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Can I measure antenna impedance with K2?

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TF3KX

Can I measure antenna impedance with K2?

Hi: I have a K2 QRP rig with a built-in ATU. Are there any suggestions on how I could use the K2 to estimate the input impedance of my antenna, or at least see if it is in resonance? Sure enough, I can simply make the ATU find its best match, but I would like to know if the antenna itself actually was close to resonance or even estimate its input impedance, as measured at the K2 antenna connector. Can I measure the antenna SWR and bypass the ATU while I am doing that, to get at least an idea? 73 - Kristinn, TF3KX

Johnny Siu

Can I measure antenna impedance with K2?

You can measure the SWR by using KAT2. The details are given on page 19 of the KAT2 manual.

73 Johnny VR2XMC

________________________________
寄件人︰ TF3KX <[hidden email]>
收件人︰ [hidden email]
傳送日期︰ 2014年07月31日 (週四) 2:26 PM
主題︰ [Elecraft] Can I measure antenna impedance with K2?

Hi:I have a K2 QRP rig with a built-in ATU. Are there any suggestions on how
I could use the K2 to estimate the input impedance of my antenna, or at
least see if it is in resonance?Sure enough, I can simply make the ATU find
its best match, but I would like to know if the antenna itself actually was
close to resonance or even estimate its input impedance, as measured at the
K2 antenna connector. Can I measure the antenna SWR and bypass the ATU while
I am doing that, to get at least an idea?73 - Kristinn, TF3KX

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Don Wilhelm-4

Re: Can I measure antenna impedance with K2?

In reply to this post by TF3KX

Kristinn,

After doing a TUNE that hopefully ends up with a low SWR, you can look
at the ATU menu parameters for Lxx.x, Cxx.x and NET x to determine the
configuration of the L network that the ATU found for the match.
You can then solve the L network equations to find the impedance of the
end of your feedline in the shack.
See page 21 in the KAT2 manual.

73,
Don W3FPR

On 7/31/2014 2:26 AM, TF3KX wrote:
> Hi:I have a K2 QRP rig with a built-in ATU. Are there any suggestions on how
> I could use the K2 to estimate the input impedance of my antenna, or at
> least see if it is in resonance?Sure enough, I can simply make the ATU find
> its best match, but I would like to know if the antenna itself actually was
> close to resonance or even estimate its input impedance, as measured at the
> K2 antenna connector. Can I measure the antenna SWR and bypass the ATU while
> I am doing that, to get at least an idea?73 - Kristinn, TF3KX
>

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TF3KX

Re: Can I measure antenna impedance with K2?

Thanks. Yes, I can determine the tuner settings (L/C-values, etc.) and from the network equations derive the impedance these settings match into. It would be neat if the K2 had enough CPU power and memory to carry these calculations out for me and just display the R+jX. Perhaps a good project for a small program or an app...

73 - Kristinn, TF3KX

vk2rq

Re: Can I measure antenna impedance with K2?

Of course you realise that the output impedance of the ATU will be the complex conjugate of the impedance at the rig end of your feedline. However, if you want to know the impedance of your antenna, you need to realise that the feedline is acting as an impedance transformer, and you need to take into account factors such as length of feedline, frequency of operation characteristic impedance and velocity factor of the feedline, etc..

73, Matt VK2RQ

> On 1 Aug 2014, at 1:02 am, TF3KX <[hidden email]> wrote:
>
> Thanks. Yes, I can determine the tuner settings (L/C-values, etc.) and from
> the network equations derive the impedance these settings match into. It
> would be neat if the K2 had enough CPU power and memory to carry these
> calculations out for me and just display the R+jX. Perhaps a good project
> for a small program or an app...
>
> 73 - Kristinn, TF3KX
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/Can-I-measure-antenna-impedance-with-K2-tp7591798p7591803.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
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John Oppenheimer

Re: Can I measure antenna impedance with K2?

In reply to this post by TF3KX

There is a program, SmithSmith http://www.ae6ty.com/Smith_Charts.html
which can also solve the impedance at the antenna. Set the ATU L and C
and the coax feed-line as constants then adjust the antenna parameters
for 1:1 SWR at the input.

