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Author Topic: Loop antenna noise  (Read 7805 times)
NB9N
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« on: March 20, 2010, 02:14:49 PM »

Just put up a loop antenna fed with 450 ohm window line.  Loop is cut for 80 meters approx 25-30 feet high.  The other end terminates into a palstar at1500cv tuner which incorporates a 4-1 balun.
Here's the deal...  Good gain BUT THE NOISE LEVEL IS EXACTLY THE SAME AS MY OTHER ANTENNAS!  I was anticipating much lower as most say.  Is there something I overlooked?

Joel/ nb9n
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N3OX
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« Reply #1 on: March 20, 2010, 04:42:56 PM »

I was anticipating much lower as most say.  Is there something I overlooked?

Yep.

You overlooked that people who insist that loops are inherently quiet don't know what they're talking about.

Noise is a signal.  It can be picked up by any antenna system, and no antenna couples differently to noise in a BASIC way independent of installation... there is no fundamentally "quiet" antenna style, it's a myth.

I think the real problem is this:

Someone hears that loops are quiet.  They put up a loop.  It happens to solve their noise issue.  Since they had heard that loops are quiet, now they're a TRUE BELIEVER and become a loop evangelist.     There are a lot of ham radio "beliefs" that continue to be popular because not a lot of people do really good tests that would *really* test the "nuggets of wisdom" behind them.

It is not a good enough test to put up a loop and find that it's quiet, because that could be an accident.  But that's what people do (the alternative is very, very hard and just no fun for a hobby endeavor)  and that's what perpetuates this kind of lore.

Consider yourself lucky.  You got a negative result.  You know loops aren't quieter.  That will save you time in the future.

I'm sorry about your noise.  Noise problems suck.   You won't fix noise most effectively by looking for "quiet" antennas, because they don't exist, but there are ways to make antennas quieter.  This isn't perfect, but it's a good starting point:

http://www.audiosystemsgroup.com/RFI-Ham.pdf

73
Dan
« Last Edit: March 20, 2010, 04:44:54 PM by Dan » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB7TDG
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Posts: 71




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« Reply #2 on: March 20, 2010, 04:47:00 PM »

Is the first antenna connected to the tuner also? I notice that When using my Palstar tuner as an antenna switch I could not get a decent a/b comparasion.
Also use an outboard balun and see what happens..
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W4OP
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« Reply #3 on: March 20, 2010, 08:06:32 PM »

N3OX is spot on. A full wave loop has a pattern almost identical to a half wave dipole. With the same pattern, why would it be any less noisey? The fact that it's a DC short means nothing at RF frequencies.

Antennas become lower noise when their pattern is such that the antenna can reject noise from specific directions- as a Beverage, K9AY etc.

The only advantage I can see to the loop is that the VSWR excursions on higher bands are not as wide as they are on a dipole- so perhaps it makes a better multiband antenna.

Dale W4OP
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N3LCW
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« Reply #4 on: March 20, 2010, 08:36:48 PM »

The only loops I have used that actually reduce noise are magnetic loops.  I've built them upwards of 1/4w circumference and the signal to noise ratio is very much improved.

Andrew
N3LCW
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N3OX
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« Reply #5 on: March 21, 2010, 08:30:19 AM »

The only loops I have used that actually reduce noise are magnetic loops.  I've built them upwards of 1/4w circumference and the signal to noise ratio is very much improved.

Nevertheless, it is NOT because they're loops.

I think there are several main reasons for "quietness." 

One of them is lack of common mode pickup, and I think magloops are often good at that (especially if they use an inductive coupling loop, less so if they don't)

Another important thing is location.  I wouldn't be surprised if a small loop situated further from the house would have less overall nearfield coupling to the noisy house than a big doublet that has a tip on the chimney or something.

The way in which antennas couple to their environment is important, but again, that's not an inherent property of loops of any description.

The most insidious thing that I notice is that people often praise lossy antennas as quiet, especially on the low bands.  And I think this is probably because people don't understand why they probably really need to judiciously use their built in receiving attenuator and RF gain control on the low bands when they listen on large, efficient antennas.

