Super Sized MW Loop Antenna Mark 3

[ChrisSmolinski]

The change here is using some scrap coax as the winding turns - using just the shield, as the conductor. I figure it will be lower resistive loss vs regular wire.  Difficult to tell if it works "better", about the same signal level from my benchmark 1620 pirate about ten miles away.   It's interesting that I can pick up 530 from Canada with a solid signal even around 3 pm in the afternoon in May.  Still 4 turns. Tunes about 570 - 1730 kHz with the varactor diode. So agonizingly close to the entire MW band.

I think the next test will be a regular variable capacitor in place of the varactor diode.

[i_hear_you]

That's cool stuff.  I've now built two tuned loops for BC AM, and the increase in gain as I increase size has me planning even bigger loops.

I recently built an RGP3 (a large loopstick made of several smaller ferrite rods) and have meant to experiment by placing it in the central axis of my largest tuned loop.  I'm not clear on the relationship between the distance of the coil to the ferrite core, or of the area ratios between their respective cross sections.  It probably wouldn't help increase nulls, but my gut tells me it would give even more gain, and if nothing else, it should allow for a turbo-charged small inductive winding for the antenna lead.

Maybe you have some ferrite rods and might experiment 

***EDIT***

I've been researching how to increase the efficiency of tuned loops and am not clear on what wire spacing on the coil does for performance (I suppose Q, in this case).  I understand the closer the wires in the loops, the higher the capacitance, but what does that really mean overall?  In particular, my RGP ferrite is about 14" long, but the coil is currently about 3" long, and I wonder if it is worth trying to spread that out over most of the ferrite length.  I'm also planning a 4' cross-arm tuned loop, I have it calculated at 9 turns.  I intend to make it out of hard wood, stain it, make it really nice like a piece of furniture.  Currently I have it planned with .5" spacing, but if it doesn't make much difference, I would like to change that to .25" for weight and size reasons.

Can someone weigh in on this?

[ChrisSmolinski]

I've read a few posts about the loops made of a bundle of ferrite rods. I've never tried it myself as I only have a few rods, and they've gotten quite pricey. Also I like to keep my antennas outside due to all the RFI from indoor appliances and such. But they do sound intriguing. I recall a youtube video by a DXer in South America that demonstrated how well they can work.

I need to play around with the pickup loop dimensions and placement, what I have right now is literally a random piece of wire thrown over the PVC cross. It's been rather humid the past few days, so I have not been motivated to work on it. Friday might offer some better weather.

I would like to replace the coax/wire for the main loop turns with something rigid, so I have a neater construction with better symmetry and likely deeper nulls.  I was thinking of small diameter aluminum or even copper tubing, but the price seems prohibitive. Anyone have any ideas? Each side is about 7 ft long, so standard 8 ft lengths would work with some cutting to size.

[i_hear_you]

If you are concerned about weatherproofing the ferrite, it's not difficult to seal the ferrite assembly inside PVC tubing.  In fact, the RGP3 construction assumes this. 

It may be worth a quick experiment using one of your rods for the inductive pickup.  The small amount of ferrite, relative to the coil area, probably doesn't measurably increase the gain of the large loop.  However, my understanding is that an inductive coil concentrates the magnetic field lines near the central axis, and I believe a rod in the center would concentrate that even further and allow for a small, neat coupling coil directly on the rod.

Have you considered refrigerator copper tubing?  With 7' sides at 4 turns, I have you at roughly 112'.  It shouldn't be too difficult to sweat them end to end.  A quick price check at Home Depot shows 20' of 1/4" OD copper tubing at about $15, so 6 of those runs you about $90.  It's perhaps a bit pricey for an experiment that may not pan out, but once you finalize your design it's something to consider.

[ChrisSmolinski]

If you are concerned about weatherproofing the ferrite, it's not difficult to seal the ferrite assembly inside PVC tubing.  In fact, the RGP3 construction assumes this. 

It may be worth a quick experiment using one of your rods for the inductive pickup.  The small amount of ferrite, relative to the coil area, probably doesn't measurably increase the gain of the large loop.  However, my understanding is that an inductive coil concentrates the magnetic field lines near the central axis, and I believe a rod in the center would concentrate that even further and allow for a small, neat coupling coil directly on the rod.

