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 <[email protected]> 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" < > [email protected]> 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 >> > >> > ______________________________________________________________ >> > Elecraft mailing list >> > Home: http://mailman.qth.net/mailman/listinfo/elecraft >> > Help: http://mailman.qth.net/mmfaq.htm >> > Post: mailto:[email protected] >> > >> > This list hosted by: http://www.qsl.net >> > Please help support this email list: http://www.qsl.net/donate.html >> > Message delivered to [email protected] >> > >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> >> Message delivered to [email protected] >> >> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
