Designing Dipole Antenna Help

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spectech2
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Designing Dipole Antenna Help

Post by spectech2 » Wed May 18, 2016 8:25 am

I am very new to designing antennas. My internship wants me to build a device that can pick up a 50Hz-2000Hz electric signal in the 1mV-25mV range underwater. Theres a fish that can do produce a 1mV/cm electric field, up to 2 meters. I can make the amplifying circuit and filtering circuit aspects of it but I would really like some help or ideas with this antenna or how to go about it. Plan on using a Diff Op-Amp and a few Op-Amps.

Thanks Mike

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Dale H. Cook
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Re: Designing Dipole Antenna Help

Post by Dale H. Cook » Thu May 19, 2016 5:28 am

A dipole is not practical at 50 Hz to 2 kHz - in order to work reasonably well in that frequency range it would need to be about 1,600 feet long.

Since you are using the antenna as an untuned antenna it just needs to be as long as feasible.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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spectech2
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Re: Designing Dipole Antenna Help

Post by spectech2 » Thu May 19, 2016 5:46 am

If possible could you point me in the right direction as to where to look this up? How to design, what shape it needs to be to pick up this electric field. I am sorry, im very new to antenna theory, im an electrical engineer who specializes in diff op-amp's and FET's and my boss threw me to the wolves. Ive been searching high and low just to find what I need and still scratch my head. Im just having trouble figuring out the shape of the antenna and length(small as possible). Since this fish creates a dipole the electric field doesn't propagate like EM waves. It falls off by the inverse of the distance cubed.

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RGORJANCE
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Re: Designing Dipole Antenna Help

Post by RGORJANCE » Thu May 19, 2016 7:01 am

I am wondering if a loop antenna might be a possible candidate. If you used a very small gauge of wire, it wouldn't be too difficult to wind enough around a form and then waterproof it. The loop will be directional, but there may be a double loop configuration that would tend to be more omni directional than the single loop.

Loop antenna design detail is all over the internet, and might give you some ideas.

Fossil

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KPJL FM
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Re: Designing Dipole Antenna Help

Post by KPJL FM » Thu May 19, 2016 7:19 am

ferrite loopstick from an old AM radio was the first thing that came to mind.
Trim to fit, paint to match, tune for minimum smoke.

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Deep Thought
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Re: Designing Dipole Antenna Help

Post by Deep Thought » Thu May 19, 2016 7:43 am

"Since this fish creates a dipole the electric field doesn't propagate like EM waves. It falls off by the inverse of the distance cubed."

You're probably going to have to be almost on top of the fish to detect it even if you can figure out a sensor that'll do it. I'm not sure an antenna is what you want. Is this salt or fresh water?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

spectech2
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Re: Designing Dipole Antenna Help

Post by spectech2 » Thu May 19, 2016 7:48 am

Ty for the advice so far everyone. Fresh water and the papers ive read said they can produce a field up to two meters, only for adults.. I know a differential op-amp can pick up very small voltages, and that's what my plan was in general and then amplify the signal with a couple op-amps. Also all the data im reading on loop antennas are in the MHz range.

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kkiddkkidd
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Re: Designing Dipole Antenna Help

Post by kkiddkkidd » Thu May 19, 2016 3:59 pm

Dale H. Cook wrote:A dipole is not practical at 50 Hz to 2 kHz - in order to work reasonably well in that frequency range it would need to be about 1,600 feet long. Since you are using the antenna as an untuned antenna it just needs to be as long as feasible.
If you are talking about a half wave dipole... That would be a little short...

A loop of some type will the the only practical antenna for those freqs. Unless you go find the Fractal antenna guy that has terrorized the ham reflectors for the last few years and then you can supposedly receive everything on a 6in long antenna...
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Dale H. Cook
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Re: Designing Dipole Antenna Help

Post by Dale H. Cook » Fri May 20, 2016 6:22 am

kkiddkkidd wrote:
Dale H. Cook wrote:If you are talking about a half wave dipole... That would be a little short...
The frequencies that he gave cover a greater than 5 octave range, so I just picked a frequency as an example. It would be about 9,800 feet at 50 Hz and about 246 feet at 2 kHz. And yes, I did mean a half-wave dipole, as when we say "dipole" without qualification in broadcast engineering that is what we mean.
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Re: Designing Dipole Antenna Help

