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Author Topic: G5RV causing RFI  (Read 2944 times)
KB5VYW
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« on: March 01, 2004, 11:58:57 AM »

I put up a G5RV but am having trouble with RFI in the house.  I am only running 100 watts max from a Kenwood 570D.  I have 100' of LMR-400 coax running out an attic vent to the antenna.  This is a regular sized G5RV (102 feet long with a 31 foot matching stub).  I have the center of the antenna raised up about 20 feet in a tree with the matching stub hanging down.  Since the stub is 31 feet and I can only get the antenna about 20 feet, the stub comes down the trunk and then loops back up about 8 feet or so and connects to the coax which is coming down from the roof.

The antenna works but bleeds RFI all over the house.  The 80 meter band turns a lamp on and off (they type you just touch to turn on or off), the 40 through 10 meter bands are coming through various speakers in the house (some like a PA system and some with just muffled noises).

I have added a home-made choke balun at the feedpoint of the antenna by winding 8 turns of coax around a 4 inch PCV pipe but that didn't seem to do much.

Is this an inherent problem witht he G5RV design?  Would I have better luck with something like the Alpha-Delta DX-CC since I would not have a matching stub hanging down?

I am not allowed to put up a "visible" antenna due to deed restrictions.

Any help here would really be appreciated.

Thanks,
Hutch
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WB2WIK
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Posts: 21836




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« Reply #1 on: March 01, 2004, 02:03:25 PM »

Hi Hutch,

It's not a "G5RV problem," it's surely a "proximity problem," i.e., your antenna is too close to your house!  (Not an uncommon problem, by the way.)

When it comes to fixing RFI, there's no substitute for "distance."  I'd bet if that same G5RV was installed 200' from your house, you'd have no problems at all.

Now, it is possible your feedline (coax) is radiating, and adding to the problem.  A G5RV is a lousy load on most bands other than 20 meters, and that's why you need a tuner.  When the feedpoint match (between line and load, up at the antenna feedpoint) is very poor, it's likely the coax outer conductor will be carrying RF current from the antenna back down to your tuner.  A very good RF choke balun (coil of coax, lots of turns, solenoid-wound, taped tightly, installed right at the coax-to-ladder line connection point) will help reduce this a lot.  How well a choke balun works depends a lot on how it's made and where it's located; you could try experimenting with this.

The fact that LMR400 is low-loss and double shielded doesn't help in this regard, unfortunately.  Any RF current carried back from load to source is carried on the outermost surface of the outer shield of the cable, so even if the cable had fifteen shields, that still wouldn't help.

I'd recommend experimenting with the choke balun at the feedpoint; and, if at all possible, locating the antenna itself farther away from your living quarters.  In lieu of that, there's always "ferrite!"  Ferrite toroidal chokes placed over electrical leads of applicances being interfered with, and using large ones so multiple turns of those leads can be wrapped through the chokes, can help a great deal in eliminating interference where the problem really is, which is with the appliances themselves.

WB2WIK/6

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W4VR
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« Reply #2 on: March 06, 2004, 02:41:04 PM »

Number 1....your antenna is to close to your house.  Number 2....on some bands the transmission line radiates as much as the dipole itself.
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K6XR
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« Reply #3 on: March 11, 2004, 02:46:18 AM »

I did use a g5rv with my ts-570dg with great success. I purchased a line isolator from Radioworks and installed it at the junction of the coax and the 450 ohm ladder line. My ladder line comes down a fiber glass push up pole which supports the apex of the antenna, to just off the roof of he house about 2 inches and connects to the line isolator. I used 70 feet of 9913 coax from there to the tuner. I used the antenna on all bands 160-10 meters with no ill effects 5 watts to 700 watts. I coiled up the unnecessary length of coax just outside the entrance to the shack.
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K8AG
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Posts: 352




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« Reply #4 on: June 10, 2004, 02:04:23 PM »

Hi,

I switched from a G5RV to a ZS6BKW that I made myself.  It solved many of my RFI problems.  Seems the ZS6BKW is optimized for lower SWR and some WARC bands as well.  Anyway, my RFI is now only a small annoyance to my daughter whose TV is fairly close to the antenna.

73,

JP, K8AG
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