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Author Topic: Radials under beam?  (Read 1884 times)
VA3GUY
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Posts: 177




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« on: November 08, 2012, 01:15:00 PM »

I currently have a Hy-Gain TH3MK3 mounted on a 4.5' roof-top tower.  The mast is approx 8' high with the bottom 3' being in the rotor inside the tower, leaving 5' of mast above the thrust bearing.  The antenna is mounted about 3-4" from the top of the mast.  Using 100W, I affect 2 of the TV's on the second floor.  I was contemplating putting some radials down on the roof to see if this might reduce/eliminate the RFI.  Any thoughts on the success of this setup?

Thank you...Guy
VA3GUY
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WX7G
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Posts: 5982




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« Reply #1 on: November 08, 2012, 01:21:24 PM »

Radials will not help.

The problem is fundamental overload and the fix must be done at the receiving end (TV). Filters.

Or reduce transmit power.

Or replace the affected TV sets with sets that are not affected.
« Last Edit: November 08, 2012, 01:25:38 PM by WX7G » Logged
AC5UP
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Posts: 3842




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« Reply #2 on: November 08, 2012, 02:20:23 PM »

Radials on the roof would be like mounting the beam 5' above ground...... Not a good plan.

What WX7G said.

The antenna is doing what it should, radiating RF into the ether. Put a house up close & personal to the antenna and there will be RF in the house. It's no coincidence the TV's on the second floor are most affected.....   Wink
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VE3FMC
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« Reply #3 on: November 08, 2012, 05:03:19 PM »

Are the TV's on cable, satellite or outdoor antennas?

I currently have a Hy-Gain TH3MK3 mounted on a 4.5' roof-top tower.  The mast is approx 8' high with the bottom 3' being in the rotor inside the tower, leaving 5' of mast above the thrust bearing.  The antenna is mounted about 3-4" from the top of the mast.  Using 100W, I affect 2 of the TV's on the second floor.  I was contemplating putting some radials down on the roof to see if this might reduce/eliminate the RFI.  Any thoughts on the success of this setup?

Thank you...Guy
VA3GUY
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VA3GUY
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Posts: 177




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« Reply #4 on: November 08, 2012, 08:19:53 PM »

The TV's are on cable.  One is an LCD panel TV and the other is an older CRT type.  For the heck of it, I covered the cables (power and data) on the LCD TV with tin foil.  It didn't eliminate the RFI but at least the TV doesn't turn off now when I key up!

Thanks all...
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WA3SKN
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Posts: 5457




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« Reply #5 on: November 09, 2012, 09:08:20 AM »

Forget radials.  You have an interference problem, probably fundamental overload.  The answers are "distance, shielding, and filtering".
Get the transmit antenna away from the eqpt recving the interference as much as practical.  Get shielding between the transmit antenna and where the signal is getting into the eqpt.  And filter the leads the signal is incoming to keep it from getting into the eqpt.  A combination of these should solve just about any situation.  I have seen a setup with a TV with no interference and a 1.5 Kilowatt transmitter within 12 feet of each other in a "fringe" area.  I have also seen 30 microwatts QRPp tear up a good TV signal!
Good luck with the project!
73s.

-Mike.
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K5LXP
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« Reply #6 on: November 09, 2012, 10:28:29 AM »

at least the TV doesn't turn off now when I key up!

Gee, I'd call that a "feature".


Mark K5LXP
Albuquerque, NM
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KF6ABU
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Posts: 351




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« Reply #7 on: November 09, 2012, 10:29:50 AM »

My 5 element yagi is about 15ft above the roof, I run 1500w often. Nothing inside is effected.

You can try filtering and ferrite cores, or different non effected tv's.

My wireless mouse was effected sometimes before I wrapped the USB and power cord that connects to the base a few times around a ferrite core.
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KE3WD
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Posts: 5694




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« Reply #8 on: November 09, 2012, 12:48:14 PM »

I'll bet that the coax is not decoupled by any sort of balun at the feedpoint of the beam, and that the radiation is coming off of the coax shield here...


73
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AA4PB
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Posts: 12793




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« Reply #9 on: November 09, 2012, 12:56:01 PM »

Any distribution amps or amplified splitters in the cable line? If so, it's possible that you are overloading the amp rather than the individual TVs.
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PD2R
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Posts: 131




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« Reply #10 on: November 09, 2012, 05:25:12 PM »

I'll bet that the coax is not decoupled by any sort of balun at the feedpoint of the beam, and that the radiation is coming off of the coax shield here...

I'll second that. Try a good BalUn or just use an ugly coax BalUn: http://www.k8dns.com/balun.html
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VA3GUY
Member

Posts: 177




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« Reply #11 on: November 09, 2012, 08:41:33 PM »

I forgot to mention that I had an ugly balun up there but removed it for a Comtek BAL-11130T balun ( http://www.dxengineering.com/search/product-line/comtek-jerry-sevick-w2fmi-series-current-baluns?autoview=SKU&keyword=com-bal-11130t )

I do have a distribution amp in the cable line, in the basement.

I am working on making a telescoping mast which should give me about another 3' up but based on everything I am reading, it is overload and I doubt the 3' will make any difference.  I guess ferrite cores may be the best bet.  Any specific specifications of the cores?  I know DX Engineering sells a mixed bag of them( http://www.dxengineering.com/search/product-line/dx-engineering-snap-on-ferrite-bead-combo-packs?autoview=SKU&keyword=ferrite )...any good?

Thank you...
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