John KN5L

On 07/31/2014 10:02 AM, TF3KX wrote:
> I can determine the tuner settings (L/C-values, etc.) and from
> the network equations derive the impedance these settings match into.
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EricJ-2

Re: Can I measure antenna impedance with K2?

The program is SimSmith (simple typo) and I agree with John that it is a
very useful one. I have used several other implementations of Smith
Charts and this one is by far my favorite. Fairly frequent updates, and
a yahoo group for support.

Eric
KE6US

On 7/31/2014 1:05 PM, John Oppenheimer wrote:

> There is a program, SmithSmith http://www.ae6ty.com/Smith_Charts.html
> which can also solve the impedance at the antenna. Set the ATU L and C
> and the coax feed-line as constants then adjust the antenna parameters
> for 1:1 SWR at the input.
>
> John KN5L
>
> On 07/31/2014 10:02 AM, TF3KX wrote:
>> I can determine the tuner settings (L/C-values, etc.) and from
>> the network equations derive the impedance these settings match into.
> ______________________________________________________________
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>

LA7NO

Re: Can I measure antenna impedance with K2?

In reply to this post by vk2rq

Hi Matt,

> Of course you realise that the output impedance of the ATU will be the
> complex conjugate of the impedance at the rig end of your feedline.

I wonder if this is correct

IMO, the task of the tuner is to make the tx happy, so it can look into the
load it is designed for. Thus, it will translate the impedance as seen from
the antenna connector into something as close to 50 ohm as possible. The
tuner will not change anything on the feeder. With or without the tuner,
the feeder will have the same VSWR.

Per-Tore / LA7NO

On 31 July 2014 22:02, Matt VK2RQ <[hidden email]> wrote:

> Of course you realise that the output impedance of the ATU will be the
> complex conjugate of the impedance at the rig end of your feedline.
> However, if you want to know the impedance of your antenna, you need to
> realise that the feedline is acting as an impedance transformer, and you
> need to take into account factors such as length of feedline, frequency of
> operation characteristic impedance and velocity factor of the feedline,
> etc..
>
> 73, Matt VK2RQ
>
> > On 1 Aug 2014, at 1:02 am, TF3KX <[hidden email]> wrote:
> >
> > Thanks. Yes, I can determine the tuner settings (L/C-values, etc.) and
> from
> > the network equations derive the impedance these settings match into. It
> > would be neat if the K2 had enough CPU power and memory to carry these
> > calculations out for me and just display the R+jX. Perhaps a good project
> > for a small program or an app...
> >
> > 73 - Kristinn, TF3KX
> >
> >
> >
> > --
> > View this message in context:
> http://elecraft.365791.n2.nabble.com/Can-I-measure-antenna-impedance-with-K2-tp7591798p7591803.html
> > Sent from the Elecraft mailing list archive at Nabble.com.
> > ______________________________________________________________
> > Elecraft mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/elecraft
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:[hidden email]
> >
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> > Message delivered to [hidden email]
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73,

Per-Tore / LA7NO

Don Wilhelm-4

Re: Can I measure antenna impedance with K2?

Per-Tore,

Yes it is correct. He stated that it is the impedance at the rig end of
the feedline.
That will not likely be the feedpoint impedance of the antenna. That
impedance can be found if you do the calculations for the impedance
transformation of the feedline (for which the length of the feedline
must be known).

73,
Don W3FPR

On 7/31/2014 4:23 PM, Per-Tore Aasestrand wrote:

