Where to place feedline choke on end fed antenna?

MTScannerNut

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I just purchased an end fed antenna from Palomar this weekend for receiving only for my Icom R8600. It is one of their "Bullet" designs with the 9:1 unun and 135' of wire. I have done a lot of reading on common mode currents and how to minimize it. I'm still confused since there is so much conflicting information and advice.

My plan is to put a coax choke in my shack just before the receiver to help with noise generated in the house. Would it benefit me at all to also put one just after the transformer say 3-5 feet down the coax? I will have about a 40-45 feet of coax feeding the antenna. There seems to be a lot of RFI in my immediate area which I am trying to knock down as much as possible. Some people say not to place a feedline choke on an end-fed antenna since the coax acts as the counterpoise, while others insist it can only help. Again, I'm only receiving so I'm sure that makes a big difference. Thanks, Ed
 

prcguy

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On the 9:1 types the coax can be part of the antenna, so I would put it away from the entry point of the house to catch any RFI riding on the coax from things in the house but far enough from the 9:1 transformer to be useful for reception. If you have 40-45ft of coax then try the choke 20-30ft from the antenna if that puts it away from the house.

I just purchased an end fed antenna from Palomar this weekend for receiving only for my Icom R8600. It is one of their "Bullet" designs with the 9:1 unun and 135' of wire. I have done a lot of reading on common mode currents and how to minimize it. I'm still confused since there is so much conflicting information and advice.

My plan is to put a coax choke in my shack just before the receiver to help with noise generated in the house. Would it benefit me at all to also put one just after the transformer say 3-5 feet down the coax? I will have about a 40-45 feet of coax feeding the antenna. There seems to be a lot of RFI in my immediate area which I am trying to knock down as much as possible. Some people say not to place a feedline choke on an end-fed antenna since the coax acts as the counterpoise, while others insist it can only help. Again, I'm only receiving so I'm sure that makes a big difference. Thanks, Ed
 

MTScannerNut

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On the 9:1 types the coax can be part of the antenna, so I would put it away from the entry point of the house to catch any RFI riding on the coax from things in the house but far enough from the 9:1 transformer to be useful for reception. If you have 40-45ft of coax then try the choke 20-30ft from the antenna if that puts it away from the house.
Thank you for the useful info.

Do you think it is still useful to have a separate choke in the shack just before the receiver? I have about a dozen Mix 31 ferrite cores I could slip over my LMR240 before I attach the PL-259 connector.
 

Hit_Factor

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I don't think common mode currents are significant problem for a receive only antenna.

Typical symptom is signal reports saying you sound like you are talking underwater.

I have them on the feedline at the balun of my OCF 80m and up antenna (ic-7300 transceiver and pw1 amp).

If you have easy enough access, I would try various locations and look for improvement in the signal. Keep a log of conditions and configurations you tried.

Think of pl259 connectors as consumable. If you need to cut one off to put on ferrite do it. If they don't help, cut it off again to remove the choke.
 

Ubbe

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Some people say not to place a feedline choke on an end-fed antenna since the coax acts as the counterpoise, while others insist it can only help.
A quality receiver will be well shielded so probably not necessary to choke at the receiver. The coax will pick up RFI from the house and along its way to the antenna and you would like to choke that before it reaches the antenna, and that will be a good distance from you house. Remember that it is radiowaves with long lengths at shortwave frequencies, so if a small choke are placed on the coax it might be at a place where the radiowave have its minumum voltage and maximum current making it a low impedance and the chokes higher impedance will not be enough to stop the RFI.

You'll have to install enough chokes so it will cover a longer length of the coax to be able to catch the RFI where it hurts it the most. If you use a ring core you can wind the coax several turns around it and there will be a higher chance that you hit the RFI at it's maximum voltage and highest impedance.




/Ubbe
 

MTScannerNut

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I appreciate the advise so far.

Dumb question time...is it possible to install too many ferrite beads on the coax, assuming you are using the correct mix? I bought a bunch during Palomar's sale over the weekend and figured I'd install them all.
 

prcguy

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The ferrite bead over coax type can reach about 20dB isolation at best and wrapping the right number of turns around a ferrite toroid can reach around 30dB isolation.

