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Author Topic: 40m antenna playing havoc with computer and tv  (Read 1087 times)
MW3HWM
Member

Posts: 11




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« on: May 16, 2009, 03:00:24 PM »

Hi,
could anyone tell me why my 40m flat top dipole is causing me major problems with my desktop pc and tv.

I have 1.1 vswr almost thru the entire 40m band.
I have a coax choke to supposedly stop rf coming down
the outside of the coax.

Dipole is just above my house roof,and i am using 100w.

Sometimes a message flashes up on pc screen saying your usb port has a problem (when i transmit))and the tv goes gaga (total wipe out).

Feed is rg58 ,tried rg213 but its to heavy on the center feed point.

My shack is in an upstairs bedroom, rig is near pc.
Surely the coax 1.1 choke balun, should help this ?.

Thanks
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G4GOY
Member

Posts: 20




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« Reply #1 on: May 17, 2009, 03:18:48 AM »

Hi

Sounds like your signal is being picked up by the leads going into your PC and TV.  Ideally, it's best to locate your antenna away from the house, but I know that isn't always possible (I've been in that situation myself).  Therefore, you need to try and prevent RF from entering the PC and TV. Get hold of some ferrite cores for RFI prevention (I believe the RSGB sells them).  Now for each lead that enters the PC (ie mains, keyboard, mouse, etc), wrap as many turns of the lead as you can in one of the cores, as close to the point of entry to the PC as possible.  These effectively create a high impedance to RF at that point.

You can try a similar trick with the mains lead entering the TV and hopefully this will cure it.  However, it may be that RF is being directly picked up by the TV itself. There are means of reducing this pickup and I would look at one of the RFI books out there for more help on this (I havent had the problem myself, so cant offer a tried solution).

Anyway, I hope that this helps.

Best 73, John G4GOY
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WW5AA
Member

Posts: 2086




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« Reply #2 on: May 18, 2009, 07:12:16 AM »

It could also be a grounding problem. A second floor shack is prone to ground loops and long grounds which act as an antenna.

73 de Lindy
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AA4PB
Member

Posts: 12990




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« Reply #3 on: May 18, 2009, 07:30:33 AM »

With a properly installed dipole you shouldn't require any type of RF ground on the radio equipment. If the antenna is close by (like 5-10 feet above the roof) then its probably coupling into wiring in the house.

Is the TV located upstairs? Is it tied to cable or are you using an antenna? Are there any amplifiers or amplified splitters in the TV line? These broad-band amplifiers are often easily overloaded by RF.

The best way to troubleshoot the TV issue is to start by disconnecting everything from it in order to determine how the RF is getting in. For example you might find no RFI with the cable not connected. You might find no RFI with the cable connected directly to the TV. No RFI with the VCR connected to the TV but RFI is present when the cable is connected to the VCR. That tells you that the RF is comming down the cable and affecting the VCR. You might find that the RFI appears as soon as you touch the cable shield to the VCR, even without connecting the center conductor. That tells you the RF is flowing on the shield of the cable. Winding a choke from multiple turns of the cable or multiple ferrite beads on the cable might help in that regard. If you can determine what equipment is being affected and how the RF is getting there then it helps in determining how to stop it. That's often easier than just putting beads on everything and hoping for the best.
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AA4PB
Member

Posts: 12990




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« Reply #4 on: May 18, 2009, 07:30:33 AM »

With a properly installed dipole you shouldn't require any type of RF ground on the radio equipment. If the antenna is close by (like 5-10 feet above the roof) then its probably coupling into wiring in the house.

Is the TV located upstairs? Is it tied to cable or are you using an antenna? Are there any amplifiers or amplified splitters in the TV line? These broad-band amplifiers are often easily overloaded by RF.

The best way to troubleshoot the TV issue is to start by disconnecting everything from it in order to determine how the RF is getting in. For example you might find no RFI with the cable not connected. You might find no RFI with the cable connected directly to the TV. No RFI with the VCR connected to the TV but RFI is present when the cable is connected to the VCR. That tells you that the RF is comming down the cable and affecting the VCR. You might find that the RFI appears as soon as you touch the cable shield to the VCR, even without connecting the center conductor. That tells you the RF is flowing on the shield of the cable. Winding a choke from multiple turns of the cable or multiple ferrite beads on the cable might help in that regard. If you can determine what equipment is being affected and how the RF is getting there then it helps in determining how to stop it. That's often easier than just putting beads on everything and hoping for the best.
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MW3HWM
Member

Posts: 11




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« Reply #5 on: May 18, 2009, 07:40:49 AM »

Thanks for the replies.
Will look into putting ferrites on the leads to pc and tv.
But I am concerned that this may be radiating into the
houses on each side of me. No knocks at the door yet though.

Also will look at the possibility of my ground wire
being an antenna on second floor.

Been thinking about a line isolator on the feed, supposed
to be equal to 200 ferrite clamps on the coax feed.

Anyone used line isolators, are they any good ?

Thanks.
73
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MW3HWM
Member

Posts: 11




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« Reply #6 on: May 18, 2009, 07:55:19 AM »

AA4PB Interesting checking procedure.
Will try this tonight to see if rfi is jumping on my cables.

But if it is, then it surely will be jumping on the houses next to me.

My tv is connected to satellite ,no external antenna
or amplifiers needed for tv.
My rig is on the desk next to pc, so on screen usb warning message may mean that my in-coming feed may be
too close to the pc.

Dont have this problem on any other antennas though.
Just wondering if i am transmitting straight through the roof into the bedroom with the above element.
If this turns out to be the case, I would have to move
the antenna further away, and turn it into a folded dipole.

Thanks
73

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KB9CRY
Member

Posts: 4283


WWW

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« Reply #7 on: May 18, 2009, 01:31:53 PM »

Dipole is just above my house roof,and i am using 100w.



Too close to the house.  Raise it up higher.
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W4ZDI
Member

Posts: 34




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« Reply #8 on: May 30, 2009, 04:58:25 AM »

I had the same problem with one of the house TVs while transmitting 40 watts to a G5RV.

Replacing a defective (poor ground shield) TV cable between wall outlet and TV corrected the problem.

Hope your problem is as simple as that.
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