Post by spectech2 » Fri May 20, 2016 8:44 am

Im thinking a inductor with a 40-gauge wire and either ferrite or iron core would work best at these low frequency's and low mV's. If I were to design this: ( this is still in theory just choose a low frequency, etc)
I know i'll need a high impedance compared to the source's impedance to pick up minute voltage changes, usually 10 times is a rule of thumb. so Z= 2*pi*f*L.
So: Z,f are known variables. And solving for L will give me my inductance.
Since I know the inductance now I can plug this into the inductor's equation: L=(u*N^2*A)/l
Where:
u is the constant of the core I use
A is the area of the whole coil and core combined
l is the length of the whole coil and core l=3.3 ft, (which I use a fraction of the wavelength here, 3.3ft is 1/3300 the wavelength of 10k ft.)
N is what I solve for correct?

The only thing im wondering is if I use a 3.3 ft inductor, wont the power be 1/3300 of the source?

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kkiddkkidd
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Re: Designing Dipole Antenna Help

Post by kkiddkkidd » Fri May 20, 2016 9:57 am

Dale H. Cook wrote: The frequencies that he gave cover a greater than 5 octave range, so I just picked a frequency as an example. It would be about 9,800 feet at 50 Hz and about 246 feet at 2 kHz. And yes, I did mean a half-wave dipole, as when we say "dipole" without qualification in broadcast engineering that is what we mean.
I was being facetious on the dipole comment.

Me thinks you are off by a few decimal places. Or maybe I have lost what little math skills I acquired from the TN public school system...

A half wave dipole for 50hz = 468(ish) / Fmhz = 468 / .000050 = 9,360,000ft = 1772.727 miles
A half wave dipole for 2khz = 468(ish) / Fmhz = 468 / .002 = 234,000ft = 44.318 miles

Or have I missed something?

Which really has nothing whatsoever to do with the OP's question...

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Dale H. Cook
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Re: Designing Dipole Antenna Help

Post by Dale H. Cook » Fri May 20, 2016 12:04 pm

kkiddkkidd wrote:Or have I missed something?
Nope, I missed something while using the RFS technical program - not surprising, as I have never used it at audio frequencies.
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kkiddkkidd
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Re: Designing Dipole Antenna Help

Post by kkiddkkidd » Fri May 20, 2016 2:46 pm

Dale H. Cook wrote:Nope, I missed something while using the RFS technical program - not surprising, as I have never used it at audio frequencies.
I understand. The reason it caught my eye immediately was that one of my college electronics instructors gave us a tech problem to design a dipole to transmit 60hz power wirelessly. The result was a dipole almost to the foot the distance between Lawrenceburg, TN (my home) and Las Vegas, NV. The feed would have been just east of Amarillo, TX. I have no clue why that high valuable factoid stuck when most of my college experience is a bit of a blur.

Later,
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COMMENG
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Re: Designing Dipole Antenna Help

Post by COMMENG » Sun May 22, 2016 4:18 pm

spectech2 wrote:Im thinking a inductor with a 40-gauge wire and either ferrite or iron core would work best at these low frequency's and low mV's. If I were to design this: ( this is still in theory just choose a low frequency, etc)
I know i'll need a high impedance compared to the source's impedance to pick up minute voltage changes, usually 10 times is a rule of thumb. so Z= 2*pi*f*L.
So: Z,f are known variables. And solving for L will give me my inductance.
Since I know the inductance now I can plug this into the inductor's equation: L=(u*N^2*A)/l
Where:
u is the constant of the core I use
A is the area of the whole coil and core combined
l is the length of the whole coil and core l=3.3 ft, (which I use a fraction of the wavelength here, 3.3ft is 1/3300 the wavelength of 10k ft.)
N is what I solve for correct?

The only thing im wondering is if I use a 3.3 ft inductor, wont the power be 1/3300 of the source?
You first need to determine (research) how far these transient signals will propagate in water, dielectric constants for water, etc.

The answer may well be, "Nada."

However, if the answer is in the affirmative, a loopstick antenna fully insulated and isolated from the water is still the best potential candidate.

What you would need is a loop with a rod of material #72.

I would also consult Polydoroff's equations for magnetic rod materials, in terms of induced voltage verses distance between source and receiver.


COMMENG.

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