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TV Interference on my HF Shortwave Receiver

11/06/2006 9:03 AM

Hi,

I am getting interference from electrical items such as my TV on my HF shortwave receiver.

When the TVs are turned on, I get a hum on my radio receiver, these are on certain frequencies from 0 to 30mhz.

What kind of filters do I need to stop the TVs interfering with my radio?

Does the filter then go on the TV or my radio?

Thank you.

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#1

Re: TV Interference on my HF Shortwave Receiver

11/06/2006 4:10 PM

It may be the TVs IF oscillator being picked up by your radio, or it may be happening over the power wires, if both devices are plugged into the same circuit. If it's the IF, I would try a directional antenna on your radio. If it's the power, there may be commercial power filters available.

Or you could just turn off the TV.

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#2

Re: TV Interference on my HF Shortwave Receiver

11/06/2006 5:43 PM

Most display devices that use a Cathode Ray Tube (CRT) create Radio Frequency Interference at multiples of the horizontal scanning frequency. In North America, the horizontal scanning frequency is about 15,750 KHz.

If your HF short wave receiver is using the "indoor antenna" you should be able to receive the harmonics of the Horizontal deflection circuit from the TV. If you tune to a multiple of 15,750 KHz on the SW radio, hear the "buzz", try changing channels on the TV and see if you hear a change on the SW signal associated with the horizontal circuit resyncing. (this probably won't show up if you are using the TV in the cable mode).

If you determine that the TV is the cause, using an external antenna, and a really good coaxial cable feed with a balun at the antenna. Grounding the receceiver (SW) to a good earth ground is needed too.

In my experience, filters do not work.

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#3

Re: TV Interference on my HF Shortwave Receiver

11/07/2006 12:54 AM

All of the above and modern TV's also have computer chips to control various systems. The problem is that your short wave receiver is very sensitive and it is unlikely that you can filter the noise without filtering the desired signal. Separate the two instruments as much as possible, put your SW antenna outside as far from the house as possible, use shielded cable to connect the antenna. If all else fails, turn off the TV. I have the same problem with AC lamp dimmers which put out a lot of broad band noise. The only solution is to turn them off when I want to listen. Then, the SW receiver only picks up noise from its own internal microcontroller. Some things you just can't win.

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#4

Re: TV Interference on my HF Shortwave Receiver

11/07/2006 6:08 AM

Talk about serendipity. I just posted the following about earth on the previous thread.

This is nothing like the capacity that you are talking of but in my home I ran a separate power and earth circuit for all my electronic equipment. The earth consisted of a 12mmØ cable with short tails going to each of the outlets. The improvement in the quality of TV reception was quite remarkable and may people commented on how clear the picture was.

The upshot is that if everything is earthed properly then the shielding will generally work well any you would be surprised ho much RF interference that you will get rid of. It costs a bit to do but the results are usually well worth the effort and cost.

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#5
In reply to #4

Re: TV Interference on my HF Shortwave Receiver

11/07/2006 9:29 AM

Yeah, I remember 100 years ago in my ham radio days that a good ground - one that actually goes into the earth - I used a cold water pipe, is essential for the best radio reception. If your electrical plugs are the three wire type, and the ground wire actually goes to a post outside that's driven into the earth, that should be good, too.

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#6
In reply to #5

Re: TV Interference on my HF Shortwave Receiver

11/07/2006 11:34 AM

The earthing system in most houses is fine for 50 or 60Hz and worked well for a long time. It was however never designed for things like PCs that have clock speed between 1 and 2GHz. The standard earth is next to useless at these frequencies and that was one of the reasons I installed the 12mmØ earth cable.

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#7
In reply to #6

Re: TV Interference on my HF Shortwave Receiver

11/07/2006 11:42 AM

Can you translate 12mmØ to American?

I was actually referencing the radio's 3-30MHz HF antenna, not your PC's clock.

