KPA1500 amplifier with built-in ATU, separate power supply

[John Perlick]

You have to remember that the built in tuner matches the amp to the transmission line.  It does nothing to improve the SWR at the antenna or to protect your coax.  

Well, it might incrementally improve the loss in the coax because the reflected wave from a high SWR antenna would not be the-reflected at the amp.  It would be fully absorbed into the amp which is well matched.  

The really good thing that the internal tuner does is make life easier for the amp.  You will get more power out because the protection circuits don't have to protect the amp.  I'd be far more protective of my amp than my coax!  

John Perlick
Aria Corporation
www.ariacorp.com

> On Apr 21, 2017, at 2:29 PM, "[hidden email]" <[hidden email]> wrote:
>
> KPA1500
>        amplifier with built-in ATU, separate power supply
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[David Woolley (E.L)]

Real PA designs are very far from correctly reverse terminating the
transmission line.  The maximum power transfer theorem is a complete red
herring in this sort of situation.  For example, on the top of page 12
of the third edition of the Art of Electronics, immediately after
setting the maximum transfer theorem as an exercise, they point out that
ordinary output stages are operated far from that condition.

The only case in which you might get reverse termination, is if there
was a (typically ferrite) isolator.  In that case, the PA wouldn't care
about the mismatch (at least not for an ideal isolator), but the
terminating resistor in the isolator might glow red or white if you
shorted or opened the output.  In practice they are normally microwave
devices, but I suppose they may be used in professional HF systems.

In real systems, most of the reflected power bounces straight back out
again when it hits the PA, to get another chance to be lost in the
feeder, or even radiated.  Of course this may cause the PA to run out of
volts, or gain, and clip or distort, or exceed the safe operating area,
and melt bond wires, or destroy the chip itself.

--
David Woolley Owner K2 06123

On 22/04/17 05:47, John Perlick wrote:
>
> Well, it might incrementally improve the loss in the coax because the reflected wave from a high SWR antenna would not be the-reflected at the amp.  It would be fully absorbed into the amp which is well matched.
>

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[Alan. G4GNX]

This reminds me of a visit to the underground BBC World Service, back in the
1980s.

There were three RCA transmitters for MF, two in use and one on standby.
They were capable of output around 1+1/2 Megawatts.

The final stages were in Faraday Cages, but they still invited my son to
wander round inside one, with a fluorescent tube lit up like Darth Vader!

I asked about tuning for low VSWR and they said they didn't bother much.
What comes back simply goes up again for another try. :-)


73,

Alan. G4GNX

-----Original Message-----
From: David Woolley
Sent: Saturday, April 22, 2017 10:06 PM
To: [hidden email]
Subject: Re: [Elecraft] KPA1500 amplifier with built-in ATU, separate power
supply

..................In real systems, most of the reflected power bounces
straight back out
again when it hits the PA, to get another chance to be lost in the
feeder, or even radiated.  Of course this may cause the PA to run out of
volts, or gain, and clip or distort, or exceed the safe operating area,
and melt bond wires, or destroy the chip itself.

--
David Woolley Owner K2 06123

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[Wes Stewart-2]

With respect to your first sentence, all I can say is, of course; is there
anyone who thinks differently?

In a signal generator, where source match is important, there will probably be
an attenuator, or at UHF and above an isolator (terminated circulator), that
dissipates returned power. Otherwise, almost all amplifiers are mismatched to
returned power and simply reflect it, as does any mismatched load.



On 4/21/2017 9:47 PM, John Perlick wrote:

> You have to remember that the built in tuner matches the amp to the transmission line.  It does nothing to improve the SWR at the antenna or to protect your coax.
>
> Well, it might incrementally improve the loss in the coax because the reflected wave from a high SWR antenna would not be the-reflected at the amp.  It would be fully absorbed into the amp which is well matched.
>
> The really good thing that the internal tuner does is make life easier for the amp.  You will get more power out because the protection circuits don't have to protect the amp.  I'd be far more protective of my amp than my coax!
>
> John Perlick
> Aria Corporation
> www.ariacorp.com
>
>> On Apr 21, 2017, at 2:29 PM, "[hidden email]" <[hidden email]> wrote:
>>
>> KPA1500
>>         amplifier with built-in ATU, separate power supply
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>

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