[K2/KX3] Tuning a magloop

[Gary Hawkins]

Dave I read your post with interest, as over the weekend I was looking at an automatic loop tuner described in the November/December isse of ARRL QEX Magazine.  This was for a MFJ-1788 Magnetic Loop working with a KX3 and based on an Arduino micro-controller.  I don't own the MFJ product but I do have an Alex Loop and with a few simple modifications the circuit should work nicely.

The loop tuner uses a DC motor but I thought a stepper might be more elegant.  Thus, I needed to determine the working bandwidth of the Alex Loop to get some idea of the angular resolution I needed from either a direct drive stepper, or a stepper using gearing.  I looked at the Alex Loop around 7, 18 and 30MHz.  3:1 VSWR bandwidths were measured using a RigExpert AA-54 antenna analyzer.  For 3:1 VSWR the measured operating bandwidths at these three frequencies were determined as 44, 65 and 190kHz respectively.  Pretty narrow at the lowest frequency.

Thus, to get a good chance of achieving a VSWR approaching minimum, I figured the angular motion should be fine enough at worst case (7MHz)to achieve an angular motion of the tuning capacitor of no more than 1/0th the 3:1 VSWR bandwith = approx. 4kHz.  Since one and a half turns of the tuning capacitor tuned the Alex Loop from resonance at 7 through 30MHz (and assuming the frequency response is linear, which it is not) then the required angular resolution is 360 * 1.5 * 4 /(30000-7000) = 0.094 degrees.  Since a typical stepper does 200 steps per revolution, then the required gearing ratio would be approximately 20:1.

With this type of performance the magnetic loop could be very close to resonance at the chosen operating frequency presenting a VSWR approaching 1:1.

73's Gary K6YOA

    Message: 3
    Date: Wed, 2 Dec 2015 20:52:18 -0000
    From: "Dave Lankshear"<[hidden email]>
    To:<[hidden email]>
    Subject: [Elecraft]  [K2] Tuning a magloop with KAT100
    Message-ID: <7B2795A9D35B4139812BF284FD02542B@DaveLLaptop>
    Content-Type: text/plain; charset="us-ascii"

    Hello, Brian.

     

    I've read your post and the replies and figure I maybe read what you said
    incorrectly, but you did say:

     

    Then use the TUNE button on the K2 to tune the loop to resonance, then
    operate.

     

    You can't use the K2's antenna tuner to tune a loop to resonance.  That's
    not how they work.

     

    The loop is a complete tuned circuit in itself.  There's the loop
    representing inductance and a capacitor in parallel with it.  These are
    resonant at a frequency and the only practical way to move that resonant
    frequency is to adjust the value of the capacitor that's in parallel with
    the loop.

     

    You can't adjust that resonant frequency using an auto ATU at the end of a
    length of coax.  It's rather like using a telephone conversation to make a
    physical change at the other end.  You can't use the phone to make the beds
    back home, when you're away!  The auto ATU will see the coax cable and the
    loop as a lump of L and C and will endeavour to match it to 50 ohms to get a
    1:1 SWR.  The coax forms part of the antenna and is not behaving like a
    transmission line and the loop is not behaving like a resonant circuit - you
    might just as well hook the coax to your automobile's fender and use the
    auto ATU to tune that!

     

    Please forgive my descriptions if I have misunderstood your question, but
    re-reading your quoted statement makes me feel like I'm the only one who has
    understood.

     

    Yes, by all means match the auto ATU into 50 ohms using a dummy load.  That
    way, the PA is looking into a load that matches the impedance of the
    transmission line, although as Don suggested, you don't really need the auto
    ATU (and its losses) as the PA should be reasonably well matched by
    bypassing the auto ATU entirely.  Now, at the loop end, there's a gamma
    match arrangement that ensures the loop, when at resonance, is a decent
    match to the 50 ohm transmission line you're using.  So the rig matches to
    the transmission line which matches to the loop that's been tuned to
    resonance with its inbuilt tuning capacitor.

     

    A loop is only a single turn coil, the resonant frequency of which is varied
    by adjustment of its parallel capacitance.  Because it's small and is a low
    loss inductor operating with a low loss airspaced (or vacuum) capacitor, the
    Q factor, or "goodness" of the single turn coil at resonance is very high.
    This means that a small excursion away from resonance, the loop's Q falls
    very rapidly and renders it pretty useless, thus it is necessary (more so
    when transmitting through the loop) to retune it for frequency shifts of
    more than a few kHz.  That means that the SWR rises rapidly away from
    resonance and the coaxial cable is more involved in becoming part of the
    antenna and less of a transmission line.

     

    The outer surface of a loop (well, outer 6%) needs to be of very low
    resistance in order to maximise Q at resonance.  RF skin effect uses only
    the outer surface of the conductor, thus the larger the surface area of the
    conductor, the more its internal resistance is in parallel and thus reduced,
    so the better performer the loop becomes.

     

    Even a soldered joint on copper piping offers resistance that compromises
    the loop's performance.  Recently a friend gave an old army magnetic loop to
    a group of collectors/militia enthusiasts.  It was in poor condition, but in
    its prime, its surface area must have been a foot across.  This makes a
    mockery of the little bits of aluminium (aluminium) joined together with
    bolts and wing nuts.  Yes, says the vendor, it is broad banded and only
    needs retuning every 100kHz or so.  What he doesn't say is that its
    resistance makesthe Q so appalling that its performance is lousy (where
    lousy is the polite word), but those devices give properly engineered mag
    loops a bad name by tarring all with the same brush.  Also, there are
    proportionally more crappy mag loops out there simply because they are
    cheaper than the "real" thing.

