Re: [Elecraft] [K2/KX3] Tuning a magloop

[Michael Babineau]

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" <r...@cobi.biz <mailto:r...@cobi.biz>>
>Date: December 4, 2015 at 4:09:18 PM GMT-5
>To: "'Elecraft Reflector'" <elecraft@mailman.qth.net 
><mailto:elecraft@mailman.qth.net>>
>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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Re: [Elecraft] [K2/KX3] Tuning a magloop

[Fred Jensen]

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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Re: [Elecraft] [K2/KX3] Tuning a magloop

[Ron D'Eau Claire]

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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Re: [Elecraft] [K2/KX3] Tuning a magloop

[Edouard Lafargue]

On Fri, Dec 4, 2015 at 1:59 PM, Fred Jensen  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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Re: [Elecraft] [K2/KX3] Tuning a magloop

[Matt Maguire]

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" 
 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 /(3-7000) = 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 -
>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 

Re: [Elecraft] [K2/KX3] Tuning a magloop

[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  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 
>
>
>
>
> On Fri, Dec 4, 2015 at 9:57 AM -0800, "Edouard Lafargue" <
> edou...@lafargue.name> 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 /(3-7000) = 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 -
>> >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. 

Re: [Elecraft] [K2/KX3] Tuning a magloop

[Ron D'Eau Claire]

I was speaking of small transmitting loops in general, not a particular
design. 

A small loop, like any other antenna, can be designed for any impedance feed
line. If a given loop happens to present a 100 ohm impedance at the feed
point, 50 ohm coax will see a 2:1 SWR. That will result in some oddball
impedance at the transmitter, depending upon the electrical length of the
feed line. If the transmitter output network (e.g. ATU) can match that
impedance, all is good providing you are happy with whatever losses occur in
the feed line.

As I said, it's a balancing act. Messing with the termination at the
transmitter end of the coax to match the impedance presented will shift the
resonant frequency of the loop; the adjustments are interactive. 

If your loop is designed so that it always presents a 50 ohm termination to
a 50 ohm feed line at all frequencies you need not adjust the rig's output
network. 

None of the homebrew loops I've built maintained that match across the
various bands. 

73, Ron AC7AC



-Original Message-
From: Elecraft [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Fred
Jensen
Sent: Friday, December 4, 2015 2:00 PM
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] [K2/KX3] Tuning a magloop

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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Re: [Elecraft] [K2/KX3] Tuning a magloop

[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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Re: [Elecraft] [K2/KX3] Tuning a magloop

[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  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 /(3-7000) = 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 -
>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
>   

Re: [Elecraft] [K2/KX3] Tuning a magloop

[Fred Jensen]

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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[Elecraft] [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 /(3-7000) = 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 -
   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