> Hi Matt,
>
>> Of course you realise that the output impedance of the ATU will be the
>> complex conjugate of the impedance at the rig end of your feedline.
> I wonder if this is correct
>
> IMO, the task of the tuner is to make the tx happy, so it can look into the
> load it is designed for. Thus, it will translate the impedance as seen from
> the antenna connector into something as close to 50 ohm as possible. The
> tuner will not change anything on the feeder. With or without the tuner,
> the feeder will have the same VSWR.
>
> Per-Tore / LA7NO
>
>
>
>
> On 31 July 2014 22:02, Matt VK2RQ <[hidden email]> wrote:
>
>> Of course you realise that the output impedance of the ATU will be the
>> complex conjugate of the impedance at the rig end of your feedline.
>> However, if you want to know the impedance of your antenna, you need to
>> realise that the feedline is acting as an impedance transformer, and you
>> need to take into account factors such as length of feedline, frequency of
>> operation characteristic impedance and velocity factor of the feedline,
>> etc..
>>
>> 73, Matt VK2RQ
>>
>>> On 1 Aug 2014, at 1:02 am, TF3KX <[hidden email]> wrote:
>>>
>>> Thanks. Yes, I can determine the tuner settings (L/C-values, etc.) and
>> from
>>> the network equations derive the impedance these settings match into. It
>>> would be neat if the K2 had enough CPU power and memory to carry these
>>> calculations out for me and just display the R+jX. Perhaps a good project
>>> for a small program or an app...
>>>
>>> 73 - Kristinn, TF3KX
>>>
>>>
>>>
>>> --
>>> View this message in context:
>> http://elecraft.365791.n2.nabble.com/Can-I-measure-antenna-impedance-with-K2-tp7591798p7591803.html
>>> Sent from the Elecraft mailing list archive at Nabble.com.
>>> ______________________________________________________________
>>> 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
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>>> Message delivered to [hidden email]
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
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>>
> ______________________________________________________________
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>

LA7NO

Re: Can I measure antenna impedance with K2?

Hi Don,

I still doubt that this is correct.

The L/C values given by the tuner, are the values as seen from the tx side.
It is the values that will make the tx happy. It does not say much about
the impedance at the rig end of the feeder.

In order to change the VSWR on the feeder itself, one has to put the
matching network (tuner) between the antenna and the feeder.

BTW, the tx is probably not conjugate matched by the tuner. Conjugate
matching does not apply to transmitters or power amplifiers. But I realize
there is a huge dissension in this area too. ;-)

Per-Tore / LA7NO

On 31 July 2014 22:42, Don Wilhelm <[hidden email]> wrote:

> Per-Tore,
>
> Yes it is correct. He stated that it is the impedance at the rig end of
> the feedline.
> That will not likely be the feedpoint impedance of the antenna. That
> impedance can be found if you do the calculations for the impedance
> transformation of the feedline (for which the length of the feedline must
> be known).
>
> 73,
> Don W3FPR
>
>
> On 7/31/2014 4:23 PM, Per-Tore Aasestrand wrote:
>
>> Hi Matt,
>>
>> Of course you realise that the output impedance of the ATU will be the
>>> complex conjugate of the impedance at the rig end of your feedline.
>>>
>> I wonder if this is correct
>>
>> IMO, the task of the tuner is to make the tx happy, so it can look into
>> the
>> load it is designed for. Thus, it will translate the impedance as seen
>> from
>> the antenna connector into something as close to 50 ohm as possible. The
>> tuner will not change anything on the feeder. With or without the tuner,
>> the feeder will have the same VSWR.
>>
>> Per-Tore / LA7NO
>>
>>
>>
>>
>> On 31 July 2014 22:02, Matt VK2RQ <[hidden email]> wrote:
>>
>> Of course you realise that the output impedance of the ATU will be the
>>> complex conjugate of the impedance at the rig end of your feedline.
>>> However, if you want to know the impedance of your antenna, you need to
>>> realise that the feedline is acting as an impedance transformer, and you
>>> need to take into account factors such as length of feedline, frequency
>>> of
>>> operation characteristic impedance and velocity factor of the feedline,
>>> etc..
>>>
>>> 73, Matt VK2RQ
>>>
>>> On 1 Aug 2014, at 1:02 am, TF3KX <[hidden email]> wrote:
>>>>
>>>> Thanks. Yes, I can determine the tuner settings (L/C-values, etc.) and
>>>>
>>> from
>>>
>>>> the network equations derive the impedance these settings match into. It
>>>> would be neat if the K2 had enough CPU power and memory to carry these
>>>> calculations out for me and just display the R+jX. Perhaps a good
>>>> project
>>>> for a small program or an app...
>>>>
>>>> 73 - Kristinn, TF3KX
>>>>
>>>>
>>>>
>>>> --
>>>> View this message in context:
>>>>
>>> http://elecraft.365791.n2.nabble.com/Can-I-measure-
>>> antenna-impedance-with-K2-tp7591798p7591803.html
>>>
>>>> Sent from the Elecraft mailing list archive at Nabble.com.
>>>> ______________________________________________________________
>>>> 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
>>>> Message delivered to [hidden email]
>>>>
>>> ______________________________________________________________
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>>>
>>> ______________________________________________________________
>> Elecraft mailing list
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>>
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>>
>>
>