Interesting idea. Many of my experiments/tests were using a portable radio sitting on the windings, so that is exactly what was happening there - coupling to the loopstick antenna in the radio. In the case of the radio it is a tuned circuit, but just an untuned set of windings might be sufficient. Certainly something easy to test. I may even have a MW radio loopstick antenna ready to use. Although one of my longer ferrite rods might work better. Lots of things to try, thanks for the suggestion.

Have you considered refrigerator copper tubing?  With 7' sides at 4 turns, I have you at roughly 112'.  It shouldn't be too difficult to sweat them end to end.  A quick price check at Home Depot shows 20' of 1/4" OD copper tubing at about $15, so 6 of those runs you about $90.  It's perhaps a bit pricey for an experiment that may not pan out, but once you finalize your design it's something to consider.

Yes, I was thinking about that. My concern (besides the price) was needing to straighten out the tubing and keep everything symmetrical. If I have to do that by hand... it probably won't be, hence the attraction of finding something that is already a straight rod.

[i_hear_you]

I haven't worked with the copper tubing yet.  I took straightening it for granted.

The design for my "furniture-grade tuned loop" includes a 9PST rotary switch in order to select between 1 to 9 (the full coil) windings.  I'm hoping I can grab some meaningful SW with less windings, but my SW experiments on the large ferrite assembly were failures. I want something clean for the splices between the loop wire and the wires leading to the switch and I settled on cutting segments of this copper tubing, placing it in the drilled hole, passing the coil wire through, and drilling a hole in the part of the tube sticking out for the wire lead to the switch. 

Now that I say all this it seems like a lot of work for very little payoff.

[Josh]

I think distributed capacitance comes into play with multiturn loops, as the loop wires can see each other.
If using copper tubing or pipe, flattening the tubing will reduce the distributed capacitance if the turns are flat to each other.
Also mebbe try litz wire but that's not cheap either.

[i_hear_you]

What's the effect of the self-capacitance, though?  I assume it's more than "it just lowers the tuning range," and I figure it must lower Q, but don't know the physics behind it. If you are trying to achieve highest Q, is it best to increase or decrease the inductance of the coil and make up the rest with capacitance?

I'm having a hard time finding answers to this online.

[Ray Lalleu]

What's the effect of the self-capacitance, though?  I assume it's more than "it just lowers the tuning range," and I figure it must lower Q, but don't know the physics behind it. If you are trying to achieve highest Q, is it best to increase or decrease the inductance of the coil and make up the rest with capacitance?

I'm having a hard time finding answers to this online.

The effect of self-capacitance is that you can't tune a loop above some frequency, so there is an upper limit on size and number of turns if you want to tune all the MW band up to 1700 kHz.

There is a way to reduce that capacitance on a multi-turn loop : use wooden stock slid between the wires (one above, one below, etc...). Doing so, the turn sides are no more straight, and the capacitance between turns is much less. That' s also a good way to get tighten the turns.

[Josh]

What's the effect of the self-capacitance, though?  I assume it's more than "it just lowers the tuning range," and I figure it must lower Q, but don't know the physics behind it. If you are trying to achieve highest Q, is it best to increase or decrease the inductance of the coil and make up the rest with capacitance?

I'm having a hard time finding answers to this online.

Pretty much distributed cap is the reason why you don't want to bunch all the turns together, completely ignoring spacing and exposure of any given wire to all others.

[i_hear_you]

The effect of self-capacitance is that you can't tune a loop above some frequency

So it is mostly just adding capacitance to the LC circuit, thus lowering the tuning range.

[ChrisSmolinski]

Small update. I was pondering why I was not getting as good of a peak in the signal at resonance (Q) and decided to check the resistance of the coax I am using as the primary winding,  It was a bit high, and the F connector on one end was suspect. I cut it off, and noticed some corrosion of the shield braid (this is junk coax I had laying around, perhaps too junk). I cut it back as far as I could, still corroded but not as bad, and crimped on a new F connector. Seems better now.  Of course I can't cover the entire MW band, I can tune from about 600-1700 kHz. Murphy strikes again.