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#10
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Re: TV Interference on my HF Shortwave Receiver

11/07/2006 10:22 PM

Sorry I keep forgetting you don't use SI units. 12mmØ is roughly half inch thick cable.

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#11
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Re: TV Interference on my HF Shortwave Receiver

11/08/2006 10:09 AM

I got the 12mm, it's the Ø that threw me.

I try to make a point of not remembering anything I learned about wave guides, but since 1-2 GHz signals travel mostly on the surface of a conductor, wouldn't 1/2 inch copper water pipe serve just as well?

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#13
In reply to #11

Re: TV Interference on my HF Shortwave Receiver

11/08/2006 11:57 AM

It's not the copper water pipe that is a problem (unless it is a very long run), it is what the water pipe is attached to at both ends, smaller diameter wire at the device end, and the actual in-ground water pipe is usually black iron or stainless steel, neither one is a great conductor for RF.

The ideal ground for RF is a long steel (for strength) rod with a thick copper coating driven into the ground at least 8 ft. or you may use up to 4 shorter 4 ft. rods to get about the same effect. Both of these are usually available at Radio Shack or home stores. I once had pretty good luck with a galvanized steel pipe, but it was 2 inches in diameter and went straight down 12 feet into fairly conductive soil with a high water table.

This should be connected with as short a run of multi-stranded (braided or woven preferred) copper strap or cable or a thick copper bus bar.

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#14
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Re: TV Interference on my HF Shortwave Receiver

11/08/2006 12:01 PM

Don't you get corrosion at the steel / copper interface?

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#15
In reply to #14

Re: TV Interference on my HF Shortwave Receiver

11/08/2006 12:14 PM

Only if the copper cladding has been pierced, allowing oxgyen to the steel surface at that point. I am not sure if the connection is a strap or a tapped hole, but it should at least have protective plating for the steel. These things are usually well-made to prevent corrosion, since the environment they will be used in is well known. Only if someone tries to "homebrew" a ground rod are you likely to find corrosion effects from galvanic action.

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#16
In reply to #11

Re: TV Interference on my HF Shortwave Receiver

11/08/2006 9:35 PM

The Ø is a drafting and mathematical shorthand for diameter. I thought the abbreviation was fairly universal so I apologize, I will use dia. In future

Yes, the half inch copper pipe would work nearly as well but it would be difficult running a water pipe form outlet to outlet and trying to make sure that there was only one point of contact with the ground. Multiple contacts means earth loop and that means noise.

That raises another point. If whatever the house is earthed to comes in contact with the ground at more than one location you will have an earth loop and it will cause noise. Water pipes contact earth all over the place so from a noise point of view would be next to useless.

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#17
In reply to #16

Re: TV Interference on my HF Shortwave Receiver

11/09/2006 10:14 AM

The standard in the US is to tie all the grounds to a metal post driven deep into the earth, at one point outside the house. It's primarily for safety, not noise reduction. One of the problems is that it's a series connection (or several parallel / series connections) from fixture to fixture and outlet to outlet, and the connection at each outlet is just wire to wire or wire to screw or clip, and these are not always the best connections, especially over time.

I think many houses will have the water pipe grounded only at one spot - where the water pipe comes in from the main. In my house, all the pipes are encased in the concrete slab, except the one that connectes to the main. In my dad's house, all the pipes run under the house, but above ground, except for the one that connects to the main.

For radio work, probably the best solution is a separate single point ground run from the earth to your radio equipment, and not used for anything else.

I should know this, since I work at NASA, but I'm a digital guy not a radio guy, but I wonder what kind of grounding scheme is used for all the big antennas we have around here?

