Confirm Antenna Choke Understanding on HF Receive Antennas

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radioetc

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I have been experimenting with receive antennas for shortwave and amateur radio and want to confirm my understanding of common mode interference and what chokes actually do and do not do. And this question is in a receive only context, not transmit.

Suppose one is dealing with a choke where the coaxial feed line is wrapped around a toroid. And let’s suppose that the toroid mix, the toroid size, and the number of turns of coax on that toroid is appropriate for the frequencies of interest and is of a sufficient degree of impedance.

So the I part of all this I think I understand is if there is common mode on the shield of the coax, this can both act as an antenna screwing up the radiation pattern of the actual antenna and also bring in noise and interference into to the radio so long as that noise and interference is not directly reaching the antenna. So putting a choke at the radio helps with this.

The part I am not sure about is when a choke is used at the antenna feed point and is the part I want to confirm.

Would it be fair to say that if local interference is directly received by the antenna itself, no choke can remove that? And that the reason for this is that if the noise is directly received by the antenna then it is differential in nature and not common mode?

Would it also be fair to say (given the above) that the reason one would use a choke at the antenna feed point is to prevent common mode noise on the outer shield of the coax from going upstream onto the antenna itself and then once there, becoming differential mode and then going back downstream inside the coax to the radio?

Any comments about whether this is right or wrong would be appreciated.

Thanks!
 

prcguy

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Yes, knowing the type of antenna would be useful here. What a common mode choke does is isolate the feedline on one side of the choke from the feedline on the other side of the choke. It will absorb to some extent, common mode RF currents flowing on the outside shield of the coax where the signals inside the coax that are instantaneously flowing equal and opposite directions will not affected by the choke.

A simple choke with ferrite beads slid over the coax will peak out at about 20dB of isolation at best then degrade to 10dB or less at 3 and 30MHz. At 1.8MHz it may only give 5dB of isolation. A single ferrite toroid of the right mix with the right amount of coax around it can peak out about 30dB of isolation and give much better isolation at the upper and lower limits of the HF band. A complex multi section choke like the MyAntennas CMC-130-3K can peak out at over 40dB isolation and have more at the band edges than some do at their most effective frequency.

I like to place common mode chokes close to the antenna whenever possible. This will isolate the antenna from the feedline and sometimes improve the radiation pattern of the antenna where the coax was influencing that. It will also strip off some or hopefully all RFI induced onto the coax from items in the house where the coax might pass close to computer cables, switching power supplies, monitors, etc. RFI traveling up the coax shield can make it to the antenna and be picked up by the antenna and go back down the coax to show up on your receiver where the coax shield would otherwise be sufficient to block direct radiation into the center conductor of the coax from the same RFI.

One of my personal experiences with common mode chokes is when I upgraded a home brew choke at the ladder line/coax junction of a G5RV style antenna. I installed a very effective choke from MyAntennas and immediately noticed on my spectrum scope that the noise floor in general and many birdie spikes were reduced. I already had a choke and just going to a better one made a very noticeable difference. Had I gone from no choke to the MyAntennas choke in one step it might have knocked me unconscious.

I later added a second choke near the radio at the point where the coax exits the radio area through a wall and it made a further but slight improvement in some noise humps and birdies on my spectrum scope. I really think the best place for the choke is at the antenna but every installation will be different and will see different results.
 

ridgescan

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I bought a CMC choke and my personal experience is it did nothing to improve noise. I tried it at both the antenna end and receiver end to no avail which tells me my antenna is picking up all the noise here. I think the main offender is an older plasma TV my neighbors have. They lived below me with that thing, then when the building owners did construction on this building, they moved to the top floor unit opposite my side, which is where the far end of my 100' wire antenna ends. I can't be rid of them!
 

wcsd45

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On end fed wire here: First 15 bead choke about 10 feet away from feedpoint to serve as counterpoise of end fed. Coax shield grounded thru house entry box to ground rod directly below. Second choke FT240-31 toroid wound with 15 turns RG400 at rig entry. If you have noise, be not shy with wall wart power cords and USB cables, more turns thru choke better. If snap-on ferrites, get the bigger sizes since the larger size enables more turns thru ferrite. Here, it’s Tx/Rx antenna.

Good luck
 
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radioetc

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What type of antenna are you using?
In terms of antennas, I am messing around with different types, loops and dipoles so far. Though I am puzzled as to why the antenna would matter. Remember, I am trying to confirm a theoretical understanding vs. any practical real world application advice (though I am open to the latter but want to nail the minutia of the theory down first).
 

popnokick

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Antennas matter. A LOT! For a theoretical (and practical) understanding of common mode interference, Common Mode Chokes, and how / why they can increase the signals you want to hear, read this -
About CMC
 

radioetc

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Antennas matter. A LOT! For a theoretical (and practical) understanding of common mode interference, Common Mode Chokes, and how / why they can increase the signals you want to hear, read this -
About CMC
I am totally confused. I do not see how that article in any way addresses the specifics of my original question. Nor does it explain why the antenna type matters relative to my question.
 
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