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Author Topic: SWR problem with antenna  (Read 1912 times)
KJ6TSX
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Posts: 123




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« on: January 26, 2013, 10:53:13 PM »

I have a vertical Moxon antenna(homebrew) that worked perfectly 1:1 at 10 feet above the ground to bottom of antenna 23 to top , I raised the antenna to twenty feet to bottom and and my SWR changed from 1:1 to 1.3:1 and I started getting RF in the shack. I am assuming that I need a Balun or ferrite beads on the coax. Another oddity is that the SWR direct through my tuner is 1.2:1 when I go through the tuner it goes to 1.3:1 at it's best I cannot tune it to anything better than that.
Assuming snap on ferrite beads are needed where should I put them? at the coax connection of the driven elements or on the coax , below the antenna. The coax travels horizontally from the driven element to the mast then down vertically.

Thanks
George
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WD8KNI
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Posts: 154




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« Reply #1 on: January 27, 2013, 01:28:45 AM »

I have a vertical Moxon antenna(homebrew) that worked perfectly 1:1 at 10 feet above the ground to bottom of antenna 23 to top , I raised the antenna to twenty feet to bottom and and my SWR changed from 1:1 to 1.3:1 and I started getting RF in the shack. I am assuming that I need a Balun or ferrite beads on the coax. Another oddity is that the SWR direct through my tuner is 1.2:1 when I go through the tuner it goes to 1.3:1 at it's best I cannot tune it to anything better than that.
Assuming snap on ferrite beads are needed where should I put them? at the coax connection of the driven elements or on the coax , below the antenna. The coax travels horizontally from the driven element to the mast then down vertically.

Thanks
George

You first comment "worked Perfectly" how did you determine this.  My dummy load works perfectly as a dummy load.  It also has a very low SWR.  SWR is not an indication of a antenna performance.  Also I don't think there is such a thing as a perfect antenna.

Impedance of an antenna is effected by the height above ground.  If you move the antenna height and the impedance didn't change, I would assume you have a dummy load on the other end.  I think this is normal.

RF in the shack tells me you have current on the outside of the coax, a balun is the correct fix for this problem.  With this situation I can guarantee you that your SWR readings are not correct.

Tuner, why would you even have a tuner inline with such low SWR readings?? 

SWR with tuner inline.  I think this is also normal as it depends on the tuner configuration and quality of the internal components.

Fred
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W5DXP
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« Reply #2 on: January 27, 2013, 06:12:04 AM »

The coax travels horizontally from the driven element to the mast then down vertically.

Feeding a Moxon vertical beam can be a real physical challenge. Sounds like you have a (conductive?) mast (to which the coax is attached) sticking up between the driven element and reflector of your antenna. That is an unfortunate configuration for common-mode problems and distorted radiation patterns. For best results, the coax needs to come away from the feedpoint at 90 degrees to the plane of the antenna, a difficult task for vertical Moxons..
« Last Edit: January 27, 2013, 07:04:58 AM by W5DXP » Logged
KJ6TSX
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Posts: 123




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« Reply #3 on: January 27, 2013, 09:06:08 AM »

Quote
You first comment "worked Perfectly" how did you determine this.  My dummy load works perfectly as a dummy load.  It also has a very low SWR.  SWR is not an indication of a antenna performance.  Also I don't think there is such a thing as a perfect antenna.

By perfectly I had a 1:1 SWR a 3 to 5 db gain over the parallel dipole connected to the same radio and tuner and no rf in the shack.

So why did I raise the antenna? the antenna was located between two houses below the roof line, By raising the antenna and adding a rotor I was hoping to improve performance and usability

Quote
Tuner, why would you even have a tuner inline with such low SWR readings?? 

I have multiple antennas which require a tuner.

Quote
Sounds like you have a (conductive?) mast (to which the coax is attached) sticking up between the driven element and reflector of your antenna.

The mast between the driven element and the reflector is PVC. The support below that point is metal, just like before. the coax runs from the connection horizontally to  the plastic mast then down, I will try relocating the coax to see how that affects the rf in the shack

Thanks for all the input
George

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K3VAT
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Posts: 763




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« Reply #4 on: January 27, 2013, 09:18:21 AM »

<snip>
Assuming snap on ferrite beads are needed where should I put them? at the coax connection of the driven elements or on the coax , below the antenna. The coax travels horizontally from the driven element to the mast then down vertically.
Thanks George 

Some kind of mitigation is definitely required if you are getting RF in the shack.
Snap-ons are good and may totally rectify the problem.  Place them at both ends: at the antenna closest to the feedpoint and at the input of the tuner.

Search this Forum for recommendations on good performing beads & mixes.

GL, 73, Rich, K3VAT
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WB6BYU
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Posts: 13580




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« Reply #5 on: January 27, 2013, 09:27:47 AM »

Ideally the coax would run down the boom of the Moxon and exit off behind the
reflector (far enough from it that it doesn't affect the tuning).  Running it down
the mast puts it in the field between the two elements.

I suspect the reason you now have RF in the shack is that you've changed the
coax length and/or how the coax is arranged between the antenna and the
shack, even if nothing else changed at the antenna.  And if the amount of
common mode current changes, that will change the feedpoint impedance of
the antenna (because the coax is part of the antenna in that case.)

If you can't route the feedline off behind the reflector, then you may need
additional decoupling to prevent the feedline from picking up RF as it passes
through the field of the antenna.  An effective balun at the feedpoint plus
snap-on beads every 1/4 wave along the coax until it is away from the antenna
might do the job.
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KJ6TSX
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Posts: 123




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« Reply #6 on: January 27, 2013, 12:51:29 PM »

Quote
I suspect the reason you now have RF in the shack is that you've changed the
coax length and/or how the coax is arranged between the antenna and the
shack,

Rich you are right! I had the coax running horizontally on top top of the Playhouse that supports the antenna, I moved the coax so it went straight vertically to the ground and the RF in the shack was minimized, Not sure why?? Maybe adding snap on may completely remove the RF from the shack??
Any other suggestions or comments would be appreciated

Thanks
George
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N1CX
Member

Posts: 138




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« Reply #7 on: January 27, 2013, 01:41:33 PM »

Moxon needs a choke balun. 8-10 turns of cable say 8" in diameter should do it. Make sure the cable comes off directly opposite where it goes into the coil.
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