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Author Topic: Magnetic Loop for noise reduction?  (Read 4193 times)
VE6EI
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Posts: 4




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« on: February 20, 2012, 02:57:31 PM »

Hello all,

I moved a year ago into a neighborhood with high voltage lines nearby. (about 6 kV) The house was otherwise perfect...

 Now that I'm playing with my radios again I'm getting very high noise levels. Tried turning off power to the house - the source is outside. I'll be trying to work with the utility company to find bad insulators etc but am hoping to get on the air soon. Noise levels can be as high as S9+10dB on the lower bands, and are still up to S7 on higher bands.

I bought a Timewave ANC-4 to reduce noise but that's been only marginally successful. Also installed a BHI audio filter in my rig's speaker. Again only slight improvement.

So now I'm looking at some alternate antenna type to minimize noise. I have heard some good things about magnetic loops (eg MFJ-1788) though I know their limitations too. (a dipole typically outperforms one, plus they're very narrowbanded) But...

Would a very narrow-banded antenna of this type, up on my roof, substantially reduce power line noise? Or would I find the same or similar noise levels once I tuned my magnetic loop?

Joel VE6EI

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WB6BYU
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Posts: 13124




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« Reply #1 on: February 20, 2012, 04:21:38 PM »

I don't think it will make much difference.

There's a lot of lore about the noise-canceling abilities of a small shielded loop, but
the truth is that "noise" is radiated RF, and no antenna can sort out "good" RF from
"bad" RF and only pass the good stuff through to the receiver.

The directional property of a loop could be of an advantage if the noise is coming
from a particular direction:  positioning the null in that direction will reduce the
noise pickup from that source compared to signals from other directions.  But the
null of a good loop is quite sharp, and if the power lines run past your house so
that you can swing the loop 90 degrees and still have it pointing at the lines,
then you'll pick up the radiation regardless of how the loop is oriented because
the noise is NOT coming from a single direction.

If you want to test the idea, small loops are quite easy to build for receiving.
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N3OX
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« Reply #2 on: February 20, 2012, 04:27:47 PM »

There's a slight chance that a magnetic loop would perform better if there's any conducted path for the noise via common mode currents on your feedlines.  A magnetic loop with inductive coupling has really excellent common-mode rejection.

But I agree with Dale that it probably won't make much difference (because the conducted noise is probably NOT your main problem).  I also agree that you should try a receiving loop first... much, much cheaper.   Plus, honestly, you can potentially set up a system where you receive on the loop and transmit on your existing antennas... best of both worlds.

If you try a loop you might try moving it to different spots.  I have lines close by on three sides and I seem to be able to walk around my yard with a small hand-held magnetic loop and map out a standing wave pattern of peaks and nulls in power line noise in my yard.  There's a 3D pattern to all of this... but anyway there are some sweet spots in my yard where on a given band the antenna will be much quieter, I guess because that spot has destructive interference from all the lines.

Someday I want to map it out in some detail and write an article but it's pretty tedious Smiley
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
VE6EI
Member

Posts: 4




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« Reply #3 on: February 21, 2012, 12:38:42 PM »

Thanks for the thoughts guys. I have a 1:1 choke (15 turns, 6" in diameter) on the coax already so I don't think there's anything much being picked up on the braid.

I am already using an MFJ-936B small loop tuner with 1/2" copper tubing loops for 20 and 15 metres, and a 10 ga wire loop for 40 metres. I've tried it in a couple of places in the house (not much yard away from the power lines) and in a couple of different orientations. I'll keep looking for any potential quieter spots on my property - that's a good idea.

The 936B has a significantly broader bandwidth than the MFJ-1788 or 1786, or the AEA Isoloop. Had hoped that would have been part of my problem. But no sense throwing away $400+. I think my next move is to start sniffing out sources of noise. I've got a handheld VHF yagi and will listen for noise on AM.

Joel VE6EI
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KW5B
Member

Posts: 21




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« Reply #4 on: February 28, 2012, 08:10:32 PM »

I'll going to disagree here. I am not going to go into why, but MAGNETIC small loops are very much quieter than "normal" antennas.

Physics supports this.  It's the magnetic properties not the narrow band properties that make it quiet.

Build one to see. 

ljg



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WB6BYU
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Posts: 13124




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« Reply #5 on: February 29, 2012, 02:01:03 AM »

Quote from: KW5B

Physics supports this.  It's the magnetic properties not the narrow band properties that make it quiet.




Would you like to give us an example of the physics that supports this claim?

All the serious electrical engineering studies I've seen don't support it (despite ham myths
and lore to the contrary), and I would be surprised if the underlying physics is any different.

