TheRadioBoard

http://s13.postimage.org/xqq53p0k7/hooploop.png

Here is my attempt at calculating a Hoop Loop. I would be very appreciative if you would look it over.

I used mini Ring Core Calculator 1.2 to do my inductive calculations.

L1 is a 18", 4-turn loop, spaced out about to 1/2" total which is 20uH. This equals the inductance of L2 which is the center tapped winding of T1. Given a 25:1 impedance ratio the 20Uh is increased to 500uH which equals L3's inductance. The total inductance is therefore 250uH which will be tuned.

After thinking about this should I calculate L1 in parallel with L2 to be stepped up by 25:1? Meaning 10uH instead of 20uH.

Also, does L2 consist of 6-turns bifilar or 12-turns bifilar? The whole bifilar/center tapped part is throwing me off.

I included D1 for ESD protection as per Silicon Labs AN555. Can we use discreet diodes or zeners for this purpose and if so what is a good recommendation?

Much thanks.

Thanks for the heads-up in the Solid State Radios forum. Somehow I missed this post.

The Hoop Loop was designed to replace the built-in antenna. So the transformer secondary (the high inductance) is in place of the radio's loopstick antenna, not in parallel.

If you put any inductance in parallel with the loopstick the total inductance seen by the tuning cap will be less than it should be, ruining the tuning range. There's no reasonable way to get around this.

What you need to do is determine the inductance of the internal antenna, remove it, and replace it with a transformer of the same inductance. If the internal antenna winding is tapped, as in my Zenith Royal 705, you'll have to accommodate that, but it probably isn't.

You can't calculate the primary until you've set the secondary inductance. Divide the secondary inductance by the inductance of the antenna and take the square root, to determine the end-to-end inductance of the primary. Add a turn or two to each end until you test it.

Testing is easy if you have a second portable radio that will sit inside the Hoop Loop. As close to either end of the band as possible, tune in a station on the radio that's directly connected to the Hoop Loop. Then tune the second radio to the same station. Rock the Hoop-Loop radio's tuning back and forth. The signal on the second radio should peak at exactly the same place as it peaks on the Hoop-Loop radio. If it doesn't, try pruning the primary evenly from both ends. Because of the high turns ratio, a couple of turns makes a big difference.

This whole experiment was made much easier in the Tecsun PL-380 because the chip will tune inductances from around 250-450 uH across the entire band, giving manufacturers some slack when designing the loopstick. With an analog radio it's harder because the inductance can't vary much from the design value.

When I use the Hoop Loop with a communications receiver, there is no internal antenna. I wound a transformer that would resonate the Hoop Loop across the band with a 365-pF cap (using a second, portable radio as above to prove to myself I really was tuning the antenna). Then I used a third winding of only a few turns to couple the transformer to the receiver antenna connector.

I don't think you need the ESD diodes. The impedance there is very low and as the loop gives a dc short circuit there isn't likely to be any static build-up. Plus, their capacitance is going to be multiplied by the turns ratio and appear across the secondary.

For others' reference, here's the Hoop Loop article: http://kr1s.kearman.com/html/hooploop.html

73,

_________________

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

Hey Jim,

I do such a poor job of explaining things sometimes. I get in a rush and it doesn't always come across clearly. I will try to do a better job this go 'round.

Were you able to see the pic I linked?
I understand that it replaces the tank's inductor.
I have stated all ratios incorrectly. I put 5:1 instead of 1:5.

L1 is the Loop with a predicted inductance of 20uH. T1 consists of primary winding L2 and secondary winding L3. From reading your article you mention matching the loop's reactance with the transformer's primary reactance.

So, if L1 is 20uH and L2 is 20uH that is a match. Now, since T1 steps up the inductance by 1:25 the loop is stepped up to 500uH correct? L3 is wound to an inductance of 500uH and it is this inductance that is in parallel with the loops 'stepped up' inductance of 500uH. (500x500)/(500+500)=250. In my example it would have been much clearer if I didn't have L3's inductance the same value as the stepped up inductance. So to make sure I have this right I will calculate your example. Loop's stepped up inductance is 16x25=400. Secondary inductance was wound to 440uH. (400x440)/(400+440)=209.52uH.

I'm pretty sure I got it right thus far.

My ignorance is really about to show though. In your example you too are using a 1:5 turns ratio. Assuming you wound the primary with 10 turns bifilar center tapped and the secondary with 50 turns. Now... The part I really don't understand is it really 10 physical turns going through the toroid core or is it 5 turns? Since it's bifilar I'm thinking that it is only 5 physical turns because its bifilar (two wires x 5 turns = 10). I know this is so rudimentary but I'm just not familiar at all with center tapped and bifilars.


Thanks for putting up with me and I hope I got my qustion out clearly enough for you to understand me.