Looking at KPA1500

[K7TV]

The KPA1500 is right now outside my budget, but there is nothing wrong with
dreaming and planning, right? The latest QST reviews the new Palstar amp,
which first seemed attractive, but I soon noticed that its continuous
carrier capacity is no better than the KPA500. Also, when you add the price
of the matching tuner, you are looking at a total price of $5k, which makes
the full legal power KPA1500 with included tuner look good at $6k. However,
what does dampen my enthusiasm is that the KPA1500 tuner handles swr only up
to 3:1 at full power. My main 40m antenna meets that requirement over a
significant portion of the band, but at band edges swr can be 4.5:1 or so.
The lower bands are where high power is most commonly needed, so it doesn't
make much sense to upgrade from my KPA500 only to have to throttle back to
500W on 40m. That begs the question exactly how much power the KPA1500 tuner
can handle on 40m. Also, it would be desirable to use higher power on the
portion of the band where swr does stay under 3:1. I could set the power to
a lower value on the K3 for 40m, but it would be helpful to be able to set
the power separately for different band segments. The actual power limit at
a higher swr probably depends on the actual impedance presented to the
tuner, and not just the swr value. One could perhaps use an antenna analyzer
to measure the complex impedance at different frequencies within the band,
and then program different drive power levels for the corresponding band
segments (if that capability were implemented). Maybe a future tuner design
could incorporate sensing of complex load impedance and signal the
transceiver to limit drive power accordingly. Or, instead of basing it on
load impedance, the design could directly measure voltages and currents
within the tuner to prevent excessive voltages and currents from being
produced (not dynamically a la ALC, but as a calibration using steady
carrier).

73,
Erik K7TV

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[Jim Brown-10]

On 10/16/2018 10:24 PM, Erik Basilier wrote:
> My main 40m antenna meets that requirement over a
> significant portion of the band, but at band edges swr can be 4.5:1 or so.

Hmmm. 40M is not very wide (only 4.2%, as compared to 80M which is
14.3%). What kind of antenna is it? Simple antennas without traps or
loading coils like resonant dipoles and verticals typically cover 40M
with SWR below 2:1 if tuned to the center of the band.

80M is the tough one, but can be tamed with some transmission line and
or stub tricks.

73, Jim K9YC

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[K7TV]

Jim, my antenna is somewhat unusual. It is a Sommer XP-708, where the
hardware added to enable 40m is very minimal: just a coil hooked up between
two points on an antenna whose dimensions say "20m and up". Given that each
antenna element is appropriate for 20m or shorter wavelength, and the boom
is no longer than the elements, the narrow bandwidth on 40  is not
surprising. Also, the antenna has no directivity at all on 40. One might
wonder if this antenna might be a compromise in other respects, but the
performance in actual use has always been a positive surprise to me. For the
cost of adding 40m to the basic design, I would say that it is a great
value.

73,
Erik K7TV

-----Original Message-----
From: [hidden email] <[hidden email]> On
Behalf Of Jim Brown
Sent: Tuesday, October 16, 2018 10:58 PM
To: [hidden email]
Subject: Re: [Elecraft] Looking at KPA1500

On 10/16/2018 10:24 PM, Erik Basilier wrote:
> My main 40m antenna meets that requirement over a significant portion
> of the band, but at band edges swr can be 4.5:1 or so.

Hmmm. 40M is not very wide (only 4.2%, as compared to 80M which is 14.3%).
What kind of antenna is it? Simple antennas without traps or loading coils
like resonant dipoles and verticals typically cover 40M with SWR below 2:1
if tuned to the center of the band.

80M is the tough one, but can be tamed with some transmission line and or
stub tricks.

73, Jim K9YC

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[Ignacy]

The low range tuners up to 3:1 are very low loss and then can be made
reliable. High range tuner for 1.5KW would be less so.

You may buy MFJ-998 as an extra. I use MFJ-998 with 1.5 KW and a
ladderline-fed dipole for a few years. Only small problems like burnt diode
in SWR circuit or smoked capacitor by the output connector. But it is better
to have such problems outside of the amp.

Another choice is to have an AB switch where the same antenna is connected
to A directly and to B via  capacitor that lowers the SWR where otherwise it
is too high.


Ignacy, NO9E



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[K7TV]

Ignacy, for many years I was using a legal limit tube amp in combination
with a manual tuner. For 20m and up, where the antenna has low swr and broad
bandwidth, I would put the tuner in bypass position. For 40m the tuner would
be active and I would manually adjust it, which didn't happen very often as
I would mostly stay in the cw portion of the band. The amp could perhaps
have handled the swr on 40m without a tuner, but I used the external tuner
so I could tune up the amp on a dummy load rather than on the air. Over the
years I did occasionally forget to switch the tuner into or out of bypass
when going to a different band. So, manual switching rather than manual
tuning was a bit of a problem. Now used to the KPA500/KAT500, I am spoiled
and don't want to go back to having to remember to switch. I was hoping that
the KPA1500 would not have the tuner built in, but I can see how most users
save money with a built-in tuner. When I said my antenna swr can be up to
4.5, I quoted the worst case I can remember. The 40m coil is copper clamped
onto aluminum elements. When I installed the antenna in 1993 I didn't apply
compound to keep the joints healthy. Over the years, when I have been
inactive for a long period such as a year, I have noticed the higher swr
values that I quoted. Some operation at high power has then restored band
edge swr readings to somewhat lower values, such as 4:1. If I recall
correctly, those readings were well under 4:1 when the antenna was newly
installed.  I intend to service the antenna some day and get back to those
better swr readings, but accessing the antenna for service is for me a big
and difficult undertaking. With the prospect of getting somewhat close to
the 3:1 limit,  I am very interested in detailed derating numbers for the
KPA1500 tuner, as well as any information that might indicate that it can
safely handle somewhat higher swr depending on the actual complex load
impedance..

73,
Erik K7TV

-----Original Message-----
From: [hidden email] <[hidden email]> On
Behalf Of Ignacy
Sent: Wednesday, October 17, 2018 7:59 AM
To: [hidden email]
Subject: Re: [Elecraft] Looking at KPA1500

The low range tuners up to 3:1 are very low loss and then can be made
reliable. High range tuner for 1.5KW would be less so.

You may buy MFJ-998 as an extra. I use MFJ-998 with 1.5 KW and a
ladderline-fed dipole for a few years. Only small problems like burnt diode
in SWR circuit or smoked capacitor by the output connector. But it is better
to have such problems outside of the amp.

Another choice is to have an AB switch where the same antenna is connected
to A directly and to B via  capacitor that lowers the SWR where otherwise it
is too high.


Ignacy, NO9E



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