M0EZP - L-Match ATU      

Mostly built from scrap/reclaimed components, the ATU is simple and built to take QRO, not that I intend to use it that way. Tony G0DLX kindly donated a number of components, such as the rollercoaster, air-spaced variable capacitor and the case. The ceramic switch, SWR meter and other bits mostly came from rallies. Tony got the rollercoaster as a set of broken bits and he and I fabricated it back to life.

After much thought I decided to build a L-Match circuit but with flexibility to match a wide variety of situations. My son James drilled the front and back panels using a CAD/CAM rig which permitted accurate hole positioning. 

The controls on the front panel (clockwise from top) are SWR sensitivity, rollercoaster tuning, Relay switch (see later), variable capacitor tuning and in the centre is the L-C circuit switch. I decided to opt for a see through front to add some interest and also make it possible to view some potential breakdown points such as if the rollercoaster mis-tracks the coil. Ultimately I intend to provide some internal illumination.

Rollercoaster

The rollercoaster is wound on a ceramic former and the coil is made of silvered copper wire. The bar for the jockey wheel is tensioned by sprung plate. The rollercoaster provides up to 25mH inductance. 

Circuit Switching

The circuit switch has 6 positions:-

1. Straight-through
2. L in series with antenna and C between RF output and ground (both capacitors in series)
3.  as 2 but using one capacitor
4. L in series with antenna and C between RF input and ground (both capacitors in series)
5. as 4 but using one capacitor
6. L and C in series with antenna

This is achieved by thinking of the L-C circuit as being essentially all in series:-

(1)---L---(2)---Ci---(3)----Cii---(4)

The RF input, output and ground connections are then switched to whatever position is required...1 to 4. Ci and Cii refer to the capacitor gangs and (3) is their 'common'.

Variable Capacitor

The variable capacitor has proved the largest challenge. With some stray capacitance one section provides a minimum of about 50pF (approx 300pF maximum) which for aerials that are near resonance is too much. As my 160m aerial is resonant at around 1860kHz I decided I needed a lower minimum capacitance more than a higher maximum (as typically needed for 160m). The two sections in series of course deliver about 25pF at minimum. I will give further consideration to being able to switch C in such a way as I can have it in parallel at at later date.

Relay Switching

For 80m and 160m, I use an inverted-L aerial which is a quarter wave on 80m. For topband, I switch in a base loading coil (30mH) and so the front panel relay switch is intended to provide a switched 12v supply to operate the relay(s). At present the choices will be:-

• straight aerial with no coil – 80m
• plus base loading coil – 160m
• alternative output for experimental use

Back Panel

The back panel has the following facilities:-

• RF input
• RF output to aerial (2 sockets but currently only one in use)
• RF output to frequency counter
• nominal 12v input 5-pin DIN Relay output

Links

L-Match settings spreadsheet

G4NSJ ATU Web Page describing L-Match theory

Last updated 2nd April 2008