Chasing RFI

  • 1
  • Problem
  • Updated 10 months ago
Chasing RFI on my station 6400M + amplifier
I have been mitigating RFI with toroids on all station cables and measured common mode currents with a RFI meter and now most of it is down to less then 20 mA when running the amplifier at CW 1kW. However on the ground lead I have still about 300 mA RF current. The antenna is a Cushcraft R8 multiband vertical and SWR and with the tuner activated the amplifier is seeing 1:1.3. I am interested in any feedback regarding ground lead RF current level in this setup as I have no idea what is reasonable.
Erik Sm0jca
Photo of Erik SM0JCA

Erik SM0JCA

  • 17 Posts
  • 5 Reply Likes

Posted 10 months ago

  • 1
Photo of Roger,  W6VZV

Roger, W6VZV

  • 273 Posts
  • 129 Reply Likes
I cured all of my RFI problems by going to a lan cable rather than wireless.
Photo of Neal Pollack, N6YFM

Neal Pollack, N6YFM

  • 287 Posts
  • 141 Reply Likes
But adding a LAN ethernet cable is not going to actually solve the base problem;  He has 300 mA of RF common mode current on his cable, and that is NOT a good thing.

First steps should be to verify at the antenna the connection to the radial mesh, or the counterpoise, make sure it is good and has not become corroded or loose, to determine why it's using the coax cable as a return.   

Second question is;  Is this 1000 watts on 40 meters, with the antenna physically close to the shack?  What frequency is the RFI?  How close (in feet) is the antenna
to your shack?

Third question is;   Do you have a proper common mode choke at the antenna end of your coax?  Proper as in not a simple torroid or snap on core, but a full size higher resistance choke such as this for example;
https://myantennas.com/wp/product/cmc-130-3k/

Fourth question: and is the shack end of your coax meeting a ground BEFORE hitting your rig?   Such as using a lightning arrestor mounted on a ground bar that has a ground rod, at the entrance to your shack?

Fifth question:  When you say 300 mA of current on "the ground lead", what are
you referring to?  The coax shield?   The ground wire connecting your flex chassis to your station ground bus-bar?   The third wire ground pin of your power supply?

Answers to the above questions will go a LONG way toward identifying the problem
source.

73

Neal
(Edited)
Photo of Jim Gilliam

Jim Gilliam

  • 960 Posts
  • 233 Reply Likes
Resonant elevated radials are wonderful for decoupling the RF at the feed point of the transmission line. Otherwise, it can be difficult to obtain a good ground unless you bury many radials fanned out. I have obtained adequate decoupling in many instances by using one elevated resonant (cut to 1/4 wave) radial.


Jim, K6QE
(Edited)
Photo of Gayle Lawson

Gayle Lawson

  • 121 Posts
  • 43 Reply Likes
Neil:

Great questions, especially number 2 and 3.  The common mode choke on the Vertical is very important. You might also ask what power level the  interference starts to occur and if any one cable  toroid  reduced the problem more than others.  It might be helpful to know if the shack in on the first floor of a house or the 10th floor of a condo.

What is being interfered with, TX audio (Mic), RX audio (powered speakers), Computer etc. .  My worst problem was on 40,  RF was getting into t he computer HDMI cables, the threshold was about 130 watts.     

Other:

1) Station grounding-- Are the grounds for the Flex and Amp separate wires going to the earth point or are the two units tied together and grounded with a common wire.  Flex has a recommended grounding configuration on its site.  

2) Common mode choke on the coax between the 6400 and the AMP?  This can also help.

It's all magic

Gayle K0FLY
Photo of Erik SM0JCA

Erik SM0JCA

  • 17 Posts
  • 5 Reply Likes
Thank you for the answer. 
I must apologize for a calculation error. The measurement was in mW not mA.

The correct current would be around 80 mA not 300 mA.
I need to clarify that the 80mA RF current is on the ground lead, a thick copper braid from my main ground plate in the shack to the ground rod outside the building, lead lenght is about 2 meters.

Common mode current on incoming coax and all other cables is between 0-20 mA at 1kW power. I have grounded surge arrestors on the coax cables and a choke directly at the amplifier output. On my LAN cable I have double 240 toroids #31 material with 5-6 windings on each, no problems. Power supply to the transciever and amplifier is via a UPS with good mains filtering. On the power cables to both of them I have big toroids.

The Cushcraft R8 vertical is about 15 feet beside the shack. On an OCF Windom over my house the current is about 100 mA on the 20 m band and 30 mA on 40m. On 80, 15 and 10 m bands on both antennas the instrument needle is not moving.

