Power Line Noise While Listening to Shortwave

You have apparently identified the problem as being associated with the power lines. What you are hearing is caused by corona discharge, leakage and micro arcing across insulators, or both. It is also possible that the power company is using a broadband communication system where the lines are used as the conductors (can't recall what it is called at the moment) and which is known to cause radio interference.

You could contact the power company and tell them about your situation. From what I have heard they may act or they may ignore you but it is worth a try.

My shortwave radio listening suffers from the electrical noise in my house produced by light dimmers and other things. It has two terminals for connecting a balanced antenna line and I connected a small loop antenna to these and the noise is practically gone. Your noise may be stronger than mine but this would be worth a try if your radio has antenna terminals. If not, changing orientation can sometimes help.

Neil

Yes,

I have noticed that changing the orientation of the antenna helps a lot.

I read somewhere that if you are near power transmission lines, your antenna should be perpendicular to the transmission lines, possibly because of the pattern of radiation. For some bands of shortwave, having the antenna flat on the floor helped, and there are certain spots in my apartment that are better than others for listening.

Some of the interference could be coming from within your or your neighbors' apartments. My problem here is that our house has many light dimmers installed. Some don't cause much problem, others do. It seems to depend on the particular dimmer. One way to track this is obviously to turn the dimmer off but another is to hold a portable radio tuned off station near any device which you suspect is generating the noise.

Being in an apartment probably limits what you can do to correct a problem.

The noise here decreases with increasing frequency on my portable SW receiver. Usually, above about 5 MHz is reasonably quiet though indoor reception doesn't compare well with outdoor reception. This is probably due to both increased signal and lower noise but there are no above ground transmission lines here.

Neil

The aluminum tube device with a gap cut its entire length sounds somewhat like the design of what I think was called a "slot antenna," once made for high power FM by RCA.

A station I worked for had a slot antenna from the late 1940s and it was extremely efficient and was still performing well all the way in 1980. Of course that antenna didn't have a ferrite loop inside, but the external shield design sounds similar. Also, the slot was not cut the entire length, but was several smaller slots I guess spaced based on the station's wavelength. Still, a possible variant of the idea.

This general string of comments this morning got me thinking about the ferrite antenna, so I searched and found

http://www.radio-electronics.com/info/antennas/ferrite_rod_antenna/ferrite_rod_antenna.php

Operation of a ferrite rod antenna (abbreviated)

Normally this type of RF antenna design is only effective on the long and medium wave bands, but it is sometimes used for lower frequencies in the short wave bands although the performance is significantly degraded, mainly arising from the losses in the ferrite. This limits their operation normally to frequencies of only a MHz.

Ferrite rod antennas are normally only used for receiving. They are rarely used for transmitting anything above low levels of power in view of their poor efficiency. It any reasonable levels of power were fed into them they would soon become very hot and there would be a high likelihood that they would be destroyed. Nevertheless they can be used as a very compact form of transmitting antenna for applications where efficiency is not an issue and where power levels are very low. As they are very much more compact than other forms of low or medium frequency RF antenna, this can be an advantage, and as a result they are being used in applications such as RFID.

My comments on that article: In the first paragraph you will note the mistake that shortwave frequencies are being called lower than long wave, which obviously is the opposite of the facts, but the rest of the item does make the interesting comment about using a ferrite antenna for low power transmission. Because the ferrite "heats up" from the transmitted energy, it seems that much of the transmission energy is therefore lost as heat. Interesting to think about.

I read that to be the lower frequencies of the short wave band rather than lower than the frequencies of the medium or long wave band. It could have been worded much better.

Many of the table top radios I repaired in the '50s and '60 had a flat rectangular loop antenna attached to the rear Masonite cover and used no ferrite. My impression was that they didn't work as well as the ferrite loop antennas.

Also, a true slot antenna has to be approximately the dimension of a dipole at the operating frequency so this is impractical for AM wavelengths. Arrays of slots are commonly used in radar antennas where the wavelengths are a few centimeters. This gives a very narrow lobe in a small space.

Neil

HI all,

Appreciate the responses and I think i'll check out the antenna suggestion!

So, I just got off work, its about 3:40am. I brought my SOny 7600GR radio with me, which I attempted to test at the radio station. Not a good idea, mostly all static under the FM transmitters. I work on the AM, and the transmitters are actually way out west from me.

