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Author Topic: Distortion on my signal  (Read 4971 times)
N7GCO
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Posts: 144




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« on: September 15, 2012, 08:53:56 PM »

Tonight I was told by a couple of people my signal was badly distorted on 80 meters. That has never happened before, in fact I usually get complements.

I did change antenna's from a 40/80 dipole to a 80 meter dipole. Since nothing else has changed, I assume it is related to the antenna some way.
I have checked all the coax and it tests fine.
My first assumption is I need to add a balun to coax coming from this antenna. I presume I would add it just before the coax enters the shack.

Any suggestions on how to trouble shoot this and get my signal clean again?

Thanks
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WX7G
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« Reply #1 on: September 16, 2012, 10:48:33 AM »

Did you make contacts and receive reports of good signal quality after the antenna change?
« Last Edit: September 16, 2012, 10:50:53 AM by WX7G » Logged
N7GCO
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« Reply #2 on: September 16, 2012, 11:49:00 AM »

No, all reports on the 80 meter dipole said I had distortion.

My reports on 10-40 meters (a StepIR antenna) were all excellent.
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WX7G
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« Reply #3 on: September 17, 2012, 08:21:37 AM »

It sounds like it could be common-mode current on the coax upsetting the radio. Yes a balun at the station would be a first step as that is the easiest spot to add one. A 1:1 ferrite current balun such as the MFJ-915 would be good.
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WB8UHZ
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« Reply #4 on: September 22, 2012, 07:17:11 AM »

The 80 meter antenna is most likely putting out a stronger RF field around your station on that band. If your station is not properly grounded you have RF getting into your station.
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AA4PB
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« Reply #5 on: September 22, 2012, 09:16:33 AM »

There are a couple of ways that RF can get back into your radio.

One is via common mode currents flowing back down the outside of the coax shield to the radio. A balun at the feed point of the dipole may help. Making sure the dipole is properly balanced (i.e. one side not significantly closer to the ground or other conductors than the other or that the two sides are not of equal length). If the feed line is run parallel to the antenna at a close distance the antenna radiation can couple directly into the outside of the shield.

A second is via direct coupling between the antenna and the station wiring. This is most likely to happen if the antenna runs close to the station. Maximum separation is the key here.
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W8JI
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« Reply #6 on: September 23, 2012, 08:48:24 AM »


Any suggestions on how to trouble shoot this and get my signal clean again?

Thanks

http://www.w8ji.com/rfi_rf_grounding.htm

Also be sure you do not have something improperly connected to your audio lines, like a digital interface.

What radio do you have???
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K4DPK
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« Reply #7 on: October 16, 2012, 08:29:50 PM »

For a coax-fed, resonant antenna, use a good quality 1:1 choke balun, and it goes at the antenna feed point, not the bottom of the coax.

Loose coax connectors, improperly assembled connectors, bad solder joints and antenna wire arcing to tree branches can all distort your audio.

Phil C. Sr.
k4dpk
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AD9DX
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Posts: 1484




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« Reply #8 on: November 04, 2012, 05:11:50 AM »

I had to use one of these http://www.balundesigns.com/servlet/the-101/1-cln-1-Isolation-Balun-/Detail when I went QRO on 160m.  I have had really good luck with this company, and they make some of the best baluns for the money. 

On another note, can you describe your grounding system to all of us.  I am wondering if this common mode interference isn't a symptom of a larger problem. 
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EX, KC9TRM, KB9IRZ
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