QUESTION

Question

hello,

currently working on a design to extend GPS signal under water. The GPS signal from the surface is amplified and sent over a coax to the receiver under water. At the end of the coax the antenna is mounted( SRFg017) the antenna is in a waterproof casing( POM/Delrin) it is a small enclusure. This enclosure is piggy backed to the recievers patch antenna. The purpose is to transfer the signal over to the receivers patch antenna. What I see s that the impedance of the antenna is verry good in free air, but verry bad in the enclosure. This due to the very near proximity of the PM/Delrin and water that surrounds this enclosure. In the water the resonant frequency is not 1575MHz, but about 1900MHz.

What would be a good design approach to have an antenna in resonance in this water envionment? Hope you can help.
Regards, Rob van Vreden

Answer 1

Hi Rob, 

It will be very difficult to pass a signal between antennas if it goes through any water.  Can you make a mechnical connection between each enclosure so there’s no water present in the path between the antennas.  If there are flat sides on each enclosure, then when you push them together the water should be squeezed out.   It would be interesting to hear how you get on, it sounds like an novel application. 

Regards,

Michael 

Comments

hello Michael,

yes it is a difficult route but I have a proof of concept. It works with a short coax cable that does not attenuate too much signal from the surface patch antenna. But a longer coax cable does not work any more( even with an extra amplifier). I have seen that the majority of the attenuation is in the antenna housing (POM/Delrin) and the distance the antenna has inside that housing irt the side walls of that housing. The antenna de-tunes when its proximity to the walls is close. It gets better when the distance is bigger, but I can not built bulky boxes.

So, to get the antenna back in its original resonant frequency, what can I do best? Is there a example of how the antenna can be tuned? It resonates inside the small housing at about 1900MHz. It should be 1575. I would expect that the antenna would lower its frequency, but instead, it goes up. Hope you can help me with a design that brings down the frequency to 1575MHZ. regards Rob BTW, I will investigate if I can build a watertight/waterproof connection between the antenna box and the receiving device.

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Hi Rob,
Have you tried with the transmit antenna close to the receive antenna (Bentoni)?  Since the antennas are close to each other, it may not be a problem that it is off-frequency.
Michael

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Hello Michael, I use at the moment a Patch antenna as receiving antenna. it is a excisting environment. This patch antenna is mounted to the plastic wall of the enclosure. The patch antenna  bandwidth is quite narrow; not so for the SRF antenna. The total distance of the plastic, stacked is about 20mm. And, inbetween a layer of water film.
not sure if I can include pictures of the setup in this mail?

kind regards,

Rob