TheRadioBoard

There's another effect as well:

The inductance of the loop antenna (or any coil for that matter) is a function of:
i. The self inductance of each turn;
ii. The mutual inductance between each turn and every other turn.

The self inductance of one turn is a function of wire diameter, but the mutual inductance between different turns is not. The mutual inductance contribution gets smaller as the number of turns decreases. And, the self inductance contribution becomes larger as the number of turns decreases.

Therefore, the wire diameter has more effect on loops having just a few turns, especially if they are well spaced (because this reduces the mutual inductance effect even more), and not as much effect on loops having many turns.

Since Loop antennas fall into the category of "few turns and well spaced," the wire diameter does have a bigger effect on these, and that's why these formulas and calculators that don't take wire size into account tend to be more inaccurate when calculating loop antenna inductance than those that calculate more standard coils geometries.

It appears that the person who set up that calculator misinterpreted the wire diameter as turns spacing. The original source of that calculator is here:
http://emclab.mst.edu/inductance/rectgl.html
and it asks for wire diameter, not spacing. However, to produce accurate numbers it should ask for both turn spacing and wire diameter.

Good info Bob.

I wondered about that calculator. It didn't come close to the inductance of my (circular) loop trials unless I used the total winding width in place of wire diameter. The formula on your site was much closer.

73, Dan

It's likely that the formula used in that calculator is accurate for some range of coils that the EMC lab was interested in, but without knowing what that range is, it's impossible to say whether it's applicable to your own project. On the EMC site there is a main page for the calculators which attributes several of the formulas to Grover, but this one doesn't appear in his book. It may be a simplified version of one of Grover's formulas though.

I compared the results of that calculator with a more general formula that's accurate for any rectangular coil or loop, and got results that were, in some cases, half the value of the EMC calculator. The general formula does come from Grover's Inductance calculations book, and the formula is traceable back to the original derivation, so I would trust it more than that calculator. I'd post the formula, but it's nearly a full page of hyperbolic trig functions. I may put it up as a calculator on my site, since there doesn't appear to be one on the internet that accounts for both wire size and spacing.


<<Edit>>
Had a bit of spare time, so I put this together:
http://electronbunker.ca/InductanceCalcRc.html

Sorry for the late reply (miserable cold) but I am going to try to make a loop for AM DX. Some of these formulas are more than I understand, but I will play with some of this info to get something working soon. Some I have tried just will not do what I thought they would. I guess I will just have to keep trying.

Thanks to all of you knowledgeable guys.

SWsenior

A tip, based on ancient experience. Making a loop that will tune 530-1710 with a broadcast-radio variable cap requires designing for as little self-capacitance as possible. Wide turn spacing, for one. In the U.S. and Canada, the high end of the band is prime territory for DXing, as there aren't as many stations on those frequencies (except for the blasted Travelers Information Stations). Sometimes, more turns on a smaller frame is easier to manage, because you can prune or add a turn without having the gross effect you get when you do that on a larger frame with fewer turns. The alternative is to design so your variable hits the high end, then switch in some fixed C to make it to the low end, but that's a nuisance.

If possible, try to arrange a tilting mechanism. For a hula hoop you could clamp the hoop, rather than drilling into it. Tilting can be very effective in nulling. It's a fine thing to be able to null a pest into the noise!

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Bob -- incredible job!! Thank you!!

SWsenior -- study & use Bob's calculators from his site referenced above and you'll be in good shape.

73, Dan

I now have a working loop that is 20" in dia. I used 5/8" plex and scramble wound 20 turns of #22 ga magnet wire. I also used a 5/8" cpvc coupling to attach the ends together. At first I had some difficulty feeding the wire through but after a cpl tries, I found that using needle nose pliers and bending over the feed end of the wire made it easier winding. I plan on mounting it on a base with a lazysuzan bearing to allow for easy rotation. I have attached a 166pf variable capacitor and am able to tune it and bring in stations louder. Thanks to all for the suggestions and help.

SWsenior

How are the nulls? For me, the best part of using a loop is nulling undesired stations and hearing something new arise on the frequency. Almost seems like magic.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

To be honest, I haven't checked that. I'll try it tonight and post a reply of how it nulls out a station.

SWsenior