By Bill Powell, W9WRP

additional notes/editing from W7DGJ

You've probably come across various antenna designs that have looked intriguing – we all have. If you are like me, among all those interesting designs there are a few that just seem to be unique enough, and feature laden enough, to stand out in the crowd. My guess is that you’d put the Magnetic Loop antenna into that category.

Whether you’re dealing with limited space, aiming for efficiency, or seeking a portable setup, magnetic loop antennas have a lot to offer.  The neat thing is magnetic loop antennas can be constructed by DIY-ers with a little ingenuity and some basic tools. There are many YouTube videos online detailing how to make them.

[DGJ: I have the Precise RF brand, and the company has had trouble providing me with a replacement part that will bring it back to operational status. As soon as they do, I will review it in a future column. My antenna was purchased from an estate sale and not directly from Precise RF.]

Let's dive into the use cases, advantages, disadvantages, and some technical insights of these intriguing antennas.

Where the Magnetic Loop Can be a Nice Addition

The first use case I think of for the Magnetic loop antenna is for those who have limited space, as these antennas can be a blessing. If you live in an apartment, have a small backyard, or face HOA restrictions, these antennas are a game-changer. Their compact size makes them perfect for small spaces where traditional dipole or Yagi antennas would be completely impractical. They’ve been used on back patios, up in attics and more . . . their ability to conform to small spaces has become legendary. You can keep one in a closet and bring it out when you need it. In short, if you’re restricted somehow by rules or by space -- or even by your spouse's gripes -- this is the antenna for you.

Another use where Magnetic Loop antennas are popular is in portable and field operations. Going out for field day? Heading out for a POTA activation, camping, or enjoying a day at the park? If so, these antennas are lightweight and easy to set up. Their portability allows hams to operate from virtually anywhere.

Lots of SOTA or POTA operators use low power operations and they benefit significantly from magnetic loop antennas. QRP is where their high efficiency and ability to operate effectively on low power makes them a good choice. You can achieve impressive contacts even with just a few watts of power, making QRP operations more thrilling and rewarding.  [DGJ: I’ve done this and it’s a blast. See my column on QRP at this link.]

For use in noisy, urban environments (where noise levels can make reception truly difficult) a magnetic loop antenna may offer you a possible solution. Their design allows for a significantly lower noise floor compared to other antennas, enabling clearer reception in noisy areas. This makes them suitable for city-dwelling hams looking to cut through the urban RF clutter.

Advantages of the Magnetic Loop Design

The first thing you might think of when you are considering this antenna design is that advantage of a very small footprint. Unlike traditional antennas that require significant space and height, magnetic loops are quite compact, and can be mounted on a tripod, a balcony, or even indoors. This compactness doesn't compromise their performance, making them highly efficient for their size.

Another advantage is that despite their small size, magnetic loop antennas are remarkably efficient, especially on the lower HF bands. They can offer performance comparable to much larger antennas, which is particularly beneficial for those operating under space constraints. If you know hams using a Magnetic Loop antenna, ask them how they feel about their efficiency, which is one of the most often-stated plusses for this design.

Also, there is a natural directional pattern in this design which can be used to your advantage. By rotating the loop, you can null out unwanted signals and interference, improving your reception. This directionality also enhances your ability to focus on specific signals, making contacts more reliable.

[DGJ: I’ve seen how this antenna design helps in a pileup, as it is possible to tune the DX station for maximum clarity while “de-tuning” that noisy ham who keeps shouting his call sign every ten seconds from one State over.]

As already mentioned, due to their design, magnetic loop antennas tend to have a lower noise floor compared to other types of antennas. This characteristic is particularly valuable in environments where electromagnetic noise can be overwhelming. A lower noise floor means you can hear weaker signals that might otherwise be drowned out.

There Are a Few Disadvantages As Well

One of the main drawbacks of magnetic loop antennas is their narrow bandwidth. This means that they need to be retuned whenever you change frequencies. While this can be managed with a good tuning capacitor, it requires constant attention, especially if you frequently change bands. Yes, this is annoying if you are a band-hopper, and certainly not the antenna to use if you are operating in a contest where you are constantly forced to tune and re-tune as you change frequencies.

Magnetic loop antennas operate at high voltages, which can be a safety concern. Proper insulation and careful handling are essential to avoid any accidents. This aspect can be intimidating for beginners, who might prefer more straightforward antenna designs. Wikipedia warns us that even a small transmitting loop will not only have a very large current going through it, but also a very high voltage across the capacitor. This is typically thousands of volts – even when fed with only a few watts of transmitter power.

