How do I make a Yagi-Uda antenna smaller?

[tackyattack]

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?

 

[berkeman]

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?

 

[tackyattack]

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?

 

[berkeman]

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?

Is is smaller than the one you calculated?

 

[tackyattack]

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.

 

[berkeman]

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)?

 

[tackyattack]

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

 

[berkeman]

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

.

 

[tackyattack]

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!