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

[Eric Swartz WA6HHQ - Elecraft]

This is not unusual when the loads presented by the antennas (or tuner)  to the 
amp are different.  This can even be true when the SWRs are identical if the 
actual +/- reactance of one is different than the other. 

This is most noticeable on lower freq bands where the amps gain is typically 
higher. 

Eric
elecraft.com
---
Sent from my iPhone 6S

> On Oct 23, 2019, at 5:00 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?
> 
> -- 
> 
> 73, Pete N4ZR
> Check out the Reverse Beacon Network
> at , now
> spotting RTTY activity worldwide.
> For spots, please use your favorite
> "retail" DX cluster.
> 
> __
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Re: [Elecraft] KPA-1500: Widely varying drive requirement - same frequency, different antenna

[K9MA]

Based on my experience, the KPA1500 is very sensitive to small changes 
in load impedance (SWR) with respect to power gain and current. The ATU 
software is much better than it started out, but still sometimes doesn't 
get below 1.2:1. That could result in the kind of power gain variation 
Pete is seeing.


73,
Scott K9MA


On 10/23/2019 12:24, Jim Rhodes wrote:

Actually the fact that you were using the internal tuner doesn't change the
chance that with the settings on the auto tuner don't tell you which
direction the tuning of a certain antenna approach unity from. So there
could be twice the variation allowed between the 2 antennas. So if one is
coming from high impedance and stops at say 1.2:1 and the other is coming
from low and stops at 1.2:1 then there is still a considerable difference
between them. Putting an analyser on them to see the starting point the
tuner is working on could explain a lot.

Jim Rhodes
K0XU

On Wed, Oct 23, 2019, 11:17 N4ZR  wrote:


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
Check out the Reverse Beacon Network
at , now
spotting RTTY activity worldwide.
For spots, please use your favorite
"retail" DX cluster.

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?





__

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

[Jim Rhodes]

Actually the fact that you were using the internal tuner doesn't change the
chance that with the settings on the auto tuner don't tell you which
direction the tuning of a certain antenna approach unity from. So there
could be twice the variation allowed between the 2 antennas. So if one is
coming from high impedance and stops at say 1.2:1 and the other is coming
from low and stops at 1.2:1 then there is still a considerable difference
between them. Putting an analyser on them to see the starting point the
tuner is working on could explain a lot.

Jim Rhodes
K0XU

On Wed, Oct 23, 2019, 11:17 N4ZR  wrote:

> 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
> Check out the Reverse Beacon Network
> at , now
> spotting RTTY activity worldwide.
> For spots, please use your favorite
> "retail" DX cluster.
>
> 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?
> >>
> >
> >
> >
> __
> Elecraft mailing list
> Home: http://mailman.qth.

Re: [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
Check out the Reverse Beacon Network
at , now
spotting RTTY activity worldwide.
For spots, please use your favorite
"retail" DX cluster.

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

[N4ZR]

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?


--

73, Pete N4ZR
Check out the Reverse Beacon Network
at , now
spotting RTTY activity worldwide.
For spots, please use your favorite
"retail" DX cluster.

__
Elecraft mailing list
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Help: http://mailman.qth.net/mmfaq.htm
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