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Old 1st Oct 2019, 3:41 pm   #106
Radio Wrangler
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Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by Bazz4CQJ View Post
I could be wrong, but I'm struggling to recall adverts for commercial ham-radio ATU's, or magazine articles for DIY ATU's, ever mentioning insertion loss. Nor is the term "Low-Loss ATU" something you hear very often. There seems to be much more emphasis on having ATU's which will load up on any 'wet piece of string' which is at hand.

I wonder what a set of rules would look like for the design & construction of a Low-Loss ATU? Perhaps one could start by looking inside an MFJ ATU for some ideas? .

B
That's the herd of elephants in the room. The efficiency or lossiness of an ATU is never mentioned in adverts or constructional articles.

Most ATUs can be looked at as a resonator with adjustable couplings to feed in power from the transmitter and to feed power out to the antenna feeder.

The resonator can be adjusted to compensate for reactive components of the antenna impedance.

The Q of the resonator depends on the coupling settings. If you tune it with light coupling the Q can be rather high. This means that the tuning is very sharp, and on receive it acts as a preselector filter handily removing large signals a bit off your frequency. However, this setting multiplies the losses of the coils, capacitors and wiring in the ATU. You get a high-loss ATU at these settings. However, the high-Q settings of the ATU are forced when you want to transform a somewhat high or somewhat low impedance to the 50Ohms your transmitter would like to see.

With stronger coupling, the antenna and the transmitter load the ATU resonator more. Losses are less and the ATUs ability to match extreme impedances is curtailed.

So ATU loss varies widely with what impedance you are using it to match.

Measuring the loss of an ATU is not easy, you need to be able to measure true power into whatever impedance it is transforming. Most power meters are dedicated to the 50 Ohm world and are useless here. You also have to handle phase shifted current and voltage and not be thrown out by power factor.

In most ATUs the inductor is the big culprit, and its Q can be spoiled by putting it in too small a box (Whoosh! there went most of the MFJ, Trio, Yaesu and Icom atus)

The power rating of ATUs is equally hilarious. Our Atlantically-challenged friends like talking of 'a full kW tuner' but they aren't power limited.... they are voltage limited and current limited. Tune 'em up for high Q and the wattage those limits translate into comes crashing down. Oddly, their best power rating is when they are transforming a 50 Ohm antanna into 50 Ohms to present to the transmitter.... um, just when you don't actually need the ATU.

When you are using an ATU to do some real transformation, the power rating will have fallen, and its losses will have risen. The universe seems rather unfair at times like these.

David
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