As I had mentioned before, I am slowly working on building a “better” Yagi antenna than my previous rough attempt. I have a lot of work related deadlines and I am also moving next week, so progress will be slow. In any case, I decided to start out with the driven element design and then construct the beam and passive elements afterwards.

I am quite fond of the Fonera wifi router, so I am going to design something that can replace its antenna. For this reason, I am going to put a female SMA connector on the end of my feed cable. I have some RG-174/U cable around the lab which is the right impedance, is fairly thin, but is also lossy. For this reason, I am going to keep the feed cable short enough to mount the Fonera comfortably near the Yagi array. The propagation velocity in the cable is 66% of vacuum, so the wavelength at 2.45GHz is about 8cm. Designing the feed cable to be an integer number of half-wavelengths makes impedance matching a little easier so I made my cable 16 cm from the tip of the connector to the loop hookup.

The signal wavelength in air is a little less than 12.5 cm, so I made my loop out of 3M copper shielding tape and soldered the ends of the tape to the shielding and feed terminals of the cable. I tapered the ends to make the loop line up nicely and took care not to heat the end too much as that will degrade the cable’s dielectric core. One side of the tape has glue on it, which will help mounting the loop on a piece of foam to give it a rigid, rectangular shape.

The helpful folks down the hall let me use their network analyzer to measure the antenna resonance, after a little bit of tuning, I got a peak at about 2.4GHz, which should work fine for lower wifi channel numbers. I also compared this to a commercial wifi antenna from SMC which resonated at about 2.43GHz. Unfortunately, I changed the window width when I tested my loop antenna so that is why the SMC antenna looks like it has more peaks.

My next step is to look up an optimal position of 10-14 director elements in literature and figure out how to fabricate the device. I am thinking of using thick copper wire since this will be an indoor unit. Might use the engraver to drill the holes to specifications.





 
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