     

    The MFJ 1782/86/88 aren't too bad and are just at the crossing point between
    good and bad, with a bias towards the good, if not too many spiders and
    other insect life are resident under the black covers. These loops have an
    airspaced variable capacitor within the black covers and that capacitor is
    tuned by a small electric motor that's attached to it.  DC power for the
    motor is fed down the coax cable itself, as well as the radio signals.  They
    are not difficult to separate, eliminating the need for a control cable.
    The DC voltage on the coax is reversed in order to make the motor turn in
    the opposite direction.  The more sophisticated MFJ controller has an
    inbuilt cross-needle SWR meter and the name of the game is to get the SWR as
    low as possible on the operating frequency.  Their semi-automatic controller
    drives the motor and detects the lowest SWR point and stops.  Of course, to
    detect the lowest point, it has to begin to increase again, so it always
    stops at a point that's not quite at resonance and this must be fine-tuned
    by the operator, hunting to and fro with up/down press-buttons.  It takes
    more effort to describe than to do in real life Hi!

     

    One last caution.  Circulating currents are very high in a transmitting mag
    loop and very high voltages (thousands of 'em) are also present when at
    resonance, so ensure that the loop can't be touched by anything that matters
    to you when it's transmitting, even at a few watts.  Give it a LOT of
    respect, indeed, powerline respect.

     

    I hope my efforts haven't missed the mark by a mile, Brian and that instead,
    the wear and tear on my keyboard has been of some small use to you.

     

    73 and early Season's Greetings.  Dave G3TJP

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[Edouard Lafargue]

Hi Gary,

   I need to set aside some time to finalize it, but I have designed a
fully automatic AlexLoop tuner that is designed to work with the KX3: you
just need to tune to the frequency you want to use, then press the "tune"
button on the KX3 (long press on XMIT), and the Alexloop will find the
correct tune in about 20 seconds worst case. It uses a tiny stepper motor
as you described.

   I have demonstrated a proof of concept for it at the Palo Alto Amateur
Radio Association, and I hope to have instructions ready by the end of the
year. The idea is to release complete instructions for whoever wants to
build one themselves, and maybe also offer it as a partly soldered kit for
people who do not want to bother with SMD soldering. Contact me direct if
you are interested!

Ed, W6ELA

On Thu, Dec 3, 2015 at 8:22 PM, Gary Hawkins <[hidden email]> wrote:

> Dave I read your post with interest, as over the weekend I was looking at
> an automatic loop tuner described in the November/December isse of ARRL QEX
> Magazine.  This was for a MFJ-1788 Magnetic Loop working with a KX3 and
> based on an Arduino micro-controller.  I don't own the MFJ product but I do
> have an Alex Loop and with a few simple modifications the circuit should
> work nicely.
>
> The loop tuner uses a DC motor but I thought a stepper might be more
> elegant.  Thus, I needed to determine the working bandwidth of the Alex
> Loop to get some idea of the angular resolution I needed from either a
> direct drive stepper, or a stepper using gearing.  I looked at the Alex
> Loop around 7, 18 and 30MHz.  3:1 VSWR bandwidths were measured using a
> RigExpert AA-54 antenna analyzer.  For 3:1 VSWR the measured operating
> bandwidths at these three frequencies were determined as 44, 65 and 190kHz
> respectively.  Pretty narrow at the lowest frequency.
>
> Thus, to get a good chance of achieving a VSWR approaching minimum, I
> figured the angular motion should be fine enough at worst case (7MHz)to
> achieve an angular motion of the tuning capacitor of no more than 1/0th the
> 3:1 VSWR bandwith = approx. 4kHz.  Since one and a half turns of the tuning
> capacitor tuned the Alex Loop from resonance at 7 through 30MHz (and
> assuming the frequency response is linear, which it is not) then the
> required angular resolution is 360 * 1.5 * 4 /(30000-7000) = 0.094
> degrees.  Since a typical stepper does 200 steps per revolution, then the
> required gearing ratio would be approximately 20:1.
>
> With this type of performance the magnetic loop could be very close to
> resonance at the chosen operating frequency presenting a VSWR approaching
> 1:1.
>
> 73's Gary K6YOA
>
>    Message: 3
>    Date: Wed, 2 Dec 2015 20:52:18 -0000
>    From: "Dave Lankshear"<[hidden email]>
>    To:<[hidden email]>
>    Subject: [Elecraft]  [K2] Tuning a magloop with KAT100
>    Message-ID: <7B2795A9D35B4139812BF284FD02542B@DaveLLaptop>
>    Content-Type: text/plain;    charset="us-ascii"
>
>    Hello, Brian.
>
>
>    I've read your post and the replies and figure I maybe read what you
> said
>    incorrectly, but you did say:
>
>
>    Then use the TUNE button on the K2 to tune the loop to resonance, then
>    operate.
>
>
>    You can't use the K2's antenna tuner to tune a loop to resonance.
> That's
>    not how they work.
>
>
>    The loop is a complete tuned circuit in itself.  There's the loop
>    representing inductance and a capacitor in parallel with it.  These are
>    resonant at a frequency and the only practical way to move that resonant
>    frequency is to adjust the value of the capacitor that's in parallel
> with
>    the loop.
>
>
>    You can't adjust that resonant frequency using an auto ATU at the end
> of a
>    length of coax.  It's rather like using a telephone conversation to
> make a
>    physical change at the other end.  You can't use the phone to make the
> beds
>    back home, when you're away!  The auto ATU will see the coax cable and
> the
>    loop as a lump of L and C and will endeavour to match it to 50 ohms to
> get a
>    1:1 SWR.  The coax forms part of the antenna and is not behaving like a
>    transmission line and the loop is not behaving like a resonant circuit
> - you
>    might just as well hook the coax to your automobile's fender and use the
>    auto ATU to tune that!
>
>
>    Please forgive my descriptions if I have misunderstood your question,
> but
>    re-reading your quoted statement makes me feel like I'm the only one
> who has
>    understood.
>
>
>    Yes, by all means match the auto ATU into 50 ohms using a dummy load.
> That
>    way, the PA is looking into a load that matches the impedance of the
>    transmission line, although as Don suggested, you don't really need the
> auto
>    ATU (and its losses) as the PA should be reasonably well matched by
>    bypassing the auto ATU entirely.  Now, at the loop end, there's a gamma
>    match arrangement that ensures the loop, when at resonance, is a decent
>    match to the 50 ohm transmission line you're using.  So the rig matches
> to
>    the transmission line which matches to the loop that's been tuned to
>    resonance with its inbuilt tuning capacitor.
>
>
>    A loop is only a single turn coil, the resonant frequency of which is
> varied
>    by adjustment of its parallel capacitance.  Because it's small and is a
> low
>    loss inductor operating with a low loss airspaced (or vacuum)
> capacitor, the
>    Q factor, or "goodness" of the single turn coil at resonance is very
> high.
>    This means that a small excursion away from resonance, the loop's Q
> falls
>    very rapidly and renders it pretty useless, thus it is necessary (more
> so
>    when transmitting through the loop) to retune it for frequency shifts of
>    more than a few kHz.  That means that the SWR rises rapidly away from
>    resonance and the coaxial cable is more involved in becoming part of the
>    antenna and less of a transmission line.
>
>
>    The outer surface of a loop (well, outer 6%) needs to be of very low
>    resistance in order to maximise Q at resonance.  RF skin effect uses
> only
>    the outer surface of the conductor, thus the larger the surface area of
> the
>    conductor, the more its internal resistance is in parallel and thus
> reduced,
>    so the better performer the loop becomes.
>
>
>    Even a soldered joint on copper piping offers resistance that
> compromises
>    the loop's performance.  Recently a friend gave an old army magnetic
> loop to
>    a group of collectors/militia enthusiasts.  It was in poor condition,
> but in
>    its prime, its surface area must have been a foot across.  This makes a
>    mockery of the little bits of aluminium (aluminium) joined together with
>    bolts and wing nuts.  Yes, says the vendor, it is broad banded and only
>    needs retuning every 100kHz or so.  What he doesn't say is that its
>    resistance makesthe Q so appalling that its performance is lousy (where
>    lousy is the polite word), but those devices give properly engineered
> mag
>    loops a bad name by tarring all with the same brush.  Also, there are
>    proportionally more crappy mag loops out there simply because they are
>    cheaper than the "real" thing.
>
>
>    The MFJ 1782/86/88 aren't too bad and are just at the crossing point
> between
>    good and bad, with a bias towards the good, if not too many spiders and
>    other insect life are resident under the black covers. These loops have
> an
>    airspaced variable capacitor within the black covers and that capacitor
> is
>    tuned by a small electric motor that's attached to it.  DC power for the
>    motor is fed down the coax cable itself, as well as the radio signals.
> They
>    are not difficult to separate, eliminating the need for a control cable.
>    The DC voltage on the coax is reversed in order to make the motor turn
> in
>    the opposite direction.  The more sophisticated MFJ controller has an
>    inbuilt cross-needle SWR meter and the name of the game is to get the
> SWR as
>    low as possible on the operating frequency.  Their semi-automatic
> controller
>    drives the motor and detects the lowest SWR point and stops.  Of
> course, to
>    detect the lowest point, it has to begin to increase again, so it always
>    stops at a point that's not quite at resonance and this must be
> fine-tuned
>    by the operator, hunting to and fro with up/down press-buttons.  It
> takes
>    more effort to describe than to do in real life Hi!
>
>
>    One last caution.  Circulating currents are very high in a transmitting
> mag
>    loop and very high voltages (thousands of 'em) are also present when at
>    resonance, so ensure that the loop can't be touched by anything that
> matters
>    to you when it's transmitting, even at a few watts.  Give it a LOT of
>    respect, indeed, powerline respect.
>
>
>    I hope my efforts haven't missed the mark by a mile, Brian and that
> instead,
>    the wear and tear on my keyboard has been of some small use to you.
>
>
>    73 and early Season's Greetings.  Dave G3TJP
>
> ______________________________________________________________
> 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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[k6dgw]

Might it also work with a K2?

