A Unique (?) variation of the Mag Loop Antenna

[ZS6BIM]

Have you ever thought of making a small loop but been discouraged by the thought of vacuum variable capacitors and plumbing copper pipe!

Well if you can you scrounge a few meters of half inch foam heliax and a largish ferrite toroid (ui 100 to 200) give this idea a try – it’s simple and you won’t be disappointed by the performance.

This idea makes use of the coaxial cable’s capacity to resonate with its own inductance – similar in a way to a conventional coaxial trap.

See schematic here

http://i620.photobucket.com/albums/tt283/mwmikep/Loopschematic.jpg


The antenna described is for 40m however the same idea can be used to make a loop for any frequency.

For the 40m band loop start with a 5m length of coax (foam dielectric) and at the exact centre open up the outer jacket and copper sheath as shown in the attached images.

http://i620.photobucket.com/albums/tt283/mwmikep/Coaxprep1.jpg

http://i620.photobucket.com/albums/tt283/mwmikep/Coaxprep2.jpg

http://i620.photobucket.com/albums/tt283/mwmikep/Coaxprep3.jpg

Take care when cutting back the outer copper so as not to damage the foam dielectric beneath.

You need to peel back about 3,5cm of the coax outer – an old pair of side-cutters does the job.
Fold back any sharp copper edges to prevent corona.

Reinforce the opened area with a length of PVC tubing and seal with heat-shrink.

Make off each end of the cable exposing approximately 1cm of inner, 1cm of dielectric and 5cm of the outer.

Wind 14 turns of insulated hookup wire evenly around a suitable toroid and slide it on to one end of the coax. (36mm Philips 4C65 or T140-61)

Bring the two coax ends around to form a circular loop and temporary solder the outer jackets to a piece of blank PCB as shown in the following image.

http://i620.photobucket.com/albums/tt283/mwmikep/Loop_couplingandtuning.jpg

Also solder a coaxial connector on to the PCB and connect the lightly twisted wires from the toroid. (To the connector inner and PCB ground)

Now it’s time to start with the trimming. Fasten the loop to a suitable mast that will allow the loop to be positioned a couple of meters above ground in a reasonably open area. The top section of the mast supporting the loop should be nonconductive material.



Connect up your RX and tune for the obvious noise peak that will most likely occur a bit below 7MHz.

You now need to cut back each of the coax ends slightly until resonance occurs about 100 kHz above the highest 40m band frequency you want to transmit on.

Once this has been achieved a small variable capacitor positioned between the ends of the coaxial inner is used to tune the loop lower in frequency.

The implementation of this tuning arrangement I’ll leave to you – you can see my rather crude homemade variable capacitor in the image of my loop.

Be aware though the voltage across the capacitor reaches 6kV at 100W input!

Being a very high Q antenna, small size and shape differences can have quite a big effect on the resonant frequency.
The final length of coax that gave me an upper resonant frequency of 7,2MHz was 4.86m with a 3.5cm wide top gap and 7.5cm spacing of the coax ends.

The home made capacitor that allows me to remotely tune the loop from 7,0MHz to 7,1MHz has a range of about 5pF and a minimum capacity of 3pF. 

Whatever capacitor design you opt for be careful not to introduce additional loss into the circuit. Any additional loss will show up as an increase in the tuned SWR.

The transformer turns ratio of 14:1 should be correct and provide a tuned VSWR < 1.2:1 across the band for a well-made loop as described and supported at least 2m above ground and away from other conducting surfaces.
If you find you do not achieve this it means either your variable capacitor is introducing loss or the loop is coupling into the surroundings.


The transformer turns ratio indicates the sum of the resistive losses (Rl) added to the loop’s radiation resistance (Rr).
At 7MHz this 1,55m diameter loop has an Rr of about 0.04 ohms and Rl of 0.214 ohms giving a total of 0.254 ohms.
A 14:1 transformer turns ratio provides a 14^2 impedance transformation ratio or 196 times.  196 x 0.254 = 49.8 ohms.
The loops efficiency is therefore Rr/(Rl+Rr)*100  or 15.7%.

The unloaded Q of the loop is 830 giving a 1,6:1 SWR bandwidth of around 2,5 kHz!
Tuning needs to be precise and may test you ingenuity in implementing the variable capacitor.
If you have to use the loop closer to ground or near conducting surfaces, for example indoors, you will most likely have to reduce the transformer turns ratio to maintain a low SWR  - this indicates some reduction in efficiency.

