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28 Ohm / 12,5 Ohm / 50 Ohm - What is this? We try to explain ...

First of all, all antenna kits are designed to work with 50 Ohm radio transceivers. The given ohms only tell you the antenna design. A lower radiation impedance has advantages like more gain and better radiation pattern that the old 50 ohm designs. They just have a little more bandwith, which is normally not needed for HAM usage. All 12,5 Ohm or 28 Ohm antenna kits will be delivered with coaxial cable for the choke to transform the 12,5 ohm or 28 Ohm design yagis to the desired 50 ohms (on the resonant frequency) for your transceiver.

Distinguishing features:
50 Ohm design: Standard design with medium gain and good bandwith. The antenna kits include only a feeding choke for elimination of the currents on the shield.
28 Ohm design: Good compromise between bandwith and gain, these antennas have more gain than similar 50 ohm types. Every kit contains 75 ohm coaxial cable for a choke (DK7ZB match)
12,5 Ohm design: Haigh gain antennas with small bandwith. These antennas have to be mounted in a clear surrounding. Every other antenna can detune your antenna! Every kit contains 50 ohm coaxial cable for a choke (DK7ZB match)

You can find more information on this on Martins (DK7ZB) website
http://www.mydarc.de/dk7zb/dk7zb-match.htm