Biquad: Optimal distance between element and reflector

[steenge]

Hi All,

I am planning to build a double biquad antenna with a centr frequency around 5.4 GHz. Several people state that the optimal distance between the driven element and the reflector should be one quarter wavelength, but intuitively I think that would mean that the reflected signal will be in opposite phase compared to the directly received signal, effectively cancelling the two signals. 

I think the optimal distance should be one half wavelength.

Could anyone tell me if I am correct, and if I am indeed wrong, the reason why 1/4 wavelength is the correct distance?

// Steen

[xaxaxa]

The waves will be in phase with a 1/4 wavelength separation because EM waves invert when reflected by a metal surface; this is because at the conducting surface the E field is zero, which means there has to be a reflected wave that cancels the incoming wave at that point. (this is a very hand-wavy explanation; the full explanation involves the fact that the E field can not be discontinuous at the boundary)

In practice the distance doesn't have to be exactly 1/4 wavelength and is often shorter; the impedance can be tweaked this way. The best way to design this antenna is using a EM simulator and running an optimization algorithm (generally optimizing for low reflection coefficient and high forward gain). I've used 4nec2 to tune biquad antennas before and the results are very good.

Hi All,

I am planning to build a double biquad antenna with a centr frequency around 5.4 GHz. Several people state that the optimal distance between the driven element and the reflector should be one quarter wavelength, but intuitively I think that would mean that the reflected signal will be in opposite phase compared to the directly received signal, effectively cancelling the two signals. 

I think the optimal distance should be one half wavelength.

Could anyone tell me if I am correct, and if I am indeed wrong, the reason why 1/4 wavelength is the correct distance?

// Steen

[steenge]

Thanks! Now it makes sense 

[hagster]

Previous BiQuads I have made at 2.4ghz have had spacing of about 10mm. Thats much closer than 1/4 lamda. I did lots of simulations in nec2 to arrive at this and the real measurements were in close agreement.

Basically i dont think the ground plane is working as a reflector in this antenna. Instead its providing capacitance to balance out the inductance of the wire elements. You can adjust the spacing to get the right impedence match.

[Richard Head]

0.18 Lambda is the figure normally used for a Yagi-Uda array.