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(Sorry about the use of "diff", but I needed the characters.) In air, a half-wave dipole can be physically trimmed to resonance and will radiate well. It can also be tuned using a pi network to resonance and will also radiate well. In fresh water, the same dipole needs to be trimmed to approximately 1/9 the length to resonate. It also can be tuned to resonate using a pi network, leaving the length the same as in air.

What would be the difference in operational range underwater between the two approaches?

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My question was more general about optimizing the ERP via trimming verus tuning and I was using the dipole as a well understood reference. Experimentally, I have trimmed a 1/4 ground plane underwater, using a VNA and it's trimmed length in air was reduced from ~3cm to ~2cm. This was done in a small air filled glass jar using the metal lid as the counter poise. Although I have tried to, I assume I could have instead tuned it to resonance, again whilst underwater using a pi network and minimizing the reflected energy. As actually measuring the operation range of either approach is quite difficult, I am wondering what the approximate difference would be in general terms?

Specifically I am developing a underwater (fresh not salt) for communicating only a short distance (50cm) using Bluetooth transceivers.

I would be most interested in learning more about magnetic loops. A big challenge with these is how to go about tuning these underwater?

Another important experimental need is what to employ for an underwater antenna that would have known or calibrated characteristics such as gain, and directionality UNDERWATER.

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A pi network, or any other impedance tuning circuit used between an antenna and a transmitter, adds or subtracts inductance or capacitance so as to null out the effects of the antenna being nonresonant (mismatched) at the desired frequency and thereby present a purely resistive load impedance to the transmitter.

This is done to prevent power that the transmitter is sending to the antenna from reflecting off the impedance mismatch and returning to the transmitter, upsetting it and possibly causing it to self-destruct.

The tuning network in essence "wastes off" the reflected power- which is now power that cannot contribute to the creation of a broadcast signal from the antenna. In this sense, it does not enhance the creation of radio waves- it only protects the transmitter from reflected power caused by off-resonance operation of the antenna.

To maximize the radiated power of the antenna, you are always better off with an antenna physically trimmed for resonance at the operating frequency.

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G'day Jay

A dipole is not a good choice for an under water antenna. The characteristic impedance of water is very low. A dipole has an impedance of approx 73 Ohms at the feed point and has the property of being able match to the impedance of free space, which is 377 Ohms. Also the ends of a thin dipole may have reactive impedance up to several thousand Ohm's. To put it bluntly, the dipole will be short circuited by the low impedance of the water. The actual impedance of water varies greatly depending on on its purity and salinity. A loop antenna is a much better choice, however it has to be designed appropriately to match your source impedance to the impedance of your water. impure water will be extremely lossy to all but very low frequencies. More details of your application are required to fine tune the answer, unless it's just a general query.

Hope this helps. Barry

The loop impedance will be quite low, one or two Ohms and will be inductive and will need capacitive reactance to be added to resonate the loop. Some form of transformer may be preferred, designing the matching will not be trivial but the end result will be far superior to a dipole. You may need access to some simulating software to design the matching circuit. I may be able to help in this area if your not in too much of a hurry for a result. What type of measuring equipment do you have access to? Silly me, just read you have access to a VNA that will make life easier particularly if you use Smith charts.

Regards Barry

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  • $\begingroup$ Barry: I just expanded the original with more details. $\endgroup$ – Jay Zebryk Apr 16 at 14:55
  • $\begingroup$ Jay: I'll edit mt post to answer your question. $\endgroup$ – barry Apr 17 at 6:16
  • $\begingroup$ I have a spectrum analyzer, (2) signal generators and a VNA. (The VNA can produce Smith Charts.) For UW testing I have a large glass fish tank filled with fresh (tap) water. Due to reflections from the glass, I just switched to a polyethene tank with my reference antenna on the outside. Happly to send you photos directly. New to this forum, you can email me Jay@Zebryk.com My website is zebryk.com My linkedin.com/in/zebryk FYI, I have been working on this for (6) months. $\endgroup$ – Jay Zebryk Apr 17 at 21:16
  • $\begingroup$ By far the biggest reflection will be from the water itself.The dielectric constant of water is about 80. If your reference antenna is a resonant dipole the electric component of the field will be almost totally reflected, only the magnetic component of the field will penetrate the water. If total path loss and or sensitivity is your aim, A resonant magnetic loop is what you need outside the tank also. A loop outside will relatively easy to design because you will not have space restrictions outside. I'm assuming that this device is to receive information from the gut of an animal? $\endgroup$ – barry Apr 29 at 17:36
  • $\begingroup$ I presume a large pill or capsule. At 2.4 GHz a physically small loop would be required. This could be a multi turn loop tuned to resonance with capacitance or a stub of semi rigid coax, this where the smith chart will be useful. If you're trying to communicate with submarines at large distances use a lower frequency say 20 KHz, and big big loop's. $\endgroup$ – barry Apr 29 at 17:52

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