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#18
In reply to #17

Re: TV Interference on my HF Shortwave Receiver

11/09/2006 10:58 AM

Bhankill having two earths works well but has one potentially dangerous drawback. When lightning strikes the current dissipates radially outwards form the point it reaches earth. The result can be potentials of several thousand volts per meter between different radii. If your' two earth stakes aren't equidistant for the grounding point of the lightning you can end up with several thousand volts between the two earths. This can cause enormous damage to equipment not to mention what it can do to people so generally two earths isn't a good idea unless they are right next to each other.

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#19
In reply to #18

Re: TV Interference on my HF Shortwave Receiver

11/09/2006 11:07 AM

You don't need a separate stake, just a separate connection to it. I like the idea of developing a multithousand volt potential between two stakes in my yard, though. Maybe I could build a lightning powered jacob's ladder.

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#20
In reply to #19

Re: TV Interference on my HF Shortwave Receiver

11/09/2006 11:39 AM

Bhankill that exactly what I did. I used the existing earth stake and ran the 12mm dia. Cable from there past each of the critical outlets then short tails coming off that to the outlets. It worked remarkably well. I thought I would mention the problem of two separate earth points though as it is potentially dangerous.

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#12
In reply to #10

Re: TV Interference on my HF Shortwave Receiver

11/08/2006 11:42 AM

Masu,

You said, "12mmØ is roughly half inch thick cable" Don't you mean "half inch diameter cable"? "Half inch thick" could also be "one inch wide" if it is rectangular or oval cross-section instead of round. Flat cables are fairly common.

Also, if you use a symbol for the diameter instead of the abbreviation (dia.), the symbol should go before the value, as in Ø 12mm.

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#21
In reply to #5

Re: TV Interference on my HF Shortwave Receiver

11/09/2006 1:01 PM

Note! for the youngsters of today... The water pipe ground has to be metal!! PVC and other plastic pipe used in many houses just dooooooon't cut it.

Bill

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#8

Re: TV Interference on my HF Shortwave Receiver

11/07/2006 2:52 PM

You would be very lucky indeed if your building contractor installed a "good earth ground" for your electrical wiring. Most merely run a wire to the nearest cold water pipe, and expect that to serve as the earth ground.

For most DC and 50/60 Hz AC this is adequate safety protection, but it does not do a lot of good for HF and above frequencies, which, as noted above, includes many TV IF and PC clock frequencies, which will generate RF noise. The noise may be either the primary frequency, if it lies in the HF band you are trying to listen, or a harmonic (multiple) of a lower frequency being generated. This noise escapes from the devices two ways, direct radiation, due to poor shielding of the device, and conduction, usually on powerlines or the outer shield of audiio and video cables. It may even be a combination of conduction to a wire outside the shielding, the radiation, as the wire acts as a transmitting antenna.

The noise can get into your HF receiver the same way it escapes from the other devices, either directly into your circuits, because the radio is poorly shielded, or being picked up when your receiver's power cord or antenna cable acts as local antenna. If you don't have an external antenna on your shortwave/HF receiver, that is the first thing you should do. Not only will it help reduce the noise from your own devices, it will also allow weaker stations to come in much stronger, with a higher s/n ratio. If you already are using an external antenna, and have coaxial cable between the radio and the antenna, try obtaining some ferrite "beads" which are usually contained by a plastic holder. You can get these at most Radio Shack stores and any indusrial electronics or Ham radio store. These are split into two halves and clamp around the cable, effectively stopping the flow of stray RF along the outer surface of the shield, without affecting the "Good" RF from your external antenna. Place these as near to your receiver as possible. You can get similar power cord ferrite beads, if your power cords do not already have them. Place them on all power cords involved, PC, TV, Cable box, receiver, etc. Notice that many PC cords already have such devices built in just for this purpose. Cheaper units may leave them out to save a little cost. Sometimes you are able to do the same thing by coiling up several 4-6 inch turns of the co-axial cable and taping them together, making an RF choke.

