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Author Topic: Isn't a "magnetic loop" really just a bent and shortened dipole?  (Read 2681 times)
VE7TIT
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« on: October 14, 2012, 08:54:08 PM »

This seems like the kind of question that could start an argument, but I don't mean it to. I've just been thinking lately, aren't our magnetic "loops" actually just capacitively loaded dipoles with the ends bent back on themselves? I guess the answer depends on how you define the word "loop," which I have always thought of as an uninterrupted DC path, but that's probably simplistic. I imagine there must be some kind of IEEE dictionary out there somewhere that defines it...

On an unrelated note, after thinking about it since 1989 and a lot of failures along the way, I finally assembled a square mag loop that's slightly less than a meter in diameter and had a successful PSK31 QSO on 20 meters this afternoon with a gent in California. Not bad for 5 watts into some badly soldered copper pipe perched in my crowded little living room. Getting the homebrew, sliding-copper-plate tuning capacitor halfway mechanically rigid without binding turned out to be a lot harder than I thought--especially with no shop facilities beyond the kitchen table. The whole mess still needs a lot of work, but it has been a lot of fun and might come together sometime before the end of this solar cycle. Wink)
« Last Edit: October 14, 2012, 09:03:59 PM by VE7TIT » Logged
N3OX
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« Reply #1 on: October 14, 2012, 09:21:25 PM »

This seems like the kind of question that could start an argument, but I don't mean it to. I've just been thinking lately, aren't our magnetic "loops" actually just capacitively loaded dipoles with the ends bent back on themselves?

You can look at it as the limit of a capacitively loaded dipole bent back on itself, but there's not a very meaningful distinction between that picture and a closed unbroken loop carrying a constant current.  It's really the same thing, at least when the loop is quite small.

The "break" in the loop to insert a capacitor is just a way to get massive currents flowing in the loop with a normal transmitter.  You could drive it directly but you'd need a special high voltage and high current transmitter to provide a huge voltage almost 90 degrees out of phase with the huge current to overcome the high inductive reactance of the loop. 

There's no difference between the radiation pattern, radiation resistance, efficiency, or any other properties of a small loop with a resonating capacitor vs. an unbroken one that is carrying a constant magnitude current driven by some other means.

Lot of words and effort wasted on arguments over verbal descriptions of identical mathematical and physical things in ham antenna discussions, so watch out Grin
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
VE7TIT
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Posts: 62




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« Reply #2 on: October 14, 2012, 09:26:43 PM »

You can look at it as the limit of a capacitively loaded dipole bent back on itself, but there's not a very meaningful distinction between that picture and a closed unbroken loop carrying a constant current.  It's really the same thing, at least when the loop is quite small.

The "break" in the loop to insert a capacitor is just a way to get massive currents flowing in the loop with a normal transmitter.  You could drive it directly but you'd need a special high voltage and high current transmitter to provide a huge voltage almost 90 degrees out of phase with the huge current to overcome the high inductive reactance of the loop. 

There's no difference between the radiation pattern, radiation resistance, efficiency, or any other properties of a small loop with a resonating capacitor vs. an unbroken one that is carrying a constant magnitude current driven by some other means.

Good answer!  Smiley
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W8JI
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« Reply #3 on: October 15, 2012, 06:54:11 AM »

.....and in addition to what Dan says, consider HOW or why any antenna radiates. They all follow the same rules.

Antennas radiate because of current, and it is all about the unopposed ampere-feet of physical distance across space between two points occupied by the current. There is no such thing as a magnetic radiator or electric radiator in real life, all of the useful radiation from any antenna is electromagnetic and comes from current.

When you think of that, you see the problems with a small loop. Exactly opposite any given point in the small loop is another area carrying opposing current. This would completely cancel radiation, except there is physical space involved. The physical space allows phase shift because of the finite speed of light. The phase error is what allows the loop to radiate. This is also why a small loop has a null through the axis, because distance through space is the same to every out-of-phase section.

Since the loop fights itself in every direction, a small loop has very high current for a given radiated power and has a low radiation resistance. 

As a matter of fact a small dipole the same conductor length as a loop has much high radiation resistance and much lower current for the same radiated power.

The advantages of a small loop over a small dipole are ability to tune with a capacitor. A capacitor might have a Q in the ten's of thousands instead of hundreds, and is much easier to vary compared to the inductor required in a small dipole.

While this offsets the small dipole's radiation resistance advantage in most applications, I'd use a small inductor loaded dipole if I wanted an efficient single frequency antenna. A small dipole would not have the ease of frequency agility, even though it could more easily be made efficient.

A small loop is electric field dominant in most of the near field distance, while a small dipole is magnetic field dominant in most of the near field distance.  You can see field ratio of a loop and dipole here:

http://www.w8ji.com/images/emfield.gif

So much for "magnetic" radiators. :-)

73 Tom
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WX7G
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« Reply #4 on: October 15, 2012, 07:10:55 AM »

Tom, I think you meant to say an electrically small loop is magnetic field dominant in the near-field and a small dipole is electric field dominant in the near-field.



