Building a magnetic loop antenna...

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Steve Kaeppler:
All-

I am thinking about building a small magnetic loop antenna for my apartment being that I cannot have an outdoor antenna.  I have read a few forum posts which suggest that indoor loops are good options if you are confined to an apartment.  I also understand they are not as efficient for transmitting as outdoor antennas, but I am willing to make that compromise.  I have been significantly reading and referencing: http://www.aa5tb.com/loop.html

1. I understand that a loop antenna is effectively a tuned RLC circuit.  I also understand that the loop antenna is more magnetically coupled than electrically coupled to the EM radio wave.  Or at least is more sensitive to the magnetic component of the radio wave(makes sense as the AC magnetic field will induce a current in the loop - thank you Faraday.)  

2. I also have some butterfly type variable capacitors on their way, which I ordered off QTH.  It is rated to 1.5 KV or so and I believe should have good range in capacitance for the bands I hope to hit.  I would like to get down to 40 meters, which I know I will have a low efficiency on.  Nevertheless, I like the  40 m band.

3. I will not operate at a power of greater than 5 watts.

4. I plan to make it a circular loop with 1/2" or 5/8" diameter copper pipe.  The diameter of the loop will be 3-4 ft (I haven't made my final decision yet), giving a circumference of 9-12 ft.

5. I plan to inductively couple.  I am not sure exactly how large the loop should be, relative to the primary loop.  Any suggestions?  I could directly couple as well, but inductively coupling seems like a good method.  Or is it?  I was also planning on connecting this loop to a SO-239 connector.

6. I will probably also need to use some wire to connect the capacitor, which I know should be as short as possible.  Any suggestion on the size?  14 gauge?  10 gauge?  solid?

Aside from that, I am just wondering if there are some other design parameters that I need to consider.  Maybe something I should do?  Or just some general advice?  Perhaps I assumed something incorrectly?  I would just appreciate whatever advice I can get!

Thank you all!  

73s
Steve
kb9pzm

Clark McDonald:
Hi Steve,

There are some good resources out on the web that can not only answer your questions, but aid you in the design of your magnetic loop. 

As for circumference, to be a true magloop the circumference should be less than 1/10th wavelength and the coupling loop at 1/5 ratio to the main loop.  But I'm about to give you a link to a very good tool for designing your magloop antennas, by AA5TB. 

Read the entire webpage and be sure to scroll way down and download the Excel file he has posted there, a great design tool, just enter the freq. or the size of the loop, power, a few other things if you desire and it will instantly give you the other factors down below such as capacitance, voltage at cap, etc.  Very handy for the design stage. 

I'm here to tell you that the magloop can work very surprisingly well, considering.  MANY EU hams and Latin American hams use them. 

http://www.aa5tb.com/loop.html

I'm the kinda guy who appreciates novel concepts in homebrewing.  Prolly because as a youngster I had not so much money to spend but plenty of time to think about using resources at hand to do things that had to be done.  (There's a long story about testing for my General the first time around, back in the day, when we had to be able to draw various schematics as part of the test and I drew the question about drawing a Copitts Oscillator - and drew it the only way I knew, using a TV tube that had a weird filament voltage for use in a series-string set.  The old buzzcut examiner looked at what I drew and said, "Wise guy."  But I passed.)

Take a look at the way PY1AHD designed his capacitor using telescoping coca cola cans with hypodermic needles and tubing filled with water for remote adjustment via hydraulics.   Elegant answer using parts at hand, Improvise and Adapt, I say: 

http://www.youtube.com/watch?v=gaJlrN3cElo

If ya don't think it works, here's another video of him in QSO with his magloop outdoors: 

http://www.youtube.com/watch?v=ry1h2d4Styk

Having to move into an apartment now, I built my first magloop to handle 100W of power.  However, I've found out that about 20W or less can do a fantastic job actually, especially since I'm a CW nut, but the darn things do a much better job than I would have imagined on SSB as well. 

Some guys seem to think that having to constantly tune the magloop when changing frequency is a challenge or a problem, I don't.  Matter of fact, the thing works because of the high Q and that's part and parcel of operation with one.  With a little practice and knowledge of your antenna on the band(s) you design it for, turning the big knob and then moving the antenna cap to suit is actually very easy IMO.  I can tune mine by just listening to the receiver noise.  Peak the noise and then it is only a matter of a slight tweak to peak it on transmit. 

No noise, which is uncanny when using indoors in this apartment building (2nd floor of a 3 story) that is crammed with flatscreen tvs, internet routers, crazy lamps with switching psu's and touch sens switches that make standard AM reception into noise city just don't show up on the magloops with the HF rig. 

BTW -  The magloop does have an Electric field.  It is just not found until you get a bit away from the antenna, my old field strength meter does show an electric field on its little vertical antenna when I get about a half wavelength or more away from the loop.  Inside that, it shows nothing.  But don't lose track of the fact that a radio wave must have both E and h fields perpendicular to each other in order for the wave to propagate.  This type of antenna just doesn't seem to have much of the E up close to it. 

Good luck with your project, be prepared to maybe build two or more magloops, sized to cover maybe three bands each at most, rather than trying to make one that will cover like 10 - 80 all by itself.  I learned that lesson the hard way. 

40 meter CW anyone?  Ant here is on the lampstand...


73

Clay Hopkins:
Loved the Vids...just wish I spoke the lingo.  I am wondering on the spacing and construction of the Capacitor.

:
Make sure you check out VK4AMZ website

Andrew Domonkos:
Steve,

Take a serious look at this site for an excellent way to get a mag loop on 40/80M:

http://g0cwt.co.uk/magloops/new_page_2.htm


I built and tuned one in an hour and was amazed how well it worked indoors.


Andy
N3LCW

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