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Author Topic: Building a magnetic loop antenna...  (Read 12722 times)
AD0AE
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Posts: 76




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« on: April 26, 2011, 04:12:33 PM »

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
« Last Edit: April 26, 2011, 04:26:16 PM by KB9PZM » Logged
KE3WD
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Posts: 5694




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« Reply #1 on: April 26, 2011, 07:21:24 PM »

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
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KC5WSU
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« Reply #2 on: April 27, 2011, 12:51:42 PM »

Loved the Vids...just wish I spoke the lingo.  I am wondering on the spacing and construction of the Capacitor.
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VK2HHS
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« Reply #3 on: April 28, 2011, 08:43:29 PM »

Make sure you check out VK4AMZ website
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N3LCW
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« Reply #4 on: April 29, 2011, 04:52:29 AM »

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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N0SYA
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« Reply #5 on: May 01, 2011, 08:57:27 AM »

related:
http://www.ti.com/rfid/docs/manuals/appNotes/HFAntennaDesignNotes.pdf
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
NH7L
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« Reply #6 on: May 07, 2011, 06:10:10 AM »

An indoor mag loop's effectiveness will depend on the construction of the building it lives in. I've transmitted PSK-31 with one inside the lower floor of a two-story wood-frame house with ceramic tile roof and could work stations 5,000 miles away. The same antenna inside my 14th-floor apartment, in a 20-floor steel-reinforced concrete building, was useless. The building amounted to a Faraday cage. By moving it to a southwest-facing window I could at least use it as a receiving antenna. But I was unable to transmit with it because steel building reinforcement and aluminum/steel window framing always was too close to allow acceptable SWR. Another consideration for indoor use is the very powerful near fields created by these antennas on transmit. Unless you live in an exceedingly spacious apartment, I'd suggest limiting transmit power to QRP levels for your health. There are reports of mag loops at 100 watts blistering paint on wood a foot or two away. I can't vouch for their accuracy, but they gave me pause. On transmit, mag loops develop very high voltages across the tuning capacitor. At 100 watts, they are in the thousands of volts. Even at QRP power, they may be enough to be dangerous. Exercise care, especially if there are kids or pets around.
« Last Edit: May 07, 2011, 06:15:41 AM by NH7L » Logged
KE3WD
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« Reply #7 on: May 07, 2011, 06:47:19 AM »

Blistering paint? 

Not hardly.

I can find NO nearfield at all on my magloop experiments.  Matter of fact, the field strength meter doesn't even read anything until I get at least one wavelength away from the antenna proper. 

If located on upper floors of a steel construction building, try turning the loop itself into the horizontal and arrange a way to hang it outside a nearby window.  If the magloop is about a half wavelength or more above ground at the frequency of interest, placing it into horizontal polarization mode like that will pretty much resemble the common dipole pattern.  Tried that from a hotel in my travels once.  Hard part is finding a tall building that still has a window that can be opened far enough to deploy the magloop out of, though.  I've had a few CW QSOs from tall hotels that had non-opening windows, though, in both horizontal and vertical antenna orientations. 

The one thing to keep in mind is that any small antenna must therefore also come under the heading of what many call the "compromise antenna".  As compromise antennas go, I have empirically found that the magloop works very well indeed. 


73
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KC4MOP
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« Reply #8 on: July 22, 2012, 06:06:08 PM »

This link back to eHam was very interesting. Helically wound mag loop antenna that can handle legal limit.

http://www.eham.net/articles/26572#comments
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W8JI
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« Reply #9 on: August 06, 2012, 07:29:17 AM »

This link back to eHam was very interesting. Helically wound mag loop antenna that can handle legal limit.

http://www.eham.net/articles/26572#comments

That article, unfortunately, is not technically sound.
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W8JI
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« Reply #10 on: August 06, 2012, 08:01:25 AM »

Hi Steve,

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.)  

Radiation comes exclusively from charge acceleration, not from inductive fields. The inductive fields don't go any significant distance, and are not involved in anything any significant distance. They simply store and return energy to the system, unless they induce current in something externally.

You can read about it here:
http://www.w8ji.com/radiation_and_fields.htm

Radiation always comes from ampere-feet of spatial area. This is why electrically small antennas, for a given EM field strength and applied power, have more current than larger antennas of the same pattern.

Quote
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.

Good capacitor style, especially if "cube dimensioned" with equal dimensions on all sides and good current paths inside. The problem is the voltage is very low for any power, if you have a small high efficiency loop.

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

OK.

Quote
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.

With 1 inch copper pipe and 3 feet diameter, assuming zero loss outside the pipe, you are right at 1500 volts peak with 5 watts applied. Assuming no capacitor or external losses, efficiency is 13%. This means if the capacitor handles 1500 volts peak from terminal-to-terminal you are OK at 5 watts in the real world.

One problem might be current through the capacitor. It is 13 amps at 5 watts. The capacitor has to be about 80 ohms or so reactance. That means you need around 400 pF maximum capacitance for a 3 foot diameter loop. The exact value will depend on your exact construction, and vary with loop size.

