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Author Topic: feedline current choke  (Read 4280 times)
KD5UYT
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Posts: 20




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« on: October 30, 2011, 04:26:48 PM »

How close to the antenna feedpoint connection should a DX Engineering current choke be? I am having tvi and rf in the shack issues and bought the choke to cure the problem. I put it right outside the window I pass my coax thru then it connects to 20 feet of rgu213 going up to the AR10 Cushcraft 1/2 wave vertical. It has helped but not enough, should it be closer to the antenna? The choke says it helps with less than perfect rf grounds and helps keep the rf in the coax. I know the AR10 has no radials so it is known to cause tvi. Someone suggested isolating the antenna from the mast might help also. I may try that with a quick tape job just for testing.
Terry KD5UYT
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G8YMW
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« Reply #1 on: October 30, 2011, 05:03:44 PM »

The choke should be as close to the aerial as poss.
The idea is that RF goes up the coax on the centre conductor and the inner of the braid. What is happening (which the choke is supposed to stop) is RF is coming BACK down the outer of the braid aka the "Common Mode Currents"

As for sorting out the root causes, I'll hand over to people that know more than me.

73 de Tony
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73 details Tony
Sent by WW2 Royal Navy signal lamp
M6GOM
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Posts: 904




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« Reply #2 on: October 30, 2011, 05:07:25 PM »

And don't waste your money on a DX engineering choke. Just buy a large FT290-31 mix ferrite ring and wrap 8 turns of co-ax through it as near to the antenna as you can get and then again where it goes into the shack.

The DX engineering choke is literally a ferrite ring with cable wrapped round it put in a nice box so they can charge you more.
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WB6BYU
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« Reply #3 on: October 30, 2011, 05:51:49 PM »

Quote from: KD5UYT
How close to the antenna feedpoint connection should a DX Engineering current choke be?

As close to the feedpoint as possible.



Quote
...I know the AR10 has no radials so it is known to cause tvi...

There is no direct relationship between those two conditions.

Most modern TVI is caused by the radiated signal simply being stronger than the
TV set can handle.  Often this is exacerbated by the outside of the TV coax
acting like an antenna, or poor shielding on the TV coax that allows the RF
signal to get in to what should be a closed system.

If the antenna coax shares a common shield connection with the TV coax, then
common mode currents could be a problem, but that usually isn't the case.
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KD5UYT
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« Reply #4 on: October 31, 2011, 05:14:59 AM »

Thanks for the info guys, do you think insulating the antenna from the mast would help anything?
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SWMAN
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Posts: 563




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« Reply #5 on: October 31, 2011, 06:03:22 AM »

 Terry,
I heard mentioned on eham here several times in the past on other forums that the best place for a choke on the coax is one quarter wavelenght down or away from the feedpoint. I am not really shure why though but do remember reading that. Maybe someone could explane why.
73 Jim W5JJG
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WA3SKN
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« Reply #6 on: October 31, 2011, 11:41:55 AM »

The problem is almost certainly "fundamantal overload".
This will not be fixed with a ferrite on your coax.
Now why don't you describe the TV and radio shack installation a little better and what transmit and receive frequencies are involved... we might be able to help.

-Mike.
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G3TXQ
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« Reply #7 on: October 31, 2011, 12:29:21 PM »

Terry,
I heard mentioned on eham here several times in the past on other forums that the best place for a choke on the coax is one quarter wavelenght down or away from the feedpoint. I am not really shure why though but do remember reading that. Maybe someone could explane why.
73 Jim W5JJG
Putting the choke a quarter-wavelength back from the feedpoint doesn't make any sense - it introduces a high impedance to common-mode current at that point, and that translates to a low CM impedance at the feedpoint. In other words it encourages conducted common-mode current to flow!

Steve G3TXQ
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SWMAN
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« Reply #8 on: October 31, 2011, 01:11:05 PM »

Steve,
I did not really understand either about putting the ferrites one quarter wavelenght away. A man at Polomar Engineering told me that and said that was the best location.Also read about it here on eham. 73 Jim W5JJG
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WB6BYU
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Posts: 13243




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« Reply #9 on: October 31, 2011, 01:15:16 PM »

Quote from: SWMAN
...I heard mentioned on eham here several times in the past on other forums that the best place for a choke on the coax is one quarter wavelenght down or away from the feedpoint...


Just goes to show that you can't trust everything you hear on the internet!

Actually, this isn't a bad location for a SECOND choke if you need one.

