Aerials/antennae for 458/459Mhz

We need to do some development work on these because aerials behave very differently underwater.

So far, I have just been using a very simple whip or rubber ducky aerial placed horizontally in the WTC.

Very neat to fit (no wire wrapped around things!) and it seems to work pretty well.

The wavelength at 458 Mhz is 0.66m so a quarter wave dipole area will be about 165mm long. And this is what I have used so far. (I am no aerial expert.)

But we have always known that you really need to substantially shorten the aerial length underwater, but we've never really bothered. (There is an article somewhere in the AMS magazine articles - I have not found it yet. Anyone know it?)

There is a very good article by Butler available online:

http://users.tpg.com.au/users/ldbutler/Underwater_Communication.pdf

This shows that the wavelength (and hence aerial length ) is affected by the frequency and the conductivity of the water.

I have used Butler's formula on page 3. I have halved the conductivity of Adelaide, Australia water for UK surface waters. (I assume less dissolved solids?, so I used a value of 0.03 mhos/m)

This gives a wavelength of  0.85m giving a quarter wave aerial length of 210mm. So that  is what I will try next.

What is interesting is that it is not much different from the aerial length in air. I don't quite understand this yet!

My reading of his article also gives some further clues why 458Mhz works at all:

-The refraction/interface losses of the signal (i.e where your transmitter signal hits the water.) reduce at higher frequencies.

- A horizontal aerial is probably better than a vertical because of refraction at the surface.

Any aerial/antenna experts available to comment on this . (Where is Paul B when you need him!)

The final article is sort of interesting and perhaps shows that underwater radio communication has been rather overlooked.

http://www.hydro-international.com/issues/articles/id697-Underwater_Electromagnetic_Propagation.html

David

Hi David,

first of all let me say that the sad news about Brian Alps have made me sad, too. I had never had the occasion to meet him face-to-face but we had been in contact by email for a while when I had sent him my former guest article for the AMS Magazine and I will keep him in mind as a friendly submarine modeller colleague.

Concerning the considerations about the antennas I must confess not beeing a real antenna expert but I can tell you from my basic knowledge that the wavelength of an electromagnetic wave depends of the material constants of the medium of propagation, such as the dielectric constant (relative permittivity) and the permeability (relative permeability) that have influence on the electric and the magnetic field component of the wave. In water the relative permeability is nearly the same as in air so the magnetic field component is nearly the same, but the relative permittivity of water is 80 (!) and so the electric field component is reduced very much. So the wavelength in water is equal to the wavelength in air devided by the square root of the relative permittivity, therefore it is only approximately one ninth of the wavelength in air. Of course a quarter wavelength antenna for water will be less effective in air.
It would be worth to do an experiment with such an antenna to prove these considerations.

Kind regards
Klaus-Dieter

Thanks Klaus-Dieter and Tim!

So Klaus-Dieter, for an aerial immersed in water you would suggest a quarter wave aerial length one ninth of that in air so, 18mm.

I have now had a chance to read Tim's excellent description of his trials on Subpirates. He uses  RX antenna lengths of 4.5,  2 and 1.25 inches with good results. (I am relieved to see that he records control at depths of 8 feet in heavily chlorinated swimming pools and 6 feet in large lakes.)

Adjusting for the higher frequency of 458Mhz and converting to mm gives RX aerial lengths of 108,48 & 30mm respectively.

Tim does not seem to make any adjustment for the fact that the aerial is immersed in water. Presumably because the aerial is actually in air within the WTC?

I think that the wavelength calculated from the formula in the paper by Butler is wrong, for some reason.

It would be worth doing some trials of an aerial immersed in water (but insulated obviously) at the length indicated by Klaus-Dieter. It may avoid the losses experienced at the air/water interface.

In the mean time, I will try out Tim's antennae lengths of 108,48 & 30mm. horizontally mounted within the WTC.

Hi David,

if your aerial is within the WTC (that means: in air) then the length must be just the same as the original length for air, that means no adaption for water shall be made. Adaptions can be discussed if the aerial is just inside the water although I am not really convinced that an aerial that is adopted for a quarter wavelength in water will be still useful when the submarine is surfaced because the effectiveness of the aerial is much worse than in original length because it is too short in electrical sense. On the other hand an aerial longer (the nine fold) than a quarter wavelength in water will also work in water.

For 40MHz RC sets I have not shortened my aerials because of the same reason. In original length they are less than a quarter wavelength in air (less than 7.5m/4 = 1.875m) and cutting down to 21cm (a ninth of 1,875m) would be far to low for reception in air when submerged.

At last your experiments will show what is best.

Kind regards
Klaus-Dieter

David,

do not go making antennas based on the lengths of the antennas posted on subpirates.

those were ones purchased on ebay, and are that length only in their external plastic casing.  the active antenna element inside the casing is not remotely similar, and in most cases has been "loaded" in some way.

use an antenna calculating website.

again, i am stymied because i haven't been posting here for more than a week, so i can't embed links.

do a google search on "antenna length calculators" and you will find a half dozen sites that allow you to enter a length in mhz (like 458 ) and they will give you a length in feet or inches, somewhere around 6.44" or 164mm for a quarter-wave antenna.   a 16th wave antenna would be 41mm, and a half-wave antenna would be 328mm.  a full wave antenna would be 656mm. an advantage to the 433/458 is that a 1/16, 1/4, and 1/2 wave antenna will all fit inside even tiny WTCs, and a full-wave antenna will fit in larger WTCs.

according to other sites on how to build your own antenna (google "make sanders style monopole antenna") the actual length for a tuned antenna (using the RSSI method) may be as short as 90 percent of what the calculator gives you (148mm instead of 164mm).

its all kind of a black science.

for a picture of antennas i have made check out the "tiny 433 receiver" thread on subpirates.  i have a picture there of both a brotronics micro rx and an orangerx rx that have homemade quarter-wave antennas.

good news is each antenna has about $3 worth of materials in them.


maybe i should make a "how to make your own antenna" thread on subpirates....

just out of curiosity, i went on ebay.co.uk, and searched for 458mhz antenna, and someone is selling quarter-wave antennas for £7.55... sadly 458mhz antennas are not nearly as common as 433mhz antennas.

Hi Klaus-Dieter and Tim,

Thanks for clearing those points up.

The photo shows my contribution, a selection of antennae - in the process of having their length revised!

I have just used stiff "hook up" wire enclosed in a 4mm OD nylon tube. (I use this for water and air connections also.)

Not brilliant probably but very easy to make and easy to fit into a WTC.



Thanks for the advice Tim. I will try a half wave and quarter wave antenna at 328 and 164mm respectively.

And Klaus-Dieter, I will later on try a completely immersed aerial. Not least because it would be an elegant solution. (A tiny stub aerial just sticking out of a bulkhead into the water!) I wonder if it would be large enough to prevent the RF chip burning out when in air though?

Thanks again, both! Will let you know how I get on.

David

PS I forgot to say that I have turned Telemetry back on in the openLRS software. Only very little servo jitter now. So I hope to be able to hear the Transmitter beeps when the RX loses signal.

PPS Tim, I can't see the photos on SubPirates - I need to register!! Also I have had a note from Kevin and Nigel. We are puzzled about the links problem. We don't try to disadvantage non-registered users in any way. Perhaps you need to use the link function? (Though I never do I just put the URL in.)