 
Quote
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.

Inductive coupling is prefectly fine. It is just as good as anything for coupling, and much better for balance if you build the loop correctly.

The size, however, is difficult to estimate. This is because small changes in loss resistance will affect loop antenna impedance and loop Q, changing the size of the coupling loop. It is fast and easy to just cut and try. I'd start with a one foot diameter loop, and squeeze or spread the shape for lowest SWR. If it is all spread out and SWR is better but does not go low enough, it needs to be bigger.

Changing the shape will not hurt efficiency, but you could cut and trim and eventually make it round. It is easy to adjust the coupling loop, since it only needs to be #10-#14 wire size solid copper wire. 


Quote
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?

You have 13 amps, and want to keep inductance and resistance as low as possible. I would use a very short lead of wide copper strip, or flatten the ends of the copper tube and bolt to them. Silver solder would be better than just bolting, if possible.

Quote
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!

Bigger diameter rapidly makes things easier electrically.


73 Tom
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JAHAM2BE
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« Reply #11 on: August 27, 2012, 07:44:46 AM »

Quote
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?

You have 13 amps, and want to keep inductance and resistance as low as possible. I would use a very short lead of wide copper strip, or flatten the ends of the copper tube and bolt to them. Silver solder would be better than just bolting, if possible.

I recall reading an earlier post where you mentioned that clamping with "thousands of pounds" of force was necessary (http://www.eham.net/articles/9527 , search for "thousands of pounds"). I inquired on another forum about how feasible it is to clamp with thousands of pounds of force and the answer was that it may be barely possible but the bolt heads will be in danger of popping off.

Could you go into a bit more detail about your recommendation for a bolted connection (or other mechanical connection) if one were to be used to connect the loop conductor to the capacitor? I suppose the idea is to maximize contact area, ensure surfaces are clean, and ensure maximum pressure in areas with maximum current flow (edges, in the case of a strip). Anything else?

And do you have any estimation for how many milliohms of loss might be realistically incurred at 7 MHz with some reasonable values for contact area and clamping force? Mechanical connections will obviously be lossy, but for portable loops or temporary loops that need to be dismantled, it may be a worthwhile tradeoff.

And, FYI, an interesting article about contact resistance: http://www.timron-inc.com/teknote_Mar2007.html . Interesting details, though it doesn't talk about RF resistance.
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WX7G
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« Reply #12 on: August 27, 2012, 12:17:31 PM »

The MFJ-933 and their similar 1/4 wavelength loop tuners work well (I have one). Using 1/2" copper tubing the radiation efficiency can be 25-50%.
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W8JI
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« Reply #13 on: August 27, 2012, 05:16:55 PM »

I recall reading an earlier post where you mentioned that clamping with "thousands of pounds" of force was necessary (http://www.eham.net/articles/9527 , search for "thousands of pounds"). I inquired on another forum about how feasible it is to clamp with thousands of pounds of force and the answer was that it may be barely possible but the bolt heads will be in danger of popping off.

I don't know who gave that answer, but that isn't true. Thousands of pounds of clamping force can easily be had with a few good grade 1/4-20 stainless bolts. Bolts can have considerable tension without failure. Although there are many sources for specs, have a quick look here:

http://www.imperialsupplies.com/pdf/A_FastenerTorqueCharts.pdf

Quote
Could you go into a bit more detail about your recommendation for a bolted connection (or other mechanical connection) if one were to be used to connect the loop conductor to the capacitor? I suppose the idea is to maximize contact area, ensure surfaces are clean, and ensure maximum pressure in areas with maximum current flow (edges, in the case of a strip). Anything else?

Yes. Remember skin effect and how current flows, and how the bolted connection applies pressure. You do not want a bolt compressing the joint away from the outer conductor edges. This also implies using more than one bolt if the area is wide.

It's actually a bit like a multiple disk clutch for a car, and stacking plates.

Also, use a suitable grease in the connection to retard oxidation.

Quote
And do you have any estimation for how many milliohms of loss might be realistically incurred at 7 MHz with some reasonable values for contact area and clamping force? Mechanical connections will obviously be lossy, but for portable loops or temporary loops that need to be dismantled, it may be a worthwhile tradeoff.

I've never measured resistance, but I have observed heat at higher power and loss of field strength. Resistance is too difficult to directly measure, but it could be backed out of field strength changes.

Quote
And, FYI, an interesting article about contact resistance: http://www.timron-inc.com/teknote_Mar2007.html . Interesting details, though it doesn't talk about RF resistance.

Interesting article!
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G3RZP
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« Reply #14 on: August 28, 2012, 12:21:46 AM »

I reckon a 1/4-20 bolt should manage 3/4 of a ton.

Some people have reported that brazing or high temperature silver solder is the best connection method. Ideally, welding would be used, but copper welding is quite difficult, and there are only a handful of people in the UK who can do it.
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