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Biquad Yagi for ISM and SRD bands?


blublob
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With the uproar of IoT I am using the TI C1110 transceiver for 315mhz, 433mhz and 868-915mhz, but I am lacking a proper antenna.

I tried a cheap Yagi from aliexpress, but it was, to put it mildly, garbage.

After going through different forums and the web I did find these two:

https://web.archive.org/web/20071023032712/http://diy.wifi-live.pl/node/8

This is something I can easily pull off I believe, but there is a problem.

I can not find any theory to help me do the proper calculations needed to create a biquad yagi for a given frequency I am using.

ie.

I test stuff on 433mhz, 868mhz and 869.5mhz atm.

(I am using an SMA connection for external antenna.)

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I have to say that is a wonderful webpage.

However it brings a few questions to light.

If I take base on the yt-video I see that instead of a reflektor there is a slightly larger element behind the driving element.

How are the measurements on that calculated, and is it important as long as it is just larger?

There is also the thing with parasitic elements.

How do they help?

How are they sized?

How is the spacing between them calculated?

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The only thing I know is that the last element in the bunch is the reflector. It just has to be larger than the rest from what I can find. If you look at Andrew McNeil's "Yagi in a can" youtube video, you'll find that he uses the entire metal bottom of the can as the reflector for his yagi.

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That is why I am wondering how to create it, and if the parasitic elements even helps me when I am on a specific frequence.

There is also how to calculate the spacings, and the parasitic elements if I would want to cover more than a given frequence.

Then there is this form of reflector, it just puzzles me how to do it.

Using this on the numbers in the video gives me this:

36 - 2020 MHz

34 - 2140 MHz

33 - 2205 MHz

32 - 2270 MHz

31 - 2350 MHz

30 - 2425 MHz

34 - 2140 MHz

31.5 - 2310 MHz

30 - 2425 MHz

29 - 2510 MHz

28 - 2600 MHz

27 - 2690 MHz

Now you know wifi is:

2412 - 30.2 mm

2417 - 30.1 mm

2422 - 30.0 mm

2427 - 30.0 mm

2432 - 29.9 mm

2437 - 29.9 mm

2442 - 29.8 mm

2447 - 29.7 mm

2452 - 29.7 mm

2457 - 29.6 mm

2462 - 29.5 mm

2467 - 29.5 mm

2472 - 29.4 mm

2484 - 29.3 mm

He mention that 31.5 mm would be in the middle of the spectrum, but the calculator gives 2310 MHz, that can't be right.

What is up with that?

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