73,

Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org

On 12/4/2015 9:56 AM, Edouard Lafargue wrote:
> Hi Gary,
>
>     I need to set aside some time to finalize it, but I have designed a
> fully automatic AlexLoop tuner that is designed to work with the KX3:
______________________________________________________________
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Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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Message delivered to [hidden email]

[vk2rq]

Does it include a resistive bridge that presents a constant 50 ohm impedance to the radio during tuning? Mag loops can be quite rough on your output transistors (that's why it is best to peak receiver noise first and only transmit to do the final tweak)
Matt VK2RQ

Envoyé à partir d'Outlook




On Fri, Dec 4, 2015 at 9:57 AM -0800, "Edouard Lafargue" <[hidden email]> wrote:










Hi Gary,

   I need to set aside some time to finalize it, but I have designed a
fully automatic AlexLoop tuner that is designed to work with the KX3: you
just need to tune to the frequency you want to use, then press the "tune"
button on the KX3 (long press on XMIT), and the Alexloop will find the
correct tune in about 20 seconds worst case. It uses a tiny stepper motor
as you described.

   I have demonstrated a proof of concept for it at the Palo Alto Amateur
Radio Association, and I hope to have instructions ready by the end of the
year. The idea is to release complete instructions for whoever wants to
build one themselves, and maybe also offer it as a partly soldered kit for
people who do not want to bother with SMD soldering. Contact me direct if
you are interested!

Ed, W6ELA

On Thu, Dec 3, 2015 at 8:22 PM, Gary Hawkins  wrote:

> Dave I read your post with interest, as over the weekend I was looking at
> an automatic loop tuner described in the November/December isse of ARRL QEX
> Magazine.  This was for a MFJ-1788 Magnetic Loop working with a KX3 and
> based on an Arduino micro-controller.  I don't own the MFJ product but I do
> have an Alex Loop and with a few simple modifications the circuit should
> work nicely.
>
> The loop tuner uses a DC motor but I thought a stepper might be more
> elegant.  Thus, I needed to determine the working bandwidth of the Alex
> Loop to get some idea of the angular resolution I needed from either a
> direct drive stepper, or a stepper using gearing.  I looked at the Alex
> Loop around 7, 18 and 30MHz.  3:1 VSWR bandwidths were measured using a
> RigExpert AA-54 antenna analyzer.  For 3:1 VSWR the measured operating
> bandwidths at these three frequencies were determined as 44, 65 and 190kHz
> respectively.  Pretty narrow at the lowest frequency.
>
> Thus, to get a good chance of achieving a VSWR approaching minimum, I
> figured the angular motion should be fine enough at worst case (7MHz)to
> achieve an angular motion of the tuning capacitor of no more than 1/0th the
> 3:1 VSWR bandwith = approx. 4kHz.  Since one and a half turns of the tuning
> capacitor tuned the Alex Loop from resonance at 7 through 30MHz (and
> assuming the frequency response is linear, which it is not) then the
> required angular resolution is 360 * 1.5 * 4 /(30000-7000) = 0.094
> degrees.  Since a typical stepper does 200 steps per revolution, then the
> required gearing ratio would be approximately 20:1.
>
> With this type of performance the magnetic loop could be very close to
> resonance at the chosen operating frequency presenting a VSWR approaching
> 1:1.
>
> 73's Gary K6YOA
>
>    Message: 3
>    Date: Wed, 2 Dec 2015 20:52:18 -0000
>    From: "Dave Lankshear"
>    To:
>    Subject: [Elecraft]  [K2] Tuning a magloop with KAT100
>    Message-ID: <7B2795A9D35B4139812BF284FD02542B@DaveLLaptop>
>    Content-Type: text/plain;    charset="us-ascii"
>
>    Hello, Brian.
>
>
>    I've read your post and the replies and figure I maybe read what you
> said
>    incorrectly, but you did say:
>
>
>    Then use the TUNE button on the K2 to tune the loop to resonance, then
>    operate.
>
>
>    You can't use the K2's antenna tuner to tune a loop to resonance.
> That's
>    not how they work.
>
>
>    The loop is a complete tuned circuit in itself.  There's the loop
>    representing inductance and a capacitor in parallel with it.  These are
>    resonant at a frequency and the only practical way to move that resonant
>    frequency is to adjust the value of the capacitor that's in parallel
> with
>    the loop.
>
>
>    You can't adjust that resonant frequency using an auto ATU at the end
> of a
>    length of coax.  It's rather like using a telephone conversation to
> make a
>    physical change at the other end.  You can't use the phone to make the
> beds
>    back home, when you're away!  The auto ATU will see the coax cable and
> the
>    loop as a lump of L and C and will endeavour to match it to 50 ohms to
> get a
>    1:1 SWR.  The coax forms part of the antenna and is not behaving like a
>    transmission line and the loop is not behaving like a resonant circuit
> - you
>    might just as well hook the coax to your automobile's fender and use the
>    auto ATU to tune that!
>
>
>    Please forgive my descriptions if I have misunderstood your question,
> but
>    re-reading your quoted statement makes me feel like I'm the only one
> who has
>    understood.
>
>
>    Yes, by all means match the auto ATU into 50 ohms using a dummy load.
> That
>    way, the PA is looking into a load that matches the impedance of the
>    transmission line, although as Don suggested, you don't really need the
> auto
>    ATU (and its losses) as the PA should be reasonably well matched by
>    bypassing the auto ATU entirely.  Now, at the loop end, there's a gamma
>    match arrangement that ensures the loop, when at resonance, is a decent
>    match to the 50 ohm transmission line you're using.  So the rig matches
> to
>    the transmission line which matches to the loop that's been tuned to
>    resonance with its inbuilt tuning capacitor.
>
>
>    A loop is only a single turn coil, the resonant frequency of which is
> varied
>    by adjustment of its parallel capacitance.  Because it's small and is a
> low
>    loss inductor operating with a low loss airspaced (or vacuum)
> capacitor, the
>    Q factor, or "goodness" of the single turn coil at resonance is very
> high.
>    This means that a small excursion away from resonance, the loop's Q
> falls
>    very rapidly and renders it pretty useless, thus it is necessary (more
> so
>    when transmitting through the loop) to retune it for frequency shifts of
>    more than a few kHz.  That means that the SWR rises rapidly away from
>    resonance and the coaxial cable is more involved in becoming part of the
>    antenna and less of a transmission line.
>
>
>    The outer surface of a loop (well, outer 6%) needs to be of very low
>    resistance in order to maximise Q at resonance.  RF skin effect uses
> only
>    the outer surface of the conductor, thus the larger the surface area of
> the
>    conductor, the more its internal resistance is in parallel and thus
> reduced,
>    so the better performer the loop becomes.
>
>
>    Even a soldered joint on copper piping offers resistance that
> compromises
>    the loop's performance.  Recently a friend gave an old army magnetic
> loop to
>    a group of collectors/militia enthusiasts.  It was in poor condition,
> but in
>    its prime, its surface area must have been a foot across.  This makes a
>    mockery of the little bits of aluminium (aluminium) joined together with
>    bolts and wing nuts.  Yes, says the vendor, it is broad banded and only
>    needs retuning every 100kHz or so.  What he doesn't say is that its
>    resistance makesthe Q so appalling that its performance is lousy (where
>    lousy is the polite word), but those devices give properly engineered
> mag
>    loops a bad name by tarring all with the same brush.  Also, there are
>    proportionally more crappy mag loops out there simply because they are
>    cheaper than the "real" thing.
>
>
>    The MFJ 1782/86/88 aren't too bad and are just at the crossing point
> between
>    good and bad, with a bias towards the good, if not too many spiders and
>    other insect life are resident under the black covers. These loops have
> an
>    airspaced variable capacitor within the black covers and that capacitor
> is
>    tuned by a small electric motor that's attached to it.  DC power for the
>    motor is fed down the coax cable itself, as well as the radio signals.
> They
>    are not difficult to separate, eliminating the need for a control cable.
>    The DC voltage on the coax is reversed in order to make the motor turn
> in
>    the opposite direction.  The more sophisticated MFJ controller has an
>    inbuilt cross-needle SWR meter and the name of the game is to get the
> SWR as
>    low as possible on the operating frequency.  Their semi-automatic
> controller
>    drives the motor and detects the lowest SWR point and stops.  Of
> course, to
>    detect the lowest point, it has to begin to increase again, so it always
>    stops at a point that's not quite at resonance and this must be
> fine-tuned
>    by the operator, hunting to and fro with up/down press-buttons.  It
> takes
>    more effort to describe than to do in real life Hi!
>
>
>    One last caution.  Circulating currents are very high in a transmitting
> mag
>    loop and very high voltages (thousands of 'em) are also present when at
>    resonance, so ensure that the loop can't be touched by anything that
> matters
>    to you when it's transmitting, even at a few watts.  Give it a LOT of
>    respect, indeed, powerline respect.
>
>
>    I hope my efforts haven't missed the mark by a mile, Brian and that
> instead,
>    the wear and tear on my keyboard has been of some small use to you.
>
>
>    73 and early Season's Greetings.  Dave G3TJP
>
> ______________________________________________________________
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[Edouard Lafargue]

