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Author Topic: 75m Magnetic Loop from Aluminum Flashing?  (Read 2600 times)
K4ZN
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« on: October 27, 2007, 12:50:33 PM »

I have not built this - yet. But, I thought I would share my thoughts.

I am attracted to magnetic loop antennas. High Q, narrow bandwidth seems to be just the ticket for some applications. Magnetic loops, being physically small look like they would make a good portable antenna. But, they get rather big when built for 80m. Also, the physical consturction is a bit of a difficulty.

I've thought about a octagonal loop consturcted of 2" dia copper pipe, 3ft to a side.This yeilds a loop of approx 7.25ft diameter (3/2^.5*2+3).

Using Steve Yates AA5TB Loop Calculator I get the following for 100w:
3.947MHz, Efficiency 36%, bandwidth 2.300kHz tuning capacitor 306pF, 4800V
7MHz, Efficiency 81%, bandwidth 10kHz, tuning capacitor 97pF, 4000V
10MHz, Efficiency 93% bandwidth 36kHz, tuning capacitor 47pF 3020V
14MHz, Efficiency 98% bandwidy 135kHz, tuning capacitor 25pF 2200v

This admittedly would be really 'twitchy' on 75m phone using all the bandwidth to do SSB. I wonder how it would work practically?

Also, the loop calculations might fall apart because at shorter wavelengths the circumference (24 ft) is more than 1/10 wavelength, which seems to define a magnetic loop.

At any rate, constructing such a thing would be expensive (has anyone looked at copper pipe prices recently!!) and Heavy. A good RF joint on 2" copper pipe would be problematic. And, any way to disassemble for portability, or even just moving would likely induce loss at electrically questionalbe joints.

Now, I have seen at one web site or another magnetic loop antennas fabricated from a flat conductor rather than a tube. I believe there is (or was) a commercial product like this. Since RF energy really only uses the surface of the conductor, why not do something like the following:

Visulize two octagons fabricated with, say, .75" PVC pipe with each side being 3ft. These two octagons are connected with 12" pieces of PVC pipe. What you end up with is a frame work that looks like a 7.25ft diameter stop sign that is one foot thick. Now, if aluminum flashing were wrapped around the perimeter this would establish a loop with no seams. From what I read, it is very important with these small loops to minimize resistance in the loop, so a seamless loop seems to be a good idea.

Further, by selective glue application the whole PVC pipe apperatus could be taken apart. One end of the loop could be detached and the aluminum flashing could be rolled up just like it came from the roofing store and stowed on the approx 1'x3'plywood base. The plywood base would be needed to mount the considerable butterfly or trombone capacitor that is required for this application.

I'm not even sure that I want to build such a thing. I might want to do so for proof of concept. I haven't seen a 75m magnetic loop that would be so light. And, I've not seen a concept that could potentially be broken down into a (relatively) small package. Now a 7.25ft diameter octagon is not itty-bitty. But, it would fit in a garage or an upstairs unused bedroom (remember RF exposure guidelines). I've seen pictures of magnetic loops in attic spaces. This consturction would be much lighter, and safer than using a torch to sweat pipe together in an attic space.

The questions I have are:

Does a wide ribbon conductor improve bandwidth? Bigger tubes have bigger bandwidth. Does the surface area of the loop govern bandwidth?

What happens to efficiency with a wide ribbon loop compared to a pipe or tube?

Copper flashing can be purchased. It's expensive. But, it is not as expensive as large diameter copper pipe. If the RF is only traveling on the skin, how thin can it be. Is copper so much better in an application like this that you would practically notice?

What problems have I not thought of?

Has anyone done anything like this, or has anyone heard of anything like this?

I look forward to everyone's comments.
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K4ZN
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Posts: 26




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« Reply #1 on: October 28, 2007, 05:07:42 AM »

OK, did some more surfing.

Re: strap vs. pipe.

Found the following on www.standpipe.com/w2bri/faq.htm

4. Is copper better than aluminum for loops?  

Absolutely and for the most part. It has over a 20% difference in efficiency according to some modeling software. More efficiency equals more performance. It's also a lot easier to solder, and can be found at your local hardware store. However, aluminum is far lighter. It's a trade-off. I would hate to know what my 12 ft x 12 ft loop with 2 inch diameter pipe would weigh if it was made out of copper instead of aluminum. Please remember, my aluminum loop works great.  

9. How do you compare copper or aluminum strap to pipe?

Modeling software claims strap is equal to one half the size of full diameter pipe.  

14. What if I exceed 1/4 wavelength circumference will the loop still work?  

Yes. I once believed that it does not seem to work as well as a loop of 1/4 wavelength circumference. Testing this theory now leads me to other conclusions, as larger loops seem to work better in some instances.

