General questions about Magnetic loops antennas

[AC2EV]

So I'm thinking of building a magnetic loop for 40m. I've read the basics about construction and theory of operation. I'll do a lot more research once I start building.

In general what is the cost involved with building one? I understand the capacitor is the most expensive part and if I want to run 100 watts I'll need a vacuum variable capacitor but how much do these cost? I'm hoping on picking one up used from a hamfest or run QRP with a standard variable capacitor.


What's the bandwidth on such an antenna?


I have access to a metal fab shop at work so I can purchase aluminum pipe/conduit that can be formed into the main loop.

Not sure if this is something I want to pursue yet. I don't have the yard for a dipole unless I buy to 40 foot masts. I have nothing substantial to anchor to. I can't put in a tower because of fall zone and flood easements. XYL isn't keen on a beam on the roof which is only 25ft high. Looking for a project.

[WB2WIK]

Assume you've already read stuff like this:

http://www.aa5tb.com/loop.html
http://www.nonstopsystems.com/radio/...na_magloop.htm
http://www.standpipe.com/w2bri/source.htm

Bandwidth for a 40m loop is normally very small (narrow), just several kHz. But that's why we make the capacitor adjustable!

Many have used motor-driven capacitors so they can adjust the resonant frequency remotely, without having to run outside to do that.

If you use aluminum, you'd probably want to heliarc (weld) the connections, as standard hardware, clamps and rivets evidently don't provide a low enough contact resistance for running any sort of power. Copper might be easier, since silver solder works well and requires no special tooling or talent.

[AC2EV]

Thanks those last two I hadn't seen.

[KO6WB]

For a magnetic loop to be as efficient as it should be then the circumference of the loop needs to be 40 feet. The diameter of the conductor should be as large as possible however the difference between 2" copper pipe and 1" copper pipe is about 4%. So if you're 94% efficient with 2" copper pipe then you'll be 90% efficient with 1" copper pipe. The difference in price is quite a bit. The use of aluminum reduces the efficiency from 90% for 1" copper down to 79% for aluminum of the same diameter. Drop the frequency down to 3.5Mhz with the same dimensions and material and the copper is 43% efficient and the aluminum is 23% efficient. That's about 3db difference. This is provided you have all connections as good as electrically possible. This is difficult to do with aluminum and a breeze with copper.
The tuning capacitor will need to be as efficient as possible to get the best performance. The best choice is a vacuum variable but a air dielectric unit can do just as well. The spacing between the plates should be 4-5mm for 100 watt operation. The rotating portion of the capacitor should either have solid direct connections or be a floating portion of the capacitance. The losses between the individual plate connections needs to be as small as possible. This can be accomplished by soldering or welding the plates to the conductive spacers that are used to separate them. A Butterfly or split stator capacitor is most effective of the air dielectric units. There are homemade units that are good as long as losses are kept to a minimum.
The bandwidth of this size antenna is over 30Khz before retuning is required for 40 meters and drops to about 4Khz at 80/75 meters and even further down to 1.6Khz on 160 meters. The size of the capacitor to get down to 160 meters is quite large.
You will want to make a way to tune the capacitor if you have more than a narrow frequency band for operations. This can be a long insulated rod but at just about 10 feet up the rod would have to be long and the tuning would be a PITA. Anything near a magnetic loop becomes coupled to it and that will effect the tuning. That includes your body, the house, the trees, the fence and anything else nearby.
The best way to tune the loop is to use a motor controlled in your shack. If you are using an air variable that has 360 degree free movement then you could use a motor that has one direct only but in the real application you'd want it to be reversible. The speed of the motor is important with slow being very good but not too slow. You don't want to wait a long time for it to tune. The use of a stepper motor is a complex approach but very effective as long at you use a gear reduction unit with it. Even the individual steps can be too much of a change in capacitance for good operation. The motor I use is a hysteresis motor that is reversible. It's a geared down unit to 10 RPM coupled to a vacuum variable that requires 30 turns to go from minimum to maximum capacitance. Takes just over a minute to tune from 40 down to 80. This type of motor was simpler to setup then a stepper. Don't forget to insulate the motor from the capacitor.
The coupling method used by just about all STL users is a small coupling loop about 1/5th the size of the main loop. The web sites listed by Steve detail this method. I used a gamma match which is very effective and produces a 1.2:1 SWR from the bottom of 80 to the top of 40. Don't forget that includes 60 meters as well.
The best metal to use for a magnetic loop is silver. Tell me if you can afford that and I'll become your newest best friend. Copper is very effective and the use of steel, although uncommon, has been done. It's not as effective but some folks have made it work. Aluminum is cheaper, lighter and not that bad in this application.
Shape, if you can make it circular it'll be just a bit better. An octagon is good and a square is still very effective. I use a square since it's simple and requires fewer solder joints then an octagon. Making a 40 foot circumference circle isn't that easy and it's not all that much better. Remember you'll have to make each and every connection as low loss as possible.
For modeling what you want to do with a magnetic loop there are some very helpful calculators. One, that requires Microsoft Excel or Open Office (free) is the AA5TB calculator and one that you can use as a program is the one by KI6GD. They'll give an idea of what you'll need to do to get the antenna you want.
As you can tell there's a lot of information you can use. Some have pie in the sky thoughts and others are more down to earth.
Height above ground isn't that important in some applications but in others it is. I have mine up 11 feet from the ground. Getting an antenna that is 10 feet on each side up at any height is a task that usually needs more than one person to get it up. Ask me how I know.
If you have any further questions that can't be answered by the stuff from the web then get a hold of me or anybody else here on QRZ.
Have fun
73
Gary