For example, if you have to reduce the transformer turns ratio to say 12:1 to get a good match this indicates that the (Rr+Rl) = (50/122) = 0.347 ohms. (0.347 – 0.254 = 0.093 ohms of additional loss has been added)
As the radiation resistance has not changed the efficiency falls to 0.04/0.347)*100  or 11.5%.

Self resonating coaxial loops can also be made for the other bands; as a starting point you can use 3.6m of coax for 30m, 2.7m for 20m and 2.2m for 17m. If you have the coax and space an 80m & 160m version should also work quite well.
Other coax lengths may be calculated using the formula y = 29.091x^-0.9071 where x is the frequency in MHz and y the length of coax required in meters. (Applicable for foam dielectric cables – velocity factor 0,81)

You can also use RG213 in place of the helix coaxial cable however the losses are considerably higher and there is also a chance of insulation voltage breakdown at 100W. For a 40m band loop efficiency falls to about 6%.

Finally an important consideration;

When transmitting at 100W the magnetic field close to the coaxial loop will greatly exceed the recommended limits of human exposure.

From what I can recall one needs to keep at least 5m away!

73
Mike
ZS6BIM
 

[W8JI]

My bet is you would be shocked if you actually measured the efficiency of the loop.

The reason designs use very low loss capacitors and large smooth conductors is the current required to cause a certain electromagnetic field level. A real radiation resistance of .04 ohms requires 50 amperes to generate 100 watts of radiated power.


In your loop design, the outside of the coax is the radiator, and it couples to the inside of the loop at the gap.  The current then turns inward and goes back to base on the inside of the cable shield. This excites the inner conductor, which then becomes a third current path, conducting RF in a loop through the capacitor. The capacitor tunes that loop.

Of all those current paths over the outside of the shield once, the inside of the shield once, and the outside of the center conductor once, the only area that contributes to radiation is the outside of the shield.

There has to be less efficiency than a simple loop made with a conductor the size of the center conductor of the heliax.

It would be much better to just use the outside of the heliax. That would emulate a traditional loop with just one current path over the circumference of the loop.

73 Tom

[ZS6BIM]

Hi Tom

Thanks for contributing your thoughts to the subject – I was beginning to think there was no interest.

“My bet is you would be shocked if you actually measured the efficiency of the loop.”

I believe I know the efficiency of this loop when used in the real world because the loop current is a function of the turns ratio of the transformer needed to match Rr plus the total of all losses to 50 ohms.

In my case this is 14:1 so the loop current is simply 14 times the primary current or 28A pk at 100W input.

From the calculated Rr, again in my instance, about 0,04 Ohms, at 100W input the radiated power would be 28^2 x 0,04/2 watts or 15,7 watts – so efficiency is about 15%.

Although this may appear low I believe the actual efficiency (when used in the real world) of most magnetic loops is considerably lower than the user would like to believe.

(How many loop users measure the loop current or the unloaded Q of their loops.)

Consider a 60mm diameter Cu pipe forming the same sized loop, 1,55m diameter, tuned with a vacuum capacitor, Q 5000, - and ignoring the environmental coupling loss – efficiency would be about 42% and the unloaded Q 1760.

An antenna with such a high Q would be unusable for SSB voice – the 1dB or 2,6:1 SWR bandwidth being only 2kHz!

I agree however with your analysis of current flow in my coaxial loop. One could achieve a slightly higher efficiency by using an optimally installed fixed high Q tuning capacitor.

However the coaxial loop at 7 MHz already runs at an unloaded Q of 830 (and this is in the real world) giving a 1,6:1 SWR bandwidth in the order of 4kHz.

Consider also that adding the environmental coupling loss, (in the order of 100 milli-Ohms) for loops close to ground, to the 60mm example described above, efficiency would fall to about 20% a fraction of a S unit better than the simple design I have described.

73
Mike
ZS6BIM

[W0BTU]

I was beginning to think there was no interest.

Well, I'm interested in STLs.    

Your design is interesting, and I see you understand that losses have to be kept low in the tuning capacitor. But if you used a solid piece of Heliax like Tom suggested, and built a different tuning capacitor, you could:

1. Make the loop work on multiple bands
2. Raise the efficiency considerably

I would also use larger diameter Heliax or copper water pipe 1" or larger.

The links on http://www.w0btu.com/magnetic_loops.html show some interesting low-loss homebrew variable capacitors, if you're interested.

[ZS6BIM]

Hi Mike

Yes the coaxial loop is a monobander however that is the price paid for simplicity.

Yes a thicker Heliax would reduce the copper loss however an unloaded Q greater than about 1000 at 7MHz will make the loop impracticable for SSB voice and extremely difficult to tune to frequency.