With the choke 1/4 the way down the feedline, you have effectively a 1/4 wave radial
wire connected to the feedpoint.  That's about the best you can do to encourage
current to flow in it instead of the antenna.  If you are using an antenna with a poor
ground anyway (such as a ground plane with inadequate radials, or and end-fed half
wave, etc.) then the added radial may improve the SWR due to increased
currents on the feedline.

Since chokes aren't perfect, if the objective is to minimize currents on the feedline,
it is best not to encourage them in the first place.  That's why the choke goes as
close to the antenna is possible.

A choke at a high impedance point in a feedline isn't as effective as one at a low
impedance point.  Putting a second choke 1/4 wave after the first means that only
one of them will be at a high current point, and the other second will attenuate
any currents that make it past the first one.  But if you have that much common
mode current that you need two chokes, you probably need to rethink your
antenna system.
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VA2FSQ
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Posts: 511




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« Reply #10 on: October 31, 2011, 02:05:02 PM »

Here's a pretty comprehensive article on common mode chokes.  I have two on each feedline.  One at the antenna and the other at the radio.  I have no rfi in my house...(the neighbours does though...)

http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf

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VA2FSQ
KD5UYT
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Posts: 20




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« Reply #11 on: October 31, 2011, 07:55:09 PM »

That is a really good article about chokes. Someone asked about my shack set up so here is what I have. I only use the 10 meter phone portion of the band since I only have a tech ticket, 28.300 thru 28.500. I use a Radio Shack htx-100 ssb radio and the AR10. The antenna is pretty close to my radio room, maybe 10 feet away and only 10 feet off the ground. I also have Uverse TV service and my coax passes pretty close to its coax. I have rg213u running up to the AR10. It is a pretty modest set up but I have recently worked all over the country with the band opened up with just 25 watts. The mast is attached to a chain link fence post and grounded at the base with a ground rod. Someone mentioned getting the antenna away from the fence post but I am very limited to options due to trees and power lines. I may try to insulate the mast from the fence and some one else said to isolate the antenna from the mast.  I know it is not the greatest set up but like I said I have limited options. I did add another line isolator I had from Buxxcom at the radio output and it greatly reduced the tvi. I think this antenna uses the mast and coax as a counterpose maybe.
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K4RVN
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Posts: 771




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« Reply #12 on: October 31, 2011, 10:10:37 PM »

Terry,
Here is a cheap way to cure your problem.
http://www.hamuniverse.com/balun.html.  This one will work and is easy to make if you can solder. Place it right at the antenna connection period.

Before you give up on your store bought current balun, Connect it right at the antenna with the shortest jumper you can
buy, make, or find from a friend. Then run your feedline in a horizontal plane away from the antenna not vertical. I would put 25 ft more of coax on the feed line if necessary to get a horizontal run away from the antenna. You can coil it if if is too long. Put some ferrite beads on your TV power cord. Radio shack may have these. Your antenna may be causing the tvi by being so close that the power line is affected by the RF.
I use a power line rf filter to power all my radios which I bought from MFJ or Ameritron. Also make sure your radio is grounded well.
I had the RF problem in my shack but finally found a solution. I do have a current balun on my half square antenna for 40 meters. It stopped my feed line radiation when I ran the feedline horizontal for about 25 ft. away from the vertical.
I have a field strength meter in the shack above my amp now which does not even register any RF in the shack.
I run up to 900 watts no problems now. Good luck

Frank
« Last Edit: October 31, 2011, 10:13:54 PM by K4RVN » Logged
G3TXQ
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Posts: 1515




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« Reply #13 on: November 01, 2011, 01:39:10 AM »

Terry,
Here is a cheap way to cure your problem.
http://www.hamuniverse.com/balun.html.  This one will work and is easy to make if you can solder. Place it right at the antenna connection period.

All air-cored chokes tend to be high-Q and narrow-band. I'm not sure which bands the OP wants to operate, but be aware that the HamUniverse design has very low choking impedance on frequencies above 20m. It manages >2kOhms impedance only on 30m and 20m.

Steve G3TXQ
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W8JI
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Posts: 9296


WWW

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« Reply #14 on: November 01, 2011, 03:25:43 AM »

Terry,
Here is a cheap way to cure your problem.
http://www.hamuniverse.com/balun.html.  This one will work and is easy to make if you can solder. Place it right at the antenna connection period.

Be careful of things you read on HamUniverse. :-)
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