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 QRO magnetic loop design question 
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Hi Guys,

I'm building a QRO auto tuning magnetic loop antenna and am figuring out the mechanics for the tuning cap.

Normally the tuning cap is placed opposite the input coupling loop resulting in a high RF voltage on both sides of the tuning cap which then results in mechanical and electrical issues to keep it isolated.

Also in my case as the vacuum variable cap is quite large it makes for a top heavy loop.

Question:

What is to stop me placing the tuning cap at the base adjacent to the input coupling loop ?
Doing so would enable one side of the cap to be at RF ground thus making it's isolation for tuning purposes much easier, less top heavy and would permit all of the control circuits to be in one place.

The down side is the loop would not be electrically balanced, but does it really matter providing the current is still flowing in the loop ?

I've googled this approach but not found anything on it - maybe there's a good reason I've missed as to why it wouldn't work.

Any comments ?

73 Tim

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Fri Oct 10, 2014 8:10 am
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
On thinking about it, I suspect the feed point matching / coupling arrangement maybe a little more challenging as it's now offset and thus lower as one end of the loop is now at RF ground.

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Fri Oct 10, 2014 9:26 am
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 QRO magnetic loop design question 
Silent Key

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Post Re: QRO magnetic loop design question
Tim,

I can't claim to have done a thorough analysis, but first thoughts are that there could be problems. The feed position - opposite the capacitor - would be at a high voltage with respect to ground, and that would likely cause some CM issues on the feedline. Maybe not insurmountable with enough choking, but challenging!

Steve G3TXQ


Fri Oct 10, 2014 9:59 am
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
Looks like I'm likely trading one problem for another !

Will most likely stick with the conventional arrangement, but it would be interesting to understand what would happen to the feed point impedance etc with an offset feed.

The vacuum capacitor I brought back from the USA is a real beast - probably overkill but at least it'll have a good safety margin. It's the 15kV model below.

http://www.rell.com/filebase/en/src/Dat ... DD-500.pdf

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Fri Oct 10, 2014 10:22 am
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
Here's the one I used when I played about with magnetic loops....it too was a bit of a beast.
At one time I had a magnetic loop in an antic around 14ft x 6ft made from 1" copper tube, fed using a Faraday loop and brought to resonance using this brute. :)


Attachments:
003 (Copy).jpg
003 (Copy).jpg [ 145.44 KiB | Viewed 5063 times ]

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Fri Oct 10, 2014 10:55 am
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 QRO magnetic loop design question 
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Looks even bigger than the one I have, although mine is ceramic sealed. Out of interest what were the specs on that one ?

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Fri Oct 10, 2014 11:07 am
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
G4WIM wrote:
Looks even bigger than the one I have, although mine is ceramic sealed. Out of interest what were the specs on that one ?


Now there's the question!
I must have it about 10 years and it's Russian military as are the markings on it but think it might be 10kV with a range of 20 - 1000 puf

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Fri Oct 10, 2014 12:25 pm
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
Tim look up K8NDS on QRZ.com he has lots of information and links that might help .

Drew


Fri Oct 10, 2014 1:07 pm
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
Tim ,what you turning the cap with .reason i ask ,my new amp I'm building I want to be remote ,I was looking at the nema stepper motors to turn the caps and band switch ,caps are comet 25kv units

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Sat Oct 18, 2014 7:48 pm
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
This is the stepper I'm using

http://www.ebay.co.uk/itm/221355161378? ... EBIDX%3AIT

This is the driver board for it

http://www.ebay.co.uk/itm/271575315558? ... EBIDX%3AIT

and my calcs indicate enough resolution when driver a 25 turn 20 to 500pF vacuum variable

But I think I'll need a 10 turn pot to help avoid hitting the end stops of th capacitor when in auto tune mode.

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Thu Oct 23, 2014 6:59 pm
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 QRO magnetic loop design question 
Silent Key

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Post Re: QRO magnetic loop design question
Tim,

Can't remember if you're using an Arduino for this project or not? If you are, you may be interested that I took delivery of a Chinese Uno "clone" yesterday - it works just fine. Price Ā£2.99 including a USB cable!

Steve G3TXQ


Thu Oct 23, 2014 7:14 pm
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
Hi Steve,

I'm using this design as a starting point as a lot of the work's been done, then adding some external RAM for storage of the tuning data along with a phase detector.

https://sites.google.com/site/lofturj/power-and-swr-meter

Already got the basic design running - just waiting for the AD8307 RF detectors.

The coupler I built also provides sampled V and I to drive the phase detector.

Written most of the code to implement the auto tuning - but waiting for the stepper motor driver.

The Teensy 2++ is quite nice bit of kit and supports USB - but may port the code to a simpler controller if it all works out ok.

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Thu Oct 23, 2014 7:24 pm
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
Will you put some pics on too Tim ? and the bit extra code you write for the phase shift will that be opensource too :blush:

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Thu Oct 23, 2014 7:35 pm
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 QRO magnetic loop design question 
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Yes, the extra code will also be open source as will be the extra hardware to detect the phase and store the settings for faster tuning.

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Thu Oct 23, 2014 8:22 pm
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 QRO magnetic loop design question 
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Post Re: QRO magnetic loop design question
Wooohooo cheers Tim I'm all excited nah

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Thu Oct 23, 2014 8:47 pm
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