     No, it does not use a bridge at all, actually - no need on a KX3,
makes everything more complicated :-) . I usually use a 2W tuning power
only on the AlexLoop. So far no issues.

Ed

On Fri, Dec 4, 2015 at 12:12 PM, Matt Maguire <[hidden email]> wrote:

> Does it include a resistive bridge that presents a constant 50 ohm
> impedance to the radio during tuning? Mag loops can be quite rough on your
> output transistors (that's why it is best to peak receiver noise first and
> only transmit to do the final tweak)
>
> Matt VK2RQ
>
> Envoyé à partir d'Outlook <http://aka.ms/Ox5hz3>
>
>
>
>
> On Fri, Dec 4, 2015 at 9:57 AM -0800, "Edouard Lafargue" <
> [hidden email]> wrote:
>
> Hi Gary,
>>
>>    I need to set aside some time to finalize it, but I have designed a
>> fully automatic AlexLoop tuner that is designed to work with the KX3: you
>> just need to tune to the frequency you want to use, then press the "tune"
>> button on the KX3 (long press on XMIT), and the Alexloop will find the
>> correct tune in about 20 seconds worst case. It uses a tiny stepper motor
>> as you described.
>>
>>    I have demonstrated a proof of concept for it at the Palo Alto Amateur
>> Radio Association, and I hope to have instructions ready by the end of the
>> year. The idea is to release complete instructions for whoever wants to
>> build one themselves, and maybe also offer it as a partly soldered kit for
>> people who do not want to bother with SMD soldering. Contact me direct if
>> you are interested!
>>
>> Ed, W6ELA
>>
>> On Thu, Dec 3, 2015 at 8:22 PM, Gary Hawkins  wrote:
>>
>> > Dave I read your post with interest, as over the weekend I was looking at
>> > an automatic loop tuner described in the November/December isse of ARRL QEX
>> > Magazine.  This was for a MFJ-1788 Magnetic Loop working with a KX3 and
>> > based on an Arduino micro-controller.  I don't own the MFJ product but I do
>> > have an Alex Loop and with a few simple modifications the circuit should
>> > work nicely.
>> >
>> > The loop tuner uses a DC motor but I thought a stepper might be more
>> > elegant.  Thus, I needed to determine the working bandwidth of the Alex
>> > Loop to get some idea of the angular resolution I needed from either a
>> > direct drive stepper, or a stepper using gearing.  I looked at the Alex
>> > Loop around 7, 18 and 30MHz.  3:1 VSWR bandwidths were measured using a
>> > RigExpert AA-54 antenna analyzer.  For 3:1 VSWR the measured operating
>> > bandwidths at these three frequencies were determined as 44, 65 and 190kHz
>> > respectively.  Pretty narrow at the lowest frequency.
>> >
>> > Thus, to get a good chance of achieving a VSWR approaching minimum, I
>> > figured the angular motion should be fine enough at worst case (7MHz)to
>> > achieve an angular motion of the tuning capacitor of no more than 1/0th the
>> > 3:1 VSWR bandwith = approx. 4kHz.  Since one and a half turns of the tuning
>> > capacitor tuned the Alex Loop from resonance at 7 through 30MHz (and
>> > assuming the frequency response is linear, which it is not) then the
>> > required angular resolution is 360 * 1.5 * 4 /(30000-7000) = 0.094
>> > degrees.  Since a typical stepper does 200 steps per revolution, then the
>> > required gearing ratio would be approximately 20:1.
>> >
>> > With this type of performance the magnetic loop could be very close to
>> > resonance at the chosen operating frequency presenting a VSWR approaching
>> > 1:1.
>> >
>> > 73's Gary K6YOA
>> >
>> >    Message: 3
>> >    Date: Wed, 2 Dec 2015 20:52:18 -0000
>> >    From: "Dave Lankshear"
>> >    To:
>>
>> >    Subject: [Elecraft]  [K2] Tuning a magloop with KAT100
>> >    Message-ID: <7B2795A9D35B4139812BF284FD02542B@DaveLLaptop>
>> >    Content-Type: text/plain;    charset="us-ascii"
>> >
>> >    Hello, Brian.
>> >
>> >
>> >    I've read your post and the replies and figure I maybe read what you
>> > said
>> >    incorrectly, but you did say:
>> >
>> >
>> >    Then use the TUNE button on the K2 to tune the loop to resonance, then
>> >    operate.
>> >
>> >
>> >    You can't use the K2's antenna tuner to tune a loop to resonance.
>> > That's
>> >    not how they work.
>> >
>> >
>> >    The loop is a complete tuned circuit in itself.  There's the loop
>> >    representing inductance and a capacitor in parallel with it.  These are
>> >    resonant at a frequency and the only practical way to move that resonant
>> >    frequency is to adjust the value of the capacitor that's in parallel
>> > with
>> >    the loop.
>> >
>> >
>> >    You can't adjust that resonant frequency using an auto ATU at the end
>> > of a
>> >    length of coax.  It's rather like using a telephone conversation to
>> > make a
>> >    physical change at the other end.  You can't use the phone to make the
>> > beds
>> >    back home, when you're away!  The auto ATU will see the coax cable and
>> > the
>> >    loop as a lump of L and C and will endeavour to match it to 50 ohms to
>> > get a
>> >    1:1 SWR.  The coax forms part of the antenna and is not behaving like a
>> >    transmission line and the loop is not behaving like a resonant circuit
>> > - you
>> >    might just as well hook the coax to your automobile's fender and use the
>> >    auto ATU to tune that!
