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Author Topic: Home brew auto ATu  (Read 2391 times)
2E0BSS
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Posts: 85




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« on: April 09, 2007, 09:09:36 AM »

Has anyone built their own Auto ATU? If so what components do I need to build mine?

Charlotte 2E0BSS
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AA4PB
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Posts: 12854




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« Reply #1 on: April 09, 2007, 01:27:21 PM »

The tuner part needs inductors, capacitors and relays. A sensor is needed to measure forward and reflected power. A microprocessor executes the code that determines what inductors and capacitors are selected by the relays for any tuning solution. Unless you are experienced in embedded processor design, the code will be a killer.

Most DIY tuners are manual remote tuners. A rotary inductor and a couple of capacitors driven by motors. You have to tune it like a normal manual tuner but it can be located outside at the antenna.
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G3RZP
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« Reply #2 on: April 15, 2007, 05:41:03 AM »

There was an article or two in the RSGB Radio Communications magazine a few years back, using a PIC microprocessor. A lot depends on how much power you are building it for, and what antennas you are going to match. For example, my vertical at 400 watts on 80m needs a tuning capacitor that can hold off 5000 volts!

For highest power, you need a vacuum variable capacitor and (ideally) a good variable inductor. But you can substitute air cored coils and a motor driven switch. You then need to decide on a tuning mechanism...I have linear feedback from positioning potentiometers. The voltage is then digitised and compared with a pre-programmed memory to decide which way to drive the servo motors - the memory has an address input derived from the transceiver frequency.

The usual L network has the problem that the working Q is a function of the impedance transformation ratio, and as the working Q goes up, so do the circulating currents and the losses. This is why all the commercial tuners are somewhat limited in impeadance matching range and power, or they're all pretty big - and most of them still have some limitations. A military 400 watt manual tuner designed to match antennas such as a pair of 16 foot whips at 2 MHz up to a long wire and operate up to 22 MHz, was manually tuned and in a box about 2 foot cube. And it wasn't that size just for fun.....
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AB9LZ
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Posts: 198




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« Reply #3 on: May 13, 2007, 08:19:53 AM »

I'm working up to one, plan on using H-bridges to drive some el-cheapo dc geared motors for the two big air variable caps as well as the roller inductor. happen to have serial A/D converter lying around that will be switched to monitor both legs of the SWR sensor.

A PIC will drive the whole thing, the programing should be straight forward.

This is the few expensive parts VS. many the many cheap ones of the switched LC type tuners. Since I plan on making a legal limit tuner, it seems to be my only practical option.

73 m/4
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