K2 QRP Version Max. Safe RF Output Power?

[Jeff-229]

Hi gang,

Now that we're at the bottom of the solar cycle, I'd like to be able to run more than five watts on occasion, but not at the expense of putting my final PA at risk.  What is the highest level of RF output power that a K2, QRP version, can run on CW without putting the finals at risk?  My K2's power, as indicated by the internal LCD display, reaches a maximum of 15.2 watts, but it's my understanding that it's not safe to transmit for any length of time with the power control turned fully clockwise.  I have the KAT2 internal antenna tuner, which is supposed to make the LCD display's power readings more accurate, but I've gotten the impression from others that, even with the KAT2 installed, the readings are not terribly accurate, especially at the high end of the range.  Is my impression correct?

I believe the instruction manual warns not to set the power to a higher level than the transmitter is capable of producing.  Am I correct in assuming that a fairly accurate external wattmeter is required to determine whether the transmitter is failing to deliver the requested power (with requested power indicated by the K2's display), or is there a way to identify this state of affairs without an external wattmeter?  Alternatively, assuming that CAL CUR is set appropriately, am I safe in cranking the power right on up, as long as doing so doesn't cause a HI CUR warning?

Thanks very much, 73 & 72,
Jeff
WB5GWB
Long Island, NY
K2 #821
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[Don Wilhelm-3]

Jeff,

The max power output from the K2 (or any transceiver) depends on dissipating
the heat from the PA as well as the length of time the PA is producing
output.  The K2 will develop more power on some bands than others just due
to the gain of the transmit stages vs. frequency.

I for one would not exceed 12 watts out for any significant length of time.
The difference between 12 watts and 16 watts is only 1.25 dB and will not
likely be heard at the receiving end.  This power limit is not 'official' in
any way and simply represents my personal opinion.

The KAT2 will properly display the power output if (and only if) it is
properly calibrated.  I have calibrated many a KAT2 within 5% and I can say
that it tracks very well at both the high and low ends as well as the center
(except for extremely low levels where the diode knee becomes significant).

If you wish to calibrate your KAT2 with accuracy, you will need a good dummy
load - known to be 50 ohms resistive at the band of measurement to within
1%, and you will need to measure the RF Voltage with an accuracy of at least
5% (you can do that with a calibrated 'scope using a 10X probe across the
dummy load and using interpolated readings).

Oh, of course before you do the forward power calibration, you will need to
adjust the KAT2 trimmer accurately - when adjusted properly, the KAT2 will
display the same SWR with an accurate (1%) dummy load of 100 ohms value and
also with a 25 ohm (1%) dummy load.  After the trimmer is balanced in that
manner, you can set the SWR indication to 2.0.

If you calibrate the KAT2 in the manner above, you will achieve an accuracy
equal to or exceeding that of the spec for a recently calibrated Bird
wattmeter (5% of full scale) - how accurate the Bird is depends on the slug,
if you have a 10 watt slug, it is accurate to within 1/2 watt, but a 100
watt slug yields an accuracy that can only be trusted to within 5 watts.

If you simply calibrate the KAT2 wattmeter with any external wattmeter, all
you can say about it is that the KAT2 calibration is almost as good as the
external wattmeter, and that is not very good at all.  Many wattmeter specs
state 20% of the full scale reading - and that is 10 watts potential error
on a 50 watt scale - and that error is the same regardless of the power
level - just what it says, percentage of the full scale reading.

I have not yet seen any information on the Elecraft wattmeter, but I can
wager that it is quite  accurate and can be used for calibration.  There are
many ICs available that will provide precision power measurements, but they
are not used in the typical wattmeters available in hamshacks.  EMRFD
contains a lot of good information about accurate power measurement, and I
refer you to that resource for further details.

73,
Don W3FPR

> -----Original Message-----
> Hi gang,
>
> Now that we're at the bottom of the solar cycle, I'd like to be
> able to run more than five watts on occasion, but not at the
> expense of putting my final PA at risk.  What is the highest
> level of RF output power that a K2, QRP version, can run on CW
> without putting the finals at risk?  My K2's power, as indicated
> by the internal LCD display, reaches a maximum of 15.2 watts, but
> it's my understanding that it's not safe to transmit for any
> length of time with the power control turned fully clockwise.  I
> have the KAT2 internal antenna tuner, which is supposed to make
> the LCD display's power readings more accurate, but I've gotten
> the impression from others that, even with the KAT2 installed,
> the readings are not terribly accurate, especially at the high
> end of the range.  Is my impression correct?
>
> I believe the instruction manual warns not to set the power to a
> higher level than the transmitter is capable of producing.  Am I
> correct in assuming that a fairly accurate external wattmeter is
> required to determine whether the transmitter is failing to
> deliver the requested power (with requested power indicated by
> the K2's display), or is there a way to identify this state of
> affairs without an external wattmeter?  Alternatively, assuming
> that CAL CUR is set appropriately, am I safe in cranking the
> power right on up, as long as doing so doesn't cause a HI CUR warning?
>
> Thanks very much, 73 & 72,
> Jeff
> WB5GWB
> Long Island, NY
> K2 #821
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>
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[Geoffrey Mackenzie-Kennedy-2]

 Jeff  [hidden email]  wrote:

> Hi gang,
>
> Now that we're at the bottom of the solar cycle, I'd like to be able to
> run more than five watts on occasion, but not at the expense of putting my
> final PA at risk.  What is the highest level of RF output power that a K2,
> QRP version, can run on CW without putting the finals at risk?

<snip>

-------------------------------------------------------------------------------------

Hi Jeff,

To tail-end Don's reply to you about the hardware, a comment on the solar
cycle. True that we are at the bottom (my crystal ball suggests that the
bottom was reached last April), but 10m has been open frequently this year,
not only summertime Es but also long distance F2, and you don't need much
power to work DX on 10m. Frequently a 10m F layer long distance path will
open but 15m will be really "dead" at the time, and there is an explanation
for this behaviour. So if you find 15m to be "dead" it does not always
follow that 10m is "dead", and it is worthwhile to check 10m and. put out a
few 'CQ' with the K2 at low power. You might not be alone! On two days last
week for example I found 10m to be well open from here to South America for
about an hour each time, while 15m was not too good, almost "dead". There
could have been openings to other places when I was not in the shack.

Good hunting!

73,
Geoff
GM4ESD



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[N8LP]

Anyone interested in accurate power measurement should take a look at my
LP-100 Digital Vector Wattmeter kit. I include free band-by-band NIST
traceable calibration for anyone who builds this kit (the LP-100 has a
built-in frequency counter to determine what band you're on). In
addition to about a zillion other exclusive features, the LP-100 will
accurately measure power and SWR from 50 mW to 2500W (autoranging), plus
it has a terminated direct input which allows measurement of power from
-15 to +33 dBm (32 uW to 2W). This is great for bench measurements
(local oscillators, mixers), for setting up transverters, etc. It also
makes a great calibrated field strength meter.

I have sold hundreds of these meters since Dayton, and am taking orders
for a new production run with deliveries well before the holidays. For
more info, check out my website.

73,
Larry N8LP
www.telepostinc.com




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