For example:

http://www.w8ji.com/magnetic_receiving_loops.htm
http://vk1od.net/antenna/shieldedloop/
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KW5B
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Posts: 21




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« Reply #6 on: February 29, 2012, 07:18:49 AM »

I think you are asking me to support my claim that the reason for the magnetic antenna being quiet is not the narrow bandwidth, but the magnetic properties.

I have two magnetic loops sitting in my front yard.  They are very similar but not exactly alike.  Both are very quiet antennas.  One is matched with a gamma match and has a very narrow bandwidth.  The second is tuned with a toroid and has a very wide bandwidth.  I can switch which one is connected to the radio as will.

It is impossible to tell from the noise level which one is connected.  Both are equally quiet.  Something is making them quiet and it is not the narrow bandwidth.

You evidently dismiss all claims that the magnetic properties make the antenna quiet as myths and lore and cite all claims to the contrary as serious electrical engineering studies.  So any study that I present is going to be dismissed by you. 

No the underlying physics is no different, but there is certainly an electrical and a magnetic property to all radio waves.  That's basic high school physics, surely you do understand this.  And it certainly follows that the magnetic properties of a electromagnetic wave can be detected.  Again high school physics. 

Are you disputing the claim that a magnetic antenna exists?  If you admit that an antenna can detect the magnetic portion of an radio wave then it certainly follows that physically the magnetic antenna will have it own properties separate from an antenna that only detects the electrical portion of a radio wave.

But again no argument of mine is going to make any difference to you since you evidently only accept 1/2 the physics of electromagnetic mechanics. 


ljg
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KW5B
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Posts: 21




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« Reply #7 on: February 29, 2012, 07:41:05 AM »

I will respond to the first of the links you provided.  Maybe the second after I review it.

The author makes several claims and has a lot of words and graphs.  Maybe some or a lot of it true.  Some of it doesn't make sense and some of it is on the surface not correct.  I'm not sure why you say this is a serious mainstream study.

The author states that a sheilded loop cannot pass the magnetic portion of a signal and not pass the electrical portion.

I think I can show that this is incorrect.  Imagine a metal non-conductive(aluminum) box, with magnets on opposed sides .  By moving the magnetic on the outside I move the magnet on the inside.

There would have been an electrical field created on the inside, but it was created by the moving magnetic on the inside, not by the electrical field on the outside.  And this electric field on the inside would be purely is response to the movement of the magnet.

I don't know, the math and graphs on the site is way beyond my understanding.  But his smoke and mirrors don't trump experience and observation.  Flawed math can be used to prove anything and graphs are easily twisted.  That's why politicians use them.



ljg
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N4UM
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Posts: 466




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« Reply #8 on: February 29, 2012, 08:39:59 AM »

To finesse (somewhat) the substance of your original question which seems to boil down to "How can one operate in a very high noise area?" - I can offer little in the way of antenna-based solutions.  A different approach to the problem might be to consider modes of operation that are less susceptible to noise in the first place.  Olivia, MFSK, CMSK and several other digital modes (not psk31) come to mind that can work rather well under high noise/weak signal conditions.  It might be worthwhile exploring the use of such modes with whatever existing antenna you have.  I wish you well in your efforts to get the power company to solve your problem but don't hold your breath.
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ZS6BIM
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Posts: 67




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« Reply #9 on: February 29, 2012, 09:14:32 AM »

As a point for further discussion check out measurements I made of noise being received by a vertical magnetic loop and 3m base tuned vertical whip both located on the same vehicle, at the link below

http://i620.photobucket.com/albums/tt283/mwmikep/Loop_Whip.jpg

The vehicle was stationary in an area of relatively high man-made noise.

The measurement was made using a spectrum analyser set to the 0Hz span mode. In this mode the S/A functions as a convenient calibrated signal strength meter that can show noise amplitude over time.

The time period on each measured frequency was 50s with half the period allocated to each antenna. The S/A resolution bandwidth was set to 3kHz.

What is interesting is the generally lower noise picked up by the loop antenna. Also shown is the S/A noise floor,  -95dBm to -105dBm and the apparent gain, based on the overall noise floor, of the loop (at 2MHz, 3MHz & 4MHz) over the 3m whip.

I would be interested to hear your opinion of the measurement and results.

73
Mike

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W5DQ
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« Reply #10 on: February 29, 2012, 09:57:35 AM »

Although I don't have a dog in this fight, I can give a real world example. I have a Pixel Pro-1B magnetic loop mounted appx 100 feet from the shack. Comparing it to my other antennas on 160M thru 40M (modified 6BTV vertical and OCFD as flattop @ 40') I find the signal levels (noise and thus signal too) drops about 2 S-units on average, sometimes more. While one might think this is not ideal since it would degrade an already weak signal if present, I find it quite to the contrary. Having the background noise lowered has the effect of increasing the signal to noise, at least to my ears. I find copy on the loop is easier for weak stations than on the vertical or OCFD. For strong signals, the vertical is just as effective as the loop.

