Thank you Skip for the link to the loop antenna radiation calculator.  I was looking for that, but couldn't find it! Are you operating your Alex Loop in a horizontal position so the null is towards you? Unless a person is operating from a second story or higher, as I understand its best to have a loop antenna in a vertical orientation. I am trying to picture your setup, and how I will do mine when I get a Magnetic Loop Antenna.

Jeff piqued my interest in Magnetic Loop Antennas, and the more I have learned the more interested I have become. Being a high Q antenna, they have a VERY narrow bandwidth, but their OUTSTANDING efficiency, small size, and lack of need for a mast make these antennas VERY appealing to me. I have enjoyed learning about and researching Loop Antennas the last few days! These antennas wouldn’t work well in a contest for Searching and Pouncing, but if Running (which I prefer to do) it would do great!

Their extremely high voltage, current and non-ionizing radiation creates real safety concerns. These issues however are a good sign that the antenna will get me the DX I want! I will simply mitigate the safety issues by securely supporting the antenna and ensuring I and others maintain enough distance from it while transmitting. The challenge before me now is how do I get one!?

Building one is possibly the most affordable option, but sourcing a capable capacitor is a significant challenge! Building the loop using 3/8 inch flexible copper pipe would be a good material as it is relatively thick and could be made out of a single piece. Perhaps cut the ends using a dremel to create flat copper tabs to directly attach the loop to the capacitor.

MFJ’s products have significant workmanship/assembly issues, so I generally avoid MFJ, but they have three High-Efficiency Loop Tuners to choose from that would solve the capacitor sourcing problem. The only difference between the MFJ 933 ($209.95) and 935B ($249.59) is that the 935B includes an Antenna Current Meter. The MFJ 936B ($299.85) is like the others, but it takes up more space, includes an SWR Meter, a Current Meter, and adds 60 Meters. For me, I think the 935B is the best option. The nice thing about going with one of these as opposed to something else is that they are good for up to 150 watts, 10-40 meters, and I can make my own loop. By making my own loop I can experiment with various materials, can have a greater variety of loop circumferences allowing for greater performance and more bands, and I can roll the loop up for greater portability. I do however see two downsides to going this route: 1) The MFJ 1788 Super Hi-Q Loop Antenna ($559.95) comes with a loop that is an inch thick so would significantly improve performance over a loop I could make myself, and 2) The MFJ 1788 comes with a controller that allows the operator to tune the Antenna from afar, something that seems like a very good idea given the extreme voltage and non-ionizing radiation that occurs with these antennas.

The MFJ 1788 is rated for 150 watts, just like the loop tuners, but it is only good for 15-40 meters. Whether the efficiency of having a thicker loop would more than make up for the ability to have loops for each band, I do not know. Aside from the added cost, the other potential problem with the 1788 is that it relies on an electronic tuner and motor, which given that its made by MFJ could fail. Still, the ability to be able to tune the antenna from afar given the amount of radiation that is involved, if cost wasn’t a factor I think I would go for the 1788 over the 935B.

Other loop antennas that were considered: The AlexLoop ($399.95) appears to be very well constructed, comes with a nice bag, and is 10-40 meters. The downside is that it is only rated for 20 watts. Still, given the amount of power Magnetic Loops put out 20 watts isn’t bad, especially if you are using it for portable. The price is very competitive, but given that I want to use my loop antenna for serious contesting in addition to portable operations, the MFJ 935B for me remains the leader.

The Alpha Antenna ($500.00), is another option. It too appears to be a well-made antenna and comes with a bag. It also comes with a small tripod. It is rated at 100 watts for 10-40 meters, and 20 Watts 40-80 meters. It is twice as much money as the 935B, but the advantage to it or the AlexLoop over the 935B is ease of use. The capacitors are smaller and lighter than that of the 935B, making them able to be supported by a small tri-pod. This however raises a very important question, given its larger and heavier size does the capacitor of the 935B perform better? I think the answer to this question determines which wins out, the MFJ 935B or the Alpha.

I think there is only one more antenna to consider, and that is the Ciro Mazzoni Automatic Magnetic Loop Antenna. It is one incredible Antenna, weighing an astonishing 44lbs is isn’t good for SOTA, but it is VERY well made, can be remotely tuned, is rated for 800 watts on 20-30 meters, and 300 watts on 40-80 meters. The rub is that it costs $2,479.99. That is a lot of bread, unless you consider the cost of the land required to erect a comparable 80 meter dipole!    

