" experiment.

Finally, depending on where the receive antenna is run inside the vehicle, it hides behind quite lot of metalwork. If you suround an antenna with enough metal you can consider it to be inside a Faraday cage and it will pick up no signal at all. Having it dangling down inside the kick panel seems like a lousy place to locate it from this point of view.

That covers the most likely points which would explain why the chin-enhancement technique works - it is, however, an imprecise science. Or, for those of you who I lost a couple of paragraphs back, it's all done with smoke and mirrors.

Hope this helps explain it without getting too technical.

Here are a couple of photos which will help you find where to modify the alarm antenna in the SI Disco if you're so inclined.

The first shows the location of the alarm box - it's the dark green box to the left in "Security Alarm Controller" and is beneath the glove box in the passenger side footwell.

The second shows detail of connector C225 which is obscured behind the front connector in the first photo - the yellow antenna wire and the connectors I installed are clearly visible. There are no issues with removing this connector from the alarm to work on it - the central locking will cycle once when you refit it which is apparently normal.

(First time I've tried uploading photos so I hope this works.)

Did I see Lucas stamped on that alarm?

Con back to the corner for that comment about Lucas.


Anyway I've had a thought about your plight, what would happen say a family outing, lock car up get everyone to the station catch train, sit down as train departs station, bumping keys in your pocket or a kiddy playing with your keys and de activating alarm 50 metres away, you notice the indicators flashing to indicate the doors have unlocked as the train rolls away. Leaving your pride and joy alone and vunerable in this dark and dangerous world.


Yes it sounds like crap but I have actually seen this happen a couple of years ago whilst working at a train station. This was however on an old commodore so I guess would have been an aftermarket alarm

dobbo most new alarms relock an reactivate if no doors are open within time limit.
Sorry hard to come out of corner again.