I don’t believe a dedicated tuning capacitor will raise the efficiency of my coaxial loop considerably – consider the 60mm OD copper pipe loop example described in my previous post.

Have you ever measured the Q of your STL deployed for use? – You may be surprised to find it considerably lower than predicted by so many of the magnetic loop calculators that don’t take into account environmental loss.

73
Mike

[W0BTU]

Have you ever measured the Q of your STL deployed for use? – You may be surprised to find it considerably lower than predicted by so many of the magnetic loop calculators that don’t take into account environmental loss.

No. I haven't actually built one yet, Mike. As you can see on that web page, I'm going by AA5TB's loop calculator and EZNEC (over real ground). A number of others who've built STLs have made very favorable comments about its accuracy, and I simply took their word for it. What you you think about it?

And is your loop mainly for receiving? 15% efficiency is fine for that.

[W8JI]

I believe I know the efficiency of this loop when used in the real world because the loop current is a function of the turns ratio of the transformer needed to match Rr plus the total of all losses to 50 ohms.

I'm having more than a few problems with that idea, because transformer ratios are not that cut and dry unless the transformer has perfect flux linkage from primary to secondary.

Generally I have not found that to be true, although it gets close with special winding techniques.

If coupling coefficient is less than unity:
http://farside.ph.utexas.edu/teaching/em/lectures/node83.html

On the surface it certainly looks like the coupling system and placement of the capacitor would greatly increase losses compared to a conventional system. It is always best to verify what we assume with an actual direct measurement, especially when we have RF transformers involved in resonant systems.

73 Tom

[ZS6BIM]

I'm going by AA5TB's loop calculator and EZNEC (over real ground). A number of others who've built STLs have made very favorable comments about its accuracy, and I simply took their word for it. What you you think about it?

Hi  Mike

Here are my thoughts on the subject.

AA5TB’s loop calculator provides correct answers but does not tell the “whole story”.

The first point is that there is no place to include the Q of the tuning capacitor.

Perhaps many believed that if a vacuum variable capacitor is used to tune the loop its losses are insignificant and can therefore be ignored.
Unfortunately this is not true – my own measurements on a small Jennings variable vacuum capacitor gave a Q around 5000 at the capacity setting I was using.
(In fact the excellent ATC case E porcelain fix capacitors are about twice as good as the vacuum variable!)

Even if your vacuum variable had a Q of say 8000 this would add 29 milliohms to your loop reducing the efficiency from 64% to 52%.

The second point is that there is no place in AA5TB’s calculator to include the “environmental coupling loss” - a fact of the real world especially if you operate the loop reasonably close to ground or any other conductor or conducting surface.

I have measured a coupled ground loss in the region 80 to 100 milliohms at 7MHz with the loop at 2m.

Ironically it is fortunate that these losses are there otherwise the bandwidth of low loss magnetic loops would be so narrow as to be unusable except for CW.

Consider your proposed 40m design and ignoring the other losses for the moment.
The Q of 880 given by AA5TB’s calculator is the loaded Q that assumes the driving source (your transmitter) has a source resistance of 50 Ohms.

This is far from being true. The transmitter will have a “pseudo” source impedance determined by the function of the ALC loop.

From AA5TB’s calculator the unloaded loop Q is 2 x the loaded Q – and in your case this is 1760.
This means that if you shift frequency by 7000/1760 or 4kHz the SWR will increase from an assumed 1:1 at resonance to 5,83:1!
Even a shift of 2kHz will result in a SWR of 2,62:1!

As a result the effect of transmitting SSB voice into such a high Q antenna will be a significant reduction in radiated power due to ALC action.

On the other hand when you use the loop in the real world other losses come to your rescue.

In your example if we use 80 milliohms of environmental coupling loss and a vacuum variable having a Q of 5000 the overall Q drops by half and the SWR bandwidth doubles.

However efficiency also halves falling from 64% to 32%.

Perhaps you think this is really bad? Try comparing it to the efficiency of a half-wave dipole supported 2m above an average ground!

Even with this additional loss you are going to find operating into such a high Q antenna difficult.
If you run high power, 1,5kW, it’s likely the antenna will rapidly detune its’ self due to local heating!

73
Mike
ZS6BIM

[ZS6BIM]

Hello Tom

Like you I also like to reaffirm assumptions with measurement.

It is fairly straight forward to add a current transformer to a magnetic loop to measure the loop current.

I have done this and feel confident that transformer coupling to a loop (if correctly done) is as efficient as any other method.

73
Mike

[W8JI]

Hello Tom

Like you I also like to reaffirm assumptions with measurement.

It is fairly straight forward to add a current transformer to a magnetic loop to measure the loop current.

