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2.4Ghz antennas do work below the waterline

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Tony Bell 201/05/2013 23:18:22
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My latest research into the R/C market has busted another myth.

2.4Ghz antennas work in any direction 50mm below the waterline at 50 and 100m. 40mm below the waterline at 185m.

The angle of the transmitter antenna seemed to have no effect.

The tests were not scientifically accurate but proved that they do work below water.

See full tests results at **LINK**

Tony

Colin Bishop02/05/2013 13:06:49
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I'm not sure just how much this proves really. The 'myth' as you refer to it suggests that 2.4Ghz signals cannot pass through water and that it is therefore not a good idea to put the antennae below the waterline as the signal can be degraded if there is not a direct 'air path' between the transmitter and the the RX antennae. If the transmitter is being held at say waist height then there will be a downward angle between it and the RX antennae, varying with distance and boat orientation which may mean that the signal does not have to pass through the water although it's strength might be degraded to some extent, not usually an issue given typical ranges at which model boats are operated.

My feeling therefore is that there is a greater risk of losing control at longer ranges if the RX antennae are mounted in the hull below the 'line of sight' to the transmitter. If you happened to be operating the boat from just a few feet away then it wouldn't matter if the antennae were in the bottom of the boat as you would be beaming the signal more or less directly down into the model.

I would still be inclined to mount the antennae at or above the waterline to minimise the risk of interference etc. Being digital, the control signal either works or it doesn't if reception falls off beyond a certain level and by mounting the antennae above the waterline you are minimising the possibility of signal dropout. Just good practice I would have felt.

Colin

Edited By Colin Bishop, Website Editor on 02/05/2013 13:08:41

Malcolm Frary02/05/2013 15:37:43
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The tests were conducted using a nice big box in calm fresh water. Cellphone technology, which 2.4GHz sets use, due to a bit of strange and unlikely maths that surfaced about 20 years ago, propogates with a spiral waveform, enabling it to reach into odd nooks and crannies better than a straight signal. An aerial mounted below waterline is relying on this, but the operative phrase is "reaches better", not "works infallibly".

Since microwaves, which also operate in this general band, work by having their energy absorbed by the liquid content of whatever they are heating, it should be no surprise that signals are lost in water. At cellphone powers, much the same as we use, the signal can penetrate 18mm of water before it is absorbed below usefulness. Reliance on the enhanced chance of a signal hitting the cavity formed in the water by the hull and finding the aerial rather than placing the aerial in line of sight is not a good pracitce, and will lead to poor reception and the problems that come with it.

Last week, a fellow club member lost signal in a fast boat on choppy salt water. The RX was mounted under the deck, but became dislodged and dropped into the bottom of the boat (about 1" below waterline when the boat stopped and settled) when between 50-100 yards out. Control was not re-established until the boat had drifted to within 10 yards of shore. The tests were an attempt at lab conditions, my mates boat was real life.

The difference between theory and practice is that in theory, there is no difference, in practice, there is.

Dave Milbourn05/05/2013 09:18:37
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Given the small size of most 2.4GHz receivers ahd the very short antennae used I wonder why one should need to mount them below the waterline of any normal model i.e. one with a freeboard and a superstructure of some description. If the general wisdom is that they don't work below the waterline then that seems good enough reason for me not to fit them there.
DM

Paul T05/05/2013 19:22:24
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There is a great deal of technical data to support the proposition of 2.4GHz not working efficiently under water.

Not to bore everyone with pages of calculations a brief synopsis of the data would be that whilst atmospheric density remains fairly constant the relative density of water is subject to far greater changes depending upon variables such as temperature and salinity and therefore water as a transmitting medium is far more likely to scatter or absorb the signal.

Tony Bell 207/05/2013 21:43:26
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Thanks for all your comments.

I wonder if you read my article “Before Buying” as nobody mentioned the antenna’s orientation which is as important as the height above water. Vertical or horizontal. I even bent one into a quarter circle. It made no difference.

In my article I looked at transmitter design and my research shows why manufactures are now placing the antenna inside the casing, it makes no difference but existing consumers might resist this idea if they are not educated into its benefits. The HobbyKing TX has the antenna inside the case in the usual direction but the Spectrum antenna was bent horizontally sideways. No difference found.

Originally I did not expect antennas to work below the waterline and pushed them to their extreme by putting them 40mm below and at a range of 180m

This information is now in the public domain as my site is searchable to anybody, anywhere, anytime and free unlike magazines, books. I use forums to extract expert advice which is not searchable. I am going to put a link to these forum posts. Forums are searchable,

Thanks again for all your help and very useful advice and I can get back to building my 1/10 Jolie Brise. www.joliebrisemodels.co.uk

I now have now found the sail winch to pull in my Main + Top Sail. The HobbyKing 11AYMB 50kg winch with a servo stretcher to adjust the sheet travel will allow me to pull the 4kg sail sheet load. At £30 it’s bigger and better than the RMG and is only running at 10% motor speed drop.

Paul T08/05/2013 15:10:03
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Tony

I read your article with great interest and fully reviewed your data before adding my comments to this thread.

The results of your experiment reveal possibilities for this equipment that have previously been considered as unachievable however I would add a note of caution.

Firstly regarding the variable condition of water as a transmitting medium I wonder if the experiment would produce the same results in seawater, fresh water with chemical additives such as a swimming pool or fresh water of differing temperatures / solids content (muddy water)

Secondly is it possible to repeat the experiment using single equipment sets only as this would eliminate any potential for signal amplification due to the presence of multiple transmitters / receivers.

Thirdly have you considered the possibility of external forces or equipment having an effect on your results, a quick look at the sky line reveals a lack of high tension cables but I have noted light industrial and residential development taking place around Watermead and it is possible that HT cables have been laid under the water. Are there any microwave or mobile phone masts in the vicinity or any companies dealing in microwave technology.

On a different note I did enjoy your website and the very interesting work that you have been doing on the Jolie Brise.

Paul

Edited By Paul T on 08/05/2013 15:10:55

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