[Elecraft] Perplexing output power behavior for K3(s)

Mon Mar 17 20:18:45 EDT 2025

Thanks Jerry.   Yes, that will work quite well.

And to all that wonder, it will make little if any difference if the 
attenuator output is open, terminated into 50 ohms or shorted.  It will 
show the characteristic impedance.   With 100 watts into the input there 
will be about 0.1 watts at the output. That reflected back to the input 
and thus attenuated 30 dB will be then about 0.0001 watts of reflected 
power.  That gives and SWR of 1.002:1.  (If I did the math correctly.)

73

Bob, K4TAX

On 3/17/2025 5:31 PM, jerry wrote:
> On 2025-03-17 14:36, Bob McGraw wrote:
>>
>> Now the reason for stressing known good 50 ohm load, I have 3
>> different brands, and power dummy loads.    Although all say "50 ohms"
>> they are not!  One is 57 ohms (air type load),  one is 48 ohms cold
>> and 53 ohms hot (Cantenna) , and the third is 50.5 ohms (Bird 100 watt
>> load).
>>
> *** And I have a 500W Bird 30dB attenuator that I use as a dummy 
> load.  Just measured it -
> 50.5 ohms, with nothing connected at the output end.
>
>                - Jerry, KF6VB
>
>
>> From the Messages files, 9/21/21, #35799 I find:
>>
>> Keith_WE6R
>> 09/21/21#35799 // <https://groups.io/g/Elecraft-K3/message/35799>
>>
>>
>> Hi Glenn;
>> I understand you got it going, but;
>> What exactly did you do when you;   "Did reset on K3.." ?
>> A Parameter Initialization (EE-INIT)  will wipe many settings and 
>> calibrations.
>>
>> *Also, one should never TX at exactly 5 or 50 watts (or 1.0MW) as you
>> might do an inadvertent manual Transmit-Gain-Calibration.*
>>
>> Keith WE6R Elecraft K3/K4 Tech.
>>
>> Hope this answers your questions.
>>
>> 73
>> Bob, K4TAX
>>
>>
>> <https://groups.io/g/Elecraft-K3/topic/85741710?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Arecentpostdate%2Fsticky%2C%2CWE6R+TX+Gain%2C20%2C2%2C20%2C85741710%2Cct%253D1&ct=1#window-192949176> 
>>
>> //
>> <https://groups.io/g/Elecraft-K3/topic/85741710?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Arecentpostdate%2Fsticky%2C%2CWE6R+TX+Gain%2C20%2C2%2C20%2C85741710%2Cct%253D1&ct=1#> 
>>
>>
>>
>>
>> On 3/17/2025 3:02 PM, Barry Baines wrote:
>>> Bob:
>>>
>>>> On Mar 17, 2025, at 11:20, Bob McGraw<rmcgraw at benlomand.net> wrote:
>>>>
>>>> Corky:
>>>>
>>>> I'd say the radio needs a transmit power calibration.   The K3 
>>>> Utility makes this easy.  Only a known good 50 ohm dummy load is 
>>>> required.  Preferably connected direct to the radio output, 
>>>> bypassing any switching, etc.
>>>>
>>>> NEVER NEVER set the power to 50 watts!  This is the point of 
>>>> automatic power calibration.  Thus avoid 5 watts and 50 watts as 
>>>> power settings.  Use either 4 or 6 watts and 40 or 60 watts for 
>>>> your normal operation.  Cold has nothing to do with it.
>>> This is a very interesting admonition and is something that I wasn’t 
>>> aware of.  Could you point me to Elecraft documentation (or where 
>>> Fred Cady, KE7X may discuss this in his K3 book) that explains 
>>> automatic power calibration and using the 5W and 50W power settings 
>>> only for calibration?
>>>
>>> Thanks,
>>>
>>> Barry, WD4ASW/5
>>> Keller, TX
>>>
>>>> 73
>>>>
>>>> Bob, K4TAX
>>>>
>>>>
>>>> On 3/17/2025 11:00 AM,elecraft-request at mailman.qth.net wrote:
>>>>> Message: 5
>>>>> Date: Sun, 16 Mar 2025 15:00:09 -0700
>>>>> From: Corky<af4pm at searlsnet.com>
>>>>> To:Elecraft at mailman.qth.net
>>>>> Subject: [Elecraft] Perplexing output power behavior for K3(s)
>>>>> Message-ID:<5f4487fc-7b18-4ab3-a2aa-354312c93a05 at app.fastmail.com>
>>>>> Content-Type: text/plain
>>>>>
>>>>> I have an older K3 that has been upgraded to a K3S with the output 
>>>>> power modification for the P3. All this works, but I find that 
>>>>> when I turn on the radio and then transmit on 20 m, the first TX 
>>>>> is always about 1/2 power compared to the second and all future TX 
>>>>> episodes. This only seems to happen when I turn on the radio and 
>>>>> it is "cold" meaning at room temperature. I only have numbers for 
>>>>> 20m, but when I did the first TX today, the output was just under 
>>>>> 40 W PEP with the power set at 50W. The second TX (about 2 min 
>>>>> later) the output power was 78 W PEP and this persisted for all TX 
>>>>> for the next hour. Nothing was changed in the radio between the 
>>>>> first and second TX episodes.
>>>>>
>>>>> I do not remember ever seeing this behavior with the original 
>>>>> boards, but it appears as though something is "warming up" before 
>>>>> it will transmit at full power. Does anyone have any insight as to 
>>>>> what might be happening and whether this is something I need to 
>>>>> address or just live with?
>>>>>
>>>>> Thanks and 73,
>>>>> -Corky, AF4PM
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