On Mon, Oct 19, 2020 at 12:17 PM Skip - K6DGW <k6dgw@...> wrote:

Jef points out a couple of advantages [and disadvantages] of small transmitting loops.  The circulating current will be high with any appreciable power.  The radiation resistance is in the milliohm ranges, and any resistance in the loop will dramatically lower the efficiency.  It is tempting to build a loop as an octagon with copper pipe since the fittings are readily available, however the added resistance in the soldered joints can be as much or more than the radiation resistance, cutting the efficiency in half or more.  Silver solder will ameliorate this a little, but a continuous length of copper pipe is much better.  The voltages across the tuning capacitor will rise dramatically with higher power, well into the kilovolt range, requiring vacuum capacitors.  Remembering that the RF current travels on the surface of the loop and is excluded from the inner region, the larger the pipe the better.

There are some decided advantages too.  The primary coupling mode for small loops is via the H-field and as a result, they tend to be insensitive to "ground" and surrounding objects so long as those objects have a permeability close to 1.0 ... I sit under my Alexloop which is on a tripod, and the tuning capacitor is just over my head.  There is little if any advantage to putting a small loop on a tower.  The loop is broadly bi-directional in the plane of the loop and the two nulls [orthogonal to the loop plane] are quite sharp.

Most loops use magnetic coupling, feeding the power through a smaller loop inside the main loop.  It is a resonant transformer, and the key word there is "resonant."  The loop must be operated exactly at resonance for power transfer to take place.  A 1:4 or 1:5 ratio between the diameter of the feed and main loops will provide close to a 50 ohm load to the transmitter.  The process is, 1) Bypass the ATU; 2) At low power, tune the loop for zero reactance; 3) Engage the ATU and let it find a match.  "Getting the loop close and letting the ATU match the rest" is a recipe for a very effective dummy load.  A phase detector is a real advantage, just tune the loop for zero phase difference between the voltage and current.  In fact, that's what is usually used for automatic, motor-driven vacuum capacitors on transmitting loops.

https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx is one of many sites with small magnetic loop calculators.  Try a few designs out and you'll see that, at 100 W, you need to keep your distance! <K6DGW>

Jef points out a couple of advantages [and disadvantages] of small transmitting loops.  The circulating current will be high with any appreciable power.  The radiation resistance is in the milliohm ranges, and any resistance in the loop will dramatically lower the efficiency.  It is tempting to build a loop as an octagon with copper pipe since the fittings are readily available, however the added resistance in the soldered joints can be as much or more than the radiation resistance, cutting the efficiency in half or more.  Silver solder will ameliorate this a little, but a continuous length of copper pipe is much better.  The voltages across the tuning capacitor will rise dramatically with higher power, well into the kilovolt range, requiring vacuum capacitors.  Remembering that the RF current travels on the surface of the loop and is excluded from the inner region, the larger the pipe the better.

There are some decided advantages too.  The primary coupling mode for small loops is via the H-field and as a result, they tend to be insensitive to "ground" and surrounding objects so long as those objects have a permeability close to 1.0 ... I sit under my Alexloop which is on a tripod, and the tuning capacitor is just over my head.  There is little if any advantage to putting a small loop on a tower.  The loop is broadly bi-directional in the plane of the loop and the two nulls [orthogonal to the loop plane] are quite sharp.

Most loops use magnetic coupling, feeding the power through a smaller loop inside the main loop.  It is a resonant transformer, and the key word there is "resonant."  The loop must be operated exactly at resonance for power transfer to take place.  A 1:4 or 1:5 ratio between the diameter of the feed and main loops will provide close to a 50 ohm load to the transmitter.  The process is, 1) Bypass the ATU; 2) At low power, tune the loop for zero reactance; 3) Engage the ATU and let it find a match.  "Getting the loop close and letting the ATU match the rest" is a recipe for a very effective dummy load.  A phase detector is a real advantage, just tune the loop for zero phase difference between the voltage and current.  In fact, that's what is usually used for automatic, motor-driven vacuum capacitors on transmitting loops.

https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx is one of many sites with small magnetic loop calculators.  Try a few designs out and you'll see that, at 100 W, you need to keep your distance! <K6DGW>

“I understand there is no free lunch, but I was unaware that one may choose two  of the three, desires, small size, efficiency, and bandwidth."

"Please tell me, Teacher, if I am willing to sacrifice bandwidth to obtain favorable efficiency and small size, what antenna(s) would be good for me?"

Aaron, K6ABJ

> On Oct 17, 2020, at 10:44 AM, Jef - N5JEF <jef@...> wrote:
>
> here is No Free Lunch, and for any antenna design one may choose two, but not all three, of small size, efficiency, and bandwidth."
>
> "But tell me, Teacher, what should I do?"