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Author Topic: Multiple antenna tuners  (Read 2605 times)
ZL2HAM
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Posts: 52




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« on: July 12, 2012, 09:12:01 AM »

I was reading in the owner's manual for an amplifier with a built-in antenna tuner that they don't recommend enabling the built-in tuner when you are also using an external tuner--and, in fact, they said that doing so could cause serious damage to the amplifier.

I understand why this isn't advisable from a basic insertion-loss perspective, but I don't get the possible serious damage aspect. Other than the inefficiency it introduces, why would it be a bad idea to chain the output of one antenna tuner into the input of another?
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TANAKASAN
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« Reply #1 on: July 12, 2012, 09:28:31 AM »

If it's an automatic ATU then there's going to be all sorts of weird problems plate and load on the amp are adjusted. An automatic ATU assumes that it's connected to a 50 ohm antenna port on one side and anything else might screw up the internal software, you'll hear it hunt as it tries to match the varying input impedance from the amplifier tank circuit.

Tanakasan
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ZL2HAM
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« Reply #2 on: July 12, 2012, 09:39:52 AM »

The manual I was reading is for an auto-tune amp with a built-in antenna tuner (the Expert 2K-FA). I would assume the amp's tank circuit isn't in-line when the antenna tuner goes into auto-tune mode, so it would only see the fixed 50 ohm impedance from the RTX. Right?
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W4VR
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« Reply #3 on: July 12, 2012, 10:09:04 AM »

Can't you turn off the auto tuner in the amp if you want to use an external tuner with a larger matching range?  Also, not a good idea to turn on the auto tuner in the exciter when you're driving that particular amp.  I use the Icom PW1 and the auto tuner in the amp can be turned on or placed in the bypass mode.
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ZL2HAM
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« Reply #4 on: July 12, 2012, 10:22:39 AM »

Yes, sure. You can turn the internal tuner off.

My question is: why is that necessary? how is it possible for the use of a tuner internal to the amp to *severely damage* the amp when an external tuner is also used?
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KA4POL
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« Reply #5 on: July 12, 2012, 10:49:56 AM »

Most likely the tuner will run into high SWR as a worst case scenario.
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WB2WIK
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« Reply #6 on: July 12, 2012, 11:17:38 AM »

I'm guessing the real issue is they want to make sure you don't try adjusting the external tuner if the internal tuner is activated, as under some tuning conditions you could place great stress on the internal tuner components.

If you adjust the external tuner first (like "barefoot") so that it presents a perfect match to the amp, and then simply put the amp in line, I can't see how that could hurt anything.
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KD0REQ
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« Reply #7 on: July 12, 2012, 11:19:41 AM »

it's going to be like hooking up two GFIs in a row... they fight with each other, and if one isn't fired off, the other will be.  nothing will work.

antenna tuners are adjustable tanks that are designed assuming there is a fixed load on one end, and they fudge it into matching on the other end.  you may or may not put any more amps into the antenna, but the objective is to present as close to a design load (50 ohms usually) to the power stage of the transmitter as possible.

the in and out of the tank circuit are interdependent... there is no hard isolation, and pulling effects will slightly change the resonance of one side as the other side is tickled closer to optimum.  that's why you tune in several stages of adjustment for minimum smoke.

your modern internal tuners are automatic and power output is often limited by the tuning software routine until the load gets within the "safe band" of lowered reverse power (max forward power, ratio of which is SWR.)

so now, we add another tuner, automatic or manual, outside the radio because we can.  as one hunts, it detunes the other.  as the other tries to catch up, it hoses up the work of the first.  ring around the rosy until the transmitter shuts down.

so don't do that.  one tuner at a time.
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WB6BYU
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« Reply #8 on: July 12, 2012, 12:49:59 PM »

In theory there is no reason why you couldn't use two tuners in series.  For example, if the
antenna is outside the matching range of the autotuner in the amp, then you could add an
external tuner to make a gross match and let the internal tuner find a more precise match
as you tune across the band.

But if you weren't careful when changing bands and fired up the amp before readjusting the
external tuner (or, worse yet, happened to tweak the knobs while transmitting) then there
may be cases where it would damage the amp or one of the two tuners, depending on the
details of the control circuitry. I suspect that condition is the one that the amp manual
is warning you about.
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N4CR
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« Reply #9 on: July 12, 2012, 02:12:37 PM »

Let's take the example of your radio with a built in tuner running into an amplifier (that is on and operational) and then into your high power tuner.

You accidently leave the tuner on in the radio and you are switched to the wrong band on the amplifier. When you transmit, your radio tunes the output to put out full power into a circuit tuned resonant on another band. And instead of transferring the power to through the tuned input on to the amplifer, it heats the tuning circuits of the wrong band and burns them up.

I've seen it happen. It can even burn up the tuning circuit of the correct band if your amplifier has a failing component, such as the cap that goes between the tuned input circuits and the grid(s).

As far as running a low power auto tuner into a high power manual tuner, it's just associated losses. Nothing is going to burn up or be damaged.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
KA4POL
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Posts: 2128




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« Reply #10 on: July 12, 2012, 09:39:36 PM »

And let's not forget, the manufacturers want to be on the safe side with such warnings. If really something happens you can't sue them as you got warned.
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