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Author Topic: RF on my audio with amp on 40m only  (Read 598 times)
VE3TMT
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Posts: 393




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« on: January 24, 2010, 07:23:46 PM »

Setup:

756PRO, AL84 amp, A3S (20-15-10) Alpha Delta DX-B sloper (160-80-40-30)

When I am not running the amp, I get no RF on my audio whatsoever. When I run the amp, I get RF back in my audio on 40m only. Doesn't do it into the dummy load so I suspect the coax going to the sloper is radiating some RF back into the rig on 40m. I can't really change out any of the cables, so I'm looking for other things to try.

Max
VE3TMT
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WB2WIK
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« Reply #1 on: January 27, 2010, 02:58:00 PM »

Try a coaxial RF choke current balun: Wind 10-12 turns of the coax feedline on a 4" diameter form, taped to hold its form, at the bottom end of the feedline and see if that helps.

A ferrite choke system would probably work better but needs to be pretty good to work at 7 MHz.

These are commercially packaged like the model T4 here:

http://radioworks.com/
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VE3TMT
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Posts: 393




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« Reply #2 on: January 27, 2010, 03:30:55 PM »

Steve,

Are you talking about the coax connecting the rig to the amp? or the antenna feedline from the amp?

Max
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W8JI
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« Reply #3 on: January 27, 2010, 09:16:30 PM »

Steve,

Are you talking about the coax connecting the rig to the amp? or the antenna feedline from the amp?

Max

Max,

A sloper excites the mast and everything connected to the mast. It is a terrible antenna for common mode RFI problems.

You can try a feedline isolator, but everything connected to the tower that supports the sloper is part of the radiating system. If you have a "half-sloper" there is at least as much current in the tower or mast as is in the sloper itself. This means the other feedlines, rotor cables, control wires and tower itself all conduct common mode.

It is unlikely a feedline choke would cure the problem, but of course you have nothing to lose and it might help. Install it at the house entrance.

You might consider a different antenna that is less susceptible to RFI problems. Otherwise work on bonding and grounding of all cables and the mast or tower.



Tom

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