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Author Topic: G5RV Installation Question!  (Read 1410 times)
KB3Z
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Posts: 38




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« on: May 10, 2013, 09:37:14 AM »

Good Afternoon, My name is Mark Griffin and my call is KB3Z.  I have had a G5RV at my home for about 5 years. And I use it primarily for 30 to 80 meters. I was reading an article this afternoon that said that the  ladder line from the center of the antenna is able to hang vertically for at least 20 feet. Ensure that the ladder line is not close to any conductive objects, such as metal, to prevent improper operation and performance of the G5RV.

The way I have my antenna installed is that it is mounted about 35 feet up buy the ladder line is not hanging vertically but is going at about a 45 degree angle from the center conductor of the antenna. Will this impact the antenna performance? I know, it's probably a dumb question but I am just curious.  Thanks!!  Mark KB3Z
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G4IJE
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Posts: 243




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« Reply #1 on: May 10, 2013, 10:30:14 AM »

When you say 45 degrees, is the ladder line angled down towards one end of the G5RV - kind of passing underneath the antenna wire? That would not be ideal, but not the end of the world. If it's angled at 45 degrees but still 90 degrees from each wire, then it's absolutely fine. I hope you see what I mean - describing stuff in three dimensions is not easy ;-)

73, Paul G4IJE.
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KB3Z
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Posts: 38




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« Reply #2 on: May 10, 2013, 12:59:13 PM »

The ladder line is basically going to the left at a 45 degree angle underneath the antenna.
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K0ZN
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Posts: 1542




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« Reply #3 on: May 10, 2013, 02:45:51 PM »

That is not good. It is closer to one side than the other and that causes "antenna currents" on the ladderline, effectively reducing the natural cancellation that exists in balanced parallel conductor transmission line. Strongly recommend you pick up or at least look at the early chapters of the ARRL Antenna Book for details and graphic explanation of the situation.
This is not a "deal killer" but with unequal currents you will have more RF on the transmission line and possibly increased chances of RF in the shack.

 Bottomline:  you want the transmission line to come away perpendicular to the antenna wire.
This is true for any balanced antenna such as dipoles, G5RV's, Extended Zepp's, etc. When you get the transmission line closer to one side, the line starts coupling to the antenna. Among other things it is possible for this to narrow SWR bandwidth or even cause increased SWR or distort the radiation pattern because it makes the transmission line part of the antenna to some degree. Usually, if possible, you want to pull the transmission line off perpendicular for at least a 1/4 wave length. You would even be better off adding some length to the line in order to maintain or improve the balance in the system.

Again, a little study time in the Antenna Book is well worth the time and effort.

73,  K0ZN
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KB3Z
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Posts: 38




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« Reply #4 on: May 10, 2013, 05:06:21 PM »

So if I understand you correctly, I would be better to take the ladder line and the cable that is attached to the ladder line and pulling it straight and in line with the dipole than leading it off to the left and then to the tower where the one side of the G5RV is attached.  Am I understanding you correctly?  Mark KB3Z
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WB6BYU
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« Reply #5 on: May 10, 2013, 05:53:13 PM »

It depends which way you are facing when you say "left".

The optimum case is that the feedline is at right angles to the antenna.  That doesn't
need to mean that it goes straight down:  it could even run horizontally, or at a 45
degree angle to the ground, as long as it is perpendicular to the antenna wire.

When the feedline runs closer to one of the antenna wires than to the other (and
especially if it the feedline runs along one of the wires) then it will contribute to
imbalance in the antenna.  That's not to say it couldn't be used, it just might not
work as expected.
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KB3Z
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Posts: 38




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« Reply #6 on: May 10, 2013, 07:26:09 PM »

Ok, lets get down and dirty.  The one end which is connected to my tower is facing North.  The other end goes South. The Ladder Line is in the center of course and is going slightly downward and going east and the coax cable is tied to a standoff and then goes back to the tower which then goes into the house. Hopefully you can picture what I am describing.  if not I will have to take a picture of it and email it to you gentlement.  Mark KB3Z
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K0ZN
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Posts: 1542




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« Reply #7 on: May 10, 2013, 10:04:00 PM »

To restate: the best situation is to have the transmission line (which means both the ladderline & coax) come away from the wire like the vertical part of "T".   i.e.  a 90 deg. angle between each side of the wire radiator and the ladderline/coax. The transmission line does not have to drop vertically; it just needs to be perpendicular to the wire radiator.  On my 135 ft. Center Fed Zepp's/Doublet, the ladderline comes off the antenna at an angle that is at 45 deg. to the ground, but it comes off dead perpendicular to the antenna wire for what mounts to a 1/4 wave length on 80 M (~ 67 ft.) then it starts a gentle bend towards the Shack.

The optimum situation with any Balanced antenna is to keep the antenna physically balanced as it relates to height of each side above ground, nearby conductors and objects, etc. AND keep the transmission line equally balanced with respect to either side of the antenna. The benefits, at least in theory are: reduced/eliminated power radiation from the transmission line and, in theory, less pick up of noise and unwanted signal by the transmission line. This is especially true with balanced parallel lines. The antenna pattern conforms more to theoretical and there are fewer issues with RF in the shack. This is true for both coax fed or balanced lines. Now!... All that said,
This is a HOBBY and we have to make do with the supports and real estate we have; we can't always do what is electrically "perfect", but we will likely get the best results by at least doing what we can with our antennas to work toward that direction.

Again, the world does not end if your ladderline or coax is not perpendicular, but it is definitely not the optimal configuration.

73,  K0ZN
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