I have been making and using mag loops for a while. These include fixed loops and portable loops. I learned much from John's presentation that is not available from other sources. The major challenge in building and using small transmitting loops comes from having a convenient way to tune them. John's idea of using a coaxial capacitor is novel and very interesting. These loops have a very high Q-factor. Tuning is also affected by temperature, humidity and other factors. Even if you tweak the loop for one frequency, it will need readjustment in at most a few hours. Pulling on paracord does not seem to be a convenient way to tune the loop accurately to resonance. Some form of motorized tuning mechanism using a lead screw or mechanical actuator would be more practical and allow you to tune/retune the loop from the operating position. The loop will generate high voltages when transmitting. Making sure that the operator is insulated from the capacitor needs to mentioned as a matter safety. Lastly, I would be interested in John's opinion, as well as the opinion of others as to the merit of feeding the loop at the capacitor as done with the MFJ-933C and its variants. Finally, in my experience, tuning the loop is best done with a NanoVNA displaying simultaneous VSWR curves and a Smith Chart.
Nice presentation and thank you for making these videos. I feel one area of optimization that is often overlooked is "portability." This is to say a mag loop design that is easily broken down and reassembled for portable field operation. Weight, size and other considerations such as the construction materials used. I think the objective of optimal portability is easier to achieve if you construct these antennas into the shape of a square, rather than a circle. - 73
The larger a loop gets relative to the wavelength the less directional it becomes. Thus the smaller loop diameter to make your loop more directional, so it helps you to null away from noise, and it makes your transmission more directional, meaning higher radiated power in the direction you want to radiate in. My question is, what was the antenna Q, of John's loop, i.e. what was the bandwidth at the 3 dB point, or 6 dB point down on each covered band? I would like to get mine as close to the bandwidth of a SSB signal as possible in order to reduce received noise and improve xmit efficiency.
The part where you use a inner loop arm to adjust the so called induction. Is there a multi wrap of coil wire going up the shoulder of the right vertical PVC pipe? Or is the adjustment clamped to the foil wrap itself? Wish we had close ups of that part.
As asked (and not answered) on another video, how do you attach the coax to the "auto transformer" loop? In addition, is there a way of including 40 meter tuning in the sliding capacitor?
The shape with the largest area with the lowest conductor loss is the circle. Not these other shapes. That is optimal. Because of the conductor diameter is the deviation. The discussion we want is how to make a large conductor jointless circle out of copper pipe. Because it would be optimized in all aspects but one. $
You say that the interior area enclosed by a loop is very important. While a square loop is easily constructed by a home builder a circle encloses the maximum area for a given perimeter length. For example a circular loop using the same perimeter length as a square loop encloses 25 percent more area than the square loop.
From experience, you scale it up the length of the loop's perimeter / circumference. I have found that the loops I have built have been good for around an octave of spectrum, so from 14-28MHz or 7-14MHz. If you take the upper frequency, you can tune reliably, when you halve the frequency, that's about as low as you want to go before the efficiency drops off significantly If you particularly want 80m, and are happy to use another antenna for other bands, I would go for a 1/6 wavelength, which is around 13m circumference as a start. I have an 8m loop for 7MHz and it works really well. Haven't gone for 80m just yet
I have been making and using mag loops for a while. These include fixed loops and portable loops. I learned much from John's presentation that is not available from other sources. The major challenge in building and using small transmitting loops comes from having a convenient way to tune them. John's idea of using a coaxial capacitor is novel and very interesting. These loops have a very high Q-factor. Tuning is also affected by temperature, humidity and other factors. Even if you tweak the loop for one frequency, it will need readjustment in at most a few hours. Pulling on paracord does not seem to be a convenient way to tune the loop accurately to resonance. Some form of motorized tuning mechanism using a lead screw or mechanical actuator would be more practical and allow you to tune/retune the loop from the operating position. The loop will generate high voltages when transmitting. Making sure that the operator is insulated from the capacitor needs to mentioned as a matter safety. Lastly, I would be interested in John's opinion, as well as the opinion of others as to the merit of feeding the loop at the capacitor as done with the MFJ-933C and its variants. Finally, in my experience, tuning the loop is best done with a NanoVNA displaying simultaneous VSWR curves and a Smith Chart.
Nice presentation and thank you for making these videos. I feel one area of optimization that is often overlooked is "portability." This is to say a mag loop design that is easily broken down and reassembled for portable field operation. Weight, size and other considerations such as the construction materials used. I think the objective of optimal portability is easier to achieve if you construct these antennas into the shape of a square, rather than a circle. - 73
The larger a loop gets relative to the wavelength the less directional it becomes. Thus the smaller loop diameter to make your loop more directional, so it helps you to null away from noise, and it makes your transmission more directional, meaning higher radiated power in the direction you want to radiate in.
My question is, what was the antenna Q, of John's loop, i.e. what was the bandwidth at the 3 dB point, or 6 dB point down on each covered band? I would like to get mine as close to the bandwidth of a SSB signal as possible in order to reduce received noise and improve xmit efficiency.
The part where you use a inner loop arm to adjust the so called induction. Is there a multi wrap of coil wire going up the shoulder of the right vertical PVC pipe? Or is the adjustment clamped to the foil wrap itself? Wish we had close ups of that part.
As asked (and not answered) on another video, how do you attach the coax to the "auto transformer" loop? In addition, is there a way of including 40 meter tuning in the sliding capacitor?
The shape with the largest area with the lowest conductor loss is the circle. Not these other shapes. That is optimal. Because of the conductor diameter is the deviation. The discussion we want is how to make a large conductor jointless circle out of copper pipe. Because it would be optimized in all aspects but one. $
You say that the interior area enclosed by a loop is very important. While a square loop is easily constructed by a home builder a circle encloses the maximum area for a given perimeter length. For example a circular loop using the same perimeter length as a square loop encloses 25 percent more area than the square loop.
I sent an email to John, hope he answers....smile...a question about building one of these
what can I do to make this loop tune to 80 meters?
From experience, you scale it up the length of the loop's perimeter / circumference. I have found that the loops I have built have been good for around an octave of spectrum, so from 14-28MHz or 7-14MHz. If you take the upper frequency, you can tune reliably, when you halve the frequency, that's about as low as you want to go before the efficiency drops off significantly
If you particularly want 80m, and are happy to use another antenna for other bands, I would go for a 1/6 wavelength, which is around 13m circumference as a start. I have an 8m loop for 7MHz and it works really well. Haven't gone for 80m just yet
Why did you cut the camera in the middle of his answer? Disapointing.
always stimulating listening to an educated ham