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KAT500 Matching Range & Power Limits

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Ed G-2

KAT500 Matching Range & Power Limits

Hello,
I'm trying to understand what the KAT500 power limits mean in real
life. If I am tuning an antenna that initially shows a very high SWR (say
14:1), and I am using high power (say 1000 watts out), the KAT500 is going
to open up the PTT line, and the tuning operation will take place with less
than 100 watts. Then I will show a 1:1 SWR, and I'm ready to go with my
1000 watts. Operationally, what is the meaning of the 3:1 SWR limit for
1000 watts as shown in the KAT500 specs? Will the KAT500 fault because it
is matching a 14:1 SWR at high power?
--Ed--

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Jim Wiley

Re: KAT500 Matching Range & Power Limits

Ed -

I think you are confusing the power handling capability of a tuner with
it's ability to match a non-50 ohm load. A tuner may be able to match a
highly reactive load at low power, but be totally unable to handle the
same load at higher power levels. The match involves selecting the
appropriate values of capacitance and inductance to transform a high SWR
load to 50 ohms resistive so the transmitter final amplifier can deliver
power efficiently. However, when substantial power is applied, the
result of the needed transformations can easily result in voltages and
currents that far exceed the ability of the components comprising the
matching network to handle them.

Let's assume your tuner is matching a load that presents a 6 to 1 SWR.
Let's further assume that you have a 100 watt transmitter. At 50 ohms,
100 watts represents about 70 volts RMS of RF signal. or about 101 volts
peak. No problem for a capacitor in the matching network to handle at
50 ohms, but that 6:1 SWR means that the voltage could now be 420 volts
RMS or almost 600 volts peak - still not too much of a problem. But,
suppose you increase the transmitter power from 100 watts to 1000
watts. The RF voltage at 50 ohms increases to 223 volts RMS, (315 volts
peak), and when the4 6:1 SWR is taken into account, that voltage is now
about 1338 RMS volts, or 1820 volts peak. The same sort of thing
happens with currents. At 1000 watts, the RF current at 50 ohms is a
little less than 5 amperes. But at 6:1 SWR, there will be places where
the current is almost 27 amperes - a significant amount to be sure.

Depending on where the tuner is physically located in respect to the
standing wave on the transmission line, a voltage or current peak could
occur directly at the output terminals of the matching network. If that
happens, the components of the matching network could easily be stressed
well beyond their ability to cope.

So, while the coupler may be able to handle lower power levels without
distress, the same is not true at high power. In your example, 1000
watts at a 14:1 SWR could result in peak voltages of about 4400 volts
and/or currents approaching 63 amperes RF. Not many components can
handle those voltages and currents, and those that can are physically
very large, certainly much larger than those that can fit inside the
cabinet of a KAT500.

- Jim, KL7CC

On 11/15/2012 12:56 AM, Ed G wrote:

> Hello,
> I'm trying to understand what the KAT500 power limits mean in real
> life. If I am tuning an antenna that initially shows a very high SWR (say
> 14:1), and I am using high power (say 1000 watts out), the KAT500 is going
> to open up the PTT line, and the tuning operation will take place with less
> than 100 watts. Then I will show a 1:1 SWR, and I'm ready to go with my
> 1000 watts. Operationally, what is the meaning of the 3:1 SWR limit for
> 1000 watts as shown in the KAT500 specs? Will the KAT500 fault because it
> is matching a 14:1 SWR at high power?
> --Ed--
>
>
>

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Dick Dievendorff

Re: KAT500 Matching Range & Power Limits

In reply to this post by Ed G-2

Ed:

The KAT500 does open up the PTT line and tunes at low power. An early step
in that tuning measures the SWR of your 14:1 antenna with the KAT500 in its
bypassed relay configuration. Then the ATU finds the reactive components
needed to match your antenna. The "bypass VSWR" is retained in each tuner
memory and is used to evaluate the power that the ATU can safely handle.
Should you exceed this power, for example 1000 watts into an antenna that
was originally 14:1, the ATU faults.

I would recommend "Transmission Lines for Windows" (TLW), a computer program
written by Dean Straw (N6BV), available in recent editions of the ARRL
Antenna Book, to evaluate tuner component stresses at various antenna
impedances. Our tuner configuration is what Dean calls a "Low Pass L
network".

High power high SWR results in some surprisingly high tuner voltages and
currents. The KAT500 is robust, but the components have limits related to
the advertised KAT500 capability.

