Magnetic loop questions.

[M6CLS]

Hi, I am wanting to construct a Magnetic loop for HF and have a few questions.

1. Does it matter if the loop is square as I will be using a wood frame and copper wire ( My old MW Receiving loop will be used)

2. And if the main loop is square should I have the smaller loop circular?

3. Does the size of the loop matter? My frame is around 83cm2 but I can increase the size if needed.

4. I'm going to be using a 2 gang 490/500pf tuning capacitor using just the 500pf side is this recommended?

I understand the coupling loop should be 5 times smaller then the main loop. Also it will be used for SWL but perhaps some QRP later.

Thanks Carl.

[WD8KNI]

Carl there are literally hundreds of posts on loops here, a search will give you much. 

As you most likely know the idea of a small loop it that it works as a tuned series resonate circuit, so currents are very high at resonance, depending on transmitter it power could be in the order of hundreds of amps, so wire size is important, the larger the better, most use copper .75 to 1.5" in size.

This also means that the capacitor is also important.  Since you will be only receiving current is not an issue as much as resistance is. Builders typically strive for resistance in the micro ohms.

If you search this forum you will find hours of reading on small loops.   
Regards.. Fred 

[WX7G]

Hi, I am wanting to construct a Magnetic loop for HF and have a few questions.

1. Does it matter if the loop is square as I will be using a wood frame and copper wire ( My old MW Receiving loop will be used)

Small loop radiation is proportional to the loop area and loss is proportional to the loop circumference. A circle has the highest area-to-circumference ratio. A square having the same area will have 4/Pi times the loss.

2. And if the main loop is square should I have the smaller loop circular?

I suspect a square coupling loop would be better. 

3. Does the size of the loop matter? My frame is around 83cm2 but I can increase the size if needed.

The maximum circumference for a "small loop" is 1/10 wavelength.

4. I'm going to be using a 2 gang 490/500pf tuning capacitor using just the 500pf side is this recommended?

It depends on the frequency and the loop circumference. This can be calculated or modeled with the free version of EZNEC (and other NEC simulation programs).

I understand the coupling loop should be 5 times smaller then the main loop. Also it will be used for SWL but perhaps some QRP later.

The "wire" diameter drives the loss and copper or aluminum tubing is best.

Thanks Carl.

[GILGSN]

A 500pf air variable capacitor will make your loop very touchy... Mine is a 6ft. diameter loop with a 9-100pf capacitor, and is already very sensitive to tune. If you need 500pf, I would suggest using a multi-turn vacuum capacitor. Otherwise, you might want to use a 100pf air variable with a regular high voltage capacitor in parallel...
There are many ways of feeding a loop, see the option I used:

http://radiopreppers.com/index.php?action=dlattach;topic=180.0;attach=148;image

Gil.

[K4SAV]

A 500 pf cap is much too large for a mag loop.  If you get a configuration and frequency where you can use 500 pf, the loop efficiency will be very low, probably less than 1%.  Also the tuning will be just about impossible without a very large gear reduction. 

The spacing on most 500 pf caps is not sufficient for a mag loop even at QRP levels.  The cap voltage varies with all the design parameters, but with a low loss loop it's possible that the cap voltage can be over 1 KV at some frequencies at 5 watts.

The capacitor is the most critical item in a mag loop design.  A poor cap can cause more loss than all the other things put together, and this is often overlooked.

Your 83 cm-sq loop is too small for decent efficiency below 15 meters.

Here is an easy to use mag loop calculator that will let you play with the loop parameters to estimate performance.
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls

Also see:
http://www.aa5tb.com/loop.html

Don't forget to estimate the resistive loss of the wires connecting the capacitor to the loop plus some additional amount for the path thru the cap, and plug that number into the calculator.  To give you a feel for that number, the ac plus dc resistive loss (mostly ac) of 4 inches of #12 wire at 14 MHz is about 0.016 ohms.

Jerry, K4SAV

[GILGSN]

A 500 pf cap is much too large for a mag loop.  If you get a configuration and frequency where you can use 500 pf, the loop efficiency will be very low, probably less than 1%.

Jerry is absolutely correct. My 30/40m loop has a 20.66 ft. perimeter, and my 15-20m loop will be a 10ft. perimeter loop. This way, the cap stays under 100pf and efficiency is great...

Also look for the RJELOOP1 program, which I think works best...

Check out this page as well: http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm

Gil.