TUNE power, low vs high

[Arno Dienhart-2]

What could be the difference for the tuning result between tuning at, say,
0.8W vs at 3W?

If there is no difference, why is the ATU tuning set to 2 or 3W, and not
lower?

 

I am thinking that a random length antenna with a potentially high SWR will
tax the PA more at higher outputs. So why shouldn't I use a low output,
always (besides causing less QRM then)?

 

73 de Arno K7RNO

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[Jack Brindle-2]

The answer is pretty simple. The higher the output power, the more resolution for both forward and reflected power measurement.
When calculating SWR we can get some pretty small reflected power values that may still produce a pretty high SWR. In order
to get something that provides reasonable results, you need a bit more power. If you tried doing the measurement with 0.8W of forward
power there wouldn’t be enough reflected power to measure.

One way to really see this is to try to tune at very low power, then increase the power. You might be surprised to see a pretty high
SWR that looked pretty good before. So, the more forward power you have, the better the tune will come out. Of course there is a limit.
You don’t want to put too much power through the tuning relays.

73!

Jack Brindle, W6FB




On Dec 12, 2013, at 4:34 PM, Arno Dienhart <[hidden email]> wrote:

> What could be the difference for the tuning result between tuning at, say,
> 0.8W vs at 3W?
>
> If there is no difference, why is the ATU tuning set to 2 or 3W, and not
> lower?
>
>
>
> I am thinking that a random length antenna with a potentially high SWR will
> tax the PA more at higher outputs. So why shouldn't I use a low output,
> always (besides causing less QRM then)?
>
>
>
> 73 de Arno K7RNO
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>
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[Don Wilhelm-4]

Arno,

If all things are perfect, there would be no difference.  But in the
practical, physical world, there are differences.  In the question you
have posed, there is the wattmeter diodes to consider.  When the forward
voltage is quite small, as would be the case at lower power levels,
there may not be enough rectified voltage for a good indication of
reverse power - that will lead to erroneous results that may cause
higher SWR indications when the power is increased.

73,
Don W3FPR

On 12/12/2013 7:34 PM, Arno Dienhart wrote:
> What could be the difference for the tuning result between tuning at, say,
> 0.8W vs at 3W?
>
> If there is no difference, why is the ATU tuning set to 2 or 3W, and not
> lower?
>
>

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[Arno Dienhart-2]

Thank you, Don and Jack, that makes sense.

 

To explain a bit more, I only operate portable, so my antenna situation is
practically never the same and I need to tune anew. For my wire antennas I
use the ATU. If I use the magloop, I bypass the ATU and use the TUNE
function. That is where I reduced power to .8W.

 

After reading your posts, next time I will tune again with .8W and then
increase tuning power to 3 and compare the SWR. If I did that for enough
different bands and frequencies, with no discernable difference, I guess I
would be safe to keep it at low power from then on, right?

 

73 de Arno K7RNO

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[Bill Frantz]

Many years ago I built a circuit which used op amps to make
diodes into "perfect rectifiers" -- rectifiers that would be
linear at very low voltages. Antenna analysers also have the
same problem. The real problem may be building a rectifier that
works at very low voltages when at low power and continues to
work at high voltages when at high power.

Cheers - Bill, AE6JV

sOn 12/12/13 at 6:21 PM, [hidden email] (Don Wilhelm) wrote:

>If all things are perfect, there would be no difference.  But
>in the practical, physical world, there are differences.  In
>the question you have posed, there is the wattmeter diodes to
>consider.  When the forward voltage is quite small, as would be
>the case at lower power levels, there may not be enough
>rectified voltage for a good indication of reverse power - that
>will lead to erroneous results that may cause higher SWR
>indications when the power is increased.

-----------------------------------------------------------------------
Bill Frantz        |Security, like correctness, is| Periwinkle
[phone]      |not an add-on feature. - Attr-| 16345
Englewood Ave
www.pwpconsult.com |ibuted to Andrew Tanenbaum    | Los Gatos,
CA 95032

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[Jack Brindle-2]

I look at it another way. When I operate QRP I want every milliwatt I can have going to the antenna. As Ron’s post noted, a 2:1 SWR may be indicated as 1.1 when low power is used. That means you are losing significant power, but don’t know it. It teas a short time to crank the power up to 5 or 10 watts, do the tune, then drop down to the power you really want to run. Then you would be sure your precious milliwatts are actually going to the antenna and being used for communications.

Realize though that even with a good match, things are still mismatched between the ATU and the antenna. Be sure to use _good_ coax here - you don’t want to lose those precious milliwatts warming the coax. In other words, at QRP, everything matters, so you have to do everything you can to make the system works properly and folks can actually hear you.

73!

Jack B, W6FB

On Dec 12, 2013, at 7:29 PM, Arno Dienhart <[hidden email]> wrote:

> Thank you, Don and Jack, that makes sense.
>
>
>
> To explain a bit more, I only operate portable, so my antenna situation is
> practically never the same and I need to tune anew. For my wire antennas I
> use the ATU. If I use the magloop, I bypass the ATU and use the TUNE
> function. That is where I reduced power to .8W.
>
>
>
> After reading your posts, next time I will tune again with .8W and then
> increase tuning power to 3 and compare the SWR. If I did that for enough
> different bands and frequencies, with no discernable difference, I guess I
> would be safe to keep it at low power from then on, right?
>
>
>
> 73 de Arno K7RNO
>
> ______________________________________________________________
> 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

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