I have done this and feel confident that transformer coupling to a loop (if correctly done) is as efficient as any other method.

73
Mike

I wouldn't. Not with the capacitor location in the drawing.
:-)

The problem is not so much the transformer, although it does add a new area of difficulty because VA power is so high. The main problem is how the currents flow in the system, as drawn.

Instead of current confined to the outside of the shield, where surface area is wide and smooth, the drawing would have all of the orignal OUTSIDE path length of a conventional tuning system, plus the inside length of the shield, plus the length of the center conductor.

All of the extra unnecessary current path  length cannot be good for anything.

[W0BTU]

 AA5TB’s loop calculator provides correct answers but does not tell the “whole story”. ... The first point is that there is no place to include the Q of the tuning capacitor. ... Even if your vacuum variable had a Q of say 8000 this would add 29 milliohms to your loop reducing the efficiency from 64% to 52%.
The second point is that there is no place in AA5TB’s calculator to include the “environmental coupling loss” ...
 

Thank you for explaining all this. You seem like a guy who understands this subject much more than I do. :-)

But I'd still like to build that loop, and do some A/B/C comparisons between it, an inverted-L, and a dipole. The people I've heard using them have much stronger signals than I would have ever imagined from a 6' diameter loop 10' high. I want to try it for myself.

[ZS6BIM]

Hello Tom
I see now my attempt at drawing the self resonant coaxial loop is rather poor – the capacitor shown at the bottom of the loop is only a few pF used to tune the loop across the band and as such carries very little loop current.

The capacity that tunes the loop to resonance is the distributed capacity of the coax inner to outer.

As such I believe the current on the coax inner will fall rapidly on either side of the gap in the shield at the top of the loop limiting additional loss.

The equivalent circuit can be thought of as two series connected capacitors at the top of the loop each having a value equal to 215pF being the capacity of half the loop circumference or 2,43m of Heliax.

I agree this method will not provide the highest possible Q however a Q much above the 830 this coax loop achieves in the real world is hardly usable at 7MHz unless you only want to operate CW.
73
Mike

[ZS6BIM]

But I'd still like to build that loop, and do some A/B/C comparisons between it, an inverted-L, and a dipole. The people I've heard using them have much stronger signals than I would have ever imagined from a 6' diameter loop 10' high. I want to try it for myself.

Absolutely - go for it Mike! Nothing better than rolling you own antenna.

Look forward to further small loop discussions as you progress with your design.

73
Mike

[KW5B]

Glad to see this topic.  

I have just built a "conventional" magnetic loop.  117 inches 1/2 in copper pipe, octagon, horizontal sliding capacitor at the top, fed at the bottom with a 240-43 toroid centered and coupled with one turn of coax.

With the capacitor I've made, it will cover 11 mhz to 35 mhz measured with an MFJ-259.  The swr at each band is less than 1.5 across a least a 1mhz bandwidth.  There is very little difference in swr when I touch the loop when using the MFJ.  By the way I didn't make a typo error.  That bandwidth is megahertz.

The same exact antenna with a 19 in Faraday loop had a very narrow bandwidth and would cover only 14 mhz to about 24 mhz.   With varying swr from 1.5 to 2.0.  

The only difference being the match.  Faraday loop vs. toroid.

I have no engineering expertise only practical experience and have no clue as to the theory.

Larry

[ZS6BIM]

Hi Larry

On 40m and above your loop is larger than what would normally be considered a magnetic loop where the loop circumference is usually less than 1/10 of a wavelength.

Although you can still use transformer coupling the high resonant impedance of your loop, especially on the higher frequencies, will necessitate careful transformer design so it might be better to stay with your other coupling method.
The narrow tuned bandwidth you get with your Faraday loop indicates far better efficiency.

The transformer you describe with one turn of coax, I presume you’re describing a 1:1 transformer, won’t do the trick and will introduce considerable loss – that is why you are measuring such a wide low SWR bandwidth.
It is also very likely that the toroid core will saturate and get really hot! (That’s where all the RF power is going!)

Because your loop is so big it will self resonate without any tuning capacitor at all at around 30 MHz. As your variable capacitor can’t go down to 0pF the residual capacity of your variable C and the high self resonance of your loop are limiting the upper frequency to 24MHz.

Also as you approach self resonance the tuned impedance of your main loop that the Faraday loop has to couple to increases rapidly making it difficult to have one size coupling loop for a number of bands.

You could consider using this loop for the lower frequencies say – 80m to 20m and build a smaller one for the higher frequencies. You would just have to increase the size of your variable capacitor or switch a fixed high voltage capacitor in parallel to tune 80m.

73
Mike