There's just too much signal available from a dipole.  This can have a real effect on the effective signal to noise ratio if your receiver starts to have problems handling strong signals.   My FT-857D is unlistenable on my 60 foot vertical on 80m or base loaded on 160.  It's harsh and nasty.  With the first preamp off and the  attenuator switched in (20dB less overall), it's nice.  Same goes if I listen on my flag, which even after the preamp is only something like -20dBi equivalent.  0dBi or even -6dBi just gathers too much noise power.

Of course, this is probably not the benefit gained from loops in general because they aren't inherently particularly lossy.  There may be some reason why many loop installations tend to pick up less noise even though they're good performers, but people often don't even bother telling you what they're comparing against.   

"Loops are quiet," whether they are big or small, is a ham lore prediction that has little direct evidence from controlled testing, and even when people A/B test, they never tell us WHY their loop was quieter.  I'm sure it works a lot, because it gets people to try something else installed differently, and sometimes you can just catch a lucky break. 

Some antennas have useful "noise reducing" properties; like W4OP says, directivity is one of them.   A great thing about a well balanced magnetic loop is the nulls.  I did find that setting up a magnetic loop in my apartment was useful to null some awful noise I had on my 80m doublet by rotating the loop until the noise was nulled.   It took the noise down below the natural noise floor even when it was indoors!

After using it for a few days, I discovered that I was nulling my computer in the other room, and if I just turned off the monitor when I was operating, my doublet was great.  This, of course, speaks to the #1 rule of noise fighting:  Kill the noise from your own personal noise generators first, then worry about your neighbors and antenna issues.

73
Dan



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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WX7G
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« Reply #6 on: March 21, 2010, 09:36:59 AM »

Your loop is one wavelength. It is essentially two half wavelength dipoles joined. It will perform as such.

The 'loop is quiet' idea is based on short loops; loops much less than 1/10 wavelength in diameter. The idea behind this is that a very short loop responds to the magnetic field and not the electric field. And that close RFI sources have an E/H ratio higher than 377 ohms. That is, the electric field is out of proportion to the magnetic field. The second part of this theory may be true for RFI sources that are much less than 1/2 wavelength from the loop. This is not often the case.

Why then use a small loop? For its directional capabilities and the ability to orient it so that the RFI source is in a null.

Why then use a large (1 wavelength loop)? Why indeed!
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AI7RR
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« Reply #7 on: March 22, 2010, 11:05:27 AM »

I find that my 1 wl horizontal 80m loop actually has a higher noise level than my dipole. That said, the loop seems to be much less susceptible to power line static or E field noise than the dipole.

73,,Roger
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N7DM
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« Reply #8 on: March 24, 2010, 06:42:31 PM »

Here in the far Northwest reaches of Seven-Land, I have specifically used loops...DC short...antennas to avoid Rain Static. Believe me, we know what Rain Static is, and a dipole gets it!  My loops, essentially One Element Quads, VERTICAL loops, do not....

Other noise I know nothing of; I live in a S-Zero QTH.

dm
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W8JI
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« Reply #9 on: March 25, 2010, 02:06:59 AM »


I have to chime in about this too. :-)


The theoretical blather by people who don't have a clue what noise is, how systems radiate, or what fields are is just disgusting, and it seems to go on forever.

The idea a loop, even a small "magnetic loop", is quieter from some kind of "loop magnetic field magic" or closed DC path is just utter nonsense.

P-static is caused by corona. I can have the very same rain, snow, or dust hitting all my antennas and ALWAYS the higher antenna, or the antenna with sharper protruding points, is most troublesome for p-static. Almost everyone in the world with stacked antennas and a way to switch them knows the highest antenna is by far the most problematic antenna for p-static. Anyone with a with a choice of antennas also knows the lowest antenna, the one with the bluntest ends, and the one shielded by taller objects is the quietest for p-static. This is because the problem is corona from sharp protruding points.

As a matter of fact my stacking switches have a p-static mode that disables the highest antenna on receive-only for use with  inclement weather!

As for locally generated noise, if noise comes from any distance beyond 1/4 wave or so, there is virtually no difference at all in ratio of electric to magnetic fields regardless of the source. At a distance of a half wave or more any small antenna cannot possible react differently to electric or magnetic fields.

Locally, in the induction or near field area, anything could be quieter by random luck.

The fascination with loop antennas and noise borders on some very strange fake science we pass along to each other, it's almost like astrology for antennas or something.