I haven't done any scientific measurements but provide this as a real world example.

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
K4FMX
Member

Posts: 17




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« Reply #11 on: March 01, 2012, 07:02:19 PM »

I will respond to the first of the links you provided.  Maybe the second after I review it.

The author makes several claims and has a lot of words and graphs.  Maybe some or a lot of it true.  Some of it doesn't make sense and some of it is on the surface not correct.  I'm not sure why you say this is a serious mainstream study.

The author states that a sheilded loop cannot pass the magnetic portion of a signal and not pass the electrical portion.

I think I can show that this is incorrect.  Imagine a metal non-conductive(aluminum) box, with magnets on opposed sides .  By moving the magnetic on the outside I move the magnet on the inside.

There would have been an electrical field created on the inside, but it was created by the moving magnetic on the inside, not by the electrical field on the outside.  And this electric field on the inside would be purely is response to the movement of the magnet.

I don't know, the math and graphs on the site is way beyond my understanding.  But his smoke and mirrors don't trump experience and observation.  Flawed math can be used to prove anything and graphs are easily twisted.  That's why politicians use them.



ljg

If you believe that a radio signal can get from the outside of a closed metal box to the inside how would you explain why a metal (box) shield used around an IF transformer will not let RF in or out of that shield? Think about a screen room used for repairing/testing radios where outside signals are not wanted inside or the radio under test signals are not wanted outside. They typically have isolation of greater than 100 db. But poke a small insulated wire thru the wall and isolation goes out the window.

73
Gary  K4FMX
« Last Edit: March 01, 2012, 07:14:24 PM by K4FMX » Logged
KF6ABU
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Posts: 351




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« Reply #12 on: March 01, 2012, 07:15:13 PM »

I have a magnetic loop from wellbrook ala-1530 on a rotator on my 2nd story roof. It IS better for rx then my verticals on 80/40/30.. but only barely. On my verticals noise is 58 on average. on the loop more like 56/57 on average. The loop has allowed me to hear stations I could not hear on the verticals due to noise floor. However If I put up a dipole at 20ft, it may be 100x better.
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W0BTU
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« Reply #13 on: March 02, 2012, 12:08:07 PM »

If you believe that a radio signal can get from the outside of a closed metal box to the inside how would you explain why a metal (box) shield used around an IF transformer will not let RF in or out of that shield? Think about a screen room used for repairing/testing radios where outside signals are not wanted inside or the radio under test signals are not wanted outside. They typically have isolation of greater than 100 db. But poke a small insulated wire thru the wall and isolation goes out the window.

Well said, Gary. :-)

Sure, a well-designed and properly placed "magnetic loop" can sometimes help us receive the weak ones by improving the signal-to-noise ratio. But not for the reasons some people claim. And a mag loop is only "magnetic" at an extremely short distance.

"Small loops are often referred to as "magnetic radiators". Folklore claims a small "shielded" loop antenna behaves like an electrical sieve of sorts, sorting "good magnetic signals" from "bad electrical noise". Nothing is further from the truth! At relatively small distances a small magnetic loop is more sensitive to electric fields than a small electric field probe. ... " -- http://www.w8ji.com/magnetic_receiving_loops.htm

" ... The gap in the shield provides a means of coupling current on the outer of the outer conductor to the transmission line formed by the outer surface of the inner conductor and the inner surface of the outer conductor. ... The source of the current flowing on the inner conductor is entirely the current flowing on the outer of the outer conductor at the gap ... The antenna is no more or less subject to the influence of electric and magnetic field components than an equivalent loop with the load connected directly at the gap. ... if attention is paid to symmetry of the loop and feed, the balance that is achieved. Best balance yields the deepest null which is important in direction finding applications for instance. ... " -- http://vk1od.net/antenna/shieldedloop/
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KW5B
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Posts: 21




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« Reply #14 on: March 25, 2012, 05:11:15 PM »

K4FMX,

Doesn't your post prove my point.  An electrical signal cannot get inside the sheilded box.  That's is the reason for using them.  No argument.

But equally obvious a magnetic signal can.  Two magnets place on opposite side of a non magnetic metal.  Move one and the other moves.  If they are electromagnets and you vary the  voltage on the outside magnet  the voltage will be mirrored by the voltage created by the electromagnet on the inside.  That's elementary physics high school physics it is not?   

And if the magnetic loop antenna does indeed operate from magnetic properties, those magnetic signal can quite easily pass through the shielded aluminum, copper or silver box. 

No one seems to realize or want to admit that although electric properties and magnetic properties are linked, they have different properties. 

ljg
kw5b
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