HOMEBREW / DIY - Let's talk about magnetic loops
...Just a couple of things to know to get your mag loop working. -by 1SFØ72
•This document is about fundamental theories of Small Transmitting Loop antennas (STL) and focuses on capacitors construction.
Let me start by saying that I don't have the haughtiness to teach you the "who-knows-what" secrets of a perfect STL (Small Transmitting Loop) construction but that day I came across mag loops I wanted to make one soon and I realized that these antennas are often underestimated; a mag loop may be the difference between being on the air - expecially on lower bands, or not being operational at all if you have no room for large HF antennas. In addition, mag loops may perform surprisingly good on reception because of their well-known rejection capabilities of nearby electric fields. A very high Q is obtained when the loop gets resonant at the frequency of interest, which means overall selectivity and high suppression of QRM or intermodulation effects of adjacent strong signals. However, the flip side of the coin is that you have to retune your STL at every small frequency change because of its very narrow bandwidth, which is the price to be paid for having a high Q factor. Furthermore, a mag loop is always a compromise between efficiency, bands of use, build quality, dimensions, ...and even its actual behavior on the air, yeah.
In fact, to be considered properly "magnetic" STLs should have a circumference between 1/8 and 1/4 wavelength because it was experimented that in that "range" we achieve uniform current distribution throughout the loop. The ideal length of outer conductor is generally fixed at 1/4 wavelength for the highest frequency of interest. This is the situation where we approach to maximum obtainable efficiency (assuming we have good positioning and high build quality with minor losses). At lambda/4 we get a slight increase of current flowing where we have low impedance (at the point opposite the capacitor) but our antenna will radiate more because we have maximized radiation resistance. Obviously things begin to change as we start moving to lower bands; efficiency is going to decrease more and more because the greater the wavelength, the more the loop length gets far from being optimal for that frequency. However, the overall conditions remain still acceptable as long as transmission wavelength won't get greater than lambda/8; if we went further, our loop would become too small. That's why for example, a mag loop optimized to work on 10 meters (circumference = lambda/4) will work with reasonable efficiency also on 20 meters (loop now is lambda/8). In relation to a given frequency and current distribution, if loop length were greater than lambda/4, you'd theoretically need less capacitance to obtain resonance but your loop would stop performing as a "magnetic" antenna. On the other hand, if loop length were less than lambda/8, you'd be forced to resonate your antenna using a greater capacitance but it would dramatically lose efficiency and if your loop were actually too small... your antenna would be useless.
Now, long story short, for a given frequency, the greater the loop length, the greater the radiation resistance (because of a larger loop area) and the greater the radiated energy. Furthermore, the greater the loop length, the smaller the required capacitance to tune your antenna and obviously, vice-versa, but bear in mind, you have some design limits fixed by the laws of physics and therefore, you can't actually make your loop too large or too small or your antenna won't work.
I'm assuming you have now all the basic information you need to build your own STL; you must shape copper or aluminum tubing into two different circles that will be the outer and the inner loop of your antenna. The inner loop has a diameter of 1/5 that of the outer loop. The outer loop length (circumference) is strictly related to the band you want to optimize it for and should be sized at lambda/4; your antenna will work good over twice its main wavelength. (10m to 20m, or 20m to 40m or again, 40m to 80m...)
It remains the most important part of all mag loops, the variable capacitor. It must stand voltages of several kilovolts at resonance, and must have a minimum capacitance of a very small value.(by the way, *NEVER* touch it when transmitting!). Vacuum capacitors are the best solution for our STLs but they're pretty expensive and fragile too. An air variable capacitor will be OK, but it's rather hard to find one that has adequate capacitance and desired plate spacing so probably you will have to build one yourself. Do not use materials with ferromagnetic characteristics, use brass, aluminum, or copper instead. I realize that many of you readers would have appreciated a "ready-to-use" project to download from here and follow as per the instructions but I don't think it's a good idea. I also realize that many of you now may be disappointed but please consider one thing, not all mag loops are made the same!
They may be constructed of various diameters for different needs and different bands, hence I won't provide you with precise step-to-step instructions to make your capacitor identical to mine because your antenna probably requires a different one. Despite this, making your own mag loop is very straightforward if you have a clue on how to proceed and know at least some basic theories on magnetic antennas (that's what this document is for). This is also true for capacitors, but depending on your loop diameter and characteristics capacitor specs must be calculated first, so have a look here because plate spacing, capacitance range and plate dimensions must be determined prior to proceed.
Use the application you downloaded and start designing your capacitor. A butterfly capacitor is electrically like a series circuit of two capacitors, and therefore it's composed of two identical sections having twice the capacitance each. In a butterfly capacitor the overall capacitance becomes half and so does the charging voltage in each section. That's the reason butterfly capacitors can stand over twice the voltage compared to other types of variable capacitors. A butterfly capacitor is by far the best choice over any other common rotary model because the former uses no wiper contacts and that means no losses and heat because of high RF currents flowing through the rotor shaft. Once you have determined the overall capacitance for your antenna along with the number of needed plates and their exact dimensions, you'll be ready to draw them on paper (use your pc!) and later, on an aluminum sheet. Avoid to cut by hand all pieces you need (like I did) because you won't have enough precision and it's rather hard and time consuming.
I made a large buttefly capacitor that has a capacitance range of 15pf-120pf and it's good for 10m-20m if used in conjunction with a loop length of roughly 2.80 meters (110in). I made it to replace my previous (and smaller) capacitor, which was mounted on a copper loop (made from a 3m length of RG213) and absolutely unsuitable for QRO transmissions. Parts assembly at times was not that easy but I had fun. I took pictures then made a short video of what I did, so have a look and...
...Just see how I made it!
I wish I had made a better capacitor, because I read a minimum capacitance of 15pf on my L-C meter that's a little bit high, but it's hard to do any better if you cut all plates by hand. However, I'm sure that you will do better than me! I'm also aware that each group of plates should have been welded and not simply bolted together in order to minimize losses. Unfortunately, in mag loops radiation resistance remains a critical paramenter to deal with because it's always too small (lower than any other antenna) and therefore, it's never a good thing to add more losses. However, my capacitor works and performed good in all the experiments I made; overall... it was worth the effort. Works to complete my new mag loop are still in progress and the best is yet to come!
73 folks, have a good time.
All the best,