73,

Per-Tore / LA7NO

vk2rq

Re: Can I measure antenna impedance with K2?

I think I see where the confusion has arisen. Here we are talking about a K2 with integrated KAT2 ATU. So, when I used the term "rig end of the feedline", I was referring to where the feedline connects to the KAT2 BNC connector at the back of the K2, which is the output or antenna side of the KAT2 ATU. In order to make the impedance where the antenna connects to the KAT2 output look like 50+0j ohms, the KAT2 will use an LC network to transform the impedance. The impedance of this LC network together with the 50ohms of the K2 final PA output will be equal to the complex conjugate of the impedance you will see looking into the shack end of the feedline leading to your antenna.

For example, to keep the maths simple, let's consider a case where the impedance at the end of the feedline in the shack works out to be 50+j100ohms. That is, the antenna is slightly inductive. In order to transform this impedance to 50ohms, you could connect a capacitor in series that has a reactance of 100ohms to cancel out the inductive reactance of the antenna+feedline. If we look at the impedance of our matching network plus radio, we see the capacitor in series with the 50ohm transmitter, so the impedance looking into the "ATU" plus transmitter is 50-j100 ohms. This is the complex conjugate of the impedance looking down the feedline towards the antenna, which we already see is 50+j100ohms.

In a more complicated scenario, the resistive component at the shack end of the feedline may not be 50ohms, and our matching network will be more complicated, and we will connect some reactance across the feedline and/or transmitter to achieve the required impedance transformation. If you go through the maths, you will again find the impedance looking into the ATU+transmitter is the complex conjugate of the impedance looking into the feedline+antenna.

73,
Matt VK2RQ

> On 1 Aug 2014, at 7:29 am, Per-Tore Aasestrand <[hidden email]> wrote:
>
> Hi Don,
>
> I still doubt that this is correct.
>
> The L/C values given by the tuner, are the values as seen from the tx side. It is the values that will make the tx happy. It does not say much about the impedance at the rig end of the feeder.
>
> In order to change the VSWR on the feeder itself, one has to put the matching network (tuner) between the antenna and the feeder.
>
> BTW, the tx is probably not conjugate matched by the tuner. Conjugate matching does not apply to transmitters or power amplifiers. But I realize there is a huge dissension in this area too. ;-)
>
> Per-Tore / LA7NO
>
>
>
>> On 31 July 2014 22:42, Don Wilhelm <[hidden email]> wrote:
>> Per-Tore,
>>
>> Yes it is correct. He stated that it is the impedance at the rig end of the feedline.
>> That will not likely be the feedpoint impedance of the antenna. That impedance can be found if you do the calculations for the impedance transformation of the feedline (for which the length of the feedline must be known).
>>
>> 73,
>> Don W3FPR
>>
>>
>>> On 7/31/2014 4:23 PM, Per-Tore Aasestrand wrote:
>>> Hi Matt,
>>>
>>>> Of course you realise that the output impedance of the ATU will be the
>>>> complex conjugate of the impedance at the rig end of your feedline.
>>> I wonder if this is correct
>>>
>>> IMO, the task of the tuner is to make the tx happy, so it can look into the
>>> load it is designed for. Thus, it will translate the impedance as seen from
>>> the antenna connector into something as close to 50 ohm as possible. The
>>> tuner will not change anything on the feeder. With or without the tuner,
>>> the feeder will have the same VSWR.
>>>
>>> Per-Tore / LA7NO
>>>
>>>
>>>
>>>
>>> On 31 July 2014 22:02, Matt VK2RQ <[hidden email]> wrote:
>>>
>>>> Of course you realise that the output impedance of the ATU will be the
>>>> complex conjugate of the impedance at the rig end of your feedline.
>>>> However, if you want to know the impedance of your antenna, you need to
>>>> realise that the feedline is acting as an impedance transformer, and you
>>>> need to take into account factors such as length of feedline, frequency of
>>>> operation characteristic impedance and velocity factor of the feedline,
>>>> etc..
>>>>
>>>> 73, Matt VK2RQ
>>>>
>>>>> On 1 Aug 2014, at 1:02 am, TF3KX <[hidden email]> wrote:
>>>>>
>>>>> Thanks. Yes, I can determine the tuner settings (L/C-values, etc.) and
>>>> from
>>>>> the network equations derive the impedance these settings match into. It
>>>>> would be neat if the K2 had enough CPU power and memory to carry these
>>>>> calculations out for me and just display the R+jX. Perhaps a good project
>>>>> for a small program or an app...
>>>>>
>>>>> 73 - Kristinn, TF3KX
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> View this message in context:
>>>> http://elecraft.365791.n2.nabble.com/Can-I-measure-antenna-impedance-with-K2-tp7591798p7591803.html
>>>>> Sent from the Elecraft mailing list archive at Nabble.com.
>>>>> ______________________________________________________________
>>>>> 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
>>>>> Message delivered to [hidden email]
>>>> ______________________________________________________________
>>>> Elecraft mailing list
>>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>>>> Post: mailto:[hidden email]
>>>>
>>>> This list hosted by: http://www.qsl.net
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>>>> Message delivered to [hidden email]
>>> ______________________________________________________________
>>> Elecraft mailing list
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>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>>> Message delivered to [hidden email]
>

