Pages: [1]   Go Down

Author Topic: Best Choke/Line Isolator to prevent RFI entering the shack  (Read 647 times)

N6RRV

  • Member
  • Posts: 19
Best Choke/Line Isolator to prevent RFI entering the shack
« on: November 06, 2020, 08:58:16 AM »

I have a end fed antenna with 100 feet of coax leading into the house.   i want to keep RFI from entering the shack from the rg8x coax.   Suggestions?    thx
Logged

W9IQ

  • Member
  • Posts: 8866
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #1 on: November 06, 2020, 09:04:05 AM »

On which bands do you operate? Are you looking to make something yourself or buy it?

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N6RRV

  • Member
  • Posts: 19
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #2 on: November 06, 2020, 09:26:03 AM »

i just bought a Nelson Antenna End fed for 10-80 meters.    i want to buy the choke
Logged

W9IQ

  • Member
  • Posts: 8866
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #3 on: November 06, 2020, 10:54:24 AM »

If you are operating 100 watts or less, consider the Balun Designs Isolation Choke with dual SO-239 connectors. They have great customer service and may recommend a different option for your situation.

Mount the choke 10 to 20 feet down the coax from the matching transformer that came with your antenna. The antenna needs to use this much of the coax as part of the radiating antenna.

It is always a good measure to install a second choke on your coax just before it enters the shack. This will also help keep RFI from your house out of your antenna and receiver.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N6RRV

  • Member
  • Posts: 19
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #4 on: November 06, 2020, 11:41:28 AM »

thx Glenn      Is there some rhyme or reason why 20 feet from the transformer would be a good place to put a choke?
Logged

K1FBI

  • Member
  • Posts: 1475
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #5 on: November 06, 2020, 11:55:43 AM »

I am running an EndFedz 20 meter antenna indoors.
I had major RF including getting bit by the mic, distorting my audio when over 25 watts and also getting into my headphones.
A pair of MFJ-915 line isolators cured everything. One is at the rig and the other is 3 feet from the matching transformer. 3 feet because that's the shortest jumper I had on hand.
I can run 100 watts with no problem.

I know, I know..MFJ but this product works and the price is reasonable.
Logged

K1KIM

  • Member
  • Posts: 1556
    • HomeURL
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #6 on: November 06, 2020, 12:36:12 PM »

I have a Hustler 5-BTV ground mounted vertical with traps that has a large ground radial field.

My 1:1 Balun is located right at the antenna with an SO-239 in and out.

After the 100' coax run to the house entry I have a polyphase on a ground rod and after that six 6" loops of coax for my second choke.

Works for me.
Logged
So Many Toys.......So Little Time!

W9IQ

  • Member
  • Posts: 8866
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #7 on: November 06, 2020, 01:11:02 PM »

thx Glenn      Is there some rhyme or reason why 20 feet from the transformer would be a good place to put a choke?

Yes, you need about that length to radiate as part of the antenna. Otherwise performance will suffer and your choke may not function well.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

W6QW

  • Member
  • Posts: 462
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #8 on: November 06, 2020, 02:05:20 PM »

The easy fix is to create a 'voltage node' at the end-fed matching network.  Otherwise, the coax to your shack becomes that counterpoise and the potential for RFI in your shack increases. 

The simple solution is to install a 1/4 wavelength radial for each band you intend to operate on and attach it to the matching network housing.  Not only will you mitigate most of the RFI potential, the antenna will radiate more effectively.
Logged

K6AER

  • Member
  • Posts: 7159
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #9 on: November 06, 2020, 05:43:17 PM »

I have a Hustler 5-BTV ground mounted vertical with traps that has a large ground radial field.

My 1:1 Balun is located right at the antenna with an SO-239 in and out.

After the 100' coax run to the house entry I have a polyphase on a ground rod and after that six 6" loops of coax for my second choke.

Works for me.

That Polyphaser will not do you any good if the Surge comes in via the AC feed to the house which happens 94% of the time.
Logged

N6YWU

  • Posts: 362
    • HomeURL
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #10 on: November 07, 2020, 03:15:55 PM »

Is there some rhyme or reason why 20 feet from the transformer would be a good place to put a choke?
The near-field of an antenna will couple to any nearby conductive objects or wires, including the feedline, if they are not centered in a balanced field.  This happens whether the wire or feed line is connected or not to the antenna (as it does for unconnected Yagi-Uda elements). 

So it's best to put a choke balun past the point where most of the near-field RF is already coupled into the feedline, a bit farther from the antenna.  If you put a choke before the RF is coupled into the feedline shield, near the antenna, it can't attenuate that coupled RF.  Where the RF near field ends depends on the antenna geometry and size.
Logged

N6RRV

  • Member
  • Posts: 19
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #11 on: November 07, 2020, 05:27:02 PM »

i have a 50 end fed....how do i figure out where the "nedar field" ends?
Logged

N6RRV

  • Member
  • Posts: 19
Re: Best Choke/Line Isolator to prevent RFI entering the shack
« Reply #12 on: November 09, 2020, 07:53:48 AM »

i have a 50 end fed....how do i figure out where the "near field" ends?
Logged
Pages: [1]   Go Up