[Elecraft] Looking to add LP700 to KPA1500 - ideas?

[Mark Goldberg]

I tend to agree. With the right calculations, the reported power, SWR and
matching inductors and capacitors from the KPA1500 will tell you more about
the antenna. In addition, the KPA1500 already has a monitoring port that
could be used for an output modulation / spectrum monitor. Of course,
something like that would have to be built that took those inputs. I expect
a K4 will do it!

The KPA500 does get calibrated at the factory for power, SWR, etc, probably
against decent, calibrated test equipment. I'll bet it is designed to hold
some sort of calibration over time. Hams tend to treat uncalibrated test
gear as gospel. An external power meter, just because it reports to the
nearest 0.1W isn't necessarily better. Any commercial operation calibrates
on a regular cycle. Being a slightly less cheap Ham than most, I do get one
scope / spectrum analyzer calibrated every year, so at least I have
something to compare the other stuff to.

Does the KPA500 report the tuner topology (if it is even switchable)? Is it
an L match? Does it report which element is the shunt element?

Does a specific L and C and shunt element match only one unique load? The
other issue is that the Ls and Cs reported are probably 20% or more off, so
the calculation would not be perfect. The C and L ladders are not even
guaranteed monotonic, so a reported LC combination may be off. Tuners often
try the next one higher and lower. I did measurements of an LDG tuner and
the inductance jumped up and down. Step N-1 was not necessarily less
inductance than step N. Two steps generally got in the right direction.

73,

Mark
W7MLG


On Wed, May 20, 2020 at 12:07 PM Rick NK7I <rick.nk7i at gmail.com> wrote:

> The KAT500 (and utility) can show the values used by the tuner to match
> the 50 ohm impedance of the radio.  Similar is possible either via the
> KPA1500 utility or the front panel of the KPA1500.
>
> While it doesn't show the complex load; it gives enough to begin to
> figure out what the antenna is presenting (for use in apps like WinSmith
> etc) so one can make intelligent changes in the antenna.
>
> I've used the utility for monitoring (and remote control) of the
> KPA500/KAT500 combo for years; now I'm using the KPA1500 utility for the
> same reason.
>
> The KPA1500 presents a lot more useful information on the front panel
> (click on the menu button then the down triangle to select the menu you
> want).  Because my antennas are resonant, I park the amp display at
> power out, efficiency, volts and amps used and temp.  The power out on
> the utility also shows the reflected power in watts, nice to know.
>
> If you put the LP sensor between the amp and the (not resonant) antenna,
> any mismatch will likely give you erroneous readings that would offer no
> value.  It would compare the antenna value (say 150 ohms of mixed
> reactance/resistance) to the tuner value (whatever is required to match
> to 50 ohms).  The worse the unmatched condition, the more useless the
> display becomes.  It wants to see 50 ohm in, 50 ohm out to provide
> accurate power readings.
>
> So if your antennas demand a matching circuit (tuner), the LP sensor
> becomes less valued at the output of the amp.
>
> 73,
> Rick NK7I
>
> On 5/20/2020 9:41 AM, Andy Durbin wrote:
> > "If it were mine, I would put the LP700 on the antenna line to monitor
> what is going on with the antenna itself."
> >
> > My LP-100A is between my KPA500 and my KAT500.  I want to know what load
> is being presented to the KPA500.   Neither the KPA500 not the KAT500 can
> display complex load values.  Since the LP-500 and LP-700 are intended to
> sample amplifier RF input and amplifier RF output I would expect the
> amplifier output sampling to be performed before the tuner.  But no, I
> don't know how to do that with a KPA1500.  It would seem to require an
> internal modification and two additional rear panel connectors.
> >
> > 73,
> > Andy, k3wyc
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