LA7NO

Re: Can I measure antenna impedance with K2?

Hi list,

Isn't an RF transmitter or PA actually more like a current generator? Its
internal resistance is kept low in order to reduce internally dissipated
power, with consequent reduction in efficiency. It is usually designed to
work efficiently into 50 ohms.

It is important to know the difference between these two statements:
(A) The transmitter has a 50 ohm output impedance.
(B) The transmitter is designed to work into a 50 ohm load

A transmitter will be able to put its rated power into a 50 ohm load, but
this says nothing about its output impedance. That is why I don't think
conjugate matching will be appropriate.

(Hope this is not getting too much OT now)

Per-Tore / LA7NO
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73,

Per-Tore / LA7NO

Don Wilhelm-4

Re: Can I measure antenna impedance with K2?

In reply to this post by LA7NO

Per-Tor,

The tuner (if properly calibrated using a 50 ohm non-reactive load) will
show close to 1:1 SWR at the tuner *input* after a TUNE - it is close
enough that it is practical to assume it is 50 ohms resistive.

After that TUNE, the Lx.xx Cx.xx and NET x ATU menu parameters will
indicate the inductance, capacitance and L network configuration that
were used in the ATU to achieve that match.

Working through the L network impedance transformation using a 50 ohm
input will produce the impedance at the output of the ATU. That will be
the conjugate of the impedance presented in the shack by the feedline.

You are correct that an ATU does *not* alter the VSWR on the feedline -
it is only capable of altering the VSWR at the *input to the tuner to
make the transmitter PA stage "happy" with the 50 ohm ATU input impedance.

I don't think the original poster nor any of the responses made any
inferences that the feedline impedance was being changed.

73,
Don W3FPR

On 7/31/2014 5:29 PM, Per-Tore Aasestrand wrote:

> Hi Don,
>
> I still doubt that this is correct.
>
> The L/C values given by the tuner, are the values as seen from the tx side.
> It is the values that will make the tx happy. It does not say much about
> the impedance at the rig end of the feeder.
>
> In order to change the VSWR on the feeder itself, one has to put the
> matching network (tuner) between the antenna and the feeder.
>
> BTW, the tx is probably not conjugate matched by the tuner. Conjugate
> matching does not apply to transmitters or power amplifiers. But I realize
> there is a huge dissension in this area too. ;-)
>
> Per-Tore / LA7NO
>

vk2rq

Re: Can I measure antenna impedance with K2?

