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Tuned loop for AM broadcast reception

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LAN_403

Conundrum

Mon Oct 02 2017, 06:25AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

What about using the glass from dead Samsung phones as it has a graphite backing with good electromagnetic properties similar to a super-ferrite rod!
Its a minor hassle to get these out and all it would need is some thin clear parcel tape to hold the glass together (I used S6) then use a printer coil former.
Limitless quantity of these so you could make a huge antenna this way.

Also it does not by any means need to be new wire, wire salvaged from an old relay would work fine.
I did this for my recent coil and it works very well indeed.

Alternate *3, take the graphene layer off the screens with parcel tape and wind it round the former.
4 layers with thin enough tape should work fine and be about 1/10 the weight of a ferrite rod but so much larger.

Proud Mary

Wed Oct 11 2017, 02:26PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Chris Trask, mentioned by Mr Russell, does have some really good designs for a varactor tuned mag loop, and a broadband mag loop, where great attention is paid to matching the very lo-Z| loop to the feeder.

Conundrum

Sun Oct 15 2017, 06:44AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

making progress.
Apart from laboriously making Litz wire *by hand* can someone suggest a source?
I found that old microwave transformers and some TVs have Litz primaries but
normally they are glued together.

klugesmith

Sun Mar 11 2018, 07:39AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Don't know if this should go on Projects forum.
I'm about to make a new loop antenna, using a laminated wooden hoop as a form.

The hoop is wound from four turns of white-surfaced hardboard about 1/8" (3 mm) thick.

The material is heavy and not very strong, flexible, or water-tolerant. I foolishly bought a sheet 'cause it was on clearance, for less than $4. Mostly bonded with polyurethane construction adhesive, from a large caulking-gun tube.

You may have seen the round form here:

At the beginning and end, I tapered the material and stuck it down with water-based wood glue.

Conundrum

Sun Mar 11 2018, 08:10AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Might have a go at this now some nice person on 4HV sent me some.

Hint: if you're linking sections of wire, try and make more than one connection. 8 seems to be the minimum but each one reduces overall noise and increases Q.
Also relevant, I can probably track down a few ferrite rods here if you wanted to make an all-ferrite coil. Just be sure to leave a gap in the foil shield!

Also relevant, try and use high silver content (Ag) solder if possible as though somewhat toxic it does work well for RF applications.
SnAgCu is adequate but sucks only slightly less than 60/40.

klugesmith

Mon Mar 26 2018, 05:05PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Had an opportunity to measure the old rectangular loop on a digitally-controlled LCR bridge.
For f=75 kHz on up, wrote down the vector impedance magnitude and angle.

Charted here, next to similar measurements on a compact toroidal inductor that might become used as a current transformer.

Vertical axis labels for angle, -180Â° to +180Â° linear, were cut off by the snip. H axis is f in kHz.

The toroid chart looks like textbook picture of L with parasitic C in parallel.
At low frequencies, with angle near +90Â°, it matches a 2.3 mH inductor (red line).
At high frequencies, with angle near -90Â°, itâ€™s like a 52 pF capacitor (green line), if my arithmetic is right.
Self resonant frequency is 462 kHz; the angle reading is a sensitive indicator of that.

The loop chart has low frequency asymptote matching 558 uH inductor (red line). This time the green line is a hypothetical 126 pF capacitor to resonate with the coil at 600 kHz. Starting in the low MHz, we see wild excursions of the impedance. I think from various resonances in the 21-turn coil due to irregular spacing, after half the turns had literally fallen off the form and only roughly been put back in place.

With that much data, it was time to sacrifice the old coil and use its wire on a nice new form. Perimeter length is about the same, but itâ€™s round so it encloses a larger area. The stubby solenoid's length and turn spacing (6 per inch) match the original rectangle.
The wire transplant was done while watching neighbor's dog, and watching Get Out on their big TV. By good luck, the old wire filled the form with about the same about left over at the finish; length per turn matched within 1%!

The turn spacers are blocks of plastic foam, each with 21 slots cut by knife.

Conundrum

Tue Mar 27 2018, 06:10AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Nice!
Wonder if anyone else has used zrzsg's graphene (aka heatsink graphite from phones) to make DIY RF antennas?
Mine seemed to work well and extended range on NFC, for lower frequencies it should be even better.

klugesmith

Fri Mar 30 2018, 02:59AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Does anyone else like the round coil's simple wooden stand?

Today's vector impedance sweep shows an inductance of 614 uH. That's 10% higher than the old rectangular coil, which had the same turns count, wire length per turn, and wire spacing.

At the high end there's now a nice simple self-resonance at 1.92 MHz, indicating effective C of about 11.2 pF. Then the complex numbers stop behaving simply, just as with the old coil.

The coil was good at detecting me.

At 1.92 MHz the angle reading was -5Â° with me sitting on a chair, close enough to push buttons. With one hand up near center of coil, angle went up to +60Â°. With me standing as far away as I could still read the display, angle was -14Â°.

Looks like the resonant C value for 60 kHz (WWVB) would be around 11.5 nF.
I figure that for AM broadcast band, this coil wants C adjustable between 16 and 140 pF. To make the most of the old 365 pF tuning capacitor, how about tapping the coil for reduced inductance, with the remaining turns left in place?

Sulaiman

Fri Mar 30 2018, 11:44AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I think that your coil is probably even better than measured - so close to earthed metal

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