160/80/40 Meter Magnetic Loop Antenna

1. OVERVIEW

This post relates to the design and build of a 160/80/40 Meter magnetic loop antenna. Loop antennas have always fascinated me. They can be highly effective as well as portable.

A friend, WA7YLI, has similar interests. He commented to me that, even if a loop is only 10% efficient, that is only down 10 dB - about 1 1/2 S units.

My initial thoughts were to build a loop for 40 meters but, after researching K8NDS's design for a helically loaded magnetic loop, I have decided to build my first antenna patterned after his design.

2. REVISIONS

1. 03/19/2016 - Initial creation of this document. Added specifications, preliminary discussions, and references.
2. 03/20/2016 - Revised design specs to reflect the K8NDS design.

3. SPECIFICATIONS (PRELIMINARY):

1. Frequency band: 160/80/40 Meters
2. Selected Design: Helically Loaded Fractional Wavelength Magnetic Loop, after K8NDS design[4].
3. Size: 4 feet diameter to 9 feet diameter, depending upon available materials, cost, and efficiency
4. Large loop material: helically wound copper strap
5. Inner loop material (if used): 1/2" coax or copper tubing
6. Efficiency: 20% minimum
7. Bandwidth: 2.5 KHz minimum
8. Power: 600 watts maximum

4. DISCUSSION

On my property, I don't have gigantic pine trees and have no tower. While I could put up a tower, it would dominate the neighborhood and is not something I want to do from both a cost and good neighbor point of view.

The Loop Skywire antenna[2] I use for daily work works well for my purposes but it suffers from not being up as high as it should be to be most effective.

This is where the magnetic loop really shines. While the Loop Skywire or dipole is more efficeint that a magnetic loop, that holds true only if they are up at the proper height. If they are not, the magnetic loop is likely to be more efficient. The magnetic loop advantages readily become apparent in the first two pages of this paper[1].

If they are, in some situations, so much more efficient, then why don't more hams use them? It is because, being small, they have a much higher Q factor than standard antennas and, therefore, must be tuned, even for short moves across the band. This can be difficult to achieve since the tuning must take place at the antenna if losses are to be kept low.

There are hundreds and hundreds of articles about magnetic loops. Rich, K8NDS, has pioneered the design of a helically loaded fractional wavelength magnetic loop. Two key references for his loop designs are [3] and [4]. Rich informed me that he has tried many different methods of driving the loop and that his favored matching method is the gamma match. Given his success, I will adopt the gamma match for the initial design. [5]

5. DESIGN NOTES

This section provides detailed design notes for each element of the antenna.

5.1 Loop Coil Form

The helical windings of the loop are wound around a coil form that is circular in shape. In short, there are no Hula Hoops that are 8.5 feet in diameter. The K8NDS design uses 3.5 inch OD corrugated plastic sewer pipe. A Google search turns up these sources:

1. 4 in. x 100 ft. Corex Drain Pipe Solid
2. 3 in. x 100 ft. Corex Drain Solid Pipe

Item (2) lists a nominal OD of 3.5 inches so this appears to be a solution. This design uses a what I call a "floppy hula hoop" design and requires an octagon support mechanism. One wonders if there are alternatives:

1. Octagon Support, per K8NDS
2. Aluminum, wood, or plastic strap supporting the corrugated pipe

A strap of some low loss material could be beneficial to supporting the corrugated pipe. This might allow construction without using an octogonal support frame used by K8NDS.

5.2 Copper Conductor

The bane of conductors carrying RF current is skin effect and, as the frequency goes up, the skin effect becomes worse. There are three configurations that I will describe here:

1. Solid copper wire - the worst possible choice for loop material because the RF current does not penetrate very deeply into the wire so, in order to reduce the overall resistance, wire size must increase.
2. Copper tubing - this is a better choice that solid copper wire because it eliminates the unused copper in the center of the conductor. In most cases, copper tubing is far more expensive that copper wire of the same diameter.
3. Copper strap - essentially, this is a "flat" copper wire wherein the area has been spread into a rectangular area rather than a round cross section. In this way, the thickness can be reduced about as far as one wishes up to the limits imposed by wire resistance. Finally, 

In the K8NDS design, 3" x 0.008" copper strap is used with excellent results. The perimeter of this rectangle is 6.016 inches. This is the equivalent of a 1.92" diameter wire, i.e., about the same as a 2" diameter copper tube. Sources for copper strap are:

1. COPPER STRAP, 3" x .012" x 100 feet    $198.00 ($1.98/foot)
   http://www.gacopper.com/012-CopperStrap.html 
2. 3" X 216'/ 10 mil/ 25lb Copper Roll $417.99 ($1.94/foot)
   http://basiccopper.com/10-mil-010-inch-.html

Other sources will be listed as they are found.

5.3 Matching Network

The loop will be matched to the feedline using a traditional gamma match.

5.4 Feedline

The feedline will be 50 ohm coax, type TBA, with PL259 connectors.

5.5 Rotor

If the sharp null of the antenna is to be useful, then the antenna "hole" should be oriented horizontal. Otherwise, the antenna could be mounted "flat" with the loop in the horizontal plane. If this were done, no rotator would be necessary.

It has been noted by Jeff NH7RO and Rich K8NDS that mounting the antenna in the horizontal plane will be exceedingly lossy unless the antenna is at least a 1/2 wavelength above ground. There are supporting statements to this rule listed in the references. [2, pg 21]. Therefore, in accordance with the advice given and credible references, the loop will be mounted with the loop in the vertical plane.

To Be Continued....

6. FODDER (pasted clips for integration into the design)

7. REFERENCES

1. "The Loop Skywire, " Dave Fischer W0MHS, QST November 1985, p20, http://srgproperties.inetusanow.net/files_custom/9467_2192.pdf 
2. An Overview of the Underestimated Magnetic Loop HF Antenna, Leigh Turner, VK5KLT, October 2015, http://srgproperties.inetusanow.net/files_custom/9467_2192.pdf
3. "Stealth Antennas for the Radio Amateur," Rish Fusinski K8NDS, http://antennastealth.com/
4. "Helically Loaded Magnetic Loop Antenna," Rich Fusinski K8NDS, http://www.hlmagneticloopantennas.com/ 
5. "An Examination of the Gamma Match," D. J. Healey (SK) W3PG, http://www.bcpink.com/KB3IFH/files/Download/Gamma%20Match%20article.pdf
6. Magnetic Loop Parts List, https://goo.gl/Sfcymt
7. WY7V Magnetic Loop Parts List
8. https://www.qrz.com/db/KV4PN
9.