Balanced ATU

atu 1

A balanced ATU
An ATU has the advantage of bringing the feeder-antenna system to resonance, act as a band pass filter (both on receive and transmit) and a low SWR. Further a balanced ATU means you can use open wire feeder. In general losses on open wire feeder are less than with coax. Below shows a schematic for a typical balanced ATU that will cover the 80m to 10m HF bands . The large coil L1 on the twin capacitor (C1) side can be 20-30 turns of 18SWG wire coiled 1 1/2" diameter while the coupling coil (L2) to the radio can be 3-5 turns ca. 2" diameter.

The second capacitor tunes out any reactance so that we can get a purely resistive load for the transmitter. Generally this is connected into the transmitter side of the coupling coil, rather than the earth side of the coil, but this means that an insulated shaft has to be used which is not so conveniant. I have found that wiring the capacitor on the earth side works well. I would be interested to hear from anyone who knows why this might be a problem.

I recently got this e-mail about the atu: and the position of the capacitor:

"To create a variable capacitor insulated from the ground (chassies) is a "piece of cake"! This task involves adding two or four ceramic pillars for the variable capacitor. The obvious advantage is that the tiny link coil is now grounded, and in addition, the radio equipment, TX/RX, is "DC insulated" from the Antenna Tuning Unit, a small price to pay for additional safety of the equipment. 73 from Bengt, SM6APQ"

A smaller version for the 20 to 10m band is also shown below. I have used wire threaded into 1/4" plastic tubing for the coupling coil. When this is wound round the larger coil it provides spacing between the two sets of coils.

Set the transmitter to low power while making all changes to the ATU while adjusting for lowest SWR. In use set C2 to half way and carefully tune C1 till there is a dip on the SWR meter. This will be quite sharp so if possible use a reduction drive on C1. Then adjust C2 and C1 alternatly to get the lowest (1:1) SWR.

Instead of using switches (which could introduce extra capacitance and loss) I used bananna (3mm) plugs and sockets for connecting the antenna and the capacitor. This gives a lot of fexibility for trying out various combinations of connections.

balanced ATU 80 to 10m

balanced ATU 40 to 20m

balanced ATU 20 to 10m

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Dr Jonathan Hare, Brighton, East Sussex. BN1 9QJ.

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