In reply to this post by LA7NO

I think we have hit the posting limit for this topic, so I'll just finish with this remark: the maximum power transfer between a power amplifier and its load will occur when the source and load impedances are complex conjugates (this can be seen using the voltage divider equation and some differential calculus). In the case of a purely resistive load, this reduces down to the source and load resistances being equal.

73,
Matt VK2RQ

> On 1 Aug 2014, at 8:24 am, Per-Tore Aasestrand <[hidden email]> wrote:
>
> Hi list,
>
> Isn't an RF transmitter or PA actually more like a current generator? Its
> internal resistance is kept low in order to reduce internally dissipated
> power, with consequent reduction in efficiency. It is usually designed to
> work efficiently into 50 ohms.
>
> It is important to know the difference between these two statements:
> (A) The transmitter has a 50 ohm output impedance.
> (B) The transmitter is designed to work into a 50 ohm load
>
> A transmitter will be able to put its rated power into a 50 ohm load, but
> this says nothing about its output impedance. That is why I don't think
> conjugate matching will be appropriate.
>
> (Hope this is not getting too much OT now)
>
> Per-Tore / LA7NO
> ______________________________________________________________
> Elecraft mailing list
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Don Wilhelm-4

Re: Can I measure antenna impedance with K2?

In reply to this post by vk2rq

Matt,

I believe it is even more 'simple' than that.
When the ATU wattmeter is properly calibrated, it is then set to measure
an SWR of 1 at 50 ohms resistive load.

When a TUNE produces an SWR=1, the impedance at the input (coax inside
the K2) has been transformed to 50 ohms, so that is a known quantity.
The ATU L and C show the values used to arrive at that transformation,
and the NET parameter indicates whether the capacitor is on the input or
the output side. So the L network values and configuration are known.

It is then a task to solve the network equations for an L network having
series L and shunt C elements to arrive at the output impedance.

Yes, the transmission line impedance (at the BNC connector) will be the
conjugate of that impedance.
Nothing has been said about the SWR on the feedline nor the feedpoint
impedance at the antenna.

73,
Don W3FPR

On 7/31/2014 5:56 PM, Matt VK2RQ wrote:
> I think I see where the confusion has arisen. Here we are talking about a K2 with integrated KAT2 ATU. So, when I used the term "rig end of the feedline", I was referring to where the feedline connects to the KAT2 BNC connector at the back of the K2, which is the output or antenna side of the KAT2 ATU. In order to make the impedance where the antenna connects to the KAT2 output look like 50+0j ohms, the KAT2 will use an LC network to transform the impedance. The impedance of this LC network together with the 50ohms of the K2 final PA output will be equal to the complex conjugate of the impedance you will see looking into the shack end of the feedline leading to your antenna.
>
> For example, to keep the maths simple, let's consider a case where the impedance at the end of the feedline in the shack works out to be 50+j100ohms. That is, the antenna is slightly inductive. In order to transform this impedance to 50ohms, you could connect a capacitor in series that has a reactance of 100ohms to cancel out the inductive reactance of the antenna+feedline. If we look at the impedance of our matching network plus radio, we see the capacitor in series with the 50ohm transmitter, so the impedance looking into the "ATU" plus transmitter is 50-j100 ohms. This is the complex conjugate of the impedance looking down the feedline towards the antenna, which we already see is 50+j100ohms.
>
> In a more complicated scenario, the resistive component at the shack end of the feedline may not be 50ohms, and our matching network will be more complicated, and we will connect some reactance across the feedline and/or transmitter to achieve the required impedance transformation. If you go through the maths, you will again find the impedance looking into the ATU+transmitter is the complex conjugate of the impedance looking into the feedline+antenna.
>
> 73,
> Matt VK2RQ
>

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LA7NO

Re: Can I measure antenna impedance with K2?

In reply to this post by vk2rq

Would not this imply a maximum efficiency of 50% ?

