Radio transmission question - spread spectrum

[wingtip]

Has anyone tried one of the new spread spectrum radios (surface or air, just for testing purposes) to compare against fm/pcm type transmissions to see if there is any better or worse performance in water at depth.....?

[aeroengineer1]

I know that David Merriman

I know that David Merriman has a friend that just bought that setup and was going to do some testing, but my guess is that the performance will not be all that great for a submarine application. The higher up you go in frequency, the lower the penetration capacity of the signal. The difference between standard 75Mhz and the 2.4Ghz is about 2 orders of magnitude (ie a lot). If I am proven to be wrong, then great, just my thoughts on the subject.

Adam

[wingtip]

thats the whole point, this

thats the whole point, this system doesnt transmitt on "just" one freq. It transmitts on one freq for an instant then looks for another "open/unused" freq and transmitts there for an instant, then again and again...
someone did the math a few months ago on one of my heli forums on this and figured that there would have to be 900+ pilots (or commanders in our case lol) before there would be a good chance of overlapping...
and from what ive read so far in the electrics field of use it has eliminated many interference problems that people were having even with pcm use...
Either way, good or bad Hope he posts some in depth results of his tests...I'd be interested in the results, as im sure others would as well...........

[JWLaRue]

I'm thinking that these systems

I'm thinking that these systems will not work for the same reason that the video transmitters won't work unless the antenna is out of the water........the water attenuates the signal too quickly.

If true, it has nothing to do with the system's ability to handle possibly hundreds of users.

-Jeff

p.s. I'd like to be proven wrong too!

[wingtip]

that was merely a statement

that was merely a statement of one of the advantages.... i didnt say the number of users had anything to do with its actual transmission thru water
This is probably one of the biggest advances in rc since the programmable radio came along lol..... hope it spawns more new ideas and technology........



Edited By wingtip on 1137555383

[aeroengineer1]

Here is a link that

Here is a link that describes the ability of a radio signal to penetrate water. Kind of technical, but very good.

http://www.qsl.net/vk5br/UwaterComms.htm

Adam

[tmsmalley]

Wow Adam - Congratulations for

Wow Adam - Congratulations for finding this gem!
Is there any way that you could do a summary of the article (for us non electrical engineer types) as it applies to the radio frequencies we use in the rc sub hobby?

[aeroengineer1]

Tim,

I will try, there are

Tim,

I will try, there are some things described there that I do not have a feel of yet, but I will study it out and try and give a nonengineer summary. The thing that struck me is that perhaps submarine antennas might be significantly longer than we need. I am not sure how this might be affecting the overall reception on our models. I will do some more research.

What I can say is this; the higher the frequency the higher the attenuation (ie the higher the antennuation the worse it is for good reception and transmission through the water). If I remember right the attenuation went up in proportion to the square root of the frequency. This means that if your frequency were 100 the multiplier would be 10, but if your frequency were 10000, the multiplier would be 100. relating this to the frequencies being discussed 75MHz (75,000,000hz) and 2.4GHz(2,400,000,000); the multiplier for 75MHz would be around 9000 and for 2.4GHz the multiplier would be about 50,000. So we see that the 2.4Ghz signal will have almost 6 times the attenuation (remember higher attenuation is bad). This is what I understand as of now of the article, and I will study it out more to see what future good information I can pull out.

Adam

[tmsmalley]

So our transmitters - which

So our transmitters - which are lower in freq that what they are talking about - are better for transmission through water?

And the antenna wire on our receivers have to be some fraction of the actual length of the wave (1/4?) or have some kind of loading coil to stay tuned in the frequency band - correct?

Discussion on RC antennas

I found a really good page on RC antenna lengths at

http://www.rc-cam.com/ant_exp.htm

He does some interesting experiments measuring signal strength comparing wrapping the full antenna length around a soda straw (as many of us do in subs) with actually cutting it to 19". The results were surprizing - for me at least!

It would be fun to see this repeated in an underwater scenario!

[aeroengineer1]

Tim,

Great links! I agree that

Tim,

Great links! I agree that it would be nice to see some experimentation with these things as well. I do not think that radio reception is much of an issue with lakes, but I do know that in a pool, I cannot get down past about 4' before my failsafe kicks in. If I had the equipment and understood it better, I would do that. Perhaps there is someone out there that has that equipment.

Adam

[KevinMC]

I'll echo Adam's comments about

I'll echo Adam's comments about the great links!

Tim, you're bang on with your understanding.

All things considered, really all you'd need equipment wise to duplicate this test is a voltmeter that can be fit and read from within a WTC. (And a pool that isn't stiff right now!)

FWIW, I'll have a go at this when the weather gets warm again.

[wingtip]

ok, here ya go!

Since i

ok, here ya go!

Since i work at IU at a nuclear radiation test facility (IUCF) I decided to go talk to our RF engineer and he's came up with something i think may help somewhat... he drew up a simple schematic for me to build that can test the "recieved signal strength"... So all i need to do now that i have built this little box, is make a small water tight container just big enough for a reciever, a battery, and this box(size of a rx), and make a pass thru stud where i can attach different length antenna leads right to the outside and i can then be able to measure voltage right while the rec is under water (multimeter leads run out the container and up a stick to the meter)...and find which antenna length actually gives the best reception for the water condition (pool,lake etc) being tested in.

i will post pics as comes available... Im still interested in merrimans friend doing the testing with the spread spectrum stuff as well...

[aeroengineer1]

Great news! It would be

Great news! It would be interesting to see what antenna lengths actually give the best performance underwater. Wingtip I would love to see the schematics for what would need to be done and if at all possible, a small explanation of the data interpretation that one would get from your schematic.

Adam

[tmsmalley]

Wingtip - Could you share

Wingtip - Could you share that schematic your buddy drew up? I'll bet a lot of guys would find a use for something like that!


This has the making of a great SubCommittee Report article too. The problem with posting stuff on the net is that all that hard work can all go away very suddenly - but when its printed hardcopy it can last forever!

[wingtip]

he sketched the parts up

he sketched the parts up on a post it note but i will have him give me a proper drawing and post it... He made it out to be a common everyday thing that he uses and knew the parts and values off the top of his head and gathered what i needed, i just had to assemble it... was just a diode, two capaciters, two resisters... but when i come back in to work tomorrow i'll have him draw it up with a basic explanation...its quittin time now so heading to the homestead to come up with a small testing wtc of some sort lol..... may end up doing these tests at the feb carmel fun run using their indoor pool....