So I get out of my car outside my apartment, its 29 degrees out, take my radio up and stand RIGHT UNDER the high voltage transmission lines.

Turn on the radio, THE FIRST THING I HEAR IS AN EMERGENCY ALERT MESSAGE FROM ANDREWS AIR FORCE BASE in the 2mhz range, CRYSTAL CLEAR! Never heard anything like it before, very exciting.

Tuning all up and down the bands, I had no static or noise like in my apartment. Beautiful signals, coming in from every direction. I picked up the time station in Hawaii on 10Mhz, they were doing Pacific weather reports.

SO, I must live in RF hell in my apartment, being on the 6th floor I suspect that's why I'm able to pull anything in at all. ANyway, time to pass out.

First thing I do when I wake up, I'm going outside to listen.

I know this isn't really Part15 related, but I can't contain myself!

One more thing:

Hey Phil, is this similar to what you describe?

http://cgi.ebay.com/SONY-AN-LP1-Portable-Active-Antenna-Lightly-Used-/260730127649?pt=Shortwave_Radios&hash=item3cb4ba0521

This is made just for my radio. WOuld this help my situation indoors? It looks like it is covered with some fabric but I can make out the loop.

Signing off,

Geoff

I know this isn't really Part15 related...

Actually it is since this information can be passed to listeners to part 15 stations who have a similar problem. Please keep us advised as you experiment with this.

The antenna you cited may or may not help since an active antenna will amplify the noise as well as the signal. The loop antenna I mentioned in an earlier post does suppress the noise and gives the ability to maximize the signal by changing the orientation of the loop. This could also function to reduce the noise if it is coming from a different direction than the signal. This seems to be the case with my loop. You might consider asking the seller about his experience with the antenna regarding man made noise.

As I read about the Faraday shield on a loop antenna I became a bit curious about its effect since a radio wave contains both electric and magnetic fields and the two are inseparable. If one field is removed so is the other so I tried an experiment. I tuned my radio to a moderate strength AM signal in the BCB and then enclosed the loop in aluminum foil. The signal disappeared confirming that the shield blocked the signal.

Perhaps a metal box with one side open could block interference (and signals) from five directions and could enclose the antenna with the open side against the window to pass signals from this direction and to suppress interference originating elsewhere in the apartment.

Neil

Most shielded loop antennas leave a small section open, unshielded. Did your "aluminium shield" experiment leave a small gap in the shielding? I think it has something to do with keeping the shield from being a shorted loop.

Geoff,

This post caught my eye because it is a subject near and dear to me.

You should consider using an external "shielded magnetic loop antenna". The basic idea is they respond to the magnetic field instead of the electric field, as is the case for whip antennas. Here is one quote that sums it up nicely:

"Most active antennas are the whip type and respond mainly to the electrostatic field portion of an electro-magnetic radio wave. The Magnetic Loop responds primarily to the magnetic field and this ensures high rejection of nearby electric fields. The intensity of the electric field is usually higher than the magnetic field when an antenna is close to interference sources such as TVs, florescent lamps, power line wiring etc. By rejecting the electric field there is a reduction in local interference compared to other types of active and passive antennas.”

I found the manual on line for your receiver. It has an external antenna jack which has a DC output voltage for supplying power to “approved” external active antennas. When connecting to a loop antenna you will short this voltage unless you insert a series capacitor (.1uF ceramic) to block the DC.

Loop antennas are of two types: tuned and broadband. For receiving across a wide frequency range, you want a broadband configuration. Broadband simply means there is no capacitance across the loop that will resonate it to the operating frequency. To do this, manufacturers make broadband loop antennas by simply amplifying the signal locally at the antenna before sending it over coax to the receiver. There is no resonating capacitor, just a very small capacitance due to an FET amplifier input.

A highly rated and expensive active loop antenna is the Pixel PRO-1A Shortwave Shielded Magnetic Loop Antenna, http://www.pixelsatradio.com/, priced at $399.99. It and covers 100 kHz to 30 MHz.

For experimenting, you can construct a shielded magnetic loop antenna with a length of any type of coax long enough for a circular diameter of about 3 ft. Use only the center conductor at each end for the loop. Connect the coax shield on one end only to ground. Leave the shield on the other end unconnected. The loop antenna is balanced and should feed a balanced antenna input. If your receiver has only an unbalanced input, then it's probably best to use an amplifier that has a balanced input and an unbalanced output. The coax shield acts as a “Faraday shield”. It blocks the electric field while allowing the magnetic field to induce current in the loop.