P-T

On 1 August 2014 00:40, Matt VK2RQ <[hidden email]> wrote:

> I think we have hit the posting limit for this topic, so I'll just finish
> with this remark: the maximum power transfer between a power amplifier and
> its load will occur when the source and load impedances are complex
> conjugates (this can be seen using the voltage divider equation and some
> differential calculus). In the case of a purely resistive load, this
> reduces down to the source and load resistances being equal.
>
> 73,
> Matt VK2RQ
>
> > On 1 Aug 2014, at 8:24 am, Per-Tore Aasestrand <[hidden email]> wrote:
> >
> > Hi list,
> >
> > Isn't an RF transmitter or PA actually more like a current generator? Its
> > internal resistance is kept low in order to reduce internally dissipated
> > power, with consequent reduction in efficiency. It is usually designed to
> > work efficiently into 50 ohms.
> >
> > It is important to know the difference between these two statements:
> > (A) The transmitter has a 50 ohm output impedance.
> > (B) The transmitter is designed to work into a 50 ohm load
> >
> > A transmitter will be able to put its rated power into a 50 ohm load, but
> > this says nothing about its output impedance. That is why I don't think
> > conjugate matching will be appropriate.
> >
> > (Hope this is not getting too much OT now)
> >
> > Per-Tore / LA7NO
> > ______________________________________________________________
> > 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
> > Message delivered to [hidden email]
>

73,

Per-Tore / LA7NO

Don Wilhelm-4

Re: Can I measure antenna impedance with K2?

P-T,

If one uses the voltage divider example, yes the maximum efficiency is 50%,
But the output of a PA stage is not a resistor, and the collector load
"resistance" is set by the designer to produce the output power desired.

The 'resistor' assumption is a simplification that can be used when
constructing equivalent circuits to analyze the voltages and currents in
the network that follows the perfect generator.

The theory of large signal amplifiers is too great for detailed
discussion on this reflector, so we usually have to settle for
simplifications. An equivalent circuit is not a full representation of
the workings of an actual PA stage. I believe that is where the
misunderstanding comes from.

73,
Don W3FPR

On 7/31/2014 6:57 PM, Per-Tore Aasestrand wrote:

> Would not this imply a maximum efficiency of 50% ?
>
> P-T
>
>
>
> On 1 August 2014 00:40, Matt VK2RQ <[hidden email]> wrote:
>
>> I think we have hit the posting limit for this topic, so I'll just finish
>> with this remark: the maximum power transfer between a power amplifier and
>> its load will occur when the source and load impedances are complex
>> conjugates (this can be seen using the voltage divider equation and some
>> differential calculus). In the case of a purely resistive load, this
>> reduces down to the source and load resistances being equal.
>>
>> 73,
>> Matt VK2RQ
>>
>>> On 1 Aug 2014, at 8:24 am, Per-Tore Aasestrand <[hidden email]> wrote:
>>>
>>> Hi list,
>>>
>>> Isn't an RF transmitter or PA actually more like a current generator? Its
>>> internal resistance is kept low in order to reduce internally dissipated
>>> power, with consequent reduction in efficiency. It is usually designed to
>>> work efficiently into 50 ohms.
>>>
>>> It is important to know the difference between these two statements:
>>> (A) The transmitter has a 50 ohm output impedance.
>>> (B) The transmitter is designed to work into a 50 ohm load
>>>
>>> A transmitter will be able to put its rated power into a 50 ohm load, but
>>> this says nothing about its output impedance. That is why I don't think
>>> conjugate matching will be appropriate.
>>>
>>> (Hope this is not getting too much OT now)
>>>
>>> Per-Tore / LA7NO
>>> ______________________________________________________________
>>> 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
>>> Message delivered to [hidden email]
> ______________________________________________________________
> Elecraft mailing list
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>
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>

k6dgw

Re: Can I measure antenna impedance with K2?

In reply to this post by LA7NO

On 7/31/2014 1:23 PM, Per-Tore Aasestrand wrote:
> Hi Matt,
>
>> Of course you realise that the output impedance of the ATU will be the
>> complex conjugate of the impedance at the rig end of your feedline.
>
> I wonder if this is correct

Well, it is, however it wasn't the original question.
>
> IMO, the task of the tuner is to make the tx happy, so it can look into the
> load it is designed for. Thus, it will translate the impedance as seen from
> the antenna connector into something as close to 50 ohm as possible. The
> tuner will not change anything on the feeder. With or without the tuner,
> the feeder will have the same VSWR.

