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Author Topic: G5RV and feedback / SWR issues  (Read 886 times)
K0JJM
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Posts: 4




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« on: November 22, 2010, 09:42:59 AM »

This weekend for the Sweeps, we used a G5RV antenna strung between three poles.  Center was up 30' and ends up 20'.  The ladder line was strung out at an angle from the center support to a cedar tree with the end of the ladder line only about four feet off the ground.  There was a 40/80 dipole running perpendicular to the G5RV using the same center support pole.  We operated in a trailer with the operating position about 30' feet from the ladder line.  We had definite issues with SWR readings during the entire contest.  However, things really got bad once the wind started blowing.  My radio, IC746, kept shutting down to protect itself from high SWR readings.  This started occurring on almost every other QSO.  I eventually shut down entirely to prevent doing damage to my radio.  We have done this same set up with crossed dipoles previously with no problems. We DO NOT transmit on both at the same time and did not do so with the G5RV this time.  Any suggestions as to what was going on?  I am pretty sure we were getting feed back through the system as I could hear myself in headphones when transmitting.  I know that the ladder line the G5RV is part of the antenna.  Have other people had this problem with the G5RV or is the antenna tuner on my radio going bad?
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K0BG
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« Reply #1 on: November 22, 2010, 09:50:49 AM »

This is sort of a pig in a poke, because there are so many things it could be. Since I wasn't there, I won't venture a guess without a lot more information.
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WX7G
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« Reply #2 on: November 22, 2010, 10:24:32 AM »

What you needed is an antenna tuner.
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KB9CRY
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« Reply #3 on: November 22, 2010, 10:26:02 AM »

This weekend for the Sweeps, we used a G5RV antenna strung between three poles.  Center was up 30' and ends up 20'.  The ladder line was strung out at an angle from the center support to a cedar tree with the end of the ladder line only about four feet off the ground.  There was a 40/80 dipole running perpendicular to the G5RV using the same center support pole.  We operated in a trailer with the operating position about 30' feet from the ladder line.  We had definite issues with SWR readings during the entire contest.  However, things really got bad once the wind started blowing.  My radio, IC746, kept shutting down to protect itself from high SWR readings.  This started occurring on almost every other QSO.  I eventually shut down entirely to prevent doing damage to my radio.  We have done this same set up with crossed dipoles previously with no problems. We DO NOT transmit on both at the same time and did not do so with the G5RV this time.  Any suggestions as to what was going on?  I am pretty sure we were getting feed back through the system as I could hear myself in headphones when transmitting.  I know that the ladder line the G5RV is part of the antenna.  Have other people had this problem with the G5RV or is the antenna tuner on my radio going bad?


Built-in radio antenna tuners are really tiny and aren't worth much excep to do very minimal tweaking of SWR.  You need a real outboard tuner to properly use that antenna.
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WB6BYU
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« Reply #4 on: November 22, 2010, 10:35:36 AM »

Quote from: K0JJM
...I know that the ladder line the G5RV is part of the antenna...

Actually it is part of the matching system - it isn't intended to radiate.

Could you watch the SWR vary on a meter?  If so, did it tend to move around smoothly, or did
it jump up and down?

The impedance seen by the tuner will change if the matching section gets close to metal or
other wires.  So if you could see the SWR varying up and down, sometimes passing beyond
2 : 1 (with the antenna tuned), then it could be the movement of the matching section (or
the wires) as they sway in the wind, etc.  Did you bring the coax for the other antenna down
parallel to the twinlead?  Changing the spacing between them may change the SWR.

If the SWR changes are more sudden, that would tend to indicate an intermittent connection: 
solid conductor wire is prone to fatigue and breaking at joints or bends due to flexing (as is
stranded wire where solder has wicked into the strands.)  Intermittent connections at the
feedpoint and/or the junction between the coax and the twinlead are not uncommon.  It is also
possible to get a break in the wire inside the insulation if it has been kinked, a poor joint
on a balun, shorts to the mast or the other antenna, etc.

The fact that the problem got worse when the wind increased would tend to suggest one of
these scenarios, though there certainly are other possibilities.  You'll have to apply your
knowledge of how the antennas were supported, their history, etc. to judge which is
more likely.  Certainly if the SWR was jumping rather than changing smoothly a close
inspection of the antenna would be in order.


You can test your tuner into a dummy load or your normal base station antenna to see if
it shows any signs of problems, but unless the tuner was arcing or has an intermittent
connection (which certainly is possible) it sounds to more more like a problem with movement
of the antenna and/or feedline, especially if you just tuned the antenna once and didn't
readjust the tuner while you were operating.
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K0JJM
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« Reply #5 on: November 22, 2010, 10:50:36 AM »

The ladder line was running out at about a 45 degree angle from the other feedline which came straight down the support.  I did try retuning regularly (especially after the problem became more frequent) but that didn't seem to help.  The G5RV was brand new.  I will be trying my IC746 out on my dipole at home but it had not had any problems that I noticed before the contest.

I do agree about the statement using the  external tuner.  I was afraid it might be an issue.  Apparently it was.  Sure made for a frustrating contest and I was only three away from a sweep!
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