Limited Power Handling is a concern for some. These antennas are typically not designed to handle high power, as most magnetic loop antennas are rated for QRP or low-to-moderate power levels. If you’re a QRO operator, you might find this limitation restrictive, while the ham with an average transceiver (no amplification) would be perfectly happy.

How easy are they to build? Unlike a dipole, EFHW or vertical antenna, building or tuning a magnetic loop antenna can be significantly more complex. The need for precise components, such as a high-quality variable capacitor (and the careful tuning required) can be challenging for those new to antenna construction. For the old hand, it’s a fun but still challenging build.

Technical Information

A magnetic loop antenna consists of a large loop of conductor, usually copper or aluminum, and a variable capacitor. The loop acts as the inductive element, while the capacitor tunes the antenna to resonance. The loop’s circumference is typically around 1/10th of the wavelength of the operating frequency.

A high-Q antenna has a narrow bandwidth, and that means the antenna can be selective and efficient at resonance, but only at that specific point. As a result, magnetic loop antennas often must be retuned when changing frequencies. The upside is the antennas perform very well at the tuned frequency. [DGJ: It’s also worth noting that you can get used to the process and it becomes less obtrusive and much faster than when starting out.]

The radiation pattern of this antenna design is unique. Magnetic Loop antennas typically exhibit a figure-eight pattern. This means that their strength is quite directional with nulls perpendicular to the plane of the loop. By rotating the loop, users can attenuate unwanted signals, thereby enhancing reception and transmission. You can work this unique radiation pattern to your advantage.

If you’ve decided to build a Magnetic Loop, use the highest-quality materials you can as that will be crucial to your success. Copper tubing is commonly used for the loop due to its excellent conductivity (while you’ll see many aluminum loops in the packaged antenna business). The variable capacitor should have a high voltage rating to handle the voltages present at resonance. Air variable capacitors or vacuum capacitors are often preferred for their performance and durability in this design.

The tuning of your Magnetic Loop antenna will be achieved by adjusting the variable capacitor. This changes the resonant frequency of the loop, allowing it to operate efficiently at different frequencies. Precise tuning is critical . . . even slight adjustments can significantly affect performance. Some advanced designs incorporate remote or automatic tuning mechanisms to simplify the process and this will in most cases bring the retail price up considerably. (But if you are installing in an attic, you’d certainly want that feature.)

The efficiency of a magnetic loop antenna depends on several factors, including the quality of the materials used, the precision of the tuning, and the environment in which it operates. Minimizing losses in the loop and capacitor, as well as ensuring proper mounting and positioning, can maximize efficiency.

While magnetic loop antennas are generally designed for low power, some designs can handle higher power levels with appropriate components. Here, it gets quite important to construct the antenna with the highest quality components and with an eye towards safety. Care must be taken to ensure that the components can handle the increased voltage and current without degrading performance or posing a risk to the operator.

Practical Tips

If you’re new to magnetic loop antennas, start with a simple design. There are many online resources and kits available that provide step-by-step instructions. Starting small allows you to learn the basics without getting overwhelmed. Grab your QRP radio and get ready for some fun out in the field or on your backyard patio.

If you decide to build rather than buy, invest in high-quality components -- especially the variable capacitor. A good capacitor is crucial for efficient operation and long-term reliability. Skimping on components like this one can lead to frustration and certainly subpar performance.

But always prioritize safety, especially considering the high voltages involved. Ensure proper insulation and grounding to prevent accidents. If you’re unsure, seek advice from experienced operators or check in with your local club to see if you can find a Magnetic Loop knowledgeable colleague. Sharing experiences, tips, and troubleshooting advice with fellow hams can be incredibly helpful. Clubs and online forums are great places to find support and inspiration.

As always, experimentation is part of the fun and learning process in amateur radio. Don’t be afraid to experiment with different designs and configurations. Adjusting the loop size, shape, and mounting position can yield different results.

In Conclusion

Magnetic loop antennas offer a unique combination of size, efficiency, and directional capabilities that make them a valuable addition to any amateur radio setup. While they come with some challenges, the benefits often outweigh the drawbacks, especially for those with limited space or a penchant for portable operations. Whether you’re a seasoned ham or a newcomer, experimenting with magnetic loop antennas can open new possibilities and enhance your radio experience. So, grab your tools, start building, and get ready to explore the magic of magnetic loop antennas!    Bill, W9WRP

DGJ: Thanks to Bill this month for his excellent contribution. Please note that if you have an idea for an article of value to other hams, QRZ has four times the readership of QST and CQ Magazine combined. We have a group of well known hams who will review the material you submit and I will edit to fit our publication. Get your articles in print! Send to my W7DGJ QRZ email address.

(Thanks also to "grouchyfarmer" blog for his photo of the Mag Loop on his farmhouse!)

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