You are correct Per-Tore. Looking into the male connector on your
feedline, you will see *some* impedance which depends on the impedance
at the feedpoint of your antenna, the characteristic impedance of your
feedline, and the length of your feedline ... with one exception:

If the feedpoint impedance of your antenna is totally resistive, *and*
is exactly equal to the characteristic impedance of your feedline, then
length only matters when calculating loss. This situation hardly ever
occurs for any real hams however. :-)

The KAT2 "antenna tuner" does not "tune" the antenna, or anything else
for that matter. It is simply an L-C network [L-network in the KAT2]
that transforms the complex impedance seen at the BNC connector to what
the PA output filter wants to work into ... nominally 50+j0 ohms. Once
you get a match and find out the L, C, and configuration of the
L-network in the tuner, you can use the L-network equations to determine
the impedance transformation taking place in the "tuner" and thus the
complex impedance looking into the feedline connector

If you know the complex impedance looking into the feedline connector,
the characteristic impedance of the line, and it's length, you can use
one of many programs to calculate the impedance of your antenna at the
feedpoint. I use N6BV's TLW which comes on the CD with the ARRL Antenna
Book, but there are many other ways, including paper and pencil on a
Smith Chart.

And yes, you are also correct that the presence of the tuner will not
affect the VSWR on the feedline, nor the loss incurred by that VSWR.
Hope this helps.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org

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LA7NO

Re: Can I measure antenna impedance with K2?

In reply to this post by Don Wilhelm-4

Hi Don,

On 1 August 2014 01:16, Don Wilhelm <[hidden email]> wrote:

If one uses the voltage divider example, yes the maximum efficiency is 50%,
> But the output of a PA stage is not a resistor, and the collector load
> "resistance" is set by the designer to produce the output power desired.
>

I fully agree.

But will not a conjugate match also imply a max efficiency of 50%

P-T
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73,

Per-Tore / LA7NO

Don Wilhelm-4

Re: Can I measure antenna impedance with K2?

P-T,

No, a conjugate match will assure 100% power transfer.

Look at it this way - what the conjugate match says is that if you cut a
transmission line at any point, looking one way at that cut point, you
will have some impedance - example is 70 + j20. Now look the other way
and the impedance will be the conjugate match - 70 - j20. That is the
condition that exists.
It also happens to be the condition for maximum power transfer.

Since that cut can be made at any and all points along the transmission
line - think what would happen if the efficiency at each of those points
(when connected together) were 50% - nothing would get to the antenna.
So we know that 50% is *not* the efficiency of any and all junctions of
any conjugate match.

Mixing the conjugate match concept with the maximum power transfer
theorem is getting us into confusion - there are 'holes' in that
combination. Yes, they work together, but not seamlessly.
If a generator has an internal impedance of 50 ohms, the maximum power
transfer will be only if the load to that generator is also 50 ohms.
That says nothing about a conjugate match.

Now to further complicate things, the internal impedance of a generator
has nothing to do with the efficiency of that generator - especially if
we are discussing a PA output stage. Bringing the conjugate match
concept into the internal design of a PA stage is in error - it just
does not work that way. The conjugate match only applies to the output
of that amplifier stage.

In other words, if we send 10 watts into a feedline (or ATU), all 10
watts will go to the load (antenna) except for losses in the feedline.

I am not about to embark on the design and efficiency of a PA stage on
this reflector, so take the conjugate match and maximum power transfer
theorem only to the terminals at the PA stage and all will make sense.
They do not apply to the internals of that stage.

73,
Don W3FPR

On 7/31/2014 7:32 PM, Per-Tore Aasestrand wrote:

> Hi Don,
>
> On 1 August 2014 01:16, Don Wilhelm <[hidden email]> wrote:
>
> If one uses the voltage divider example, yes the maximum efficiency is 50%,
>> But the output of a PA stage is not a resistor, and the collector load
>> "resistance" is set by the designer to produce the output power desired.
>>
> I fully agree.
>
> But will not a conjugate match also imply a max efficiency of 50%
>
> P-T
> ______________________________________________________________
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> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>