[Elecraft] KPA-1500: Widely varying drive requirement - same frequency, different antenna

[N4ZR]

Thanks to everyone who replied, either directly or on the list.  I'm 
sorry, but I forgot to mention one key fact - I was running the amp's 
ATU with both of these antennas.  Assuming that the amp's power output 
measurement is done at the input of the ATU and not the output, and that 
the ATU settings for the two antennas ( ATU rRetune SWR and ATU Stop Tun 
SWR ) are identical, this really does look to me like a difference in 
amplifier gain rather than measurement error.

The antennas are clearly a lot different - in fact the Windom will often 
throw a Reflected Power fault when I first switch to it - but it seems 
to me that after the ATU they should look the same.

73, Pete N4ZR
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On 10/23/2019 9:55 AM, Bob McGraw K4TAX wrote:
> Pete:
>
> The fact it requires more drive with one antenna as opposed to another 
> says the two antennas do not have the same impedance. This is not at 
> all unusual.  Power measurements, with the methods used in ham radio 
> applications, are based on voltage measurements with the impedance 
> presumed to be 50 ohms.   Ohms law says P = E²/R thus any change in R 
> {impedance} will cause a change in Power indicated or measured at a 
> given point.
>
> A second factor is SWR which is an indication of the relative 
> impedance between the source {amp} and the load {antenna}.  In this 
> case, as example, a 1.5:1 SWR can be 75 ohms or it can be 33 ohms.  
> SWR and Power meters are calibrated for 50 ohms and are based on 
> voltage on the feed line.  Again we see that the impedance or the R 
> part being different will affect the Power. In order to deliver the 
> same power into a 33 ohm load, the amp is required to deliver more 
> current and thus more drive is required.
>
> From an RF measurement at a given point with different impedance's we 
> find:
>
> 1500 watts into 75 ohms is 335 volts with a current of 4.47 amps
>
> 1500 watts into 50 ohms is 273 volts with a current of 5.47 amps
>
> 1500 watts into 33 ohms is 222 volts with a current of 6.74 amps
>
> From the above one can see the amp is required to deliver more current 
> into a lower impedance and to do so will require more drive power. And 
> from the above, one can see the voltage on the feed line is different 
> with different loads.  In this regard, in as much as we measure power 
> as a voltage with a known resistance the power measurements can be in 
> error.   Our power indicating devices are calibrated for 50 ohms.   
> Any deviation from 50 ohms will thus cause an error in power indication.
>
> A third component of SWR and Power measurements would be Common Mode 
> Current on the feed line. This is usually current induced on the 
> outside of the feed line from the power radiated from the antenna.  
> This common mode current is not measured by our power indicating 
> instruments.   As a side note, Windom antennas are noted to exhibit 
> high Common Mode Current conditions.  The solution for CMC is to have 
> a good Common Mode Choke at the feed point of the antenna, where the 
> feed line connects, and also at the station end.
>
> Jim, K9YC, has and excellent paper on the topic: 
> http://audiosystemsgroup.com/2018Cookbook.pdf
>
> Also, Rick, DJ0IP has a lot of valid information on his site:
>
> http://www.dj0ip.de/balun-stuff/
>
> http://www.dj0ip.de/rf-cmc-chokes/
>
> Now after all of this is said and done, I'd say your results with your 
> KPA1500 are normal.   Hence the reason for the variation in drive is 
> due to the load impedance presented to the amp and the power 
> measurement method.
>
> 73
>
> Bob, K4TAX
>
>
>
> On 10/23/2019 6:59 AM, N4ZR wrote:
>> I have two 40M antennas, a Carolina Windom and a full-wave parasitic 
>> sloper.  I have just noticed that on any given frequency, my K3 must 
>> deliver considerably more power on one than the other, in order for 
>> the amp to produce 1500 watts.  The difference is 27 watts on one vs 
>> 42 watts on the other.  Is this normal?
>>
>
>
>