>> >
>> >
>> >    Please forgive my descriptions if I have misunderstood your question,
>> > but
>> >    re-reading your quoted statement makes me feel like I'm the only one
>> > who has
>> >    understood.
>> >
>> >
>> >    Yes, by all means match the auto ATU into 50 ohms using a dummy load.
>> > That
>> >    way, the PA is looking into a load that matches the impedance of the
>> >    transmission line, although as Don suggested, you don't really need the
>> > auto
>> >    ATU (and its losses) as the PA should be reasonably well matched by
>> >    bypassing the auto ATU entirely.  Now, at the loop end, there's a gamma
>> >    match arrangement that ensures the loop, when at resonance, is a decent
>> >    match to the 50 ohm transmission line you're using.  So the rig matches
>> > to
>> >    the transmission line which matches to the loop that's been tuned to
>> >    resonance with its inbuilt tuning capacitor.
>> >
>> >
>> >    A loop is only a single turn coil, the resonant frequency of which is
>> > varied
>> >    by adjustment of its parallel capacitance.  Because it's small and is a
>> > low
>> >    loss inductor operating with a low loss airspaced (or vacuum)
>> > capacitor, the
>> >    Q factor, or "goodness" of the single turn coil at resonance is very
>> > high.
>> >    This means that a small excursion away from resonance, the loop's Q
>> > falls
>> >    very rapidly and renders it pretty useless, thus it is necessary (more
>> > so
>> >    when transmitting through the loop) to retune it for frequency shifts of
>> >    more than a few kHz.  That means that the SWR rises rapidly away from
>> >    resonance and the coaxial cable is more involved in becoming part of the
>> >    antenna and less of a transmission line.
>> >
>> >
>> >    The outer surface of a loop (well, outer 6%) needs to be of very low
>> >    resistance in order to maximise Q at resonance.  RF skin effect uses
>> > only
>> >    the outer surface of the conductor, thus the larger the surface area of
>> > the
>> >    conductor, the more its internal resistance is in parallel and thus
>> > reduced,
>> >    so the better performer the loop becomes.
>> >
>> >
>> >    Even a soldered joint on copper piping offers resistance that
>> > compromises
>> >    the loop's performance.  Recently a friend gave an old army magnetic
>> > loop to
>> >    a group of collectors/militia enthusiasts.  It was in poor condition,
>> > but in
>> >    its prime, its surface area must have been a foot across.  This makes a
>> >    mockery of the little bits of aluminium (aluminium) joined together with
>> >    bolts and wing nuts.  Yes, says the vendor, it is broad banded and only
>> >    needs retuning every 100kHz or so.  What he doesn't say is that its
>> >    resistance makesthe Q so appalling that its performance is lousy (where
>> >    lousy is the polite word), but those devices give properly engineered
>> > mag
>> >    loops a bad name by tarring all with the same brush.  Also, there are
>> >    proportionally more crappy mag loops out there simply because they are
>> >    cheaper than the "real" thing.
>> >
>> >
>> >    The MFJ 1782/86/88 aren't too bad and are just at the crossing point
>> > between
>> >    good and bad, with a bias towards the good, if not too many spiders and
>> >    other insect life are resident under the black covers. These loops have
>> > an
>> >    airspaced variable capacitor within the black covers and that capacitor
>> > is
>> >    tuned by a small electric motor that's attached to it.  DC power for the
>> >    motor is fed down the coax cable itself, as well as the radio signals.
>> > They
>> >    are not difficult to separate, eliminating the need for a control cable.
>> >    The DC voltage on the coax is reversed in order to make the motor turn
>> > in
>> >    the opposite direction.  The more sophisticated MFJ controller has an
>> >    inbuilt cross-needle SWR meter and the name of the game is to get the
>> > SWR as
>> >    low as possible on the operating frequency.  Their semi-automatic
>> > controller
>> >    drives the motor and detects the lowest SWR point and stops.  Of
>> > course, to
>> >    detect the lowest point, it has to begin to increase again, so it always
>> >    stops at a point that's not quite at resonance and this must be
>> > fine-tuned
>> >    by the operator, hunting to and fro with up/down press-buttons.  It
>> > takes
>> >    more effort to describe than to do in real life Hi!
>> >
>> >
>> >    One last caution.  Circulating currents are very high in a transmitting
>> > mag
>> >    loop and very high voltages (thousands of 'em) are also present when at
>> >    resonance, so ensure that the loop can't be touched by anything that
>> > matters
>> >    to you when it's transmitting, even at a few watts.  Give it a LOT of
>> >    respect, indeed, powerline respect.
>> >
>> >
>> >    I hope my efforts haven't missed the mark by a mile, Brian and that
>> > instead,
>> >    the wear and tear on my keyboard has been of some small use to you.
>> >
>> >
>> >    73 and early Season's Greetings.  Dave G3TJP
>> >
>> > ______________________________________________________________
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[k6dgw]

