Homebrew T-Match Antenna Tuning Unit (ATU)

One of the rewards of being a Radio Ham is building your own equipment and actually getting to use it. At present most of my gear is commercially made including my ATU. I bought an auto ATU off E-Bay about 3 years ago to get me on the air and by and large it’s been quite good. The MAC 200 ATU is designed to cover 1.8 to 30 MHz and is capable of handling ¬†200W however on the 160m band it battles to match the impedance and becomes unstable. With this in mind I decided to build a new ATU that would cover 160m and also handle much more power.

I decided that a T-network would be the best option and I would also include a built in VSWR meter. Most of the components for this I had in the shack. I picked up a lovely roller-coaster inductor from Bruce at GB Airspares last year and Paul Bennett donated a couple of 1500 pF variable capacitors.

The capacitors were high quality but a bit on the large size. Luckily they came apart easily and after removing a few vanes I converted them to 350 pF and increased the voltage rating to about 2kV. Thanks to John, G3LYW for the idea!

The circuit for the ATU is nothing new. I am using a standard T-network that many commercial ATU’s use. The circuitry for the SWR ¬†comes from my old (1985) ARRL Handbook. Here are the details here:

ATU

After a bit of hardware bashing the ATU starts to take shape. The chassis is actually from an old charge amplifier rack that I have turned into a box.

The next stage is the wiring. The SWR sampling unit is a yellow toroid with about 30 turns of wire. It is crucial that this is screened off from the rest of the box. I’m using a spare piece of copper clad PCB for this purpose.

After a bit more tinkering and calibrating the SWR meters it’s ready to go.

 

Well it all works and I’m very pleased. I think it may struggle with the higher bands above 20m but I can always add a high frequency inductor in series with my roller-coaster to help with this.

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