[Elecraft] [K2/KX3] Tuning a magloop
Edouard Lafargue
edouard at lafargue.name
Fri Dec 4 15:14:48 EST 2015
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 <matt.vk2rq at gmail.com> 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" <
> edouard at 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 /(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 at 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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>> >
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