On 12/4/2015 1:09 PM, Ron D'Eau Claire wrote:

> IMX I tune the loop for maximum noise in the receiver. Properly
> designed small loops have very high Q, so the peak is very "sharp".

Yes.  While the RX noise peak *is* quite narrow, especially on 40 and
30, it is still quite a bit broader than the resonance point.  I've
occasionally gotten the look to resonance with the RX noise peak, but
it's pure luck.
>
> If I then transmit and find that the SWR is high, I'm sure the
> coupling between the loop and the feed line is not correct. That is,
> the loop at resonance is not presenting a 50 ohm impedance to the
> feed line.

Ummm ... not my experience with either the Alex or MFJ, nor does that
match theory.  The Alex is designed to present 50 ohms *exactly at
resonance*, nothing else is required. Also true for MFJ.  Once I've
peaked the noise, I put the K2 into TUNE and adjust for an SWR of 1.0:1.
  It is much narrower an adjustment than the RX noise peak.  No ATU, ever.

> If you are not worried about feed line losses, I would expect the
> loop to work just fine using the ATU at the rig to compensate for the
> odd impedance presented to it by the feed line, but be sure to check
> the loop tuning after the ATU does its job since it will interact
> with the loop and shift its resonant frequency somewhat.

Uhhh ... no.  No ATU, ever.  Everything is done with the little knob on
the loop while watching the SWR.  Conveniently, my K2 displays the SWR.
  No ATU, ever.

This is an amazingly simple antenna to adjust, I'm a little surprised at
the length of the thread.  Many years ago [OK, decades ago :-)], I built
a little analog phase detector from a QST, CQ, or 73 article.  It
powered a small DC motor connected to a variable cap that drove the
reactance on the 75 m antenna on Dad's car to zero.  Worked real good.
Not sure why it wouldn't work with the loop since "resonance" = zero
reactance.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org

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[Edouard Lafargue]

On Fri, Dec 4, 2015 at 1:59 PM, Fred Jensen <[hidden email]> wrote:

> On 12/4/2015 1:09 PM, Ron D'Eau Claire wrote:
>
> Uhhh ... no.  No ATU, ever.  Everything is done with the little knob on
> the loop while watching the SWR.  Conveniently, my K2 displays the SWR.  No
> ATU, ever.
>
>
   Precisely... this is exactly what I do on my system: I read the KX3
display on the serial port and use the SWR reading that it displays while
in tuning mode for tuning the loop automatically. If the K2 has got a
serial port, it will work too.

