How do I make a Yagi-Uda antenna smaller?

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Discussion Overview

The discussion revolves around the construction of a Yagi-Uda antenna for 900 MHz, specifically focusing on methods to reduce its size while maintaining functionality. Participants explore formulas for antenna dimensions and spacing, as well as the implications of size on efficiency and gain.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents formulas for the lengths and spacings of a Yagi antenna based on wavelength calculations.
  • Another participant argues that reducing the size of the antenna could compromise its efficiency, particularly for high-gain applications.
  • A participant mentions finding a smaller Yagi antenna that works for 800 MHz and questions how it achieves functionality despite its size.
  • Concerns are raised about a potential error in calculations, with a participant suggesting that the reflector length calculated appears excessively large.
  • Participants discuss the wavelength at 900 MHz, noting it should be approximately 1/3 of a meter, which leads to further calculations regarding the dimensions of the antenna elements.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of reducing antenna size without losing efficiency. There is no consensus on the best approach to achieve a smaller Yagi antenna while maintaining its performance.

Contextual Notes

Participants reference specific formulas and calculations, but there are indications of potential errors in the application of these formulas. The discussion includes unresolved mathematical steps and assumptions regarding the relationship between size and efficiency.

tackyattack
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How do I make a Yagi-Uda antenna smaller?

I am trying to build a Yagi antenna for 900mhz and I am using this formula:

http://www.skyscan.ca/Antennas.htm

Length:

Reflector length = 0.495 x wavelength

Dipole radiator = 0.473 x wavelength

Director D1 = 0.440 x wavelength

Director D2 = 0.435 x wavelength

Director D3 = 0.430 x wavelength

----------------------------------------------

Spacing:

R-DR = 0.125 x wavelength

DR - D1 = 0.125 x wavelength

D1 - D2 = 0.250 x wavelength

D2 - D3 etc = 0.250 x wavelength

--------------------------------------------------------------------------------------------

But the problem is, when I use this formula with 900mhz the antenna ends up being huge. What I need to do is make it smaller. Would it work if I took all of the lengths and spacings then divide it by, say four?
 
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tackyattack said:
I am trying to build a Yagi antenna for 900mhz and I am using this formula:

http://www.skyscan.ca/Antennas.htm

Length:

Reflector length = 0.495 x wavelength

Dipole radiator = 0.473 x wavelength

Director D1 = 0.440 x wavelength

Director D2 = 0.435 x wavelength

Director D3 = 0.430 x wavelength

----------------------------------------------

Spacing:

R-DR = 0.125 x wavelength

DR - D1 = 0.125 x wavelength

D1 - D2 = 0.250 x wavelength

D2 - D3 etc = 0.250 x wavelength

--------------------------------------------------------------------------------------------

But the problem is, when I use this formula with 900mhz the antenna ends up being huge. What I need to do is make it smaller. Would it work if I took all of the lengths and spacings then divide it by, say four?

Nope. The size of the antenna is what makes it efficient. And since you are talking about a Yagi antenna, it sounds like you are aiming for high gain, correct?
 


berkeman said:
Nope. The size of the antenna is what makes it efficient. And since you are talking about a Yagi antenna, it sounds like you are aiming for high gain, correct?

Yes for high gain.

I found a small Yagi for 800mhz:

https://www.amazon.com/dp/B0006H4FVM/?tag=pfamazon01-20

This is a small antenna and it works. How did they get it to be small and still work?
 
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berkeman said:
Is is smaller than the one you calculated?

Way, way, way smaller. The reflector ended up being about 12 feet long, that is wwwaaayyyy to big.
 


tackyattack said:
Way, way, way smaller. The reflector ended up being about 12 feet long, that is wwwaaayyyy to big.

Nah, there must be a typo in your math. A wavelength in free space at 900MHz is 1/3 of a meter, so a half wavelength is 1/6 of a meter. That is the basic size of each of your elements.

Can you show your math where you plugged in numbers into the formulas in your original post (OP)?
 


berkeman said:
Nah, there must be a typo in your math. A wavelength in free space at 900MHz is 1/3 of a meter, so a half wavelength is 1/6 of a meter. That is the basic size of each of your elements.

Can you show your math where you plugged in numbers into the formulas in your original post (OP)?

Reflector length = 0.495 x wavelength
wavelength = 900mhz
0.495 x 900 = 445.5
there is 37.125 feet in 445.5 inches
 


tackyattack said:
Reflector length = 0.495 x wavelength
wavelength = 900mhz
0.495 x 900 = 445.5
there is 37.125 feet in 445.5 inches

900MHz is the frequency, and the speed of light c is the velocity (300*10^6 m/s)

Wavelength = velocity/frequency.

So what is the wavelength at 900MHz? Should be about 1/3 of a meter.

http://en.wikipedia.org/wiki/Wavelength

.
 


berkeman said:
900MHz is the frequency, and the speed of light c is the velocity (300*10^6 m/s)

Wavelength = velocity/frequency.

So what is the wavelength at 900MHz? Should be about 1/3 of a meter.

http://en.wikipedia.org/wiki/Wavelength

.

Ahh I see! So at 900mhz the reflector would be 6.48956682 inches or 0.164835 meters long?

Thanks!
 

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