Also, there is a big difference between a "good ground" for DC and low-freq. AC versus RF at HF and above. For this you need a good RF ground, which is also grounded for DC to prevent static buildup. A good RF ground can be a large strip of copper several feet long, or large copper plate, or a long strip of very wide (1 inch or better) flat and thick copper braid. Some ham operators try to get by with stripped out co-axial cable shielding braid, but this is often inadequate, especially if there are strong local RF sources. MFJ also sells an "artificial ground" device, MFJ-931, but bear in mind that is for RF only, not DC. They also sell various filters and other radio accessories. However, beware that MFJ quality is hit-or-miss. It is not for no reason they are known in hamdom as producing Might Fine Junk! Fortunately, they have excellent, "no hassle", return policy. Many local industrial electronics and CB or Ham radio shops also sell MFJ products.

If all else fails, then you will just have to put HF filters on your antenna cable. If the noise is lower than your listening frequency and generating harmonics in your receiver, a high-pass filter may work. If it is higher, but overloading your circuit anyway, a low-pass filter may help. If you are getting noise from multiple sources, high and low, a band-pass filter may work, but if the noise is right inside the band you are listening, you may only be able to tune it out with a notch filter, which can be either tunable or specially desigined for one or more undesirable frequencies. Some DC power supplies, especially those intended to power HF radios, include a tunable notch filter, putting a deep null on any stray signal that comes in over the powerline.

By all means, be sure your radio is well-shielded and well-grounded, or nothing will help!

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#9

Re: TV Interference on my HF Shortwave Receiver

11/07/2006 9:23 PM

Just try to relocate the equipment & TV. You will get by trial a location where minimum of interferrence is sensed

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#22

Re: TV Interference on my HF Shortwave Receiver

11/09/2006 4:56 PM

Cover all the ceiling walls and doors of your house with copper plate. Use copper mesh over all the windows. Make sure to use RF tight gaskets at the doors and windows. Actually why not go one better and use copper plated mu-metal (high nickel stainless) and TIG weld the seams. Grid the land around you house with burried heavy gage copper (like they do at AM broadcast sites). Let me know how that works out for you.

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#23
In reply to #22

Re: TV Interference on my HF Shortwave Receiver

11/09/2006 10:12 PM

Better still move to "Cooberpedy" they live underground because it's too hot to live on the surface. An added bonus it that while digging your house you are likely to find opals. I know of one person that came out $200,000 in the green after building his house.

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#24
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Re: TV Interference on my HF Shortwave Receiver

11/10/2006 2:52 AM

I always wanted an underground house though you still need the shielding between rooms but it does make the "grounding" easier. Of course at some point short wave radio may be completely replaced by internet "radio". It won't be as much fun though. By the way, where is Cooberpedy?

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#25
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Re: TV Interference on my HF Shortwave Receiver

11/10/2006 5:19 AM

Coober Pedy is 29ºS 134.7º E or about 780Km (480 Miles) North North West of Adelaide Australia. It is fairly close to the center of the state of South Australia. Its an opal mining town You can find out more at the following link

http://www.opalcapitaloftheworld.com.au/

and they even have an international standard underground hotel

http://www.desertcave.com.au/

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#26
In reply to #22

Re: TV Interference on my HF Shortwave Receiver

11/13/2006 9:41 AM

Just move underwater. You'll be shielded from all but the lowest (ELF) frequencies. And you'll never have to buy an aquarium.

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#27
In reply to #26

Re: TV Interference on my HF Shortwave Receiver

12/05/2006 3:18 PM

The only real solution is an aeriel system well away from the sources of interfence fed in by good quality co-ax cable.

If you can bury the cable so much the better , an amplifier at the aeriel end of the cable would help but you may then require an attenuater at the reciever end to avoid overloading the reciever with resultent cross modulation.

A good ground connection on the reciever would also help, the amplifier will require a source of power care should be taken that interference is not fed into the system via this, an auto type battery a the aeriel end charged when reception is not required might be a solution

HF band reception is difficult, one of the reasons NASA are going to the Moon!

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