TheRadioBoard

Finally I became curious about how a loop crystal receiver might perform. This way my latest afterwork project was born. From the schematic, the radio is rather simple:

Finding an equation to calculate the necessary number of windings was less easy, since I wanted to do a flat loop. In the end I did not got more than an estimation which later results in having two windings to much. The outermost winding of the loop forms a square of 0.8m x 0.8m. Making the windings of the loop was the most difficult task. It tooks me three times longer than expected. I luckily managed to do the loop with the first 30m of litz wire without ending up in a big bend.

For the loop, 150 x 0.04mm litz was used. The air variable is a good quality three-ganged device with ceramic insulators, but just one gang is used. Tuning is done with help of a 6:1 vernier dial which is coupled by a plastic extension to the shaft of the variable cap. For mounting the cap to the box the originally provided stand-off is used. The FO-215 and a Überformer from Dave provide the necessary matching to the phones. Currently I am still using my Baldwin Type C's. The box is mounted head-down on a wooden base. The loop construction is also attached to the base although mounting all together is a bit more difficult this way.
After all was done, I was able to test the performance of my new radio - and I was surprised. There was no problem to copy about 40 station within a few hours, including stations from North Africa and UK (longest distance yet: 1300km). Selectivity is good, just at the high end of the broadcast band it could be better. This improvement is left for version 1.1...
Now I can here to a crystal radio anywhere in my flat, assuming enough room for the loop Thanks for all the inspirations I got here in this nice forum!

Good looking looper. I am in the mood to build another myself.

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What would our world be like, if Tesla was alive in our time?

Well done, Broesel!

There are many calculators for loop antennas - but only few of them are good. An excellent one has been made by our friend Hagen, you should ask him for a copy of his Excel program. Another very good one can be downloaded here: http://www.user.fh-stralsund.de/~dl0hst/software/magnetloop105.zip

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73 de Fritz

--
<b>Radio geht ins Ohr, Fernsehen ins Auge.</b>
<i>Robert Lembke (1913-1989)</i>

Mein Blog

Nice job - I like loop antennas.

Have a look at the one I made here:
http://theradioboard.com/rb/viewtopic.php?t=1539

Cheers
alpal

Thanks for your nice comments.

@DB4IW: the program looks nice. Anyway, it calculates the values for an edge-wound loop, while my one is spiral-wound. I found another program somewhere which was also intended for edge-wound loops. I used it to get something like a first estimation, and the program in your link gives rather similar results. In the end it seems the spiral-wound loop has a somewhat larger inductance.
I estimated the inductance of my loop as it was finished, just tuned-in a station with know frequency, disconnecting and measure the cap and do some easy calculations. From that, inductance is around 210µH. Actually it might be somewhat lower since I did not care about the self capacitance of the loop in my rough estimation.

@Alpal: Nicely done loop. From the dimensions given, the covered loop area is very similar to my one. This is a good idea for the future, maybe I can spice-up my vintage tube radio by a loop as well. Just need to find a way to make a multiband-loop to cover all, longwave, mediumwave and shortwave range. Probably this will be an edge-wound loop, this kind of loop will save me from running the whole wire length through many small holes as done with my loop.

Broesel, I have the square spiral loop inductance formulas which are suitable for spreadsheet calculations, and are quite accurate. I will dig them out and post them later.

Broesel ... Very nice set.

I have unfortunatly not the room to build one. Maybe one day when I get the attic fixed.

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regards,

George ..
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I thought I had amnesia, I just forgot I didn't.

Broesel, how about calculating with the mean length of one edge ? Probably, Bob Weaver's programm will do a good job, too. Bob, would you mind to send me a copy as well?

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73 de Fritz

--
<b>Radio geht ins Ohr, Fernsehen ins Auge.</b>
<i>Robert Lembke (1913-1989)</i>

Mein Blog

Loops do not have to be huge. 2x2 ft is the minimum before they become too lossy. You can get by with a smaller loop if you basket weave it. 2 ft wide by however high you have room for also works. You can get more wire and better directional pick up on a round footprint loop like the old RCA console antennas. I have a old RCA loop that I occasionally hoist up on top of my TV antenna mast. Does not require rotation. It works quite well.

There is no need to calculate the turns required.

Just use some scrap enamel wire to find the correct number of turns ie
take some off if too much, or solder more on if not enough.

When you know how many then remove the scrap wire & wind on
your good litz wire.

Here are my spiral loop inductance calculation spreadsheets:

Open Doc spreadsheet format:
http://electronbunker.ca/Extras/SpiralLoop.ods
<<Edit: some browsers try to open and display ods files as text. If your browser does this, then right click on the link and select the 'download to file' option>>

Excel format:
http://electronbunker.ca/Extras/SpiralLoop.xls

These calculations use F. W. Grover's spiral coil formulae with Rosa's round wire correction factors. The example data entered in the spreadsheet are the parameters of my own hexagonal loop antenna. The formula produced an inductance value of 152.8µH, while the actual measured value is 152.1µH, which is pretty good agreement. If you measure the dimensions accurately, you should be able to predict the value very precisely. It may in fact be more accurate than your measurement instruments.

To measure the inductance of my loop accurately, I had to take resonance measurements at eight different frequencies, across the BC band, resonating with different known capacitance values, and do a least squares fit to find both the self-capacitance and inductance of the loop.

Thank you very much, Bob!

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73 de Fritz

--
<b>Radio geht ins Ohr, Fernsehen ins Auge.</b>
<i>Robert Lembke (1913-1989)</i>

Mein Blog

Thanks a lot for the Excel/OO sheet. I will use it this evening, already curious about the results. Do you expect deviations in case litz wire instead of solid wire is used?

I think the only problem with litz will be the difficulty in getting an accurate wire diameter measurement. And a slight error in the wire diameter won't likely have much effect when the windings are spaced as much as they are in your loop.

falling in love with the first pic think ile try that! i seen a directional loop coyle on secret life of macheans in you tube it was my favoret child hood tv show http://www.youtube.com/watch?v=3ehVVpY6XE4
one of the radios in this clip looks exactly like it enjoy

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31 years old now playing with crystal sets since 12 years old tweaking and tinkering and sharing ideas ever since...