Multi-band common mode choke

Rick - W5FCX

After fighting with RFI from my transmitter for many weeks, I finally solved it and want to share the solution, in the hopes it will help someone else avoid the angst I have been through.

The problem was RFI from the 100W transmitter that kept tripping GFCI breakers during SSB conversations on different bands, as well as causing the capacitance touch lights on a wall unit to go crazy (that one took an extra effort, as well).

What I found was that to properly choke different bands required a range of choking action.  What worked for me is shown in the attached photo - 3 toroid chokes in series, each with a different number of turns, placed within about 1 foot of the antenna feed point (a 4:1 balun matching 450 ohm ladder line that feeds inverted vee ZS6BKW).

Works great!  So happy I can operate now without turning off the lights and driving the XYL up the walls.

Hope that's useful to someone.

Jim Gilliam

Very good to use materials to give effective loss at various bands. Nice going Rick!

Jim, K6QE

Norm - W7CK

I don't understand.  The ZS6BKW does not use a 4:1 Balun.  All you need is a 1:1 current choke.   I've even run the BKW without any choke at all and have never had any issues.  I'm currently using a ZS6BKW with a 1:1 current balun at full legal limit, without any RFI issues.  Maybe I am misunderstanding what your doing?

Norm - W7CK

Rick - W5FCX

Hi Norm,

Thanks for asking.  I'm using a home-built design for a stealthier inverted-V version of ZS6BKW. I used the Ni4L commercial version of the ZS6BKW as a starting point for this design, using similar components:

- 2 x 48.5' #26 polystealth insulated wire
- WA1FL Ladder-Loc strain relief (connects the wires to ladder line)
- 37.9' 450 ohm ladder line (MFJ)
- W1AU 4:1 LL - Ladder line to coax transition 4:1
- 12' RG8X coax with CMC chokes

The antenna is tuned for 1.2 SWR at 14.2 Mhz and works extremely well.  I found the ladder line had to be a bit shorter than expected and the smaller polystealth wire a bit longer, which I assume is due to a lower velocity factor than the usual larger wire.  These lengths were determined experimentally using an antenna analyzer.

Since my shack is on 2nd floor, my ground strap is about 15' long running to nearby 8' grounding rod, which connects via #6 solid copper ground wire that runs about 58' underground around the house to join with the main house ground connected to the breaker box.

The only issue I have been having (with every antenna, not just this one) is RFI with the very sensitive GFCI breakers on various circuits (including all bedrooms like the one in my shack room), and the triggering of capacitance touch sensor in a cabinet downstairs.  To squelch the capacitance sensor, I installed a 100 mH coil in series with a 2.4K resistor on the 4' wire that leads to the human touch point, but this wasn't enough.  I had to also use a snap-on ferite choke on the A/C feedline coming into the touch sensor.  With both of these choked, the extremely sensitive capacitance touch sensor was neutralized.  This confirmed I had RF on the A/C circuits...

As is typical, the issues were caused by common mode current flowing back up the outer braid of the coax feed line, which has now been suppressed by the chokes. It was especially bad on bands outside 20m, where the tuner had to make up the difference more to match SWR (e.g., 40m, 17m).

Perhaps there was an easier way to resolve this?

Rick

EA4GLI

Have you tried without the ground strap?

Bill -VA3WTB

Check out Howard's work on How To Build a Quiet Station.  I don't think you have an RF ground at all. If it is tied to you electrical box, it is not an RF ground and may even be a ground loop.

https://www.dropbox.com/s/kffp92esffo3zy5/How%20to%20Build%20a%20Quiet%20Station%20V2.pdf

Norm - W7CK

That's basically the same as the antenna I built except that I'm using a 1:1 Current Balun which is the 5kw model from Balun Designs.   I don't quite understand why your using a 4:1.  None of the articles I have read mention the use of a 4:1 but only recommend a 1:1 as a choke.

At the 1:1 choke, you can wrap a clamp and ground wire around the outside of your coax connector.  This might help reduce RFI.

Using a 1:1 choke, I find I do not need a tuner for the lower ends of 6, 10 or 40m.   I also get all of 12, 17, and 20 meter without a tuner.  I do need the tuner for the upper end of 40, and all of 80.

I'm sure the radiation pattern is a bit crazy on 6 meters, but I have had a great time with it on 6.  It consistently out performs all of the other simpler home built antennas I've used on 6.   My rotatable dipole for 6 does occasionally outperform it when the station is in one of the nulls of the ZS6BKW. 

