loop antennas

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Addressing noise issue of 'out of band' signals.

compare ROD to LOOP Mainly consider DC to RF Band of interest: ROD starts out high impedance and being capacitively coupled picks up everything out there, AC mains [harmonics], etc LOOP starts out low impedance and pretty much shorts out every signal you don't want, until you're up into the band. These facts may have been the basis for the comment in your first URL.

From experience, it is far easier to keep noise out of getting into the LOOP signal path than it is to keep noise out of the signals from a ROD antenna, because of that high pass effect. like first stage of filtering is free.

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OK you didn't mention the frequency range. My only experience is with low frequency loops (~100 Hz to maybe 1 MHz.) In that range I found it was essential to have a shield, else the AC electric fields overwhelmed the magnetic fields I was looking for. This was a single ended input.

George H.

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Yep. I made a 60kHz loop (for WWVB) wound inside 1/2" copper pipe as the shield (shorted turn effect broken with a fiber joint... fittings at your local Home Depot :-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

1: The reason they're called "EM" waves is because they're electromagnetic -- the energy in traveling wave oscillates between electrical and magnetic as it travels through space. So you can't shield against one and not the other. 1a: But you are going to be shielding against waves with a particular polarization -- probably vertical. 2: Shielded loops are reported to work well, so _something_ good must be going on. 3: Low frequency radio reception is dominated by ground waves -- this is energy that is refracted by the curved surface of the earth, and tends to "stick" to it. There's only one polarization that "sticks" (and no, I can't remember which, but I think it's the one with the E-field parallel to the ground). A shielded loop as pictured may not reject the E-field

-- but it's going to be highly polarized.

4: As mentioned, the antenna is going to be selective. If you tune it, it's going to be _really_ selective. That'll clean up a lot of crap before it ever hits the front end of your receiver, which is all to the good.

I suspect that the improvement you get from one (if any) is a combination of the polarization selectivity and frequency selectivity. Certainly there are other LF antennas that are reported to work well because they are sensitive to the polarization inherent in ground waves.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

Not necessarily. That's true either for plane waves, or the more general long wave case. Near fields can have widely varying ratios of E and M, other than Zo. For example, Tesla coils generate a heck of a lot of volts, but the magnetic field doesn't extend very far, so they make terrible antennas.

A loop for low frequencies is only going to intercept a small, local portion of a wave, and more importantly, the fields that are interfering are most likely near fields. Best example is electric induction from 60Hz line noise, which is typically unbalanced (0-120).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Go back to huge, sharp EM spikes and Morse code. Probably pick it up around the world.

Pretty annoyingly slow data rate though. The bomb would be hitting right about the time you get the first line of the warning message completed.

I even have a name for it.

BullyIT®

Wellbrook claims their antennas favor the magnetic part of the EM wave. All I can say is they work great and they cost too much money. I have the original ALA100. You add your own wire loop. It doesn't need to be shielded.

You can DF with them at low frequencies (beacons for example). At higher frequencies they are more omni. They are quite directional in the BDB.

The old Kiwa loop (truly overpriced) wasn't shielded either. Since Kiwa stopped making them, the price went crazy.

The Wellbrook works much better in the comparison test I made, but Kiwa has it's fans.

I think most readers of this NG saw your posting..

Good points -- I hadn't thought of the near-field/far-field dichotomy, nor had it pointed out to me in this context. And there's certainly a lot of potential low-frequency interference that starts out as high voltage, low current phenomena.

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I was using a loop to try and measure the local magnetic fields in the lab. (Mostly 60 Hz AC and it's harmonics) I always assumed the interfering electric fields were near fields. (Dang fluorescent lights)

George H.

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And then one day the antenna is gone, on account of that nice shiny copper pipe ...

--
Regards, Joerg

http://www.analogconsultants.com/

case:

We've had a number of deaths (deservedly) here... yo-yo's trying to steal copper from SRP power stations ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

case:

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You got a problem with Darwin?

Yes. Many thanks to all respondents.

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I then presume that you were using them as broadband antenna. My experience is with AM BCB receivers where they were resonated for selectivity and image rejection.

?-)

Coax is good:

What do you mean "Dang fluorescent lights?" If I'm ever out in the middle of a field someplace with an oscilloscope, at some point I'm going to touch the probe with my finger to get the Universal Test Signal, and I'm going to think that something is broken when I don't get a whopping big 60Hz hum.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

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Hi Tim, The fluorescent lights dominate =91electrostatically=92 at my lab bench. Sticking my finger onto the 'scope input I see ~ 3.5 Vp-p of 26 kHz from the lights. Turning the lights off, I can see the =91universal=92 60Hz. less than 1 Vp-p.

(It's actually a pretty good test for electro-static leakage into a circuit. I trun off the room lights and see if anything changes.)

George H.

Not strictly true. The E field and H field are in phase, else no net energy would flow. They are *spatially* at right angles, and mutually at right angles to the direction of energy propagation.

A simple dipole antenna responds to E field only, hence it could be said to "shield" (actually discriminate) against the magnetic component. Align it to the H field, you get nada (unless the E field is circularly polarized, in which case you can't).

Fields in quadrature, where the energy returns each half-cycle, are what give rise to the "near field" component, which does not radiate energy. Simple induction.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)