And then there is this:

13. Do radials help?

Radials added to the ground feed of a loop is supposed to lower Q and raise efficiency (see the ARRL Antenna Book, section 5). I know this sounds anti intuitive, but I have tested this many times and it seems to be the case. The bandwidth of the loop opens up by about 10%, the efficiency seems to be raised (this is indicated by observations I have made in antenna feed arrangement in relation to loss), but I have not noticed improved performance. It would seem to make sense that with higher efficiency there be better performance, but I have not seen a measurable difference. This needs more testing. When possible, I do add radials. Radials do not need to be cut to operating frequency, but can simply be twice the diameter of the loop.  

Does anyone have any other experience? This sounds like a project worth developing for experimentation.
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K4ZN
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Posts: 26




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« Reply #2 on: October 28, 2007, 05:11:07 AM »

Found another site, can't recall where, but someone used a magnetic loop with a calculated bandwith of 800Hz (between 1:2 swr) and I was reported to work quite well on SSB, so, maybe a loop with a calculated bandwidth of 2300Hz would not be too bad on the local 75m phone net.
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N3OX
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« Reply #3 on: October 28, 2007, 02:44:08 PM »

I think you should try it.

2300Hz bandwidth isn't much but your rig probably doesn't care if some frequencies at the output see an SWR over 2:1, so if you're a little off center or whatever... plus you're unlikely to have it be quite as narrow as calculated once you've put it together in the real world.

I think it's a great idea to try aluminum flashing.  It's light and cheap and probably works pretty well.  It has less inductance than round wire with the same surface area so you'll need to increase the perimeter or capacitance to get it to resonate, I think.

I like the idea of using it for a portable loop... just roll it up...

I'm thinking of trying it now and what I thought to do at the ends of the aluminum flashing will be to bolt maybe 20 to 40 star washer solder lugs to the ends of the flashing, 10 to 20 per side (I have a whole lot of them) and solder flat copper strap to them so I can solder a capacitor in.

Dan


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

Monkey/silicon cyborg, beeping at rocks since 1995.
NO9E
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Posts: 382




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« Reply #4 on: October 29, 2007, 09:14:50 AM »

2.3 KHz BW means that tuning will be tricky and any physical movement is likely to be seen on the SWR meter.
Still may be good as a curiosity and to experiment, especially on CW.

With small loops, the lowest frequency determines perhaps 90% of costs.  In my experiments with loops, adding coverage from 17 to 20m needed changing from gauge 12 wire to a 1/4 inch pipe.

Ignacy, NO9E
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K4ZN
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Posts: 26




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« Reply #5 on: October 29, 2007, 07:11:51 PM »

Dan, N3OX
Ignacy, NO9E

Thanks for the input.

Playing with numbers on MAGLOOP4.exe from G4GFQ, and looking at the calcuated results it looks like bandwidth is really affected by soil type. The more conductive the soil, the wider the bandwidth and the lower the efficiency.

Looks like I'll need a vacuum variable of about 500pF that can handle 10KV, or I need to build a really big butterfly. I'm now inclined towards the vacuum variable because I want to test an antenna concept, not my meager metal fabrication skills.

One concern I have is that 12in wide flashing will have to handle 35 to 55 amps of RF current at the top of the loop (the capacitor being at the bottom).

Ignacy, your observation about physical rigidity is taken under advisement. A cheap light weight form of diagonal bracing (fishing line?) must be devised if the PVC frame does not have adequate rigidity. But in an indoor, out of the way installation the tension of 12in wide flashing is likely to significantly firm up the assembly.

This will be an interesting experiment. I don't see anything on the web about a loop with a thin and wide (like flashing) loop material. Most seem to be of the opinion that a strap type loop will behave like a tube of diameter 1/2 the width of the strap. But, so often in the real world things don't behave as expected - there are often variables not considered.

It occurs to me that the flashing should be new and not oxidized. Resistance from oxidation could increase resistance. I hope to build and test against a 80m inverted "V" 46' at the peak, 20' at the ends. It will take me a while to put it together.

Rod, K4ZN
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N3OX
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« Reply #6 on: October 29, 2007, 07:42:03 PM »

Rod,

Max Gain Systems has good prices on vacuum variables.

mgs4u.com

No connection, just happened to notice it.

A relatively small-capacitance vacuum variable in parallel with a fixed bank of heavy transmitting-type doorknob capacitors could make for a very nice single-band magnetic loop on a budget.  

By the way, I think 55A RF current is nothing at all for wide flashing... I wouldn't give that a second thought.  That's kind of the point of using a wide conductor.

I've never tried a 75m loop but my 5-21 MHz version has me thoroughly convinced that they're worthwhile antennas.