I think that I have mostley cured the common mode RFI to the radio system and what remains to evaluate is the current on the braid to the ground rod outside the shack. 

73
Erik


Photo of Neal Pollack, N6YFM

Neal Pollack, N6YFM

  • 291 Posts
  • 145 Reply Likes
Eric, you mention that the antenna is only 15 to 20 feet from your radio operating point, and you were running a kilowatt. This puts you and the radios in the extreme near field. This will also explain the high rfi common mode currents, since that much RF that close will be coupling into all metal objects at a kilowatt.

Is there no option to put more distance between the antenna and the equipment, if you are going to run high power?

I know that the RF safety calculator thinks you are just about maybe safe if you switched up to the 15 meter band, but personally, I would want a little more safety margin at a kilowatt. It would be good for the RFI and also good for you.

Finally, is there a specific RFI problem occurring, or are you simply trying to reduce the 80mA on the ground cable?



Cheers

Neal
(Edited)
Photo of WA2SQQ

WA2SQQ

  • 534 Posts
  • 134 Reply Likes
With the antenna being only 15 ft away from the shack, you are in a fairly strong RF field. I'm not sure any amount of filtering would be capable to totally eliminating the possibility of RF interference.

OCF antennas do require a very good RF choke to keep RF off the coax and out of your shack.

I would be very interested in learning how you are measuring the current, using what instrument? I personally do not have a problem, but I would like to learn more if I ever have the problem.
Photo of Erik SM0JCA

Erik SM0JCA

  • 17 Posts
  • 5 Reply Likes
Very good questions from you all, I really appreciate your help.
Some more details:

- Amplifier is tripping after 2-5 minutes operation for no apparent reason over 300 W output on digital modes (i.e. FT8 or RTTY). This is my problem.

- No RFI problems on my signal or modulation.
- Phone and CW output up to 1kW are fine.
- Coax to the amplifier has big choke from RadioWorks rated at 1.5 kW, similar to the one mentioned above.
- All equipent in the shack is grounded to a common point on a ground bus in the shack, from there a thick braid goes outside to an antenna tuner with integrated antenna switch and Array Solutions lightning arrestors on a copper plate in a steel cabinet. The Ground cable goes from that copper plate to a single ground rod.
The coax is thus meeting ground before entering the house
- My shack is on the ground floor of a 100 year old wooden house.

- When writing this down I realize that I probably need to improve the ground rod system. However I am still curious abot the current level on the ground rod braid.

73 de Erik

Photo of Erik SM0JCA

Erik SM0JCA

  • 17 Posts
  • 5 Reply Likes
The instrument for measuring RF current is a DIY mikro Amp meter with a coil in a ferrite clamp and a germanium diode plus a couple of resistor and a capacitor.
I clamp the ferrite around the cable to be measured. There are several designs on nthe internet, look for RFI metering.

MFJ-805
73
Erik

Photo of Neal Pollack, N6YFM

Neal Pollack, N6YFM

  • 291 Posts
  • 145 Reply Likes
MFJ, in their usual "brilliance", has named the  above product wrong! :-)
That is really an RF current meter, and not an RFI detector.  RFI is not measured
as an RF current.

While common mode current on a coax shield may result in RFI, a lot of times it
does not.  And, you can measure zero common mode current on the coax shield,
and still cause RFI for a neighbor, due to power level, proximity, and the quality (sic) of the neighbor's consumer electronics.  There is no direct correlation between the two.

[A more recent version of the above MFJ product has been re-labeled as
an RF current meter.]

Neal
(Edited)
Photo of Erik SM0JCA

Erik SM0JCA

  • 17 Posts
  • 5 Reply Likes
I have an ongoing project to move the vertical antenna about 50 feet further away from the shack as I have realized the situation with the near field radiation. Thank you for heads up.
73 de Erik

Photo of Neal Pollack, N6YFM

Neal Pollack, N6YFM

  • 295 Posts
  • 150 Reply Likes
Excellent choice!
Photo of Gene Duprey

Gene Duprey

  • 130 Posts
  • 19 Reply Likes
Did you say you have braid going to the outside ground rod?  If so, rip it out and use a solid copper wire.  Braid weathers and becomes a huge noise generator.  Also braid has much higher resistance than solid wire.