I don't think common mode currents are significant problem for a receive only antenna.

Typical symptom is signal reports saying you sound like you are talking underwater.

I have them on the feedline at the balun of my OCF 80m and up antenna (ic-7300 transceiver and pw1 amp).

If you have easy enough access, I would try various locations and look for improvement in the signal. Keep a log of conditions and configurations you tried.

Think of pl259 connectors as consumable. If you need to cut one off to put on ferrite do it. If they don't help, cut it off again to remove the choke.
A quality receiver will be well shielded so probably not necessary to choke at the receiver. The coax will pick up RFI from the house and along its way to the antenna and you would like to choke that before it reaches the antenna, and that will be a good distance from you house. Remember that it is radiowaves with long lengths at shortwave frequencies, so if a small choke are placed on the coax it might be at a place where the radiowave have its minumum voltage and maximum current making it a low impedance and the chokes higher impedance will not be enough to stop the RFI.

You'll have to install enough chokes so it will cover a longer length of the coax to be able to catch the RFI where it hurts it the most. If you use a ring core you can wind the coax several turns around it and there will be a higher chance that you hit the RFI at it's maximum voltage and highest impedance.




/Ubbe
 

MTScannerNut

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If I were to attach a single 20-25' wire to the 9:1 transformer would that then effectively replace the coax as the counterpoise? Said wire would run vertically along my roof line roughly perpendicular to the main wire. Or am I better off leaving the coax un-choked to serve as the counterpoise? Or perhaps both the coax and a separate wire to serve as counterpoise would work best?
 

prcguy

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I don't know for sure but I think its not a bad idea. At frequencies where the counterpoise is a 1/4 wavelength or multiples of 1/4 wave it would become the dominant counterpoise. At other frequencies RF currents would divide between the counterpoise and coax shield depending on their resonant lengths. A counterpoise wire will not keep RFI in the shack from riding up the coax and being picked up at the antenna like a good ferrite choke can do.


If I were to attach a single 20-25' wire to the 9:1 transformer would that then effectively replace the coax as the counterpoise? Said wire would run vertically along my roof line roughly perpendicular to the main wire. Or am I better off leaving the coax un-choked to serve as the counterpoise? Or perhaps both the coax and a separate wire to serve as counterpoise would work best?
 

prcguy

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The ferrite beads slid over coax can reach about 20dB isolation maximum and coax wound through a ferrite toroid ring can reach about 30dB isolation. Its hard to beat a $7 FT-240-31 ferrite core for making a broad band HF choke and 9 turns of coax through it makes an effective 3 to 30MHz choke.

A quality receiver will be well shielded so probably not necessary to choke at the receiver. The coax will pick up RFI from the house and along its way to the antenna and you would like to choke that before it reaches the antenna, and that will be a good distance from you house. Remember that it is radiowaves with long lengths at shortwave frequencies, so if a small choke are placed on the coax it might be at a place where the radiowave have its minumum voltage and maximum current making it a low impedance and the chokes higher impedance will not be enough to stop the RFI.

You'll have to install enough chokes so it will cover a longer length of the coax to be able to catch the RFI where it hurts it the most. If you use a ring core you can wind the coax several turns around it and there will be a higher chance that you hit the RFI at it's maximum voltage and highest impedance.




/Ubbe
 

N4GIX

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It can help strip off any RFI riding on the shield of the coax picked up by junk in the shack and reduce the chance of it being received.
Okay, thanks for the information. I've never had any such problem myself, but perhaps I've just been lucky!
 

k8niv

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I'm not a expert, I'm learning as I go, but my friend said he had his CMC Choke a 1/4 wave length down the coax from his transformer and it worked well there for him.........well, I didn't do that because I didn't like the idea of the choke been up in the air and put stress on the PL connectors, this is why I have mine laying on the ground about 50-60ft down the coax, and I have ferrite cores in the shack on the coax as it goes into the radio....so far there's no signs of any RFI in the shack or the house......... the CMC Choke is on antenna 2 now because Frontier ran the coax run for phone /internet over on that side of the house ....

Before I bought this choke I used to have issues with RFI in the shack it affected the desk top computer mouse and speakers as well...but don't have any problems like this now...
 
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