Such a loop antenna should be aligned vertically and it is very directional. Expect to be constantly fiddling with the orientation.

The reason this subject is near and dear is that I once owned a little GE alarm clock radio that had amazing AM reception. I could pick up noise-free DX at night with it sitting on a table in the basement. Other radios produced huge noise levels mostly due to the overhead fluorescent lights. Out of curiosity I opened it up and found it had a shield over the ferrite loopstick antenna. The shield consisted of an aluminum tube surrounding the loopstick with a gap cut along its entire length. The aluminum tube was connected to ground. This slotted aluminum tube was the Faraday shield. I always wondered why such a simple thing wasn’t standard fare for all AM radios. Maybe it was patent related or just deemed unnecessary by the industry.

--------------------------------------------
Edited on Feb. 4, 2011. Modified paragraph 7, "For experimenting, you can..." to clarify how inner conductors and coax shield are connected.
Phil

Hi Geoff,

Bingo! The Sony AN-LP1 is an active loop antenna and is recommended by Sony for use with the 7600GR receiver (according to the user manual for the receiver). You won't even need batteries for the antenna because it takes its power from the external antenna jack on the receiver.

I found this little review of the antenna: http://stephan.win31.de/sony76-4.htm#an-lp1 Click on "The AN-LP1 active antenna" in Page Contents at the beginning.

Here is one source for the antenna manual. It even has the schematic for the amplifier! http://www.electronica.ro/schematics/sony/antennas/an_lp1.pdf

Phil

I say just toss a Discone out there, sure they are nearly deaf at the lower SW frequencies (they don't really work till 25mhz!)
but there wont be noise either 😉

I've found the noise reduction by just sticking a discone in my back yard running a coax to my receiver inside cuts the noise well enough, that although the built in antenna provides a "stronger" signal, the cut in noise is worth the cut signal, an even trade.

The question was asked: "Did your "aluminium shield" experiment leave a small gap in the shielding?"

No, so I repeated the experiment but this time I tuned to a strong local station so I could hear a signal with the foil in place. With the foil totally covering the loop the signal meter (S) indicated 4.

With approximately a 3/8 inch gap, S = 5.

With the foil removed, S = 9.

There is a big difference between 5 and 9 on this receiver.

With the foil in place the signal may have been picked up by the unshielded lead wires from the loop to the receiver. Also, the foil was not grounded to the receiver ground.

This is not a very scientific experiment and the capacitance effect of the foil on the loop may have been a factor but at the test frequency of 610 kHz it should not be too much.

The increase from 4 to 5 without then with the gap could be due to exposing the loop to the RF field.

Theory tells us that a propagating wave will have both an electric and magnetic field and if one is removed so is the other. I expect a more carefully controlled experiment would validate this. It is possible that the field will get through a metal shield due to eddy current effects but there will be attenuation. Since my experiment was not carefully done I cannot say this explains the data.

Neil

Neil,

I think maybe your results even with the gap in the foil are clouded a bit by a few things.

1. The foil shield must have a gap to prevent it from just shorting across the loop. Your second experiment was correct in that it had a gap.

2. The gapped shield must be connected to the receiver ground to be a true Faraday shield.

3. The foil wrapped around the loop will change the capacitance of the tuned circuit. A more accurate evaluation would involve re-tuning the antenna stage in your receiver.

Theory says that a loop antenna or loopstick antenna responds only to the magnetic (H) part of the electomagnetic (EH) signal. But, shielding will improve the rejection of the electric (E) component of the signal, and further decrease local electrical interference which is mostly electric (E).

The way to really evaluate the effectiveness of a shield on a loop antenna is to test it in the presence of significantly high local electrical interference and see if there is a change in the interference level with vs. without the shield (make sure you adjust tuning to resonance). This comes from my understanding of what I have seen on the web about SHIELDED loop antennas, and from the dramatic performance of that long lost RCA clock radio that had the gapped aluminum shield surrounding the loopstick. BTW, I loaned that radio to my stepdaughter, and she ended up throwing it away. Retro-yikes.

Phil