Gene, K1GD
Photo of Neal Pollack, N6YFM

Neal Pollack, N6YFM

  • 295 Posts
  • 150 Reply Likes
We don't care about DC resistance, this is RF.   We care about RF impedance of the ground.   Remember, at RF frequencies, current in a conductor experiences significant skin effect (most of the current will be very close to the surface, and not
in the center of a conductor).   Braid has much more surface area than a solid conductor.   But if you use a half inch diameter solid copper wire,  say like;
AWG Gauge 000, or 0000, then you will have a lot of surface area, but you will
also need a pipe bender to work with it :-)

Braid is very popular, and sold at all the ham radio shops, for indoor station use
in grounding and bonding.   Solid copper flat strap is very popular for the short outdoor runs to your ground rod.   If you can't buy solid copper strap for the outdoor
run, the braid will work but does eventually corrode worse.  You can also use
a 3 to 5 foot run of 000 or 0000 gauge multi-conductor copper cable, which usually has several 12 gague copper strands, but again, it is tough like iron to bend.

Neal

Photo of Gene Duprey

Gene Duprey

  • 130 Posts
  • 19 Reply Likes
Yes we are talking about ref no dc, however braid is still poor as it weathers and becomes a noise generator. Copper wire, stranded or otherwise and copper strap is far superior. Yes braid is popular, unfortunately promoted back in the early days.
Photo of Erik SM0JCA

Erik SM0JCA

  • 17 Posts
  • 5 Reply Likes
@Gene
Good idea, the braid goes fthrough the wall to the outside metal cabinet, from there it is a copper wire to the ground rod, however not solid. I have also got 4 inch copper band that is intended to be used to the ground rod. My shack is under renewal and a lot of rebuilding is underway.

73 de Erik

Photo of Gene Duprey

Gene Duprey

  • 130 Posts
  • 19 Reply Likes
Ground wire does not have to be solid, but most use solid. Stranded of appropriate size will work.
Photo of Bobby

Bobby

  • 10 Posts
  • 6 Reply Likes
Try an mfj-915 isolator inline ... that should do it.
Photo of FISHULA X

FISHULA X

  • 111 Posts
  • 31 Reply Likes
 I must ad to the above comment. I have an in line line isolator. I always had all kinds of RF when I transmitted. I came through my speakers and I even messed up the image on my monitor. And it got much worse when running my Amp. Here is some helpful info. I tried everything from grounding to snap on beads on everything. This is what I stumbled on by accident and may fix your RF issues in your shack. I took the line isolator in my hands just as if I was water skying. I held it with both hands with the coax attached to the left and right.  Then I took it and like I say I held it like a water ski handle and I twisted it a few times and tucked it away. I keyed my radio and talked and noticed  it got much better. I then pulled it back out and twisted it one more time. No more RF, None, completely gone. even with the amp on. Now ever since I learned that trick it has always worked 100 percent for me. It was an accident and it worked. Hope this helps you. Try it, 
  
Photo of Reggie

Reggie

  • 97 Posts
  • 8 Reply Likes
When you twisted the line isolator, did the coax on both ends of the line isolator twist together?

Reggie 
Photo of FISHULA X

FISHULA X

  • 111 Posts
  • 31 Reply Likes
  Reggie, Yes they did. And I still use this method today.
(Edited)
Photo of K9SO

K9SO

  • 128 Posts
  • 31 Reply Likes
Erik,

You mentioned that you are using a Cushcraft R8 antenna. Be aware that there is a coax choke inside the black matching box of that antenna. My experience with that antenna is that the core that choke is wound on overheats and cracks when running a KW on CW. This has happened to me on multiple R8 antennas. 

This is especially true in cold climates. Starting up your station in cold weather, that core heats up very quickly and that causes it to crack. You will not notice this in antenna  VSWR, it will only manifest itself as "RF in the shack". 

That "no radial" antenna depends on a good internal common mode isolator in order to work. I would suggest opening up the match box and inspecting that core. If broken, repair it and add another good choke just outside the matching box too.

73,

Fred 
K9SO


(Edited)
Photo of Ray Sylvester

Ray Sylvester

  • 12 Posts
  • 2 Reply Likes
try a simple Collins style choke on both feed lines at the shack end of both feed lines 8 turns of rg8u 10 inch in diameter .  
Photo of Ray Sylvester

Ray Sylvester

  • 12 Posts
  • 2 Reply Likes
w8ji web site on station grounding for lightning protection is a must read.    
Photo of Neal Pollack, N6YFM

Neal Pollack, N6YFM

  • 297 Posts
  • 151 Reply Likes
While chasing RFI is indeed one aspect of sport, most of us prefer to chase QSO's.   After setting up and installing several different types of antenna's, I have learned that which of the above sports we spend most of our time with, will depend on the choices we make while purchasing antenna's, how close to the radio position and house we place the antenna (vs. how much power we transmit), along with the design of our grounding and bonding system, the quality and types of chokes used (if needed at all, due to the above choices) etc.

