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?
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