[N7GH]

I've a friend that loads up his gutters on his house (66 ft long on each side), uses a versatuner 2, and works the east coast (we're west coast) and also the pacific and Russia. There are simple ways to get an antenna to get on the air and do well.

[KO6WB]

Given good conditions anything will work DX or across the country. In comparisons between antennas nearby my QTH the magnetic loop has done very well. How well did the gutter antenna do? A simple dipole works quite well if you take the time and effort to place it properly. It's cheaper then a magnetic loop and easily made. In applications where you cannot do the things necessary for a dipole to do it's best then the magnetic loop fills in well. Mine does very well and if you're going to use DX as a reference then the magnetic loop performs exceptionally well. I've gotten half way around the world and short of moon bounce you can't do much better. It also performs well on the short haul stuff. If you put a magnetic loop too high it's performance drops. If you use the magnetic loop in a horizontal position and not high enough it does poorly. It works best vertical at low heights.
Have fun
73
Gary

[KM1H]

For 40M only whats the efficiency difference between that loop and a 40M shorty dipole such as the Leeson modified Cushcraft which can cover the full band at 1500W + ?

Carl

[WX7G]

The MFJ-933 loop tuner is a good introduction to loop antennas. While these are not "small" loops (defined as less than 1/10 wavelength in circumference) they are still small.

For 40 meters the circumference can be 20 to 33 feet. The 20 footer will work both 40 and 30 meters. Made of 1/2 copper tubing the radiation efficiency can approach 50%. Tuning is sharp and you must tune it as you move about the band.

I bought one to experiment with and am having fun.

[KO6WB]

Hi Carl, let's see....Huh?

Leeson modified Cushcraft which can cover the full band at 1500W +

I'm not certain what W6NL did to a Cushcraft dipole antenna. I do know what he did to the two element beam for 40. So if you could clarify what was done it would make it easier to come up with something approaching an answer. Also, what height are you going to put the dipole?
Currently I can only state that my magnetic loop is just over 90% efficient according to the calculator by KI6GD. The dipole will vary according to height above ground. At 65 to 70 feet up the shorty dipole may do well and the difference between them may be difficult to tell. That's just a guess and that would remain to be seen. Lower the shorty dipole to the height of the magnetic loop and you'll have a contest that will normally favor the mag loop.
The cost of the Cushcraft dipoles that will cover 40 meters isn't what I would call economical but neither is the construction of a mag loop. The dipole requires a substantial vertical support to get it up to a favorable height and that adds to the overall costs. It all depends on how you want to spend your hard earned money.
BTW in no way am I stating a dipole has to be up at a half wave length for it to work, I'm just saying to equal the mag loop you would want to get the dipole up that high.
73
Gary

P.S. The definition most used for a magnetic loop is a circumference of 1/10 wave or less. There are others that state it is much larger. I do know as a fact any loop smaller then a full wave is considered a small transmitting loop and that may be a better name then a magnetic loop. Much more descriptive.

[AC2EV]

Thanks for the great information. Looks like I've got a lot of work ahead of me.