« Last Edit: October 15, 2012, 07:14:23 AM by WX7G » Logged
N3OX
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« Reply #5 on: October 15, 2012, 07:16:40 AM »

While this offsets the small dipole's radiation resistance advantage in most applications, I'd use a small inductor loaded dipole if I wanted an efficient single frequency antenna. A small dipole would not have the ease of frequency agility, even though it could more easily be made efficient.

Yep.  This is the problem I was trying to solve with this: http://www.n3ox.net/projects/n3oxflex

A small inductor loaded antenna can easily handle high power without the need for an expensive capacitor.

But even so, I only have continuous agility across a single band and have to switch taps to cover 20/30/40m.  Eventually I will add a remote tap switcher, but the high voltage across the coil is going to need a pretty decent switch.  Probably not vacuum relays at the 500-600W I can run, but something high voltage which makes it harder to actuate remotely.  It should also have low contact capacitance and low contact-to-coil capacitance if I use a relay.  And I'd sort of like a tap switcher that's as cheap and simple as the rope-tuned hat. 

I got two-band 30/40m switching at the 100W level using a microswitch pulled by a second string but it arced over at higher power Smiley  A rotary wafer switch would be a good coil tap changer probably.

And that's the issue with only a couple of bands.  My magloop with vacuum variable covers 5-21.5MHz with no gaps Cheesy  It's some dB worse than my short vertical on 40m but still very, very usable.

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WX7G
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« Reply #6 on: October 15, 2012, 07:50:47 AM »

N3OX, multibanding your antenna looks like a job for a screwdriver antenna.
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PA1ZP
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« Reply #7 on: October 15, 2012, 09:03:14 AM »

Hi to you all

I have build 3 of those miniloops in the past.
I was not very happy with them
I used copper water pipe of half an inch and I used 5/8 inch for an other loop.
I used a very large remote controlled motor driven air-capacitor in this loop.
With this cap I could use 100 W into the loop and the cap. didnt spark.

The resuls of a 2 meter (6 ft) dia loop on 20 mtrs were worse then my GP on 20.
My rotatable 2 x 7 mtr dipole with thick open wire for feedline and an enourmes balanced tuner does a far better job on 80 mtrs as the 2 mtr dia magloop (miniloop because there is nothing more Magnetic about it as a dipole or a big loop) I had

At this moment I am building a new specialy designed ATU that is balanced and handles low Z very well and even high Z   ok .

This tuner uses a high Q all ceramic Russian surplus roller inductor with extree high eficiency and very big air capacitors with welded plates for high Q and low losses.
This tuner has no switches at all for getting R losses als low as possible.
Wiring is done with red copper strips of 5/9 x 1/25 inch (15 x 1 mm)
The balun at the transciever side of the tuner is wired with 6 x .5 mm square paralel teflon insulated wire

This is all done to get as much power s possible in the 2 x 7 mtr V-dipole with open wire on 10-80 mtrs.
Because of extreme size and cabinet of brushed stainles steel and the need to balance the rotors of the capacitors because the rotors would tumble down on their heavy weight.
This tuner is 20 inch wide 16 inch deep and 10 inch high (inside measure of cabinet without knobs and connectors) and weighs more then 20 Kg.

When it is finished an article nd pictures will be at my blog at:

http://blogs.hamstart.net/blog02

the style is retro in brushed stainless steel and plexiglass.
All lettering on top and front are in Russian.
The name of this tuner is "Soyuz"
And the design is dedicated to Andre Kuipers our Dutch long stay in space  record holding cosmonaut.
He was in ISS from december 2011 - juli 2012.
He travelled to and from Iss in a Coyus TMA 3M



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N3OX
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« Reply #8 on: October 15, 2012, 09:40:30 AM »

N3OX, multibanding your antenna looks like a job for a screwdriver antenna.

That is one option for sure.  But it's an option that rules out pure homebrew for most people, and reduces the number of improvisations that are possible.  It raises the cost a huge amount in a lot of cases.  A roller inductor would be cheaper but still hard to find and more work to weatherproof.  I also originally envisioned this antenna as a dipole, doubling the adjustable coil costs.  I switched to a vertical because it was what I actually wanted for a 40m antenna, and because I could easily put up a 1/4 wave vertical as a reference for my tests.

I made my antenna coil pretty fancy because I have access to a machine shop and  I use this one as a primary antenna for 40m at least part of the year.  But an electrically equivalent and mechanically adequate version could be made from scratch completely with hand tools and Home Depot parts.

A screwdriver-coil load with a huge hat would be a great fixed-station antenna but it didn't fit my design constraints (or my budget Smiley )
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WX7G
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« Reply #9 on: October 15, 2012, 11:21:59 AM »

MFJ has roller inductors starting at $59.
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W8JI
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« Reply #10 on: October 15, 2012, 04:43:03 PM »

Tom, I think you meant to say an electrically small loop is magnetic field dominant in the near-field and a small dipole is electric field dominant in the near-field.





Hi Dave,

The small  loop is only magnetic field dominant within 1/8th wave, then, still in the near field, it is electrical field dominant from about 1/8th wave to a full wave out. So the majorty of near field area in a small loop is electric field dominant.

A small dipole is electric field dominant out to 1/8th wave. Then, still in the near field, it becomes magnetic field dominant.

At about 1 wave out any small antenna is the same.

You can see the field impedance change in the graph I posted.

73 Tom
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