As for DC grounds or shorts, my yagi's all have dc grounds. The highest antenna have p-static, the lower ones do not. My high folded dipoles have just as much p-static and noise as my regular dipoles at the same or similar height do, as do my shunt fed towers and other antennas.

Blunt ends, non-protruding ends, lower height, shadowing by other taller objects, all are true reasons for an antenna being quieter in storms. Feedline issues, directivity, and pattern are reasons one antenna is better for electrical or distant storm noise.

There isn't any magic with a loop.

73 Tom
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VK2FXXX
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« Reply #10 on: May 02, 2010, 06:29:17 AM »

aw cmon Tom ,not even just a little bit of magic,just a little ,huh ,please? 
I like my lil magnetic loop thats 1% efficient on 80m. Its very quiet! Whats - 20db between amateurs?
73
Brendan.
« Last Edit: May 02, 2010, 06:41:30 AM by brendan » Logged
VK4AMZ
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« Reply #11 on: July 06, 2010, 06:59:49 PM »

Apology for the late post but this is an interesting discussion and one that seems to go on forever.

Before I go on I would like to say that after three years of testing small antennas on a tiny suburban block and 30 years of having a ¼ 160m vertical, large dipoles and receive loops at a better location, I have to agree with almost everything that Tom W8JI and others have said.

This small block involved my final choice between a copper loop and a small “inefficient” vertical to operate on 80m and 160m. Now a vertical is not the antenna of choice for local nets. Because of height restrictions and the shape, size and type of buildings on this site there is no way to get a dipole up to any height, let alone get any horizontal aerial on 160m. Besides, as I found out when testing a few trial dipoles on 80m, they are noisy when mounted low amongst all this noise, yes, quieter than the vertical and if very low even quieter than the loop (but not too good for transmitting at that height).

If I could have erected a dipole up high and in the clear I would not be writing this. Now please don’t insult me with assertions about feed methods, common mode noise and the multitude of suggestions as to why I didn’t get a vertical or dipole to work on this tiny block.

The copper Loop mounted in the same location as the vertical was in reality about 10db quieter on average conditions than any vertical configuration I tried. I feel the reason for that is twofold AND not because of the loops supposedly “magic noise reducing properties”, actually you could say three fold as the environment is the third factor.

When not set to null the local interference the loop was still removing SOME of the noise coming from two directions at any given time so it was quieter in “MY” location. Where it really came to the fore was its ability to remove strong “directional” local industrial noise and power line noise from power pole leakage.  This interference can be taken from +20 down to S7 on a calibrated S meter, that is a significant reduction; my noise floor is now S6 to S7 instead of a constant S9 to + 20 on the vertical (when power line discharge and local carriers are at their worse) and the Loop receives local net signals that the vertical by design could not – the reason for the loop being up to 30db better on RX to local stations just as a dipole would be.

My conclusion is that there is nothing magical about the loop compared to other antennas with respect to noise.

It is the application and location of different aerials that allows the loop to “SOMETIMES” give a better signal to noise ratio than other aerials that have been seriously compromised by the location and less than favourable installation restrictions for those aerials. I have seen many situations where the loop aerial has a “worse” SN ratio than other horizontal serials. It comes back to trying various antennas until you find one that works in your situation.

Now before I get hysterical responses and an effigy is set on fire please remove you loop filter and try not to misconstrue my meaning.

Soooooooooo in case you missed it, I said “I have found there is nothing magical about the loop compared to other antennas with respect to noise”
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W8JI
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« Reply #12 on: July 10, 2010, 08:08:52 PM »

Of curse there isn't anything magical, there can't be. Noise is a signal just like any other signal, and a MAGNETIC loop has the same ratio of electric to magnetic field as any other antenna does outside of the near field. Up to about 1/8th wave distance the magnetic induction field is dominant, and beyond that up to around 1/2 wave the ELECTRIC field dominates.

So the very idea the magnetic loop is "magnetic" is nonsense, let aloe the idea that noise is "electric" field dominant.

As for dc shorts, that's another myth. The only reason a dipole has more noise under inclement weather is the high impedance ends of the dipole stick out and are a target of corona, and more closely matches the very high impedance of corona discharges. A loop has blunted HV areas that do not normally protrude, and so offers much less chance for corona.

