Antenna Matching Transformer

A number of LF experimenters have used link coupling to the main loading coil with great success.  But link coupling can be difficult to set up. Often, series capacitance is required to tune out the inductance of the link winding - adding yet another frequency-conscious component.  Also, the link winding will tend to 'look' like a short circuit when the antenna goes off-resonance - hence adding to the risk of (another) failure of the power amplifier!

At GW4ALG, I have sought to use untuned (aperiodic) stages wherever possible.  I therefore decided to remove the link winding and use a wideband matching transformer instead.

Construction

To make the transformer, I first wound a primary winding of 10 turns on a 58 mm OD 3C85 core using insulated conductors salvaged from some old 15 A mains flex.  The primary winding was held in place with two layers of PVC insulating tape.

The first 10 turns of the secondary winding were wound over the top of the primary winding, and further turns added to provide the required  transformation ratios.  Finally, the turns were held in place using cable ties.

Note that the transformer provides isolation between the antenna earth and the screen of the feeder.  I believe this helps to prevent an earth loop which might otherwise introduce mains-borne noise on receive.

Interconnections

Interconnections between the transformer secondary; the tapped loading coil; the variometers; and the antenna itself, are all via 'banana' plugs and 4 mm sockets.

In use, I simply select the secondary tap that provides the lowest SWR as measured at the transmitter end of the coaxial feeder.  If the ground is wet, I need to use the 160 ohm tap; on a dry day, I can use the 100 ohm tap.