If I were stuck with a limited space antenna, this would be the type I'd stay with, for sure.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #7 on: October 29, 2007, 07:43:40 PM »

Oh yeah, higher band version:

http://n3ox.net/projects/magloop/magloop1_lg

http://n3ox.net/projects/magloop/goodjunk_lg.jpg

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

Monkey/silicon cyborg, beeping at rocks since 1995.
K4ZN
Member

Posts: 26




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« Reply #8 on: October 30, 2007, 04:07:34 PM »

Dan,

Thanks for the heads up on the vacuume variable and door knob combo for single band operation. I'm always thinking of the 'finished' product (as if anything is ever finished).

For one location I'm actually thinking this might be an appropriate fixed freqency solution - a check into the net kind of antenna. If it's indoors it would be the one that will not come down in the ice storm, etc.

Thanks for sharing the pictures.
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N3OX
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« Reply #9 on: October 30, 2007, 04:19:38 PM »

"For one location I'm actually thinking this might be an appropriate fixed freqency solution - a check into the net kind of antenna. If it's indoors it would be the one that will not come down in the ice storm, etc"

Yep...

I've always thought that would be good for diehard PSK ops too, who never really stray more than a kilohertz or two in either direction.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KZ1X
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Posts: 3227




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« Reply #10 on: October 31, 2007, 02:26:59 PM »

My 10' diameter mag loop is made of 1 5/8" LDF7 Heliax, tunes via remote control with a 1 degree stepper motor turning a vacuum variable, and has a 2' diameter Faraday loop exciter.  It's built into a wooden cross-brace.  Works great!  Just 6' above the ground.  VERY directional.  Bandwidth is a percentage of the operating frequency, and usually a couple kHz.

I have built many one-band, fixed-tuned loops for use on PSK31.  Perfect application, especially for stealth operation.  Antennas have no noise (they are effectively a very narrow front-end filter) and can be constructed to be 90%+ efficient.
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NR9R
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« Reply #11 on: November 04, 2007, 08:54:29 PM »

I have one suggestion to add that I have come up with in building a similar loop.  For a PVC frame, I would go circular.  You can take thin walled 1/2" or 3/4" PVC and bend it into a circle, closing the loop with a glued coupler.  for 10-15 ft, 1/2 will bend easily, for a 24' loop I would move up to the 3/4" or 1" pipe.  Use the white pluming stuff.  The black, grey, or beige pipes are not as RF-transparent and may lead to losses.  1-1/2" or 2" PVC can be used to form a cross-frame on which to mount the circular loop.  

Here is another advantage you get from using metal strip over pipe:  Efficiency of a magnetic loop is achieved by minimizing resistive losses.  The designs which use an octagon of copper pipe requires soldering of the 8 pipes together and connecting grounding braid from the pipes to a capacitor, which is ultimately a mechanical connection.  All of these are potential points of loss.  Using copper strap or aluminum flashing offers the advantage of a continuous conductor which can terminate directly to the capacitor.  I wonder if the losses which occur due to these added mechanical and soldered connections may fail to produce the advantage one would expect by using pipe over an aluminum strip.

N0HC describes his experience with using aluminum flashing for a loop on his site:http://www.geocities.com/n0hc/loop
He warns, "One thing that the loop programs (like mloop32) do not take into account is the fact that if the material is too wide it starts acting like a capacitor not just an inductor." I have not verified this myself but is worth taking into consideration. The hardware store near me has flashing available from 4."  If problems arise, it should be easy enough to roll the flashing up and cut off an inch with a hacksaw.

Please report you experience if you go through with it.

73,
Anthony
AA9OC

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K4ZN
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Posts: 26




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« Reply #12 on: November 06, 2007, 05:56:39 PM »

AA9OC de K4ZN

Thanks for the heads up on the white pvc vs. others.

I think the octagon vs. circle issue from most of what I have read is not going to change performance too much. But I agree a circle is better. The problem I foresee with wide flashing is that it were to be tensioned on a circular frame without many crossbraces it might get kind of 'crinklely'.

I too read on someone's page that a wide loop is going to begin to act like capacitor. But I wonder if all things are relative. 8" or 12" wide flashing may not be 'wide' on a 288" loop.

It is suggested that bands act approx. like a tube of diameter 1/2 the width of the band. So, I guess the sweet spot will be the maximum width (to gain effiency, bandwidth, and current handling) without too much capacitance effect. This might have to be determined by experimentation.

The biggest problem is the capacitor and the connection to it. I guess the ulitmate in loss minimization would be to get each end of the loop 'welded' to the appropriate side of a butterfly! If I had extra $$ I would be tempted to build the whole thing of copper sheet (loop & butterfly) all polished. It would look like NASA made it! (plus it would solder nice.
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