My low yagis and low dipoles are stone quiet during p-static like we are having today. for example on 40 meters, my identical yagi at 180 feet is unusable for receiving. Noise gets as high as 40 over nine on it. The same rain hitting the same style antenna at 85 feet is dead quiet for p-static. The whole difference is the charge gradient from the higher antenna to the sky is much higher, so the higher antennas all get corona first and much worse.

On a dark night in nasty weather I can sometimes see the faint glow off the upper antenna's tips with my night vision glasses. If it was a quad, it would not have the sharp points stuck way out and would have less or no corona, and less or no corona noise.

73 Tom
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M0SVA
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« Reply #13 on: August 04, 2010, 06:20:04 PM »

Hi folks,

My two cents re whether magloops are quieter than big wire antennas.

Anyone who has ever used a Wellbrook 1530 for example, knows they (and other similar magloops) are indeed exceptionally quiet, turn the antenna if vertically fixed, left or right till you null the noise out, especially on the low HF bands, MF and LF.

Place it horizontally and make use of the  >20-30 dB nulls from directly beneath it, thus getting rid of loads of noise if the antenna is placed over your house and noise producing appliances.

It really works like a notch with the only difference that it can do so much more for the SNR than a conventional IF-notch.

Have been using Wellbrook magnetic loops (they are active antennas made in Wales, UK and cover LF, MF, HF) about 15 years in urban environments in the UK and abroad.

With the small magloop, I can receive distant MW stations from across Europe, that I can't hear with  other wire antennas, because with the magloop I can null out e.g. domestic noise that usually propagates via vertically polarized groundwave.

Since the magnetic and the electric part of the same signal are perpendicular to one another, by using a magloop, (Wellbrook, MFJ, Capco, baby loop etc) to a significant extent, you can pick and choose what you want (or not want to) hear by how your magloop is orientated.

Joe Carr also explains in "Antenna Toolkit", "...(small) loops operate on the magnetic  component of the electromagnetic wave, so the loop can be shielded against Voltage signals and *electrostatic* interactions".

It is the natural properties of these antennas that are so impressive, but one could easily get ecstatic at the achieveable quietness and use the word "magic"!

I also have a 1 wavelength antenna for 20m supported by a conifer tree.  Like any large loop, it responds to the electric field component of a signal. Also because of its size it cannot be rotated, turned Vertically or Horizontally. So it's liable to all the crap of urban noises, not much one can do about it, other than its directivity and nulls.
It may be useful to remember for testing purposes, that both nulls and direction of maximum response are opposite when we consider a large loop and a small (magnetic) loop!

Flip antenna switch from large loop to magloop, and for the reasons mentioned above, most usually there is a big improvement regarding how quiet it becomes re noise and hence Signal To Noise increases dramatically, very cool for receiving MW DRM/IBOC for example !

As an added bonus, fading is far less pronounced with the magloop, this I systematically have observed on and 80m when comparing A/B with an Alpha Delta DX-CC.

Use two magloops and you can phase in or out the stations you don't want to hear but are on the same frequency e.g. doing Medium Waves DX-ing.

So good (quiet) do I consider magloops, that for a period of time, I used to have five of them (RX+TX), till  I relocated to a smaller property where there weren't enough tree-branches to hang all of them, so I downsized.

For those of us who have been involved professionally with telecoms, some may remember that so good are the characteristics of this technology of loops, that they have replaced in many cases Rhomics and/or Log Periodics. Arrays of 2-32 MHZ broadband, aperiodic loops, for example, are used for Oceanic Air Traffic Control and not only.

On the flip side, *transmitting* magloops are very-very narrow band.
Wellbrooks (RX only) may be fascinatingly efficient and hugely broadband -they cover some 30MHz spectrum- but transmitting loops are efficient only for a few kHz, then need retuning, so consider that if you need fast band hopping or using a spectrum display like SDR radios provide.

As room, various costs and issues with planning permissions for big antennas become more and more of a challenge and as industrial or domestic noise rises to horrific levels for our hobby, we have an ally on our side, magloops!

My most used antenna for general listening for ham, utility and broadcast stations has been my faithful Wellbrook ALA-1530.

73 es gd dx de M0SVA (Tim)

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