73 de Dick, K6KR

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Ed G
Sent: Thursday, November 15, 2012 1:57 AM
To: [hidden email]
Subject: [Elecraft] KAT500 Matching Range & Power Limits

Hello,
I'm trying to understand what the KAT500 power limits mean in real
life. If I am tuning an antenna that initially shows a very high SWR (say
14:1), and I am using high power (say 1000 watts out), the KAT500 is going
to open up the PTT line, and the tuning operation will take place with less
than 100 watts. Then I will show a 1:1 SWR, and I'm ready to go with my
1000 watts. Operationally, what is the meaning of the 3:1 SWR limit for
1000 watts as shown in the KAT500 specs? Will the KAT500 fault because it
is matching a 14:1 SWR at high power?
--Ed--

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Phil Salas

Re: KAT500 Matching Range & Power Limits

In reply to this post by Jim Wiley

The voltage or current can be as bad as the 1:1 voltage or current multiplied by the square root of SWR – NOT multiplied by directly by SWR. So voltage or current at a 6:1 SWR could be as bad as about 2.5 times the 1:1 SWR voltage or current, and at 10:1 SWR a little over three times the 1:1 SWR values. Note that these are worst-case numbers – and as voltage goes up current goes down and vice versa for a given power level.

Phil – AD5X

“Let's assume your tuner is matching a load that presents a 6 to 1 SWR. Let's further assume that you have a 100 watt transmitter. At 50 ohms, 100 watts represents about 70 volts RMS of RF signal. or about 101 volts peak. No problem for a capacitor in the matching network to handle at 50 ohms, but that 6:1 SWR means that the voltage could now be 420 volts RMS or almost 600 volts peak - still not too much of a problem. But, suppose you increase the transmitter power from 100 watts to 1000 watts. The RF voltage at 50 ohms increases to 223 volts RMS, (315 volts peak), and when the 6:1 SWR is taken into account, that voltage is now about 1338 RMS volts, or 1820 volts peak. The same sort of thing happens with currents. At 1000 watts, the RF current at 50 ohms is a little less than 5 amperes. But at 6:1 SWR, there will be places where the current is almost 27 amperes - a significant amount to be sure.”
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N5GE-2

Re: KAT500 Matching Range & Power Limits

In reply to this post by Dick Dievendorff

If you have a KAT500, there is a good way to understand how the
tuner works in high power modes by running the KAT500 Utility while
operating the tuner and watching the Last Observed section of the
Operate tab.

The Bypass VSWR value is shown near the bottom of the Last Observed
section. If that value is above 10.0 the tuner will fault when you
try to tune at high power. I believe there is another plateau in the
power range that causes a fault if the Bypass VSWR is above 3.0, but
I'm not sure what the power limit is for that because I only use the
KPA500 here.

Amateur Radio Operator N5GE
ARRL Lifetime Member
QCWA Lifetime Member

On Thu, 15 Nov 2012 05:42:07 -0800, "Dick Dievendorff"
<[hidden email]> wrote:

>Ed:
>
>The KAT500 does open up the PTT line and tunes at low power. An early step
>in that tuning measures the SWR of your 14:1 antenna with the KAT500 in its
>bypassed relay configuration. Then the ATU finds the reactive components
>needed to match your antenna. The "bypass VSWR" is retained in each tuner
>memory and is used to evaluate the power that the ATU can safely handle.
>Should you exceed this power, for example 1000 watts into an antenna that
>was originally 14:1, the ATU faults.
>
>I would recommend "Transmission Lines for Windows" (TLW), a computer program
>written by Dean Straw (N6BV), available in recent editions of the ARRL
>Antenna Book, to evaluate tuner component stresses at various antenna
>impedances. Our tuner configuration is what Dean calls a "Low Pass L
>network".
>
>High power high SWR results in some surprisingly high tuner voltages and
>currents. The KAT500 is robust, but the components have limits related to
>the advertised KAT500 capability.
>
>73 de Dick, K6KR
>
>
>-----Original Message-----
>From: [hidden email]
>[mailto:[hidden email]] On Behalf Of Ed G
>Sent: Thursday, November 15, 2012 1:57 AM
>To: [hidden email]
>Subject: [Elecraft] KAT500 Matching Range & Power Limits
>
>Hello,
> I'm trying to understand what the KAT500 power limits mean in real
>life. If I am tuning an antenna that initially shows a very high SWR (say
>14:1), and I am using high power (say 1000 watts out), the KAT500 is going
>to open up the PTT line, and the tuning operation will take place with less
>than 100 watts. Then I will show a 1:1 SWR, and I'm ready to go with my
>1000 watts. Operationally, what is the meaning of the 3:1 SWR limit for
>1000 watts as shown in the KAT500 specs? Will the KAT500 fault because it
>is matching a 14:1 SWR at high power?
>--Ed--
>
>
>
>______________________________________________________________
>Elecraft mailing list
>Home: http://mailman.qth.net/mailman/listinfo/elecraft
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:[hidden email]
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html
>
>______________________________________________________________
>Elecraft mailing list
>Home: http://mailman.qth.net/mailman/listinfo/elecraft
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:[hidden email]
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html