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Author Topic: putting a number on background noise level; also effect of loop-antenna.  (Read 690 times)

Posts: 397

« on: November 28, 2017, 08:21:30 AM »


I'm setting up shop in a new location (adult retirement) that has lots of noise sources, which is a problem I did not have in my old home.
In this new place, I now have neighbors separated only by cement block walls (single story row-house setting).

When I walk around here with an AM-radio tuned to 580 KHz, I pick up lots of static from the appliances, cable-modem, lights, computer, etc.

What procedure could I use to put a number on the quiescent noise level here?
I'd plan on measuring several times during the day/night to make comparisons; also at other times during the year. 
I have an Elecraft KX3 and a Yaesu FT-950; my antenna at this time is only a 15' wire strung around the room.

Additionally, for a "real" antenna, I plan to buy a W4OP loop-antenna.
If I understand correctly how that antenna operates, I'm depending on its very narrow band-width to greatly reduce the effect of these local noise sources.
Could someone comment on that opinion?

TIA for any replies.
73 Jerry KM3K

Posts: 1002

« Reply #1 on: November 28, 2017, 09:50:48 AM »

Tuned loop antennas have a narrow bandwidth and a deep null off the sides. You can reduce noise or unwanted signals by mounting them vertically and rotating them to take advantage of the null. I have used one horizontally mounted at 25 feet and it was fine really. I have read that they work in vertical mode when mounted only a foot or two off the ground.

Novice 1958, 20WPM Extra now... (and get off my lawn)

Posts: 182

« Reply #2 on: November 28, 2017, 07:23:09 PM »


I think you'd be disappointed with any noise metric that attempted to collapse the complex picture involving frequency, time, location, etc to one number.  Probably the most practical thing to do, lacking a spectrum analyser (or good SDR) and a more formal site survey approach, is to choose a few of your favourite frequencies and then use your S-meter to log background noise levels in a few locations at various times of the day.

Regarding the loop, the main benefit of small receive loops comes from the excellent nulling ability of a well-balanced system.  The nulls are perpendicular to the plane of the loop, with maxima in the plane of the loop ("off the ends").  The pattern comes from simple geometry involving a loop very small in wavelength terms, and a varying angle of arrival of signals. It's well described in a large number of references.  A high-Q tuned loop does reject out of band interference, noise etc. and, while that is useful in keeping large signals out of your receiver, it doesn't affect the in-channel signal-to-noise ratio to any great extent in a decent receiver.

These days you don't have to use a tuned loop for receive.  For example, I use a rotatable, broadband Wellbrook loop mounted vertically about 1 loop diameter (1m) above a garden bed.  It's a symmetrical, well-balanced system and I have no trouble getting noise reductions of 25 dB or so relative to my large doublet and 80m transmitting loop, with better results on some signals. It's important to locate the loop well: it's all about reducing the noise pickup with these small aperture antennas since the signal component is often many dB below what you will receive with a big antenna.  To win in the s/n stakes, which is usually possible, the noise component has to be more than proportionately lower.  In your case I guess you might settle on breaking even, but you get the idea. By the way, there are other options apart from the Wellbrook loop, including a DX Engineering model or homebrew alternatives.

The small loop you mention is both a transmit and receive version, if I read the catalog correctly.  I can see the attraction of that but you may find the manual tuning a pain.  It will be very sharp: a few kHz at most.  If you locate the loop indoors to allow access to the tuning control you'll sell the loop short on its receive s/n potential, since it will couple to the mains wiring etc in the building.  If you put the loop outdoors, you will be trekking in and out to re-tune.  If you are set on using the one antenna for transmit and receive, I would go the extra distance and get a remotely tuned loop.  It's certainly possible to use a loop indoors but, in a semi-permanent installation, you can probably do better without vast effort.

Alternatively, and maybe somewhat more attractively, you could use a separate receive and transmit antenna. You can then have a broadband receive loop located optimally, and oriented/steered for best s/n.  The transmit antenna could be anything you can fit in: e.g. a vertical with a manual or automatic tuner.

73, Peter.


Posts: 17171

« Reply #3 on: November 28, 2017, 07:33:01 PM »

Quote from: KM3K

...Additionally, for a "real" antenna, I plan to buy a W4OP loop-antenna.
If I understand correctly how that antenna operates, I'm depending on its very narrow band-width to greatly reduce the effect of these local noise sources.

While it has a narrow bandwidth, that doesn't reduce the noise on the frequency you are
listening to (unless the rig is being overloaded to the point of distortion).

The noise reduction is because the loop has a directional pattern:  a sharp null broadside to the
plane of the loop.  If your noise is coming from a single location, you can turn the antenna so
to null out the interference.

Assuming, of course, that it is only coming from one direction.  On 80m noise can propagate down
powerlines for some distance, and a loop doesn't help when you get close because from the side
a power line is not a point source, and you can't get a null.  Similarly if the noise is radiated from
your electrical wire, alarm system, computer network, etc. where the wiring extends throughout
your house.  But if it is a noisy plasma TV two houses away, or a bad streetlamp down the block
the loop may work very well.

Posts: 182

« Reply #4 on: November 28, 2017, 08:15:07 PM »

It's definitely useful to experiment with loop placement, partly for the reasons Dale mentions.  I've previously noted how my carefully thought-out first location for the Wellbrook loop turned out to be hopeless - too close to the house, and not able to null out some of the local noise.  The final location (in the garden) allows pretty good nulls, including neighbours' solar inverters, HV distributors at a km or so, etc.

There is also a hard to describe, somewhat useful, intermediate regime in which the noise is not completely nulled, nor the signal completely peaked.  In general, you won't always be able to win in s/n but I've been surprised at how much value I get from having the option of switching to the loop for 30m and below.  Quite a few hours of WSPR fun, too Smiley
« Last Edit: November 28, 2017, 08:26:06 PM by VK6HP » Logged

Posts: 8138

« Reply #5 on: November 29, 2017, 03:44:23 AM »

There is also the point that a lot of local noise sources are vertically polarised mainly electric field radiators and the loop, especially if screened, doesn't tend to react to them. Once they are about a half wavelength or more away, they are tending to be producing a far field where the electric and magnetic fields combine at right angles to give a true electromagnetic wave. At that stage, field strength decreases as the square of the distance: closer in, field strength decreases as the cube of the distance, so doubling the distance between noise source and receive antenna reduces noise by 8 times.

Posts: 480


« Reply #6 on: November 29, 2017, 04:05:28 AM »

... producing a far field where the electric and magnetic fields combine at right angles to give a true electromagnetic wave. At that stage, field strength decreases as the square of the distance: ...

Isn't it the received power that decreases by the square of increased far-field distance in free space, whereas the received E-field intensity there changes at a rate of 1/r?

Posts: 372

« Reply #7 on: November 29, 2017, 04:55:56 AM »

Small transmitting loops are a good choice for operating in cramped conditions.
As for the receiving issues mentioned above you might need to use a noise canceler such as a Timewave ANC-4 or the MFJ noise canceler which has a built in whip antenna, and it reported to work better than the timewave. I have a Timewave ANC-4, and it works, however it does not have a built in whip antenna which may be more useful in your situation.

Unfortunately modern life is hell on the HF bands with all the appliances and crap switching power supplies.

Take Care

Posts: 6517

« Reply #8 on: November 29, 2017, 05:33:56 AM »

You have local noise sources, probably lots of them!
 The best plan would be to get your antenna outside and away from them. Is there any possibility of getting permission to do that?  It would resolve a lot of potential RFI problems. Getting your antenna outside, up, and in the clear is the best way to go.

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