Design and testing of an inexpensive HF (up to 30MHz) antenna tuner for reception

Circuits and details to be uploaded soon! For now have a look at my constructed circuit above.

The matching network has beed designed for a Pi- orientation: Input and output ports see (each) a capacitor in parallel and between these two is in series a capacitor or an inductor (selectable through a switch).

In the middle it’s a single sided copper PCB with the copper removed in some places to make 3 separate copper ‘islands’. The copper strip on the left is the ground part. At the top and bottom you can see the (plastic white housing) male BNC connectors (connected with cables in this case) used as an input and output for the tuner (signal direction does not matter).

The semitransparent ‘cubes’ (shown 4 in total) are common FM/AM receiver variable capacitors soldered to yield 2-660pF range (capacitance depends on the way you connect the internal capacitors included, but I picked this combination to maximise range of capacitance). Each variable capacitor has integrated two 2-33pF and two 1-330pF capacitors, so there are 4 capacitors in total to select for soldering. I chose to solder the two’s 1-330pF in parallel, to make capacitance range from 2-660pF, as said.

In the middle you can also see a 12-position switch, although I’ve only used 8 positions so far. I use this switch to select the value and the type of the series component in the pi-network. I have soldered 6 inductors of various values from 100nH to 10uH, and 2 variable capacitors soldered in parallel, so the effective capacitance ranges from 4-1320pF (2 x 2-660pF). I also have one position available to short-circuit the series component, i.e. the tuner only works with the parallel capacitors at the input and output in this position.

The parallel capacitors at the input and output (2-660pF each) can be isolated (i.e. taken out) through the small switches (black in colour) next to each BNC connector. So effectively, these switches can be used to convert the Pi-network in an L-network (of twin-type, i.e. first series or first parallel depending of which switch you turn ON-OFF), or simply keep the series component in place only when both parallel capacitors are isolated.

So, this antenna tuner is effectively a very versatile in terms of network type.

I am using this tuner when listening to SDR reception. My receiver antenna is an ordinary wire dipole, tuned for the 14-14.35MHz band (20m), as this appears to be the most ‘busy’ here. With the help of the tuner I have also managed to improve signal reception (i.e. matching) for the 3.5 MHz(80m) and 7MHz (40m).

I wouldn’t recommend making this circuit if you intend to use the tuner for transmission. The capacitors and inductors are of low voltage ratings and they won’t be able (i.e. spark!) to withstand the current rating during high power transmissions. I guess for small wattages, up to maybe 5W, this tuner could be used both for reception and transmission.

(tests and more results to be posted soon)