G4WIM Auto Tune QRO Mag Loop 3.7MHz to 14.6MHz
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- เผยแพร่เมื่อ 30 พ.ย. 2024
- This video shows a magnetic loop I've built which uses a stepper controlled Jennings vacuum variable capacitor along with a Teensy++ controller to automate the tuning and measure SWR plus forward and reverse powers.
The controller is based on a power meter originally designed by TF3LJ / VE2LJX which uses a couple of AD8307 to measure fwd and rev powers.
I've added hardware and code to extend its functionality into an auto mag loop controller.
It can auto tune with powers as low as 4mW or as high at 400 watts.
Briefly, it measures the applied frequency, drives the stepper motor to a calculated tuning point then applies an algorithm to fine tune the loop to a VSWR of less than 1.5, usually less than 1.2.
It does all this in a matter of seconds, much faster for small changes in freq, effectively making the loop appear to have a much greater bandwidth thus overcoming one of the major problems with a magnetic loop - ie narrow bandwidth and difficulty with tuning.
The next steps are to design a proper PCB and write up the whole design for publication.
That's a very nice idea. This kind of antenna is very sharply resonant and doesn't have a whole lot of bandwidth, so this makes it a lot more convenient to use!
Since I published this video I've made some improvements to the original design - please see related video showing how two stepper motors are now employed.
The full design has been written up and will likely appear in RadCom before too long.
Are there schematics and program available? The powermeter and phasebridge I found, but to combine the rest....could you help me out ? '73 de PA3U
Congratulation for this very nice achievement!
F4GEQ
Since making this video, I've now designed a PCB for the controller which also has a second stepper motor output which will rotate the input coupling loop to ensure a good match across a much wider frequency range.
Hi. Are details available for.me.to build one for my loop I am making please ? Also. Are the protocols known in order to drive it all from the radio to tune as you tune from vfo ? Or do you select the F from the controller ?
Hi Ross, this design requires no radio interface or control as it measures the incoming frequency and acts accordingly. The design was published by Radcom but I don't have any PCB's available. Regards Tim
Tim, this is awesome work. I read your article in RadCom and, that is even more impressive. Thank you for sharing. I do have a question, as I was pursuing as similar but less elaborate approach. When you initially powered up the control box, at around 3:07 into the video, the calibration sequence initiates and shows the motor's steps on the LCD. How does controller know where in the VVC's 24 turns it is starting from? I didn't see any limit switches... Where is it spinning the VVC from and where is it going to to in the initial calibration process? I am hesitant to connect a motor to my VVC and over spinning it in either direction. I realize that once you have a correct starting point, it is just counting steps going forward. Do you manually spin the VVC back to an end point before you power up the controller? Thanks again and my apologies for my rambling on.
Tim, I would be very interested in more information on the controller design. Could you tell me which issue of RSGB this design was published in, or any online resources pointing to it? A quick search shows that you produced a PCB for the design. Is it possible to purchase the PCB or to get the design files for the PCB?
-Freeman, N5FPP
The wood parts now can be replaced with PVC, which will make it more resistant to the elements.
I want one of these boxes! So far, I haven't found anyone who offers ready to ship loop tuners like this, hard to find someone who will even build one.
Hi, have you watched my latest video with the new hardware and firmware etc? I do have a complete set of prototype pcbs fully populated but would still need building into a case and the loop / vacuum variable capacitor would need to be assembled so far from a ‘turn key’ ready to use solution.
Do you do a kit for this project
Id be really interested in building this. I have a pair of big vacuum caps. Will soft/firmware be available?
Hi Tim great design.
Very Nice job F5JWX 73...
Demonstration starts at 3'. My calculations show than 500 W input requests 25 kV variable capacitor rating! Who's right?
I am currently rigging up an arduino to a gear motor and a potentiometer to my variable cap. Instead of doing a proper setup I'm just going to program in the positions found by testing and use up down buttons to set the freq i the arduino which snaps the motor to the stored position which is coupled also to the pot where 1 revolution of the pot covers the entire range of the cap. Basically its a dumb blind controller that just has presets for certain freqs and interops between them. It wont have an swr bridge so it will be blind and will require the user to set the freq in the arduino to match the transceiver.
Its super cheap though.
you may find that using the system you describe is very difficult to accurately tune due to the back lash in your gear box and the fine degree of control required - especially on the lower bands where the loop bandwidth may only be a few kHz. I found that I had to write the code to accomodate the very small about of back lash in the shaft couplers as I needed to be accurate to within 0.25 degree. A single turn of the pot would imply a count of 1440 and at least a 12 bit a/d to get anywhere near, then you'd still have the back lash to contend with etc. No doubt you've figured this yourself and decided it's ok for you needs :-)
Tim Forrester I use a 6rpm motor. I have been just manually pushing buttons to turn the motor. I'm going to use plastic gears instead of a shaft couple ( currently using a stiff straw and two hose clamps).
The gearmotor is so low geared that it doesnt ever turn on its own during vibration. The straw flexes though so there is a small lag time before the cap follows the motor.
So currently its just a gearmotor coupled to the cap by a straw and then 2 boshe automotive type relays to implement an h bridge and then a 3 position switch near the operator. Also replacing the copper tubes with copper foil wrapped around plastic tubing with heatshrink to make it durable enough for mobile. The goal is to survive brutal tree branch impacts at highway speed.
hi Richard, most VVC's take about 20 revolutions, thus a 6rpm motor would take 3-4 minutes to go from min to max which is why I use a stepper whose speed I can control at will and so perform controlled accel and de-accel to minimize transit time. Guess I must not be seeing the full picture of your system, but good luck and have fun with your project !
To be honest Tim I don't think birds sitting on the antenna would be a real problem. obviously the Capacitor and braid connections would be waterproofed to stop any rain or snow causing arching? Also change in temperature will also change the tuning when sun comes out! If it ever does in the UK? I assume a high vswr alert maybe a Beep etc could warn of any problems?
Cheers Mark GØNMY
Wow, if a bird perches on it will it auto tune while transmitting? I guess he would move pretty quick if you were running 1kW.
Ha Ha - I hadn't thought of that :-) In practice it's most likely to be installed in the attic but if it were installed outside the auto tuning can be turned off to prevent the system 'hunting' caused by external effects.
Tim Forrester Wouldn't it be though that if a Bird landed on it outside that it wouldn't be affected because the Bird isn't stood on the ground/floor as you are. Just a thought Tim as of course Birds can sit on power lines. You have replied to my post asking for info in constructors corner on hrd.co.uk.
Fred196407 Thats because the line frequency of power lines is extremely low so the reactance of the bird is tiny. At rf frequencies the bird will have enough reactance to draw enough current for a nasty RF burn. Though at higher power levels the small reactance will draw more current which is why birds do not land on power lines of 40KV or higher. The reactance causes pain to the bird when the bird gets close enough so they dont land.
Hi Ross, firmware is available and there's a group purchase of pcb's being arranged by GD1MIP - you might like to contact him. 73 Tim
Do you have a link to your RadCom write up? This is an elegant design. 73! W4CWL
Send me an e mail and I'll send you a copy of the article as it's not available on line Address is good on qrz.com, please note I don't have any PCB's available.
Salve ottimo lavoro ,mi iscrivo al canale è attendo l'intero progetto grazie per il video e 73 de livio iz6awp
A very nice project neatly exceuted, if you don't mind me saying so? Most impressive! 2E0ILY Shropshire
Mmm.