[Elecraft] Magnetic Loop Performance
Morgan Bailey
mbaileycrna at gmail.com
Fri Oct 28 16:18:59 EDT 2016
First off, Is your loop 48 inches diameter or total circumference of 48
inches?
Second, is your cap a butterfly cap. Those camps make a huge difference in
performance. Although I have successfully used a double ganged cap with
excellent results.
Using a toroid to connect the loop with the turns on the toroid and the
pipe going through the loop may be a better set up.
Spacing on the coil turns and contact with the loop will change
dramatically and I would highly recommend a faraday loop for
feeding/driving the loop. make this coil/loop a circle with diameter 1/5
the diameter of the loop. For a 48 inch diameter loop that would be about
9.5 inches diameter. My loop worked great with a 1/4 inch copper tube 1/5
the diameter of the main loop fed directly across the coax, that is, braid
on one side center on the other. I adjusted the matching by moving the loop
up and down on the central axis. It worked great and would take 350 watts
with out heating up, providing the resonant frequency of the loop was
matched with the capacitor. My cap was rated at 2500 voltage break down.
You might consider a gamma match with your loop, Although this will make
the loop favor one side over the other but only marginally.
Personally, I have never had much luck with just turns on wire on the loop
to make it work. I know it is in the literature but it never worked
repeatability for me because of the variability of the construct.
I never used a 1to1 balun on any of my loops. And never got kissed by RF
coming back at me. I could see that a wire wound choke balun may introduce
inductance in the match and cause problems where as a bead balun may not
cause problems.
Also you are expecting way to much from this loop. Loops only work well for
1 maybe 2 bands at a time. This is limited by the variability of the
capacitor and the inter capacitance of the construct. As you go up in
frequency your need way way less capacitance and soon your loop will be
unmatchable. Your 48 inch loop makes the conductor length of 12.5 feet.
This is way big for 15 meters because it is greater than a 1/4 wave at that
frequency and loops only function as a loop if they are a fraction of a
quarter wave in frequency. You loop, if it is 48 inches in diameter is
ideal for 30 and 20 meters and with the right capacitor will be usable on
40 with reasonable efficiency. A 2.5 foot diameter loop for the high bands
is more in the ideal range for 20-10 meter operation. Because the total
circumference of the loop is only 8 feet which is shorter than a 1/4 wave
at the lowest frequency, 28 mhz. A loop this size will best work on 15-10
meters and will be matchable to work with decreased efficiency on 20. It
calculates to about 48 % on 20 with all the other values much higher in
efficiency as the frequency goes up. The Problem is with the matching
capacitor minimum and maximum values and the intrinsic capacitance of the
total system changing the resonance. Finding capacitors has always been the
weak point in Loop construction. Vacuum variables with high Q values are
the best widest frequency solution and can run the most power to the
antenna because of their high voltage break down. The problem is the
fragility and the cost factor of the capacitor. I shopped EBAY and found
air variable double gang or butterfly caps that would work over a wide
value of frequency for pennies compared to the dollars spent on vacuum
variables, and for less than 100 watts, worked quite well.
This website has helped me immeasurabley when I was constructing loops. For
me this was the best calculator and guide to construction.
http://www.66pacific.com/calculators/small_tx_loop_calc.aspx
Vy 73,
Morgan Bailey NJ8M
On Mon, Oct 24, 2016 at 7:22 PM, johnpierce <ja-pierce at verizon.net> wrote:
> I could use some help with a MagLoop. I constructed a 48" loop from 3.8"
> copper pipe. I have a 51 pf capacitor across the open end of the loop. I
> am feeding the loop with a 4 turn coil around the copper pipe opposite of
> the capacitor. Before checking the loop I tested a carbon resistor on the
> balun (ant side). Using a 50ohm resistor on the ant side of a 1:1 Balun
> which was fed with a 2' piece of 50 ohm coax and with the loop disconnected
> I get the following readings from a MFJ 259B antenna analyzer:
>
>
>
> Freq Resistance X
>
> 9.7mhz 66ohm 0
>
> 10 68 0
>
> 12 77 0
>
> 14 89 0
>
> 17 109 0
>
> 21 126 0
>
> 27 128 0
>
> 27.6 78 0
>
>
>
> Reattaching the feed coil and the balun I get the following readings
>
> Freq Resistance X
>
> 9.7mhz 18ohm 77
>
> 10 20 81
>
> 12 43 119
>
> 15 163 203
>
> 16 303 282
>
> 16.5 406 120
>
> 16.56 440 0
>
> 17 547 0
>
> 21 >650
>
> 21.3 467 114
>
> 24 95 191
>
> 27 38 129
>
> 27.6 34 120
>
> 29.1 21 104
>
>
>
> I ran all these test trying to find out what the resonance is for my 51pf
> capacitor. Rather
>
> than helping me, I am more confused. I was not finding anything like the
> normal response
>
> I have been led to believe I should experience with a magloop. E.g. peek
> noise at the resonance frequency.
>
>
>
> John
>
> AD2F
>
>
>
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