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Question about ferrite core material and common mode RF Reduction.




 
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Author Topic: Question about ferrite core material and common mode RF Reduction.  (Read 541 times)
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WA2SQQ
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« on: January 28, 2019, 01:02:23 PM »

I’m making two coaxial chokes for a 160m antenna. I found two cores that are suitable, each using a different “mix”. The type 77 states that is has an AL value of 3155. The type “J” states an AL value of 6845. Both have the same suggested frequency operating range. Am I correct that the type J with the higher AL rating should provide more attenuation to common mode RF?
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W1ITT
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« Reply #1 on: January 28, 2019, 01:08:11 PM »

This may have been posted here previously, but if so it bears repeating.  The author just finished a revision in the last few weeks, with more information.  It's the comprehensive work on chokes, ferrites and methods to keep RF where it belongs.

http://www.audiosystemsgroup.com/RFI-Ham.pdf
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WA2SQQ
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« Reply #2 on: January 28, 2019, 03:02:29 PM »

I've read the document, but it does not get into a specific mention of the AL rating. I'm thinking, the higher the AL rating, the higher the attainable inductance for a given number of turns. Just trying to verify this.
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KK4YY
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« Reply #3 on: January 28, 2019, 06:22:50 PM »

Not an answer to your question, but a comment...
I put an FT-240-77 at the feed point of a 75M full wave loop with as many turns of RG-59 around it as I could neatly fit. My purpose was to reduce common-mode noise, coming from within the building, from reaching the antenna. It reduced the noise from the broadcast band up through 75M quite nicely, even better than I had hoped, about -20dB.

Don
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« Reply #4 on: January 29, 2019, 12:56:08 PM »

Oh ya! I live 2 miles rom WABC (770) 50kw. Never realized how much they raise the ambient noise untuil one night when they went off the air. So you had it at the feedpoint of the loop - how about what the feedline picks up?
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KK4YY
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« Reply #5 on: January 29, 2019, 05:56:03 PM »

I live in a multi-unit building. There are noise sources from within that I cannot control. The coax feed-line, routed through the building, was picking up the nearby EMI. Putting the ferrite at the antenna feed point reduced the noise significantly. Imagine the noise coupling to the outside of the coax and traveling up to the antenna feed-point. There, it makes a quick U-turn and travels down the inside of the coax and into my receiver. Placing the choke at the feed-point was like a 'No U-Turn' sign to the EMI. Wink

You're probably experiencing front-end overload from WABC. You might try a tuned notch filter to reduce the level. Or a 'T' type antenna tuner (C-L-C) to act as a high pass filter. I don't think a ferrite will help with that problem. Grin

Don
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« Reply #6 on: January 29, 2019, 06:59:14 PM »

Contrary to what intuition tells us, choke design is not merely a matter of looking at the A-L number and winding on lots of turns for a given inductance.  Interturn capacitance complicates the matter.  I have two two-port network analyzers and have seen choke isolation peak and then turn downhill for a given frequency range when adding too many turns.  It's not a case of more-is-better.  Unless you have access to a network analyzer, it's probably best to go to a source like K9YC and adopt one of his cookbook designs, which he has wound and tested on the bench. G3TXQ (sk) also has some online cookbook designs which he has bench tested.   Or, if you can justify the money, it's a fine opportunity to buy an analyzer.  My Array Solutions VNA-2180 is (relatively) inexpensive, quite capable, and often travels overseas with me in preference to my large HP unit.
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« Reply #7 on: January 30, 2019, 08:29:50 AM »

You're probably experiencing front-end overload from WABC. You might try a tuned notch filter to reduce the level. Or a 'T' type antenna tuner (C-L-C) to act as a high pass filter. I don't think a ferrite will help with that problem. Grin

Don
I have the situation under control. My Flex 6500, by itself does a very respectable job with very minimal degradation on 160, and virtually none on the other bands. To enhance the performance I have one of the ICE broadcast band filter which attenuates WABC @ 770 down to an S3. For weak signal work, such as FT8, I use a magnetic loop, nulling out 770 even further. Last night my noise level on 160 was S2. I use a similar filter that kills everything above 540 khz when listening down to 10 khz (with my Flex or RSP 1A).

Knowing that I enjoyed DX'ing the AM broadcast band, several years ago the CE of WABC tipped me off that they were going dark for a few hours to do maintenance. That's what motivated me to start the aggressive filtering. I logged a station in Bogota Colombia and sent them a reception report. I received a personal letter from their CE telling me that my report was the 1st he received  from the NE US, but had no idea why. I guess he was not aware of WABC!
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« Reply #8 on: January 30, 2019, 09:15:02 PM »

I'm glad you have solved the problem with WABC. And a great story about DXing their frequency!

As a belt-and-suspenders tactic, I had also added a 43 mix ferrite, where the coax enters the building, to the same loop that has the 77 mix at the feedpoint. My thought was to break-up the coax length while covering a different frequency range at the same time. I didn't notice any improvement when I added the 43 mix. I think the 77 mix did all that could be done.

If calculations should fail, a shotgun approach of using different mixes together like that may get it done for you. I had given up trying to figure out ferrites after too much reading and too little understanding.

Solving EMI problems seems to be more like a black magic than a science. Roll Eyes

Don
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