antenna-theory.com

[Stormpetrel]

Hi all

As it is the first time I participate to this forum I would like to thank the people who have created this website. It is a gold mine for a low frequency minded guy like me.

Regarding my problem, I'm trying to make a small device to evaluate the interaction of seabirds with boats for my wife studies. Those small devices are attached on the back of the seabirds for a short period. As most of the boats are equipped with radar, my idea is to use a RF detector and record the radar level with a µc. We just want to know if the animals approach a boat (<100m) or not. The sea radars operate at 9.4Ghz.
Please note, I have no experience at all in antenna design and HF but anyway I have decided to try something.

The RF detector is a LTC5532 from Linear Technology. It operates up to 12Ghz. (The datasheet states 300MHz to 7Ghz, but it is said that the lead-less version works up to 12GHz with reduced performance). http://www.linear.com/product/LTC5532

My antenna is a "CPW-Fed Ultra wide band slot antenna with triangular tuning stub". It is based on the following paper: http://www.ijcee.org/papers/229-E277.pdf

I have modified the dimensions for a better detection at 9.4Ghz. The antenna is made using FR4 0.8mm thick substrate.

Here are the results of the simulation:

Return loss:


Input impedance:


As those results looked good to me, I have made a PCB based on this model.

PCB:


Detailed view of the coupling capacitor


As you can see the distances between the Rfin pin of the LT5532, the coupling cap and the antenna are as short as possible (2mm including the cap )

Unfortunately my first try to measure some radar signal are unsuccessful. Today, I was 50m away from a ferry with its radar rotating and I could not measure any signal at all on my scope except the EMI of my cell phone when placed in the vicinity of my circuit... All the components seems to work properly. As I don't have any HF analyzer to check my antenna, I have no clue why it does not work.


Do you guys think I am on the right track with this antenna/detector solution? Do I miss something in the antenna theory?

Regards, Dom

[bigSteve]

At 9.4 GHz, the difference between a good antenna and a bad antenna is probably only 10 dB in terms of antenna efficiency. Things like to radiate at 9.4 Ghz.

At 9.4 GHz, your pathless (power falloff with distance) will be quite large at 50 meters - is probably about 70 dB [see Friis Transmission Equation http://www.antenna-theory.com/basics/friis.php]


This means that if you see nothing, you could be too far away already or there could be no signal at all. Before you rule out your antenna, you should try to find a known source at 9.4 GHz and move closer and closer to it. If you still see no response it is most likely not the antenna - probably the circuit is screwed up in some way.

I don't see anything fundamentally wrong with your design.

Here is a simple antenna to try to put together, if you want to do another design. The vivaldi antenna is very wide bandwidth and so it shouldn't be very sensitive to getting the details right

http://www.antenna-theory.com/antennas/aperture/vivaldi.php

[Stormpetrel]

Thank you very much for your feedback. It's a relief that there is no major flaw in the design.
I will replace the LTC5332 with a new one and try to approach a marine radar closer. Some supermaket automatic door works also with a 10Ghz radar. I might have a try.