Don't use your house ground as an RF ground.  You will certainly experience RFI issues. I'm sure I will hear a lot of grief over this one, but I do not use any ground in the shack.  The only grounds I have are at the base of the antennas and at the 1:1 choke for my ZS6BKW.   I have absolutely NO grounding on any of the equipment in the shack.  My station is battery powered and is absolutely quiet.

Your use of a 4:1 confuses me.

Norm - W7CK

Rick - W5FCX

I copied the 4:1 balun from the Ni4L design. It's a voltage balun - perhaps that's a problem?

My ground is to an 8' grounding rod 12' below the 2nd floor.  From there, as I said there's also an 84' #6 solid copper wire that runs around the house, which joins the house grounding rod (that goes to the breaker box).  This is supposed to eliminate the grounding loops - no?

Perhaps I should also try grounding the shield side at the balun (ladder line side) to the earth ground, as well.

Will ponder all of this and give it a try... however, for now, I do not seem to have a problem to solve... 

James Whiteway

I would not ground the coax shield. You would be grounding out part of your dipole antenna. Your station ground for your equipment (bolt or nut on the case of your radio) should connect to your ground Rod. The A/C cord from your power supply should have a three prong plug that grounds thru your house wiring. No need (unless there's city code that requires it) to connect the station ground rod to your A/C mains ground. Such a long run as you have may, cause the RFI issues you are having. Disconnect the long wire from the ground rod and move it away from the ground rod and see if that doesn't help. Antennas are fun to experiment with. And simple and less complicated is better. I use a 120' doublet (just a dipole) as an inverted V fed at the center with 450 ohm ladderline(40') 37' outside and the remaining 3 feet inside connected to a balanced tuner. No RFI and tunes 80 to 10 meters easily. I use a ground rod outside for an equipment ( radio and tuner case) ground only. No connection to the A/C mains ground other than that provided by my power supply's power cord. Good luck! James WD5GWY

Stan VA7NF

This stream bothered me all night - The extensive choke SHOULD not have any effect UNLESS there was already significant RF on the outer coax shield.

The answer was your balun had to be a voltage type and that, by design, put the RF on the coax.

Solution:  Use ONLY a current balun, otherwise look up the Caroline Windom and the intentional RF on the coax design.

Tim - W4TME

Please read this article for a comprehensive description of the different types of grounds.
https://helpdesk.flexradio.com/hc/en-us/articles/204779159-Grounding-Systems-in-the-Ham-Shack-Paradi...

Rick - W5FCX

Yes, that's one of the issues, the Voltage Balun instead of Current balun. Should be easily resolved. Thank you very much.

Rick - W5FCX

I see conflicting information about grounding that says the station ground should be cross connected to the house ground to eliminate ground loops. I discussed my configuration with one of our local club elmers who confirmed I have done it right by connecting station ground and house grounding rods. Now it seems that was a waste of time and money?

Tim - W4TME

I recommend reading the link I posted to the article about station grounding.  It isn't about what is connect to what.  It is about using the proper ground for its intended purpose.

The AC electrical safety ground is a poor RF ground at frequencies above 60 Hz.  If you have a good, low impedance RF ground and it is bonded to the AC safety ground, the that is OK, because the high-frequency currents will have a low impedance path to ground (earth) rather that traveling through your house wiring looking for a path to ground.  

And as per the NEC, they should be bonded together.  But, if you have a poor RF ground connected to your AC safety ground, then what you have effectively done is make your AC wiring an antenna.

Most people cannot establish a good low impedance RF ground unless the ground point is within several feet of the radio.  In these cases, not having an electrical ground connection between the radio and a ground is better.  I worked with a customer who had a 2nd-floor shack that had serious problems with RFI.  I told him to disconnect the ground connection, which to him seemed counter-intuitive.  He did as I asked and the problem abated a lot, but because he used an un-balanced antenna, he still had RFI issues.  Then I told him to make a counterpoise for the RF ground (1/4 wavelength "radials").  At that point, his RFI issues were resolved,

Rick - W5FCX

Makes a lot of sense. My station ground is apparently insuffient according to this article, as it's only a single grounding rod instead of four rods connected by copper pipes. Given I'm on 2nd story, I'm stuck with a 15' ground wire run (and copper pipe ain't happening). Will give no ground a try, after replacing the voltage balun with a current balun. Very helpful advice from everyone. Thank you! Rick

EA4GLI

Tim's comments are right on. I was asking about the ground because we create our own problems inadvertently. You are on the right path, and you will not only solve your rfi issues, you will also achieve a better radiating system to boot. It is a long journey but getting there is half the fun. Good luck Rick.