# shackspace wiki

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project:hgg:antenna_design:omidirectional

# Omnidirectional Antenna

I've started experimenting with this antenna type because it was proposed by some people as a good sollution as long as we are in Receiving mode. Here's what I found.

# Groundplane Antenna

The antenna should be a <m>lambda / 4</m> antenna and thus Wavelength is ~ 275.04 mm, a quarter wavelength is ~ 68,76 mm.

Thus the antenna should look like this:

The lenght of the antenna should be lambda/4, the lenght of the ground-wires should also be.

German wikipedia (article Groundplane Antenna 2011-01-01, 3am says, the ground wires should be in a 135 degree angle from the antenna wire to match 50 Ohms.

TODO: test the antenna.

# More Gain

I found out that comercial antennas with high gain often are very long like in 1.2 to 3.5 Meters.

Omni antennas by definition offer 0 dB gain (1).

To generate higher gain, one must increase the directionality of the antenna, and therefore make it directional (ie not omni directional).

The gain of the antenna is given by:

G = η(πD/λ)^2

Where: η = efficiency (0.65 good initial value) D = Diameter λ = wavelength of signal being received/transmitted.

In the antenna Manuals (eg see Different Types of Kathrein Antennas, in the section VPol Omnidirectional Antenna 1087 to 1093 Mhz) they talk about decouploed Dipoles. As far as i get it, that means that you put more than one Dipole into an Antenna. To make the antenna not have different gains for different directions, the dipoles are stacked. Since the signal is vertically polarized, the antenna won't become direction-dependent if the dipoles are above each other.

It's not clear if that also works for groundplane antennas.

Need plan to build that.