    Why make things complicated !

Ed
W6ELA
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[Gary Hawkins]

The QEX featured circuit designed by WB0OEW takes a slightly different
approach.  It monitors serial communications on the ACC1 of the KX3.  
 From this it pulls the KX3 operational frequency (AUTO INF must be set)
that it compares to the frequency the loop is currently tuned to.  If
these frequencies are sufficiently different and the loop needs
retuning, the KX3 is disconnected from the Alex Loop using a coaxial
switch and a Fox Delta DDS then excites the loop at the KX3 frequency
and an algorithm varies the angular position of the variable cap to
minimize VSWR - similarly takes about 10-20secs.  When the loop is
returned to resonance the coaxial switch reconnects the KX3/disconnects
the DDS and a logic line allows the KX3 to go into TX if required.  This
circuit does include a balanced bridge at 50 ohms and can't place any
strain on the KX3 as it's not connected during the tuning process.  I'm
currently implementing the serial read of ACCI, after I've got that done
I'm going to look at DDS options.   All the components are controlled by
the Arduino Uno - which is a fun device to play with.

On 12/4/2015 9:56 AM, Edouard Lafargue wrote:

> Hi Gary,
>
>    I need to set aside some time to finalize it, but I have designed a
> fully automatic AlexLoop tuner that is designed to work with the KX3:
> you just need to tune to the frequency you want to use, then press the
> "tune" button on the KX3 (long press on XMIT), and the Alexloop will
> find the correct tune in about 20 seconds worst case. It uses a tiny
> stepper motor as you described.
>
>    I have demonstrated a proof of concept for it at the Palo Alto
> Amateur Radio Association, and I hope to have instructions ready by
> the end of the year. The idea is to release complete instructions for
> whoever wants to build one themselves, and maybe also offer it as a
> partly soldered kit for people who do not want to bother with SMD
> soldering. Contact me direct if you are interested!
>
> Ed, W6ELA

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[Gary Hawkins]

I found out the manuscript of the QEX article is available online from
WB0OEW at http://www.clearskyinstitute.com/ham/LoopTuner/

73's Gary K6YOA
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[Michael Babineau-2]

Ron makes a very important point here. Using an ATU in place of
properly resonating a Magnetic Loop is a no-no.

If the loop, at resonance, doesn’t provide a good match to 50 ohms, then it is ok to use the ATU to “fix” the mismatch. Note the use of “fix” in quotations. You are really not
fixing anything, just making the rig see a resistive 50 ohms or something close to it
so that it is happy to deliver full power to the loop.  You will incur some additional losses in the tuner, but they are likely minor. Just make sure you properly resonate the loop before kicking in the ATU.

One additional  point worth mentioning.  A Magnetic transmitting loop fed via a coupling loop works like a transformer.  The 1 to 5 ratio between the size of the coupling loop as compared to the main loop provides an impedance transformation that should normally give a good match to 50 ohms.  If, at resonance, you are consistently seeing an SWR higher than what you would like then it is possible to tweak things by either slightly moving the position of the smaller coupling loop, or by slightly deforming the coupling loop in the vertical plane (i.e. making it rounder or more oval).  It is usually easier to just bend the coupling loop. The process is trial and error but usually you can find a configuration that will yield a pretty good match on all bands covered by the loop.

I had to do this with my MFJ-1786 Hi-Q Loop as the best match I could get at resonance
was around 2:1 SWR. With a bit of tweaking it now typically around 1.5 : 1 SWR on most
of the bands.

Michael VE3WMB

P.S.  Manually resonating a loop by ear is an acquired skill. With practice, peaking band
noise will get you very close to resonance without transmitting. Then with a second
or two of applied RF, aided by feedback from an SWR meter,  you should be able to hit the loop resonance point pretty much dead-on.


>From: "Ron D'Eau Claire" <[hidden email] <mailto:[hidden email]>>
>Date: December 4, 2015 at 4:09:18 PM GMT-5
>To: "'Elecraft Reflector'" <[hidden email] <mailto:[hidden email]>>
>Subject: Re: [Elecraft] [K2/KX3] Tuning a magloop


>There are two parts to tuning a small transmitting loop antenna: Resonating the loop at the >operating frequency and matching the loop at resonance to the impedance of the feed line.

>The Alexloop tuner sounds like a great idea, provided it does both at the same time.  

>IMX I tune the loop for maximum noise in the receiver. Properly designed small loops have very >high Q, so the peak is very "sharp".

>If I then transmit and find that the SWR is high, I'm sure the coupling between the loop and the >feed line is not correct. That is, the loop at resonance is not presenting a 50 ohm impedance to >the feed line.

>So the challenge for optimum power transfer is to adjust the coupling and tuning to get both peak >noise and low SWR at the same time.

>If you are not worried about feed line losses, I would expect the loop to work just fine using the >ATU at the rig to compensate for the odd impedance presented to it by the feed line, but be sure >to check the loop tuning after the ATU does its job since it will interact with the loop and shift its >resonant frequency somewhat.

>It's like two people standing up in a rowboat. You both work together or you both get wet.


>73, Ron AC7AC
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