The Double Zepp The W3EDP Longwire with MTFT

Feeding the Double Zepp with Baluns

Fan-Dipoles Vertical-L

Switchable MTFT for

 Wire Antennas (by DK7ZB)

Rate this site

(with 10 = top)
Wire Antennas have a random impedance at the feedpoint. Especially for the higher bands the impedances are often in a range of a middle radiation resistance with a reactance of +/- J X Ohm.

Often a MTFT (Magnetic Transformer for Transmitting) is used to match the impedance down to a value which can be handled by an ATU. In the ideal case a built-in-ATU of a transceiver can match the antenna. The normal used MTFTs have a ratio of 1:9.

Here is a solution of a switchible Un-Un which is tapped for different impedances. The toroid core is a FT240-43 and can handle up to several hundreds of Watt output power.

The wires must have PTFE insulation if you use higher power > 250 Watts. The coupling coil has 3 turns, the output coil has 12 turns and is tapped from 3-12 turns for switching to the best SWR and a value which can be matched by an ATU.

In most cases 3, 5, 7or 9 turns are sufficient and you can use a simpler version which fixed bushes for the connection of your wire.

Here is an example of the SWR with a 20 m long wire with a counterpoise of a wire fence and 5 turns of the outcoupling coil (left picture). As you se you could change lengths to get a better SWR in or near some amateur band. It is recommended to use any metalic structures as a counterpoise.

The right picture shows the SWR when used a 220 Ohm resistor and the 5th turn.