Switchable MTFT for
Wire Antennas (by DK7ZB)
|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
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.