As Ray points out, if we have made compromise choices on antenna system (like some of mine also), then W8JI has great reading material, as do other sites that google will provide.

In the end, an understanding of proper pre-entry grounding, equipment bonding & grounding,
quality common mode choke models with high resistance, and proper antenna installation,
will result in undetectable levels of RF in the shack (assuming we have also studied the near field,
and don't expect to place a kilowatt antenna within 20 feet of the radio while running on 40m :-)  )

[DISCLAIMER:  If you are situated on a moutain-top shared commercial transmitter site
with commercial radio and TV stations, you WILL have detectable RF in the shack despite
all of the above design accomodations.   There is "near field" and then there is "EXTREME near
field"  :-)   ]

Cheers,

Neal
(Edited)
Photo of K9SO

K9SO

  • 128 Posts
  • 31 Reply Likes
Everything you guys are saying is true and excellent advice but as I said above, I'll bet he has a broken antenna choke inside the R8 matchbox. All 3 of my R8's went into the scrap heap because of this problem. 

K9SO
Photo of Stan - VA7NF

Stan - VA7NF

  • 482 Posts
  • 120 Reply Likes
Generally I do not trust MFJ to supply a quality tested (and corrected) product.
If the action of a boxed filter changes by twisting then there is an unsoldered joint inside.
Past history - two (out of two) MFJ power line filters required soldering the interior components.  Open it up for your own quality inspection.
Re grounding.  Not mentioned is copper pipe.  A length of 2" copper pipe has 6.8" of ground touching surface width, or 10" for 3" diameter pipe and it will safely carry your coax and rotator lines.  My choice here is copper pipe buried several feet deep for 6' from your rf ground area, joined to plastic pipe, then another 6' of copper connected to your tower lightening ground.  Use 3" strap (6" surface area using both sides) between grounding 3/4 pipes 4' deep at the rf ground and 8' deep at the lightening ground. 
Note 1: Standard 5/8" ground rods have just over 2" of grounding surface. 
Note 2: Don't use the water drilling method for placing the pipes as it washes the conducting soil away from the pipe 
Note 3: The lightening ground is not bonded to the rf ground except by the coax braid which should have a type 31 torroid and small loop at the tower end after the lightening ground
Note 4: Most electrical codes require bonding your tower and house electrical ground but NOT your rf ground, and don't run unshielded bonding wire near your rf ground area.
Note 5: Your 12V power supplies require building safety grounds but may be isolated from your shack rf grounding system.  If at all possible do not join your shack ground to the safety ground, if necessary there is no restriction on having that safety ground wrapped around a type 75 or 31 torroid at the outlet!  Consider lightening emi as a high power 1Mhz signal
Photo of Erik SM0JCA

Erik SM0JCA

  • 17 Posts
  • 5 Reply Likes
Hello all again, back after some chores, hi :)

Thank you all for good comments and advice.

I have read up on most of the available papers on common mode currents,grounding, lightning protection and what not. The meter I have made is measuring the common mode current or any HF current in a line.

I checked the condition of the toroids, cables and connections in the Cushcraft R8 matching box and all is fine, no damage. The antenna SWR has not changed and is below 1:2 on all operating frequencies, the amplifier matchbox takes it down below 1:1.2

I just tested 1000 W CW carrier for a quick burst on 20 m band into my Cushcraft R8 and measured 300 mW in the ground braid to the ground rod. 
Second test: 500 W - 1 minute CW burst, 150 mW in the ground braid, no change in SWR , no tripping of amplifier, everything is nice and dandy!

Measured all cables going in and out to the transcievr and amplifier, RF current = 0 everywhere, I have plenty of ferrite chokes on them.

However, to be sure I changed the isolation choke on the coax after the amplifier to a similar one I have, a 1,5 kW choke from Radio Works. Put the RF meter on the ground braid and run 1 kW to the antenna.  RF current gone! The culprit must be the old choke.

I will now test the station to see if the digital modes work as they should, fingers crossed.

73 de Erik





Photo of WA2SQQ

WA2SQQ

  • 534 Posts
  • 134 Reply Likes
I was looking for information on the meter that you're using or plans to build one online. Anything you can share would be appreciated. Not sure what to search for regarding plans to build one
Photo of Erik SM0JCA

Erik SM0JCA

  • 17 Posts
  • 5 Reply Likes
There are several building projects for RF current measurement instruments online but the best one I have found is this one:
https://owenduffy.net/module/icm/index.htm

73 de Erik


Photo of WA2SQQ

WA2SQQ

  • 534 Posts
  • 134 Reply Likes
Thanks so much! I really appreciate this info